Half-Bridge vs Full Bridge SMPS?

[grigson]

I think that Offline Half-Bridge converters are a complete waste of time....am i right?

The full-bridge is cheaper and better......

FET RMS current will be much higher with a half-bridge than a full bridge, and the half-bridge fets will need to be more pricey with bigger heatsinks.
-this is due to the unfortunate rail-splitting capacitors in a half-bridge, which divide the input voltage by two.

Also, the Half bridge needs these highly expensive , extremely low dissipation factor, rail-splitting film capacitors.....these are more expensive than a mosfet....so the extra mosfets in a full bridge, would be payed for by the fact that the full bridge doesn’t need the expensive film capacitors.

FULL BRIDGE COSTS:
4 FETs, Two NCP5181 Bootstrap fet drivers.

HALF-BRIDGE COSTS SO FAR
2 FETs, one NCP5181 Bootstrap fet driver, 2 Pulse rated film capacitors



...Already the half-bridge is looking awry...........but there's more woes for it........

the half bridge cannot utilise low-side resistive current sensing, because when the high side fet conducts...most of the power current would not go through the low-side current sense resistor.......so the half-bridge must use an expensive current sense transformer in order to be able to sense any primary overcurrents.

..the Full-Bridge can just use a cheap current sense resistor.


Also, in the event of overload, the half-bridge’s rail-splitting capacitors will end up with one discharged , and the other charged all the way up to the rail........which means that each rail-splitting capacitor must be rated for the full Vin of the DC Bus.....so thats two, expensive 400V rated pulse capacitors for the half bridge.

Also, the film capacitors for the half-bridge take up a large amount of space in the half bridge.


Is anyone seriously going to defend the half-bridge?


Even more woes for the half-bridge in that it cannot do peak current mode control without blowing itself up, or at the very least running in an unstable way...this is mitigated by adding a balancing winding, but thats more expense and complexity.

The full-bridge might need more heatsinks…but actually, no it won’t…because if you use Insulated tab fets you can simply fix the four fets of a full bridge to the same heatsink, side-by-side.

Its already been said that primary current is much higher for a half-bridge than a full-bridge……so there’s more woes for the half-bridge at the EMC testing stage.

So, Is anyone seriously going to defend the Half-Bridge vs the Full-Bridge?...Surely not?

 

[VEBill]

 

[Comcokid]

I like VE1BLLs response. It says it all about the fine points and trade-offs of choosing topologies in DC/DC and AC/DC conversion.

At one time I did RF and electrically small antennas, and I though it was somewhat a artform. Now (through no choice of my own) I design DC/DC and DC/AC power inverters, and at times it is a black-artform. At times, you need more insight and experience into 'magic' than Harry Potter.

Grigson - your issues are AC/DC or "off-line" converters which I'm not familiar with. The off-line half-bridge is the most popular, and the off-line full-bridge is considered more complex.

Resistive current sensing is a pain-in-the-butt. Noise generally swamps the current-ramp signal. If you have two sets of switches connected through the same current sense resistor, then layout symmetry drives the whole design lest you get unbalanced sensing from your two sets of switches. Current transformers (if you deal properly with the flux relaxization) can provide an almost pure text-book current ramp signal. But yes - they're more expensive.

 

[grigson]

Dont get me wrong, the half-bridge is ok if done with USA mains and a voltage doubler....because then your centre tap is a hard centre-tap....

this avoids the dreadful problems that exist with a half-bridge for example, when it is overloaded and goes into cycle-by-cycle current limiting.

..these problems can be mitigated by adding in the bells and whistles........but then the cost benefit is against this.

-Time to get out the plain, simple, full-bridge...no balancing windings, no 400V rated film capacitors, Lesser current rated FETs, no expensive avergage current mode controllers ever needed.

 

[benta]

One thing you have to beware of in full-bridges is "flux-walk" in the transformer.
This occurs if your FETs don't have 100% similar switching characteristics. The transformer flux will then not be symmetrical around zero, but start walking to one side, eventually resulting in saturation.
One way of dealing with this is to use a capacitor in series with the primary (expensive). The other way is to actively monitor the flux in the transformer and compensate. Again, not easy.

The half-bridge does not have this issue, as the splitting capacitors compensate.

Benta.

 

[grigson]

The thing is, half-bridges also need a capacitor in series with the primary....and in a half-bridge, the current in this capacitor is an awful lot higher than for a full bridge.

The full bridge may have four fets, but theyre rated for much lower current than a half bridge , and are cheaper and need less heatsinking.
...each fet in a full bridge will dissipate four times less than a fet in a half bridge.

 

[benta]

"The thing is, half-bridges also need a capacitor in series with the primary."

No, the voltage-dividing caps provide this function.

Apart from that, it's understood that you love full-bridges, so no more needs to be said

Benta.

 

[Skogsgurra]

grigson

Is this some kind of religious war you are fighting?

I think you should be in politics and not engineering.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Is anyone seriously going to defend the half-bridge?

Who can be bothered when you've already condemmed them?

Maybe you should check out Eng-Rants.

 

[grigson]

QUOTE
"The thing is, half-bridges also need a capacitor in series with the primary."

No, the voltage-dividing caps provide this function.
UNQUOTE

..However, ...and i am definetely not after anyone's vote here....but Raymond Mack's book "Demystifying switch mode power supplies", on page 160, states that for a half bridge, we either need a capacitor in series with the primary, or a Balancing winding, which keeps the voltages on the rail-splitting caps equal.

Mack states that half-bridge can be done without a balancing winding or series capacitor, but only if the rail splitting caps are the output caps of a mains voltage doubler rectifier...(because then, he says, we get a hard centre point voltage)