Calculation of output capacitance of flyback converter

The example of a flyback converter is used to explain the calculation of output capacitance. This example is an RCC topology with an output power of 6W, an output voltage of 5V, and an output voltage of 1.2A. At the minimum input voltage, the duty cycle is 0.5 and the operating frequency is 100KHz. (For simplicity of data, the frequency is taken as an integer)

Principle analysis:

First: In the flyback (RCC topology), the capacitor at the output is used to store energy. When the switch is turned on, the output capacitor is responsible for power supply. So we can start with the energy storage of the capacitor.

Second: In AC-DC power modules, we generally use electrolytic capacitors as energy storage devices. We should not only start from the energy storage of the capacitor, but also calculate from the EMR of the capacitor.

The first option:

1. Capacitor supply ripple current

There are three currents at the positive electrode of the output capacitor: one is the current supplied by the output winding, which is an AC current (changing); one is the current flowing through the load, which is a DC current (unchanged); and the other is the current flowing through the capacitor.

From the above, we can know that the current peak of the output winding is the current ripple of the capacitor.

2. Calculate the power supply time of the capacitor

From the duty cycle, we know that the peak value of the output current IP2=4*1.2A=4.8A;

As we all know, the output current of the output winding is a triangular wave, so the time ratio of the output winding power supply current less than 1.2A is D2=1-D+1/4*(1-D), and D2=0.625 is obtained.

Here, D2 is considered to be the time ratio of the output capacitor supplying power to the load. When the winding output current is less than 1.2A, the power supply time of the winding is ignored.

Suggestion: When reading the above explanation, in order to make you understand better, it is best to draw the circuit diagram yourself, and also the waveform diagram of the output current and the current waveform diagram of the output winding....

In critical mode, when the duty cycle is 0.5, the input current peak is 4 times the average value.

Then we can know it by mapping from primary to secondary. The following is the diagram used in the working process:

Winding output current waveform

Setting: The maximum output voltage ripple is 50mV,

What we will talk about next is when aluminum electrolytic capacitors are used at the output end.

Here we use the experience that "the equivalent series resistance ESR of an aluminum electrolytic capacitor is twice the capacitive reactance XC". This is mentioned in the book "Introduction to Switching Power Supplies".