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

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.

• Why is the 24 V value critical? You haven't explained that and we're missing the link to the switching controller datasheet. Please add the info into your question. – Transistor Mar 19 '18 at 17:32

## 3 Answers

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.

• Note that some multi-output flybacks have their feedback derived from a ratioed combination of several (or even all) outputs. For example, for a +5V, +12V supply, the duty cycle could be adjusted depending on by the error at the +5V output weighted by 80% and the error at the +12V output weighted by 20%. So, the +5V will be pretty accurate, but the +12V will still be somewhat regulated, although less accurately. This improves cross-regulation and avoids some outputs drifting too much when loads are very unequal. See eetimes.com/author.asp?section_id=183&doc_id=1321744. – dim Mar 19 '18 at 16:15
• You also see secondary side saturatable reactors used to provide independent voltage control (Usually in power supplies a sight bigger then that one however), so it can be done. – Dan Mills Mar 19 '18 at 16:59

You simply can't control the output voltages of that transformer separately. If you have an uneven load on its secondary, then the "unregulated" output voltage will go up or down depending on the situation, just like you already mentioned.

Do you really need the 24V to be that accurate? A voltage multiplier's output goes down quite a lot under load so you might need additional regulation there anyway (sensing the 1kV high voltage line).

If both of your voltages have to be accurate, you can design the converter to deliver 6V instead of 5V and use an additional linear regulator to get regulated 5V.

Your load regulation Error and Relaxation voltage time constant Will both be dependent on the load resistance ratio with source impedance or the percentage of maximum current rating.

Therefore your control feedback should be from the highest power load which is probably your 24 V boost regulator . Since the 5V load is small, The overvoltage can be regulated with an LDO.