The RC peak absorption problem of Flyback's secondary rectifier diode 2

1. Choose a larger capacitor (such as 100nF) as capacitor C, and select an ultrafast recovery diode (such as 1N4148) with a withstand voltage greater than 1.5 (Uin/N+Uo) for capacitor D;

2. You can choose a smaller resistor of 10K or 1W to absorb R;

3. Gradually increase the load and observe the voltage at the C-terminal of the capacitor and the peak voltage of the rectifier tube:

If the voltage ripple on C is greater than 20% of the average value, the C value needs to be increased; diode

If the voltage at the C terminal is too much higher than Uin/N+Uo at full load (more than 20%, depending on the withstand voltage of the rectifier tube), it indicates that the absorption is too weak and the resistance R needs to be reduced;

If the voltage on C is lower than or equal to Uin/N+Uo when fully loaded, it indicates that the absorption is too strong and the resistance R needs to be increased; diode

If the voltage on C is slightly higher than Uin/N+Uo (5%~10%, depending on the withstand voltage of the rectifier tube) when fully loaded, it can be considered that the design parameters are reasonable;diode

Under different input voltages, verify whether the parameters are reasonable, and finally select the appropriate parameters.

Let's take a look at the absorption loss issues corresponding to two types of absorption circuits (using Flyback as an example):

Adopting RC absorption: The voltage on C after the primary MOS is turned on to steady state is Vo+Ui/N (Vo is the output voltage, Ui is the input voltage, and N is the initial stage turn ratio of the transformer). Because the time parameter of the RC we designed is much smaller than the switching period, it can be considered that RC charging and discharging can reach steady state within one absorption period. Therefore, the energy absorbed by each switching period is: secondary leakage inductance peak energy+RC steady state charging and discharging energy, approximately RC charging and discharging energy=C (Vo+Ui/N) ^ 2 (energy consumed on R, charged and discharged once per cycle), so the energy consumed by RC absorption is fswC (Vo+Ui/N) ^ 2. Taking DC300V input, 20V output,diode transformer turn ratio of 5, switching frequency of 100K, and absorption capacitance of 2.2NF as an example, the energy loss is 2.2N (20+300/5) ^ 2100K=1.4w; diode