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　　Detailed explanation of problems encountered in switching power supply related projects and related solutions

　　Project: A power supply in a laboratory was broken. After taking it apart, it was found that the full bridge controlled by UC3875 needed to be repaired.

　　Phenomenon: Preliminary inspection showed that the power tube was broken. Since there are no tubes of the same model, all tubes were replaced with tubes of the same power level. After powering on, everything is normal when the input voltage is low. When the input voltage is high, the drive is chaotic and the frequency jitters.

　　Solution: Increase the driving resistance of the power tube, the phenomenon disappears, everything is normal, and the power supply is repaired.

　　Analysis: The parasitic parameters of the new tube are different from those of the old tube. Under the same drive circuit, the switching speed will be faster, resulting in greater interference. At high voltage, the interference will be large enough to affect the work of the control circuit.

　　Just write a few simple sentences:

　　1. Components must be soldered carefully to avoid false soldering, which is very dangerous and not easy to see. The direction cannot be reversed, especially the direction of the diode. I once soldered the bridge rectifier diode in the wrong direction, which directly caused back voltage to the filter electrolytic capacitor, which was very dangerous.

　　2. If flying wires are required during debugging, and they are round-trip signal wires, the outgoing wires and return wires should be twisted together. Because if the lines and return lines are removed to form an enclosed area, it is equivalent to an antenna, and it is easy to cause interference.

　　3. The bus power supply must not only have large filter capacitors, but also high-frequency filter capacitors. The same goes for filtering on output.

　　Project: UC3845 dual-tube forward

　　Phenomenon: After the two tubes are turned off, the voltages DS bears are very different, not half of each other in theory. I guessed it was caused by inconsistent MOS parameters. I welded the upper and lower tubes off and swapped their positions. The result was still the same. It seems to have nothing to do with MOS.

　　Solution: Adjust the two-tube drivers so that they turn off at the same time. The situation improves slightly, but the voltage still cannot be divided equally.

　　Analysis: This should be caused by two reasons. One is caused by the difference in PCB parasitic parameters. The actual capacitance of the DS of the tubes at the two positions is different. Another is that the driver is not turned off very synchronously.

　　Project: UC3845 controls flyback with auxiliary winding feedback

　　Phenomenon: The main circuit output voltage has a large overshoot when starting up. However, the voltage of the auxiliary winding participating in the feedback does not overshoot.

　　Solution: In order to adjust the regulation rate, a resistor is connected in series to the auxiliary winding. By reducing the resistance of this resistor, the main output overshoot is significantly reduced.

　　Analysis: Since the feedback samples the auxiliary winding, and a resistor is connected in series with the auxiliary winding, there is a voltage difference between the voltage of the auxiliary winding and the voltage at the feedback at startup, which is coupled through the transformer, causing the output voltage to overshoot.

　　Project: NCp1014, optical coupling feedback flyback

　　Phenomenon: After re-soldering a mature board that has been made by others, it was found that the output voltage regulation was incorrect.

　　Solution: I was smart and replaced the 431 in the original BOM with another 431 of the same standard. Just change it back.

　　Analysis: The original zetex 431 was used, and its minimum operating current was uA level, so the minimum operating current was basically not considered during the design. Later, it was replaced with TI's 431. The minimum operating current was 1mA, which caused it to work abnormally.

　　Project: ICE1pCS01 controls boostpFC

　　Phenomenon: In the whole voltage range, when adjusted with a voltage regulator, the input current waveform is very good and the high-frequency ripple is very small. Only when the input voltage is around 220V, the high-frequency ripple of the input current suddenly becomes larger. Both greater than 220V and less than 220V are very small.

　　Solution: Just use ACsouce. The high-frequency ripple is relatively large at any voltage, haha.

　　Analysis: An auto-coupling voltage regulator is used. The self-coupling voltage regulator has leakage inductance. The leakage inductance can filter out the input high-frequency ripple current. However, when it reaches 220V (network voltage), the output of the self-coupling voltage regulator The terminal is actually directly connected to the input terminal, so naturally there is no leakage inductance.

　　Project: UC3845 dual-tube flyback

　　Phenomenon: The drive is unstable, constantly vibrating, and the transformer hisses. The adjustment loop is useless. Use an oscilloscope to observe the sawtooth waveform of the uc3845 oscillation pin and find that the frequency of the sawtooth wave is jittering. UC3845 is a fixed frequency, so there seems to be interference.

　　Solution: Strictly separate the control circuit ground and power ground, and then connect them at a single point. The drive signal is stable, the frequency is fixed, and the transformer no longer screams. But the damn thing is that the conduction has actually gotten worse. Maybe the legendary frequency jitter is indeed good for conduction.

　　Analysis: Layout is very important in power supply design, especially the layout of the ground. The power ground and signal ground are separated and grounded at a single point. It is to prevent high-frequency power current from flowing through the signal ground plane, otherwise it will interfere with the control circuit.

　　The ground of the IC and the ground of the MOS must be strictly separated and then connected at a single point.

　　The auxiliary winding supplies power to the IC, so the ground of the filter capacitor of the auxiliary winding must be formed independently and then connected to the signal ground at a single point. In this way, the high-frequency current on the auxiliary winding will be absorbed by the capacitor and will not be connected to the signal ground.

　　Project: UCC3895 current mode control phase shift control full bridge, double current rectification

　　Phenomenon: The transformer has magnetic bias

　　Solution: Make one PCB power trace of the secondary power circuit thicker. This PCB trace is connected to a certain inductor of the current doubler rectifier circuit. The bias magnet disappears~~~~

　　Analysis: The current doubler rectifier circuit has a unique problem, that is, the average currents on the two inductors will be inconsistent. If current-type control is used, the control signal will ensure that the positive and negative current peaks of the transformer primary are the same. Then if the positive and negative current peaks of the transformer secondary If the negative current is inconsistent, magnetic bias will occur.

　　The average current of the inductors is inconsistent because the DC impedances of the two inductors are different. But in fact, the difference between the same batch of inductors is not that big. On the contrary, the PCB traces connecting these inductors are quite different, resulting in a relatively large difference in the actual DC resistance of the two inductors (plus the resistance of the PCB traces).

　　Project: 431 plus optical coupling feedback flyback

　　Phenomenon: The output voltage regulation rate is very poor, and the voltage drops significantly as the load increases. The voltage difference between the measurement voltage sampling point and the output pin is not large.

　　Solution: Place a small capacitor between the reference pin of 431 and the cathode. The adjustment rate immediately improved.

　　Analysis: The reference foot of 431 is disturbed.

　　Project: IR1150boostpFC

　　Phenomenon: The switching frequency is 100K, but the input actually has a ripple current of 1Khz. The X capacitor still squeaks.

　　Solution: Adjust EMI filter parameters.

　　Analysis: The EMI filter resonates by itself.

　　Project: Flyback Synchronous Rectification

　　Phenomenon: The voltage spike of the synchronous rectifier is very high and cannot be absorbed no matter how hard it is.

　　Solution: Replace the synchronous tube with a tube with a fast recovery body diode

　　Analysis: Because the reverse recovery time of the body diode of the synchronous tube is too long, a large reverse recovery current results. causing severe voltage spikes

　　Project: IR1150pFC

　　Phenomenon: During the high-temperature test, the shell temperature of the MOSFET was only 80 degrees, which was already fried chicken. In the previous few units, the shell temperature of the MOS reached 110 degrees, and everything was fine.

　　Solution: Get it out and find out the cause. The driver resistor was welded wrongly. It was originally 10R, but ended up being welded to 100R.

　　Analysis: The driving resistance is too large, resulting in large MOS losses. For the same junction-to-case thermal resistance, large power consumption will lead to large temperature differences. Although the case temperature is only 80 degrees, the actual junction temperature has exceeded the tolerance range of the MOS.

　　If the driving resistance is large, the driving power will be seriously insufficient and the tube will be heated to death!

　　If the driving power is large enough, it will not cause fried chicken.

　　If the inductance caused by the PCB trace is large enough, it will resonate with the capacitance Cgs of the GS terminal of the MOS, and spikes will be superimposed on the driving signal line. In severe cases, it will cause fried chicken. The purpose of adding resistors is to attenuate this oscillation.

　　Project: L4981pFC

　　Phenomenon: When powered on with no load, the drive becomes extremely chaotic and the oscillation frequency changes significantly. The higher the input voltage, the more powerful it is. At first I thought that the ground wire was not laid out properly, and the PCB was cut again and again, but nothing could be solved.

　　Solution: Carefully inspect the PCB and find that there is a power line relatively close to the control circuit. This power line is connected to the D pole of the MOSFET. It's useless to cut off the power line and let the power current flow away from the control circuit. Make the PCB copper wire close to the control circuit into an island so that it becomes dead copper and the interference disappears.

　　Analysis: Electric field interference, the D pole of the MOS is a place with a large dv/dt, producing a large common mode interference. Therefore, the control circuit should be as far away from this point as possible.

　　At the Technology Zone Mu Exhibition, SiC, GaN, and three-level products brought by Shiqiang will bring your efficiency to the highest point. How to use a secondary output filter to prevent switching power supply noise. Welding precautions for ceramic vertical mount packages (CVMp) and What are the common methods for generating reference regulated power supplies for average small signal mathematical modeling and loop compensation design of DC-DC converters?

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