Flyback switching power supply output capacitance calculation

1. Set the switch operating frequency: f=60kHz, output current Io=1A; according to the transformer, input and output voltages, calculate the actual maximum duty cycle Dmax=0.457;

2. Calculate Toff and Ton:

Toff=1/f*(1-Dmax)=9.05 Ton=1/f*Dmax=7.62

3. Calculate the output peak current:

From the waveform above, we can see that: Io decreases, Uo also decreases, that is, the output electrolytic capacitor mainly maintains the voltage from t1 to t2. Assuming the output ripple is 120mV:

5. Ripple current is generally taken as 5% to 20% of the output current, that is, Inppl=20%*1=0.2A. The actual ripple current of each electrolytic capacitor is 0.2A, so it meets the design requirements.

6. Actual maximum value

7. Empirical formula

Note: The ESR value needs to be determined according to the actual ripple current. The actual usage value should be much smaller than the calculated value; it should be about 20% or less of the maximum value.