Electronic – In a flyback transformer with multiple windings, how are voltages controlled for different current loads

flybacktransformer

This is the flyback transformer I will be referring to: Wurth 760871543

There are three output windings as follows, 24 V at 0.30 A, 5 V at 1.15 A, 14 V (unspecified but in mA range aux winding).

The voltage which I need to control precisely is the 24 V winding using secondary side sensing.

Here is my question: If the output I am sensing has a very low current load (1 mA), but the 5 V winding has a very high current load (1 A), won't the voltage at the 5 V winding drop much faster than at the 24 V winding?

If this is correct, how would I then go about controlling all the voltages simultaneously?

For example, if I instead sense the 5 V output winding for feedback, would switching the flyback transformer more frequently (to compensate for the larger current load) cause the voltage at the 24 V output winding to increase more than the regular 24 V?

Just some additional background on my design…
I am using a Power Integrations TinySwitch-4 switching controller. The 24 V will get increased to 1 kV through a converter and voltage multiplier. The 5 V will power a Raspberry Pi. The 14 V auxiliary winding will power the switching controller IC.

Best Answer

A multiple output flyback converter (with only one controller, i.e. PWM circuit/feedback loop) can only precisely control one voltage. If you need multiple precisely controlled voltages, you will need multiple independent controllers.

That being said, in theory the output voltage of a flyback converter is dependent ONLY on the duty cycle and transformer ratios - NOT on the load current. Unfortunately, this simple relationship breaks down because of losses, but in a well designed converter with fairly low loss components, the other output voltages should be close. Whether they are close enough depends on your allowable voltage range and the detailed design of the converter.

Typically, multi-output flybacks make the assumption that they know approximately what the current draw of each of their outputs will be, and set the turns ratios to get close enough to the intended output voltage over the expected range of load currents. However, all output voltages except the master output (whose voltage is being controlled) will vary over load conditions.