A question about SMPS

Thread Starter

safa373

Joined Jul 20, 2023
2
Hello,

I have a question about SMPS.

I haven't seen an SMPS or a schematics of it using bridge rectifier on the output section, Why?
It's either a half-wave rectifier or a center tapped full-wave rectifier.
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Is there a reason or any reason to not use bridge rectifier on the output section of an SMPS?

Thanks.
 

BobTPH

Joined Jun 5, 2013
9,173
Yes, you don‘t need a bridge when what it rectifies does not alternate in polarity. In fact, using a bridge would push current back toward the input and reduce or even totally squash the output.
 

BobTPH

Joined Jun 5, 2013
9,173
Actually, ignore my answer, when the output is through a transformer, a bridge rectifier would be fine. My answer applies to non-isolated buck or boost converters.
 

Ian0

Joined Aug 7, 2020
10,054
If the bobbin is full then there is a choice between a single winding and a bridge rectifier, and a split phase winding and two diodes.
The former gives half the copper loss and twice the diode loss, the latter gives twice the copper loss and half the diode loss.
For low voltage windings, the diode loss dominates, so the split phase circuit is preferred. For high voltage windings, the copper loss dominates, so the bridge rectifier is preferred.
 

Hymie

Joined Mar 30, 2018
1,288
I don’t think I’ve ever seen a switch mode PSU incorporating a bridge rectifier, the primary reason for this is the switching frequency of the transformer.

In a linear power supply operating at 50/60Hz without a bridge rectifier, the reservoir capacitor(s) would be being topped up 50/60 times a second, and at 100/120 times a second with a bridge rectifier. Being topped up every 100/120 times a second reduces the voltage discharge drop between cycles, halving the capacitance value required for a given supply ripple voltage.

But with a switch mode PSU switching at 40kHz without a bridge rectifier the reservoir capacitor(s) will be topped up every 25µs, allowing the use of a significantly lower reservoir capacitance for a given supply ripple voltage.
 
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