1. Schematic
2. Analysis: The function of the absorption circuit in the figure is to absorb and discharge the energy stored in the transformer leakage inductance when the main switch U1 of the power supply is turned off.
According to the formula: P = 1/2* LK * I (t) ^ 2, LK = 40uH,
I(t) is the charging current flowing through D3 to the absorption circuit. From Figure 1, the actual maximum value tested is about 45V/100R = 0.45A.
The time when the voltage in Figure 1 is positive is the charging time and corresponds to the time when the absorption capacitor voltage increases in Figure 4, which is about 320ns;
Assuming that the maximum current is always during the capacitor charging time, P = 0.5* 0.000004* * 0.45*0.45 *0.00000032=0.000000405W, the power is very small, and the discharge is very fast. The time for the absorption capacitor voltage to decrease in Figure 5 is about 520 ns. From the VDS waveform and the waveforms at both ends of the absorption capacitor, the peak charging and discharging time is about 840ns, which is much shorter than the switching cycle of 14 us. That is, the energy stored in the transformer leakage inductance can be discharged through the RCD in each cycle, and there is no energy accumulation and superposition phenomenon.
Figure 1 R1 voltage
Figure 2 CH1 is the voltage of the positive electrode of VBuck on the D pin of the MOS tube; CH2 is the voltage on the absorption capacitor C3
Expanding the waveform in Figure 2 yields the following waveform:
Figure 3
Figure 4
