DC Transformer?

[meteor]

There have been past posts, like mine asking about "parallel" balancing Li-Po chargers, that mention the use of "DC transformers".

The transformer(s) in the balancing charger case is obviously needed to separate the ground references amongst each individual charger, when powered from a single DC source, but I'm curious how it's done...


A "DC transformer" is obviously an oxymoron, anybody like to (simply) describe how they actually work?



Thanks! :)

[jeffs555]

They work just like any other switching power supply. A normal AC powered switching supply has a rectifier and filter on the input to convert the AC to DC. This DC is then switched on and off, usually at a high frequency. It is then fed through a real AC transformer. It is then rectified and filtered back into DC. The high frequency switching allows the transformer and filter components to be small.

For a "DC transformer" you don't need the rectifier and filter on the input, but everything else is still the same.

[meteor]

...It is then rectified and filtered back into DC. The high frequency switching allows the transformer and filter components to be small.



OK, we have a square-wave from 0V to Vcc on the input side, and I assume it's exactly the same thing on the output side, albeit isolated. (is that correct?)

Does the output gain an AC component somehow? IE, why do you need a rectifier?

Sorry if this is basic transformer physics, but I've never gone there...


Thanks.

[Probedude]

There's some pictures of the guts of a DN/CSRC charger on this board somewhere. If you look carefully at the toroids you can see there are 2 sets of winding them.

The voltage may be a square wave on the input, but due to the inductance of the transformer/toroid, the current ramps on the primary - it looks like a saw tooth. When the primary is switched off the magnetic field collapses and the energy gets dumped into the secondary through a high speed diode into a capacitor and then your battery. Without the rectifier, the energy would be able to flow back from your cap and battery, directly across the secondary of your transformer, burning up components.

The first CSRC charger, without reverse polarity protection, failed exactly this way when the battery was connected backwards. This diode would now be in conduction.

As you can see from pictures, the toroids have very few turns of wire on them. DC resistance is very low.

[jeffs555]

You are correct in how it works. It is more properly called an "isolated DC to DC converter".
And, any time you switch a DC voltage, you add and AC component to it.