In flyback converters, specifying the output voltage ripple determines the output capacitance. Let’s look at some of the considerations for specifying the output voltage ripples.

Figure 1 shows a simplified flyback converter composed of input capacitor CIN, power transformer T1, and secondary side rectifying diode D1. The circuit in the blue box is the simplified capacitor model, which incorporates the ESR. RL is the output load resistor, M1 is the primary side switching FET, and RCS is the current sense resistor.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 3 Figure 1

Figure 1. Simplified flyback converter

Figure 2 gives the voltage and current waveforms for the relevant components.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 3 Figure 2

Figure 2. Waveforms for critical components in the simplified flyback converter

The output voltage ripple includes the voltage drop on ESR caused by the charging and discharging current, and the capacitor voltage variation caused by the charging and discharging process.  To lower the output voltage ripple, you can use a capacitor with a lower ESR. Be careful, though, because some capacitors may have a higher ESR value, even if the capacitance is large. That is, the output voltage ripple may still be high, even if you use a capacitor with higher capacitance.

Another way to lower the output voltage ripple is to use several electrolytic capacitors in parallel, and with ceramic capacitors. This lowers the ESR while also increasing the total equivalent capacitance. An example of this is shown in Figure 3. The C2 and C3 capacitors are in parallel.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 3 Figure 3

Figure 3. Using capacitors in parallel

Using a parallel configuration can become expensive if there are more than two or three capacitors. In this case, you can use a second-stage LC filtering circuit, shown in Figure 4, to reduce output ripple without adding too much cost to the converter.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 3 Figure 4

Figure 4. Addition of an LC filtering circuit

The blue box labeled “1” is the first-stage filter, which only uses filtering capacitors. The blue box labeled “2” is the second-stage filter, which uses an LC filtering circuit.