Regulation in flyback converter ...HOW??

Thread Starter

supermankid

Joined May 26, 2013
54
Flyback converter can be used to create multiple output using multiple secondary transformers. I am wondering how is the regulation maintained....for eg.....if we have 5V, 3V3, 1V8 and 1V5 as output from 50V input. If the load changes on one of the output...does this not effect the other rail....I don't understand how the controller maintains the reulation on all the rails if load on all rails is changing....or is there a clever idea I might me missing?:confused:
 

Alec_t

Joined Sep 17, 2013
14,280
Only one rail is regulated usually. The other rails are assumed to track the regulated rail. If the tracking isn't accurate enough for your purpose you might consider using individual post-converter regulators for the rails which need it.
 

ian field

Joined Oct 27, 2012
6,536
Flyback converter can be used to create multiple output using multiple secondary transformers. I am wondering how is the regulation maintained....for eg.....if we have 5V, 3V3, 1V8 and 1V5 as output from 50V input. If the load changes on one of the output...does this not effect the other rail....I don't understand how the controller maintains the reulation on all the rails if load on all rails is changing....or is there a clever idea I might me missing?:confused:
A really good PSU will have a chopper transformer for every rail, otherwise there are various methods, the cheap ones have tightly coupled secondaries so as long as the regulation sampled rail is within spec, the others won't be far off.

Common in TV set top boxes, several rails are sampled and summed together in a resistor network.
 

#12

Joined Nov 30, 2010
18,224
The basic principle is about voltage being proportional in every winding. You pretty much can't have one winding with a different volts per turn ratio than the other windings in series with it. If the load makes any single voltage sag, they all sag and the drive is increased until that voltage is right. The proportionality of the transformers windings forces voltage tracking even if the current loads are different.
 

ian field

Joined Oct 27, 2012
6,536
The basic principle is about voltage being proportional in every winding. You pretty much can't have one winding with a different volts per turn ratio than the other windings in series with it. If the load makes any single voltage sag, they all sag and the drive is increased until that voltage is right. The proportionality of the transformers windings forces voltage tracking even if the current loads are different.
They're usually high frequency switching, so the secondary windings will have few turns and very little DC resistance - but you can't count it out altogether when you go down to 3.3, 2.7 and 1.8V outputs.

At those voltages, having to add extra turns to compensate for rectifier Vf doesn't help.
 

Thread Starter

supermankid

Joined May 26, 2013
54
They're usually high frequency switching, so the secondary windings will have few turns and very little DC resistance - but you can't count it out altogether when you go down to 3.3, 2.7 and 1.8V outputs.

At those voltages, having to add extra turns to compensate for rectifier Vf doesn't help.
Hi, could you please explain me what it means by this can't count it out altogether when you go down to lower voltage.......thanks in advance
 

ian field

Joined Oct 27, 2012
6,536
Hi, could you please explain me what it means by this can't count it out altogether when you go down to lower voltage.......thanks in advance
At very low voltage *ALL* unwanted resistances in the circuit become significant.

At very low voltage; the Vf of the rectifier amounts to a significant percentage of the intended output voltage, so the winding length on the secondary has to be increased to compensate for it.

Its only a few milli-Ohms here and there, but at several amperes or more the volt drops add up.
 
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