Switching PSU Theory of Operation / Repair Tips & Techniques

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Wed Jul 26 10:56:43 2000

> > it's some kind of voltage doubler. Since I can't imagine why they'd need
> to do a doubling act
> > for the primary side of a SMPS I'm inclined to suspect the former.

In HVSMPS (High voltage switchmode power supplies) the input voltage is
converted to DC at some high voltage and stored in capacitor prior to
being converted to AC (via chopper) and so on... The reason for high
voltage is that twofold. Same thing can be applied to 240/480 supplies.

For powersupplies that must (or can) run off 115V/230V you can use voltage
doubled 115VAC or halfwave(or fullwave) rectifed 230VAC to get nearly the
same voltage in the capacitor bank making multivoltage operation
convenient.

Also for a given volume of capacitor higher voltage means lower current
for a given power. Lower current means less loss to internal (and
external) resistances in the circuit, hence higher effientcy.

Once you have the high voltage DC you than chop it into square waves
using various possible circuits so that it can be applied to a transformer
for conversion to lower AC voltages. One of the hallmarks of HVSMPS units
is small size and this is accomplised by useing higher frequencies that
nominal 50/60HZ. Typically frequencies from 5khz (old designs) to as high
as 250 or even 500khz (very latest designs) are used to allow using very
compact transformer cores. A Ferrite core at 100khz for 400 watts is far
smaller than the common EI core for 50/60hz, lighter too!

I hope this gives some insight to SMPS.

I've not mentioned how regulation is achieved but the common way is to
vary the width of the pulse appled to the transformer, The narrower the
pulse the less power it contains. The circuit monitors the rectified and
filtered DC from the output side of the transformer (torid, cup core or
other form of ferrite core transformer) output side and uses that
information (voltage and current limits) to modulate the chopper and
therfor by servo action regulate the output.

> > The Prime techie mentioned a bridge, so I assume it's not a doubler
> circuit,

In the quote that likely ment the switching chopper circuit, commonly
used in higher powered is a 4 (or more) transistor(or HEXfet) bridged
circuit.

Allison
Received on Wed Jul 26 2000 - 10:56:43 BST