Cold knowledge: the magical use of slow recovery tubes in switching power supplies

　　If this is the case, can slow recovery diodes not be used in switching power supplies? In fact, the reasonable use of slow recovery diodes in switching power supplies will bring unexpected surprises. The following will explain the analysis of two examples.

　　Here are two examples from my work:

　　Case number one

　　The slow recovery power frequency rectifier tube 1N4007 is used for rectification of the main control IC power supply winding to solve the problem of high bias voltage in multi-winding systems.

　　When using a certain IC as a 5-channel DVB power supply, during the mass production process, it was found that the defect rate was high, and the symptoms were that the power supply did not work or hiccuped. After going to the factory for actual measurement, it was found that the IC's power supply voltage was too high, and the IC's overvoltage protection mechanism was triggered.

　　As we all know, for a multi-output power supply, achieving a good cross-regulation rate is a great test of the transformer design skills, and it is inevitable that the bias power supply winding voltage is high. The customer has mass-produced multiple sets of 1W power supplies, and redesigning the transformer is obviously not a good solution. Increasing the resistance in series with the rectifier diode has limited effect. After all, its main function is to filter out spike voltages, and what causes IC protection is the high bias winding voltage. At this time, the charm of the slow rectifier tube is reflected. The final solution is to replace the fast recovery diode HER107 in the customer's original solution with 1N4007, and the problem is perfectly solved. See Figure 1 for details:

　　figure 1

　　Some people may ask, will there be any safety risks when using the slow tube here? Is it appropriate?

　　Indeed, a slow tube cannot be used for the rectifier tube of a switching power supply, but it is indeed suitable here. Because the IC power supply current is basically at the mA level and the load is not large, there will be no problem using a slow tube.

　　Case 2

The RCD absorption circuit in Flyback uses a slow tube 1N4007 to solve the leakage inductance spike voltage stress and EMI radiation problems 　　on the main switch .

　　figure 2

　　The common RCD absorption circuit structure is shown in Figure 2 (D1 is generally a fast recovery diode).

　　If the transformer is not designed properly and the leakage inductance is large, when the switch tube is turned off, the leakage inductance voltage will be large and the oscillation time will be long, resulting in large MOS voltage stress and excessive EMI radiation.

　　image 3

　　Figure 3 is the measured waveform of D1 using the fast recovery diode UF4007.

　　The yellow line is the waveform of C1 in the RCD, the pink line is the waveform of the drain of the switch tube, and the blue line is the voltage waveform of R1. Obviously, the drain oscillation time is longer and the peak value is higher. What will happen if D1 is replaced with a power frequency rectifier tube 1N4007?

　　Here is the performance of 1N4007:

　　Figure 4

　　Obviously, the drain oscillation is perfectly suppressed and the peak value is greatly reduced, thereby reducing the voltage stress of MOS and greatly improving EMI .
　　Careful friends will find that the voltage peak of R1 becomes larger. Why is this? Because the reverse recovery time of 1N4007 is long, so the electricity of C1 will flow back.

　　Some literature points out that it is this energy return that reduces the loss of R2 and will improve the efficiency of the power supply. However, actual measurements have not found any improvement in efficiency, so I would like to reserve my opinion here.

　　Insufficient energy return does exist, which has been shown in theoretical analysis and actual test results. It is also for this reason that the 1N4007 will generate a lot of heat, so this solution is suitable for low-power Flyback and is not recommended for high-power.

　　If you encounter large MOS voltage stress and EMI always exceeds the standard during design, you may want to try this solution.

　　Although it is not recommended to use slow-reacting diodes in daily switching power supply design, it does not mean that they are useless in design. On the contrary, this type of diode can solve some more difficult problems. So when you encounter problems in learning and design, it is better to think in a different way, and perhaps the problem will be solved.