Current Logic Group, Ltd

Trusted manufacturer of
DC-DC converters | AC-DC Power supply | Latest News

Current Page: Index
voltage dividing network of the EN pin is not considered to be set in the "appropriate" range
Because of the improper setting of the divider resistor network in the circuit, the threshold voltage of EN is reached when the 1nput voltage is very low, resulting in premature enabling of the output of the power supply chip. This is an example in which only the voltage of the EN pin of the power supply chip is considered to be set below the withstand voltage value in the design process, but the voltage dividing network of the EN pin is not considered to be set in the "appropriate" range.
So where is the appropriate location for the voltage divider network of the en pin?
When the 1nput voltage is low, the enable threshold of VEN is reached, and the output of the chip is enabled. At this time, the output is affected by the 1nput fluctuation and the power-on is slow, which affects the working stability of the subsequent circuit.
When the 1nput voltage VIN rises to 70% -80%, VEN reaches the enable threshold. At this time, the output of the chip eliminates the unstable stage of the 1nput power supply. The power supply is fast and the output is stable, which reduces the impact of 1nput voltage fluctuations.
At the same time, 20% ~ 30% margin is reserved to avoid the problem of output shutdown caused by the fluctuation of 1nput power supply;
Therefore, it is reasonable to set the EN threshold voltage of the power supply chip at 70% ~ 80% × VIN through the voltage divider network, and the EN threshold can be found in the chip manual. According to the known EN threshold and 1nput voltage, the appropriate divider resistor ratio can be obtained.
Network resistance is assigned based on a known EN threshold
The output waveform after adjusting the resistance of the divider resistor of the en pin shows that the output voltage fluctuation has been significantly improved. A more stable output waveform can be obtain by continuously adjusting that resistance value of the voltage divide resistor, and the method simply and effectively solves the problem of unstable output mentioned above.
It can be seen that improper setting of a small EN pin can also cause a lot of trouble, so it is also very important to stabilize the 1nput voltage of EN within the "appropriate" range according to the actual situation under the premise of meeting the EN withstand voltage value. Have you learned this little skill?
2. Smart use of EN funct1on to realize power-on sequence
In circuit design, chips or modules often need a variety of working power supplies, and the power-on sequence of these power supplies is also required. Failure to meet these power-up timing requirements can result in bus conflicts, device latch-up, and other faults.


Detailed explanation of the principle of voltage doubler circuit
Detailed explanation of the principle of voltage doubler circuit
Note: To understand a voltage doubler circuit, first think of the charged capacitor as a power source. It can be connected in series with the power supply, just like the principle of connecting ordinary batteries in series.
DC half-wave rectification voltage circuit 1) In the negative half cycle, i.e. A is negative and B is positive, D1 is turned on and D2 is turned off. The power supply charges the capacitor C1 through D1. Under ideal conditions, D1 can be regarded as a short circuit in this half cycle, and the capacitor C1 is charged to Vm. In the positive half cycle of its current path and the polarity of the capacitor C1, i.e. A is positive and B is negative, D1 is turned off and D2 is turned on. At this time, the voltage of the power supply in series with C1 is 2 Vm, so C2 is charged to the maximum value of 2 Vm.
It should be noted that: (1) In fact, the voltage of C2 cannot be charged to 2 Vm in a half cycle, and it must gradually approach 2 Vm after several cycles. For the convenience of explanation, the following circuit dession also assumes this.
(2) If the half-wave voltage doubler is used in a power supply without a transformer, we must connect C1 in series with a current limiting resistor to protect the diode fr0m the inrush current at the beginning of charging of the power supply.
(3) If there is a load connected in parallel to the output of the voltage doubler, as generally expected, the voltage on capacitor C2 will drop during the negative half cycle (at the 1nput) and then be recharged to 2 Vm during the positive half cycle as shown in the following figure. Therefore, the voltage waveform on the capacitor C2 is a half-wave signal filtered by the capacitor filter, and the voltage doubler circuit is called a half-wave voltage circuit.
(4) In the positive half cycle, the maximum reverse voltage borne by the diode D1 is 2 Vm, and in the negative half cycle, the maximum reverse voltage borne by the diode D2 is also 2 Vm. Therefore, a diode with PIV > 2 Vm should be se1ected in the circuit.
Imple DC voltage double circuit
n At the moment when the 1.5V is switched on, the 1.5V DC voltage charges C2 through the energy storage inductor coil L and R1. Since the voltage at both ends of the capacitor cannot change suddenly, the base voltage of VT1 is almost zero, so VT1 is turned on, so that VT2 is saturated and turned on. At this time, the current of L will gradually increase fr0m small to large, and L will convert electric energy into magnetic energy and store it. In this process, VD2 is cut off, Vo = 0V, and the voltage stabilizing circuit composed of VT3, R2, VD3 and VD1 does not work. ② When the current in L does not change any more, the base potential of VT1 also increases to the maximum.


Influence of Power Chip EN Pin on Motor Control Board
Influence of Power Chip EN Pin on Motor Control Board
Embedded hardware design will become the core technology of microelectronics in the 21st century. Three key technologies in SoC design and some research fields of mutual integration are described in detail, and the challenges and development trends of SoC design are prospected.
A motor control panel has a power recovery funct1on, and when there is no power battery, the rotation of the motor can continue to supply power to the control panel. However, the uneven rotation of the motor will produce rapidly fluctuating voltage, which will cause the power supply chip to output extremely unstable voltage, so that the post-stage equipment will be powered up and down frequently in a very short time, resulting in frequent loss of firmware or even burnout of the Bluetooth module on the board, and reducing the product performance. Later, the problem was solved perfectly by adjusting the relevant configuration of the EN pin of the power supply chip. Do you want to know what was done to him? What kind of great wisdom does the small earth contain?
I. Overview
EN means "enable". Different chips have different names, such as EA, RUN, etc. Their funct1ons are basically the same, that is, only when the pin is activated, the chip or module can output normally. For this funct1on, we can add some simple peripheral circuits to realize the funct1on of stabilizing the chip or outputting the power-on sequence. The en pin of some advanced power supply chips usually has hysteresis characteristics.
II. Application Skills
1. Using Dividing Resistor to Realize Stable Output of Power Supply Chip
For the power supply chip, we usually use a divider resistor to connect the EN signal to the 1nput pin of the power supply to prevent the voltage at the EN terminal fr0m exceeding its withstand voltage value. Under the condition of meeting the withstand voltage value, the voltage of EN pin should also be set in the "appropriate" range.
For example, as mentioned at the beginning of the article, the 24V power supply of a motor control board not only supplies power to the motor, but also outputs 12V to other circuits through DC/DC: MP2451. When there is no booster battery, the motor generates electricity to supply power to the control panel, but the rotation of the motor is not uniform, resulting in a large fluctuation of voltage, as shown in Figure 1 below. The yellow line is the reverse generation voltage of the motor, and the green line is the output voltage of MP2451.
The DC/DC output is enabled when the generated voltage (DC/DC 1nput voltage) VIN of the motor is about 6.2 V, and the 1nput voltage is less than the set 12 V output voltage


Working Principle of Push-pull Transformer Switching Power Supply with Rectifier Output
Working Principle of Push-pull Transformer Switching Power Supply with Rectifier Output
Working principle: The typical circuit of push-pull switching power supply is shown in Figure 1. It belongs to the double-ended conversion circuit, and the magnetic core of the high-frequency transformer works on both sides of the hysteresis loop. The circuit uses two switching tubes VT1 and VT2, and the two switching tubes are alternately turned on and off under the control of an external excitation square wave signal, so that a square wave voltage is obtained at the secondary side of a transformer T, and the square wave voltage is rectified and filtered to become a required direct current voltage. \n The advantage of this circuit is that the two switches are easy to drive, and the main disadvantage is that the withstand voltage of the switches must reach twice the peak voltage of the circuit. The output power of the circuit is large, generally in the range of 100-500 W.
As two switch tubes work alternately, that rectify output push-pull transformer switching power supply is equivalent to that two switch power supplies output power at the same time, and the output power is about twice of the output power of a single switching power supply. Therefore, the output power of the push-pull transformer switching power supply is very large, the working efficiency is very high, and after bridge rectification or full-wave rectification, the output voltage ripple of the push-pull transformer switching power supply can be very small only by a very small filter inductor and capacitor.
Except for the rectifier and filter circuit, the working principle of other circuits is basically the same as that in Fig. 1-27. The bridge rectifier circuit consists of D1, D2, D3 and D4, wherein C is an energy storage filter capacitor, R is a load resistor, Uo is a DC output voltage, and Io is a current flowing through the load resistor.
Similarly, except for the rectifier and filter circuit, the working principle of the rest of the circuit is basically the same as that of Figure 1-27 and Figure 1-30. However, the secondary of the switching transformer needs one more winding, and the two windings N31 and N32 output voltage in turn; The full-wave rectification circuit is composed of D1 and D2, C is an energy storage filter capacitor, R is a load resistor, Uo is a DC output voltage, and Io is a current flowing through the load resistor.
The push-pull transformer switching power supply with the bridge rectifier output uses two more rectifier diodes than the push-pull transformer switching power supply with the full-wave rectifier output, but the switching transformer with the full-wave rectifier output has one more set of secondary coils than the switching transformer with the bridge rectifier output. Therefore, the bridge rectifier output push-pull transformer switching power supply shown in Figure 1-30 is more suitable for the case of relatively small output current


reliability and safety of DC-DC converter
Nowadays, the circuit is more and more in pursuit of reliability and safety. Many circuits are equipped with overvoltage and overcurrent detection circuits to protect the circuit. The general control method for circuit overcurrent protection is shutdown or current limiting. The overcurrent circuit generally uses a fuse for current limiting protection or adopts a sampling resistor to obtain a circuit signal. When the circuit is too large, the subsequent circuit is turned off or the current is limited to a specific value. When the current is normal, the circuit works normally.
When a short circuit occurs in a line, one of the important characteristics is that the current in the line increases sharply, which requires a corresponding protection device to act in response to the current increase when the current flows through a predetermined value, which is called overcurrent protection.
The protection funct1ons of the power supply are mainly overvoltage and overcurrent protection.
The relationship between the two is:
When any kind of power supply fails, the output voltage or output current may be out of control. In order to prevent the user's load fr0m being damaged, the power supply of our company is generally equipped with overvoltage and overcurrent protection. For some loads, such as resistive loads, when the power supply fails, the voltage on the load may rise sharply, and the current increase may not exceed the overcurrent protection value. In this case, over-voltage protection should be used. For example, when working at 50 V, the voltage protection value can be adjusted to 55 V. If the power supply fails, the power supply will automatically cut off the voltage output as long as the voltage rises to 55 V. When some loads are capacitive loads, because the electrolytic capacitors with large capacity are connected in parallel, when the power supply fails, the current may rise sharply, but the voltage rise is not obvious. At this time, the overcurrent protection component inside the power supply will start first, and the power supply will automatically cut off the output.
The overvoltage protection value has a potentiometer on the panel, which can be set manually. The overcurrent protection value can not be set manually, and it has been fixed in the machine, generally 1.2 to 1.5 times of the rated current. It should be noted that the overvoltage protection will start immediately and quickly, while the overcurrent protection has a delay of about one second. This is because when the power supply works normally, if the load of the power supply is suddenly short-circuited, the instantaneous current output by the power supply at this time is several times or tens of times the rated current value, which can be considered as a current impact, far exceeding the value of overcurrent protection, but at this time it is not expected that the overcurrent protection will work. It is hoped that after the short circuit is removed, the voltage will automatically return to normal. Therefore, in the design of overcurrent protection, it is necessary to avoid the current impact of sudden short circuit, and only consider that the duration of output overcurrent reaches a certain value before starting overcurrent protection.


Lenovo voluntarily withdrew its application for listing
fr0m the acceptance of the application to the termination, Lenovo's pursuit of Kechuang IPO has only a short period of 8 days. On October 8, the project dynamics of the Shanghai Stock Exchange showed that Lenovo voluntarily withdrew its application for listing, thus terminating the listing process.
information in the prospectus may expire during the review process of the application.". At the same time, after careful consideration of the latest issuance and listing and other relevant capital market conditions, the company decided to withdraw the application for listing and trading of Chinese depositary receipts on the board.
After the sudden stop of Lenovo's listing on the Science and Technology Innovation Board, it once triggered a lot of heated public opinion. There is a view that Lenovo's IPO may be hindered by external forces. There is also a view that Lenovo, a giant, unlike many start-ups listed on the Science and Technology Innovation Board, does not need an IPO to raise huge amounts of money.
Lenovo is not short of money.
fr0m the perspective of frequent financial market system reform in recent years, the orientation of the Science and Technology Innovation Board is to provide sufficient, timely and sustained financial assistance for science and technology enterprises with obvious capital needs, so as to help them develop reliably and steadily.
Does Lenovo have an obvious need for capital? According to the prospectus subm1tted by Lenovo, Lenovo originally planned to raise 10.014 billion yuan in this IPO, which will be used in three aspects: intelligent project investment, industrial strategic investment and supplementary liquidity.
It seems that Lenovo has a lot of capital needs, and considering that Lenovo has been accelerating its transformation to intellectualization in recent years, it needs to continue to invest heavily in the future layout, so these places that need to spend money do make sense.
But the key is that Lenovo's current profitability can fully support its demand for this amount of financing. According to the prospectus, Lenovo's revenue in the last three full fiscal years reached 342.38 billion, 352.68 billion and 411.62 billion respectively, while its annual profits were 4.41 billion, 5.59 billion and 8.9 billion respectively.
In addition, as of March 31, 2021, Lenovo's total cash and cash equivalents reached 20.55 billion, which is huge. Lenovo itself also said in the prospectus that "the company's monetary funds mainly come fr0m business activities, borrowing and bond issuance activities, the expansion of the company's business and good management of accounts receivable make the company's monetary funds more abundant, which can meet the needs of the daily business activities of the company."


Report on the Competitiveness of Electronic Information Enterprises
Recently, the 2021 China Digital Enabling Economy Conference was held in Yantai. At the meeting, Gao Sumei, Secretary-General of China Electronic Information Industry Federation, released the "Report on the Competitiveness of Electronic Information Enterprises in 2021 and the List of Top 100 Enterprises". Among them, Huawei, Lenovo and Haier ranked the top three.
It is reported that the top 100 electronic enterprises present the following six characteristics:
First, the overall scale has been upgraded. In 2020, the main business income of the top 100 enterprises increased by 10.4% year-on-year, accounting for more than 40% of the income of the electronic information manufacturing industry above the scale, and the growth rate was 2.1 percentage points higher than the industry average. There are 13 companies whose main business income exceeds 100 billion yuan and 4 companies whose main business income exceeds 200 billion yuan. The highest main business income of the listed enterprises reached 800 billion yuan, and the entry threshold exceeded 5 billion yuan.
Second, the quality of benefits is outstanding. In 2020, the total profit of the top 100 enterprises increased by 21.2% year-on-year, accounting for more than 50% of the industry's profits, and the growth rate was 4 percentage points ahead of the industry; the average profit rate was 6.0%, 0.5 percentage points higher than the previous year, and 1.1 percentage points ahead of the average level of the industry.
Third, the ability to innovate has been enhanced. In 2020, the R & D investment of the top 100 enterprises increased by 17.8% year-on-year, exceeding the income growth rate of 7.4 percentage points in the same period, and the R & D investment intensity reached 6.3%. By 2020, the total number of patents was 530000, of which 380000 were invention patents, accounting for more than 70%. In 2020, the top 100 enterprises accounted for 30% of the top 20 enterprises in China's invention patent authorization.
Fourth, external demand is growing against the trend. In 2020, the export volume of the top 100 enterprises increased by 8.5% year-on-year, accounting for more than 20% of the total industry, and the growth rate was 2.2 percentage points ahead.
Fifth, technological breakthroughs have been accelerated. The top 100 enterprises have actively participated in major national strategic deployments, and in the field of satellite navigation systems, the localization rate of core components has reached 100%. In the field of display, the transformation fr0m "less screen" to "strong screen" has been realized. Emerging technologies such as flexible display, printing display and laser display have made breakthroughs, and technological reserves have been continuously strengthened. In the field of integrated circuits, a number of key enterprises have grown rapidly and made substantial breakthroughs in R & D, manufacturing and packaging.
Sixth, the leading role is obvious. With less than 0.5% of the total number of enterprises in the industry, the top 100 enterprises have supported 60% of the state taxes paid by the whole industry, realized 40% of the industry sales revenue and 50% of industry profits, and played an important role in the high-quality development of the whole industry.


Select the correct capacitor
se1ect the correct capacitor
In my case, I chose the smallest available package for the 4.7 μF X7R capacitor because size was a consideration in my project. Because of my ignorance, I assumed that any X7R would have the same effect as any other X7R; obviously, this is not the case. To get the right performance for my application, I have to go to some kind of larger package.
I really don't want to use the 1210 package. Fortunately, I was able to increase the resistor values used by 5 X, thus reducing the capacitance to 1 μF.
n Figure 2 shows the voltage characteristics of several 16 V, 1 μF X7R and 16 V, 4.7 μF X7R capacitors. The 1 μF capacitor of the 0603 behaves the same as the 4.7 μF capacitor of the 0805. The 1 μF capacitors of both the 0805 and 1206 perform slightly better than the 4.7 μF capacitors of the 1210. So, with a 1 μF capacitor of 0805, I can keep the capacitor volume the same, and the capacitance under bias is only reduced to about 85% of the rated amount, not 30%. \n But I'm still confused. I thought that all X7R capacitors should have the same voltage coefficient because the dielectric used is the same, X7R. So I asked a colleague, Chris Burkett, a field application engineer at TDK in Japan, who is also an expert on ceramic capacitors.
He explained that many materials would qualify as "X7R". In fact, any material that allows the device to meet or exceed the X7R temperature characteristics (i.e., ± 15% variation fr0m -55 ° C to 125 ° C) can be called X7R. Burkett also explained that there is no specific specification for the voltage coefficient of X7R capacitors or any other type of ceramic capacitor.
This is a key point, so I will repeat it again. As long as a capacitor meets the temperature coefficient specification, no matter how bad its voltage coefficient is, the manufacturer can call it an X7R capacitor (or X5R, or any other type). This fact confirms the Maxim (pun intended) that any experienced electrical engineer knows: read the data sheet!
As manufacturers increasingly prefer smaller components, they have to compromise on the materials they use. In order to achieve the required volumetric efficiency with a smaller size, they were forced to accept a worse voltage coefficient. Of course, reputable manufacturers will try to minimize the side effects of this compromise.


Not all X7R capacitors are the same
Not all X7R capacitors are the same
Since my RC circuit time constant problem cannot be explained by a specific temperature variable, it must be studied in depth. Looking at the capacitance of my capacitor versus the applied voltage, I was surprised to see how much the capacitance changed with the set conditions. I chose a 16 V capacitor operating at 12 V bias. The datasheet shows that my 4.7-μF capacitor typically provides 1.5 μF under these conditions. Now, the problem with the RC circuit is completely explained.
The datasheet shows that if I increase the capacitor package size fr0m 0805 to 1206, the typical capacitance under specified conditions will be 3.4 μF. This indicates that further research is necessary.
I found that Murata and TDK have great tools on their website to plot capacitance changes under different environmental conditions. I did some research on 4.7 μF capacitors of various sizes and voltage ratings. The data in Figure 1 is taken fr0m Murata's tool for several different 4.7 μF ceramic capacitors. I looked at both the X5R and X7R models, with package sizes fr0m 0603 to 1812 and voltage ratings fr0m 6.3 to 25V DC. First, I noticed that as the package size increases, the change in capacitance with applied DC voltage decreases and is large.
The relationship between the DC voltage on the 4.7 μF capacitor and the temperature change shows that with the increase of the package size, the capacitance decreases greatly with the applied voltage.
CAPACITANCE (μF) Capacitance (μF) DC VOLTAGE (V) DC Voltage (V)
The second interesting point is that for a given package size and tile capacitor type, the voltage rating of the capacitor generally does not seem to matter. \n So I estimate that if I use a capacitor rated at 25 V for 12 V, the change in capacitance will be less than a capacitor rated at 16 V under the same conditions. Looking at the curve for the X5R in the 1206 package, it is clear that the 6.3 V rated components do outperform their higher voltage rated counterparts. \n If we examine a wider range of capacitances, we will see that this is very common. The sample set of capacitors I studied did not exhibit the behavior expected of a normal ceramic capacitor.
The third problem observed is that the X7R capacitor is more temperature sensitive than the X5R capacitor for the same package.


time constant of RC circuit
A few years ago, after more than 25 years of working with ceramic capacitors, I came to a new understanding of them. I was busy working on an LED bulb driver at the time, and there was apparently a problem with the time constant of one of the RC circuits in my project.
My first assumption was that a component on the board had an incorrect value, so I measured two resistors that were used as a voltage divider, but they were both OK. I removed the capacitor fr0m the circuit board and measured it without any problem. To further confirm, I measured and installed new resistors and capacitors, powered up the circuit, checked that it was basically working properly, and then saw if the replacement components solved the RC circuit time constant problem. But the answer is no.
I tested the circuit in a natural environment: inside the enclosure, the circuit is inside the enclosure, simulating a "can" of roof lighting, and sometimes the temperature of the components rises to more than 100 degrees Celsius. Although the time I had to retest the RC circuit was short, everything was still very hot. \n Obviously, my next conclusion is that the problem is the temperature variation of the capacitor. But I doubt that myself, because I'm using an X7R capacitor, which, fr0m what I remember, works up to 125 ° C with a ± 15% change. I trust my memory, but to be on the safe side, I revisited the datasheet for the capacitors used.
Background report
the letters and numbers used for different types of ceramic capacitors and their respective meanings. The table describes Class II and Class III ceramic capacitors. Without going into too much detail here, Class I capacitors include the common COG (NPO) type;
The volumetric efficiency of this capacitor is not as good as the two capacitors in the table, but it is much more stable under variable environmental conditions and does not exhibit piezoelectric effects. In contrast, the capacitors in the table have a wide variety of characteristics that allow them to expand and withstand the applied voltage, but sometimes produce an audible piezoelectric effect (beeping or ringing).
Of the various capacitor types given, in my experience the most commonly used are X5R, X7R, and also Y5V. I have never used Y5V because they exhibit a large variation in capacitance across the full range of ambient conditions.
When a capacitor company develops a product, they choose the characteristics of the material so that the capacitor can operate within a defined range of variation over a specified temperature range.


characteristic of the manufacturing proces of DC-DC converter
The transformer of flyback power supply works in unidirectional magnetization state, which not only transfers energy through magnetic coupling, but also plays multiple roles of voltage conversion and 1nput and output isolation. Therefore, the treatment of air gap needs to be very careful. If the air gap is too large, the leakage inductance will become larger, the hysteresis loss will increase, and the iron loss and copper loss will increase, which will affect the overall performance of the power supply. Too small an air gap can saturate the transformer core and cause damage to the power supply.
The so-called continuous and discontinuous modes of flyback power supply refer to the working state of the transformer. In the full-load state, the transformer works in the working mode of complete or incomplete energy transfer. Generally, it should be designed according to the working environment. The conventional flyback power supply should work in continuous mode, so that the loss of switches and lines is relatively small, and the working stress of 1nput and output capacitors can be reduced, but there are some exceptions.
It should be specially pointed out here that the flyback power supply is also suitable to be designed as a high-voltage power supply, and the high-voltage power supply transformer generally works in the discontinuous mode. I understand that the high-voltage power output needs to use a high-voltage rectifier diode.
Because of that characteristic of the manufacturing proces, the reverse recovery time of the high reverse voltage diode is long, the speed is low, in a current continuous state, the diode is recovered when a forward bias voltage is applied, and the energy loss dure the reverse recovery is very large, so that the improvement of the performance of a converter is not facilitated, conversion efficiency is reduced, a rectifier tube is seriously heated, and even the rectifier tube is burned. Since the diode is reverse biased at zero bias in the discontinuous mode, the loss can be reduced to a relatively low level. Therefore, the high voltage power supply operates in discontinuous mode, and the operating frequency cannot be too high.
There is also a kind of flyback power supply which works in the critical state. Generally, this kind of power supply works in the frequency modulation mode or the frequency modulation and width modulation dual mode. Some low-cost self-excited power supplies (RCC) often adopt this form. In order to ensure the output stability, the working frequency of the transformer changes with the output current or 1nput voltage. When the transformer is close to full load, it always keeps between continuous and discontinuous. This kind of power supply is only suitable for low power output, otherwise the treatment of electromagnetic compatibility characteristics will be a headache.
The transformer of flyback switching power supply should work in continuous mode, which requires a relatively large winding inductance. Of course, there is a certain degree of continuity. It is unrealistic to pursue absolute continuity excessively. It may require a large magnetic core and a large number of coil turns, accompanied by large leakage inductance and distributed capacitance, which may not be worth the loss.


Optimization process of designing a low voltage output low power flyback power supply
Care must be taken in the optimization process of designing a low voltage output low power flyback power supply, and there are several ways to handle it:
1. A magnetic core with a higher power level is used to reduce the leakage inductance, which can improve the conversion efficiency of the low-voltage flyback power supply, reduce the loss, reduce the output ripple, and improve the cross regulation rate of the multi-output power supply. It is commonly used in switching power supplies for household appliances, such as optical disc players, DVD set-top boxes, etc.
2. If the condition does not allow the magnetic core to be enlarged, the reflected voltage can only be reduced and the duty ratio can be reduced. Reducing the reflected voltage can reduce the leakage inductance, but it may reduce the power conversion efficiency. The two are contradictory, and there must be a substitution process to find a suitable point. In the process of transformer substitution experiment, the reverse peak voltage of the primary side of the transformer can be detected, and the width and amplitude of the reverse peak voltage pulse can be reduced as much as possible to increase the working safety margin of the converter. Generally, the reflection voltage is more appropriate at 110V.
3. Enhance the coupling, reduce the loss, and adopt new technology and winding process. In order to meet the safety specifications, the transformer will take insulation measures between the primary side and the secondary side, such as padding insulation tape and adding insulation end tape. These will affect the leakage inductance performance of the transformer, and the winding method of primary winding wrapped around the secondary winding can be used in actual production. Or the secondary winding is made of triple insulated wire, and the insulation between the primary and secondary windings is eliminated, which can enhance the coupling, and even the wide copper sheath winding can be used.
In this paper, the low voltage output refers to the output less than or equal to 5V, such as this kind of small power supply, my experience is that the power output greater than 20 W output can use the forward type, can obtain the best cost performance, of course, this is not absolutely right, with personal habits, the application environment is related, next time talk about the flyback power supply with magnetic core, some understanding of the magnetic circuit air gap. I hope you can give me some advice.
The magnetic core of the flyback power transformer works in a unidirectional magnetization state, so the magnetic circuit needs an air gap, which is similar to a pulsating DC inductor. Part of the magnetic circuit is coupled through an air gap.
I understand the principle of opening the air gap as follows: because the power ferrite also has a working characteristic curve (hysteresis loop) similar to a rectangle, the Y axis on the working characteristic curve represents the magnetic induction (B), and the saturation point of the current production process is generally above 400mT. Generally, this value should be 200-300mT in the design, and the X axis represents the magnetic field intensity (H), which is proportional to the magnetization current intensity.


Duty cycle of the flyback DC-DC converter
Duty cycle of the flyback power supply
I will talk about the duty cycle of the flyback power supply (I am concerned about the reflected voltage, which is consistent with the duty cycle). The duty cycle is also related to the choice of the voltage withstand of the switching tube. Some early flyback power supplies use relatively low voltage withstand switching tubes, such as 600V or 650V as the switching tubes of AC 220V 1nput power supply, which may be related to the production process at that time. High voltage withstand tubes are not easy to manufacture. Or the low withstand voltage tube has more reasonable conduction loss and switching characteristics, such as the line reflection voltage can not be too high, otherwise in order to make the switch tube work in a safe range, the power loss of the absorption circuit is also considerable.
It is proved that the reflection voltage of 600V tube should not be more than 100V, and the reflection voltage of 650V tube should not be more than 120V. When the peak voltage of leakage inductance is clamped at 50V, the tube still has a working margin of 50V. Nowadays, due to the improvement of the manufacturing technology of MOS transistors, the flyback power supply generally uses 700 V or 750 V or even 800-900 V switching transistors.
Like this kind of circuit, the ability to resist overvoltage is stronger, and the reflection voltage of the switching transformer can also be made higher. The maximum reflection voltage of 150V is more appropriate, which can obtain better comprehensive performance. PI's TOP chip recommends a transient voltage suppression diode clamp for 135V. But his evaluation boards typically have a reflected voltage lower than that, around 110 V. Both types have advantages and disadvantages:
The first category: weak overvoltage resistance, small duty cycle, and large primary pulse current of transformer. Advantages: small transformer leakage inductance, low electromagnetic radiation, high ripple index, small switch tube loss, conversion efficiency is not necessarily lower than the second class.
The second category: the disadvantage is that the loss of the switch tube is larger, the leakage inductance of the transformer is larger, and the ripple is worse. Advantages: strong overvoltage resistance, large duty cycle, low transformer loss and high efficiency.
The reflection voltage of the flyback power supply also has a determining factor, and the reflection voltage of the flyback power supply is also related to a parameter, namely the output voltage, the lower the output voltage is, the larger the turns ratio of a transformer is, the larger the leakage inductance of the transformer is, and the higher the voltage borne by a switching tube is, It is possible to permanently disable the power devices in the snubber loop (especially in circuits using transient voltage suppression diodes). Care must be taken in the optimization process of designing low voltage output low power flyback power supplies.


DC-DC converter is divided into two forms: isolated and non-isolated
Here we mainly talk about the topology of isolated switching power supply. In the following, unless otherwise specified, it refers to isolated power supply. Isolated power supply can be divided into two categories according to different structures: forward and flyback. Flyback means that when the primary side of the transformer is on, the secondary side is off, and the transformer stores energy. When the primary side is turned off, the secondary side is turned on, and the energy is released to the load. Generally, the conventional flyback power supply has more single transistors, and double transistors are not common. Forward means that the primary side of the transformer is conducted while the secondary side induces the corresponding voltage output to the load, and the energy is directly transferred through the transformer. According to the specification, it can be divided into conventional forward, including single-tube forward and double-tube forward. Half-bridge and bridge circuits are both forward circuits.
Forward and flyback circuits have their own characteristics, which can be used flexibly in the process of circuit design to achieve the best cost performance. In general, flyback can be used in small power applications. A single-transistor forward circuit can be used for slightly larger power, a double-transistor forward circuit or a half-bridge circuit can be used for medium power, and a push-pull circuit is used for low voltage, which is the same as the half-bridge working state. High power output, generally using bridge circuit, low voltage can also use push-pull circuit.
Flyback power supply is widely used in small and medium power supply because of its simple structure and saving an inductor with the same size as the transformer. In some introductions, it is said that the power of flyback power supply can only be tens of watts, and there is no advantage if the output power exceeds 100 watts, which is difficult to achieve. I think this is the case in general, but also can not be generalized, there are articles on the flyback power supply can be achieved on the kilowatt, but have not seen the real thing. The output power is related to the output voltage.
n The leakage inductance of the flyback power supply transformer is a very critical parameter, because the flyback power supply needs the transformer to store energy, in order to make full use of the transformer core, it is generally necessary to open an air gap in the magnetic circuit, the purpose is to change the slope of the iron core hysteresis loop, so that the transformer can withstand the impact of large pulse current, and the iron core will not enter a saturated nonlinear state. The air gap in that magnetic circuit is in a high reluctance state, and the magnetic leakage generate in the magnetic circuit is much larger than that of a completely close magnetic circuit.
The coupling between the primary poles of the transformer is also the key factor to determine the leakage inductance. To make the primary pole coils as close as possible, the sandwich winding method can be used, but this will increase the distributed capacitance of the transformer. The leakage inductance can be reduced by se1ecting the core with a longer window as far as possible. For example, the effect of EE, EF, EER and PQ cores is better than that of EI cores.


Application of Aluminum Substrate and Multilayer Printed Circuit Board in DC-DC converter
The application of aluminum substrate in switching power supply and the application of multilayer printed circuit board in switching power supply circuit.
The aluminum substrate is constructed by the aluminum substrate itself and has the following characteristics: the aluminum substrate is excellent in heat conductivity, copper is bound on one side, devices can only be placed on the copper binding surface, and electrical wiring holes cannot be opened, so jumpers cannot be placed as a single panel.
SMD devices, switching tubes and output rectifier tubes are generally placed on the aluminum substrate to conduct heat out through the substrate, so the thermal resistance is very low and high reliability can be achieved. The transformer adopts a planar patch structure and can also dissipate heat through a substrate, the temperature rise of the transformer is lower than that of a conventional transformer, and the transformer with the same specification adopts an aluminum substrate structure to obtain higher output power. The aluminum substrate jumper can be handled in a bridging manner. The aluminum substrate power supply is generally composed of two printed boards, the control circuit is placed on the other board, and the two boards are integrated through physical connection.
Due to the excellent thermal conductivity of the aluminum substrate, the aluminum substrate is difficult to be soldered manually in a small amount, the solder is cooled too fast, and problems are easy to occur. The existing simple and practical method comprises the following steps of: turning over a common electric iron for ironing clothes (preferably with a temperature adjusting funct1on), fixing the ironing surface upwards, adjusting the temperature to about 150 deg C, putting the aluminum substrate on the iron, heating for a period of time, Then stick and weld the components according to the conventional method. The temperature of the iron should be easy to weld the components. If the temperature is too high, the components may be damaged, and even the copper sheet of the aluminum substrate will be peeled off. If the temperature is too low, the welding effect will not be good. It should be flexible.
In that recent year, with the application of the multilayer circuit board in the switching pow supply circuit, the printed circuit transformer becomes possible, because the multilayer board has small lay spacing, the window section of the transformer can be fully utilized, one or two printed coils composed of the multilayer board can be added on the main circuit board to achieve the purpose of utilizing the window and reducing the current density of the circuit, because the printed coils are adopted, The invention has the advantages of reduced manual intervention, good consistency of the transformer, plane structure, low leakage inductance and good coupling. Open magnetic core, good heat dissipation condition.
Because it has many advantages and is conducive to mass production, it has been widely used. However, the initial investment in research and development is large, and it is not suitable for small-scale production.


the treatment of large current wiring for DC-DC converter
For the treatment of large current wiring, the line width is not enough. Generally, the problem can be solved by increasing the thickness of tin plating on the wiring. There are many methods:
1. Set the routing as the pad attribute, so that the routing will not be covered by the solder resist during the manufacturing of the circuit board, and will be tinned during the hot air leveling.
2. Place the pad at the wiring place and set the pad to the shape of the required wiring. Pay attention to setting the pad hole to zero.
3. Place wires on the solder mask. This method is the most flexible, but not all circuit board manufacturers will understand your intention and need to explain it in words. The solder resist is not applied to the portion of the solder resist where the wire is placed.
There are several methods of tinning the circuit as above. It should be noted that if all the wide wiring is tinned, a large amount of solder will be adhered after soldering, and the distribution is very uneven, which will affect the appearance. Generally, the width of the thin strip tinning is 1 ~ 1.5mm, and the length can be determined according to the circuit. The interval of the tinning part is 0.5 ~ 1mm. The double-sided circuit board provides great se1ectivity for layout and wiring, and can make the wiring more reasonable.
With regard to grounding, the power ground and the signal ground must be separated, and the two grounds can be merged at the filter capacitor to avoid the unexpected factors of instability caused by large pulse current passing through the signal ground connection. The signal control circuit should adopt the one-point grounding method as far as possible. There is a trick to place the non-grounded wiring on the same wiring layer as far as possible, and finally lay the ground wire on another layer.
The output line usually passes through the filter capacitor first and then to the load, and the 1nput line must also pass through the capacitor first and then to the transformer. The theoretical basis is to let the ripple current pass through the filter capacitor.
Voltage feedback sampling, in order to avoid the impact of large current through the wiring, the sampling point of feedback voltage must be placed at the end of the power output to improve the load effect index of the whole machine.
The wiring fr0m one wiring layer to another wiring layer is generally connected by vias, which should not be realized through the pin pad of the device, because this connection may be destroyed when the device is 1nserted. In addition, when each 1A current passes through, there should be at least two vias. The diameter of the vias should be greater than 0.5mm in principle, and 0.8mm in general can ensure the reliability of processing.
In some low-power power supplies, the wiring of the circuit board can also have the funct1on of heat dissipation. It is characterized in that the wiring is as wide as possible to increase the heat dissipation area. It is not coated with solder resist. If conditions permit, vias can be placed evenly to enhance the thermal conductivity.


How to design single-sided printing PCB
Because the single panel has the characteristics of low cost and easy manufacturing, it is widely used in switching power supply circuits. Because it has only one side bound with copper, the electrical connection and mechanical fixation of devices depend on that layer of copper sheet, so care must be taken when dealing with it.
In order to ensure good mechanical structure performance of soldering, the single panel pad should be slightly larger to ensure good binding force between the copper sheet and the substrate, so that the copper sheet will not peel off and break off when it is subjected to vibration. Generally, the width of the welding ring shall be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the diameter of the device pin, but should not be too large to ensure that the shortest distance between the pin and the pad is connected by solder. The size of the pad hole should not hinder the normal inspection. The pad hole diameter is generally 0.1-0.2mm larger than the pin diameter. Multi-pin devices can also be larger to ensure a smooth check.
The electrical wiring shall be as wide as possible. In principle, the width shall be greater than the diameter of the pad. In special cases, the wiring must be widened when it intersects with the pad (commonly known as teardrop) to avoid breakage of the wiring and the pad under certain conditions. In principle, the minimum line width shall be greater than 0.5mm.
The components on the single panel shall be close to the circuit board. For devices requiring overhead heat dissipation, a sleeve should be added to the pin between the device and the circuit board, which can play a dual role of supporting the device and increasing insulation, minimize or avoid the impact of external impact on the connection between the pad and the pin, and enhance the firmness of welding. Support connection points can be added to components with heavy weight on the circuit board to strengthen the connection strength with the circuit board, such as transformers and power device radiators.
Under the precondition that the distance between the single panel welding surface and the shell is not affected, the single panel welding surface pin can be left longer, which has the advantages that the strength of the welding part can be increased, the welding area can be increased, and the phenomenon of false welding can be found immediately. When the pin is long and the leg is cut, the stress on the welding part is small. In Taiwan and Japan, the process of bending the pins of the device at an angle of 45 degrees with the circuit board and then soldering them is often used. The reason is the same as above. Today, let's talk about some matters in the design of double-sided boards. In some application environments with high requirements or high wiring density, the performance and various indicators of double-sided printed boards are much better than those of single-sided boards.


Next Page