Full-Bridge SMPS is cheaper than Half-Bridge?

[grizedale]

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

the FETs in a half-bridge only switch half of vin (due to the rail-splitting capacitors), so FET RMS current will be much higher with a half-bridge than a full bridge, and the fets will need to be more pricey with bigger heatsinks.

Also, the Half bridge needs highly expensive , pulse rated 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 doesnt 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-bridges 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.

Also, since the half-bridge has more primary current flowing (due to the rail-splitting capacitors gaiving a low primary voltage when the fet switchs on)...it will require more filtering components, including capacitors, and will be harder to get through EMC.

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

 

[Orson Cart]

There are plenty of half bridges in manufacture that use feedforward of the mains rectified line to eliminate the 100/120Hz ripple, and that use a shunt-resistor/opamp on the sec side to do average current mode control - which gives a good fast current limit and protects the mosfets, literally hundreds of thousands have been made and sold for power levels up to 1800 watts... Regards, Orson Cart.

 

[grizedale]

I haven't found these Half-Bridge PWM controllers that do mains ripple feedforward.....the MC34025 doesnt have it.

Half bridge controllers for offline use are few in number, but i havent seen the ones you describe.

......nor have i found controllers for offline use which facilitate the average current mode....i am not sure how useful that would be for us because our load is highly transient in nature.

And in any case, if you start up into a short, then nothing on the secondary side ever gets powered up....so you rely on primary current sense to curtail current on a cycle by cycle basis.....this results in the centre-point of the rail-splitting capacitors wandering off to one of the rails....and bang!.........that is, explodes if we didnt use 400V rated film capacitors in the rail-splitter.

And in any case, why bother with all those extras that you describe......just use a full-bridge and job done, , at a cheaper price


...Another point, maybe those hundreds of thousands of converters that you describe were for USA mains when a voltage doubler is used.............in this case, the centre point is a rock-solid, hard centre-point....and in that case........your half bridge is a good idea.

I put it to you all , that in any other circumstance, the half-bridge is a bogus topology.

 

[ALERTLINKS]

One instant i apreciate is a 12V supply for 50 watts spotlight chinamade is available two for 1$

 

[grizedale]

i think you are talking about a resonant half-bridge topology, and this is totally different than a hard-switched half-bridge.

I dont think anyone would ever do a half-bridge for 50W.....thats flyback territory...or , in the case of lighting, resonant half-bridge or self-oscillating half-bridge type of thing.

Lighting is the oasis of resonant , or ZVS half bridge converters....all florescent bulbs are driven by ZVS half bridges...but these are not the same as the hard-switched half bridges that is under discussion here.

 

[ALERTLINKS]

In terms of cost you may be right. Highly priced filter capacitors.But in mass production, it may be a different story. And why every ps supply is made with Half-Bridge converter.
By the way as Pakistan is a dump place for old monitors ,computers etc. The are disasembled here and recycled here. I can get 20 470uF/200V capacitors for 1$ and they are still better than chinamade capacitors available. Similarly 20 pieces of irf740 for 1 $.

 

[Orson Cart]

Ripple feed forward is usually implemented additionally to make the chips perform better, same for average current mode (except PFC i.c.'s), the 1/2 bridges I mentioned were for 230Vac, quite often a small cheap CT is used for pri side overcurrent protection. The same control IC's are used for half bridge or full bridge - there is no intrinsic difference unless the full bridge is phase shifted. Regards, Orson.

 

[electrophysics]

As we see computer ATX power supplies for 230VAC supply operation are made with half bridge. for a novice full bridge is better choice.

 

[mtwieg]

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

In a decent full bridge design, you'll still have at least one film capacitor on the DC bus to suppress switching transients. Also each half bridge should have a snubber of some sort.

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.

I don't think CTs are actually that expensive, at least not in bulk. Probably not much more than a good current sense resistor.

Also, in the event of overload, the half-bridges 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.

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.

No, this is wrong. With proper control and sensing you can prevent flux walking and drift under fault conditions while using peak current mode control. It does take a well designed controller and some cleverness though.

Also, since the half-bridge has more primary current flowing (due to the rail-splitting capacitors gaiving a low primary voltage when the fet switchs on)...it will require more filtering components, including capacitors, and will be harder to get through EMC.

This is true.

I think the short answer is that at certain power levels and certain applications, the benefits of full bridge converters aren't really needed, and the disadvantages of half bridge converters aren't really relevant. Half bridge converters have issues such as flux walking and rail splitting, but these are all solvable for competent engineers. Full bridge, on the other hand, is easier to design but has a higher parts count, which no amount of cleverness can change.

 

[grizedale]

Thankyou mtwieg,

No, this is wrong. With proper control and sensing you can prevent flux walking and drift under fault conditions while using peak current mode control. It does take a well designed controller and some cleverness though.

......does the "well designed controller" that you speak of cost a lot of money?.....surely it would.?

....And the need for "cleverness"?.....are we talking lots of mathematics?, or a good feel for the principles of physics?, or just being "in the know"?

 

[mtwieg]

......does the "well designed controller" that you speak of cost a lot of money?.....surely it would.?

It certainly doesn't have to. One approach to make current mode control feasible it to detect the average current from the primary CT and use that signal to change the pulse widths delivered to each of the FETs and balance the volt-time product applied to the primary, which prevents flux walking over long periods of time. There's no reason such functionality can't be built into the control chip at no extra cost, but I'm not sure if anyone has done so.

....And the need for "cleverness"?.....are we talking lots of mathematics?, or a good feel for the principles of physics?

No, not really, but whoever came up with protection methods like that was certainly clever.