Cross Regulation of Flyback Power Supplies

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Cross Regulation of Flyback Power Supplies [Copy link]

1. Flyback power supply

When choosing a system topology that can generate multiple outputs from a single supply, a flyback power supply is a wise choice. Since the voltage on each transformer winding is proportional to the number of turns in that winding, each output voltage can be easily set by the number of turns. In an ideal case, if one of the output voltages is regulated, all other outputs will scale with the number of turns and remain stable.

2. How to improve the cross regulation rate of flyback power supply

In real life, parasitic components work together to degrade the load regulation of an unregulated output. I will further explore the effects of parasitic inductance and how to use synchronous rectification instead of diodes to significantly improve the cross regulation of a flyback power supply.

For example, a flyback power supply can generate two 12V outputs at 1A each from a 48V input, as shown in the simplified simulation model of Figure 1. The ideal diode model has zero forward voltage drop and negligible resistance. The transformer winding resistance is negligible, and only the parasitic inductance in series with the transformer leads can be modeled.

These inductors are the leakage inductance within the transformer, as well as the parasitic inductance within the printed circuit board (PCB) traces and diodes. When these inductors are set, the two outputs track each other because the full coupling of the transformer forces the two outputs to be equal when the diodes conduct during the 1-D portion of the switching cycle.

Figure 1. This simplified flyback model simulates the effect of leakage inductance on output voltage regulation.

Now consider what happens when you introduce 100 nH of leakage inductance into the two secondary leads of the transformer, and 3 μH of leakage in series with the primary winding. These inductances create parasitic inductances in the current path, which include leakage inductance inside the transformer and inductance in the PCB and other components.

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