In designing a CCM flyback converter, ceramic capacitors alone are suitable, but seven would be required to realise 83µF after DC-biasing effects.
Loss calculations associated with Coss are somewhat nebulous, as this capacitance is quite non-linear, decreasing with higher Vds, and for this design is estimated to be 0.2W.
Capacitor requirements generally consist of calculating the maximum RMS current, the minimum capacitance necessary to obtain the desired ripple voltage and holdup for transients. Output capacitance and IOUTRMS are calculated as:
Ceramic capacitors alone are suitable, but seven would be required to realise 83µF after DC-biasing effects. Therefore, I only chose enough to handle the RMS current and followed with an LC filter to reduce output ripple voltage, as well as improve load transients. If large load transients exist, additional output capacitance may be required to reduce voltage droop.
The input capacitance is equal to:
Again, you must consider the capacitance-robbing DC-bias effect. The RMS current is approximately:
Figure 2 shows the prototype converter’s efficiency while Figure 3 shows the flyback evaluation board.
Figure 2: Converter efficiency and losses dictate package selection and thermal requirements.
Figure 3: 60W flyback evaluation hardware measures 100mm × 35mm.
This design example covers basic component calculations of a functional CCM flyback design. However, initial estimates often make it necessary to iterate the calculations in order to fine tune it. Still, more detail work is often necessary in areas such as transformer design and control-loop stabilisation in order to obtain a well-working, optimised flyback.
First published by EDN.