Electronics > Power/Renewable Energy/EV's

Reducing size of isolated power supplies

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5PNdcc0Jety0:
Hello world,

I've been recently musing about how is it practically possible to make isolated 220V to 20V AC-DC power supplies without bulky passives.

The most obvious one is the size of isolation transformer, followed by safety capacitors.

Can one make a power supply for a < 100W application using a smaller transformer if instead of flyback topology, you drive the transformer with high frequency AC, and rectify after it?

Is it practical to forego the transformer altogether, and instead use something like piezo transformer, or capacitive isolation?

David Hess:
The usual solution is to operate at higher frequency so that the passive parts including the transformer can be smaller.

Piezoelectric transformers exist but are commonly used only where very high isolation is required.  I do not think they have very high power density.

5PNdcc0Jety0:
It's clear that they do get smaller with frequencies, but ones going to simple flybacks are already much smaller today that say 20 years ago.

Would the size of passives needed to do AC-AC conversion to drive a very high frequency transformer be small enough to not to offset the gain from a smaller transformer size?

David Hess:

--- Quote from: 5PNdcc0Jety0 on May 14, 2021, 03:23:23 pm ---It's clear that they do get smaller with frequencies, but ones going to simple flybacks are already much smaller today that say 20 years ago.
--- End quote ---

The advantage of high frequency operation applies just as much to flyback designs as the others.


--- Quote ---Would the size of passives needed to do AC-AC conversion to drive a very high frequency transformer be small enough to not to offset the gain from a smaller transformer size?
--- End quote ---

I am not sure what you mean.  The advantage of higher frequency applies to the passives and the transformer up until the point where losses increase which require physically larger parts to dissipate increased heating.

ocset:
if you want 100W and small, then its more difficult because you need PFC if you are in some countries.

Also, SMPS for offline connection are usually with switching frequency <150khz as thats where the EMC limits start.
I mean, the smallest way to do it would probably be to use a Integrated transformer....like a transformer in a PCB.
But they are quite expensive.
Otherwise, i reckon the smallest way for you would be to use a power.com innoswitch, which gives a small solution because the isolation is in the control chip which sits nicely under the transformer.

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