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Old 10th July 2008, 07:52 PM   #1
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Default Full Bridge X Half Bridge: What's The Best?

Hi guys

Tell me: what's the best topology for a 13.8VDC/60A smps? HB or FB?
If a use 2+2 mosfet in paralel (half bridge) have the same power that 2 + 2 mosfet in full bridge configuration?

Thank you very much.
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Old 11th July 2008, 01:36 AM   #2
areza is offline areza  Bangladesh
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FB is better then HB, phase shift FB may be best , 13.8V x 60A can be done with HB allow low cost and less complexity, why not use one MOSFET with proper power ratting,
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Old 11th July 2008, 06:05 AM   #3
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Hi,
Full is always better than half and it is applicable in this case also.
Yes, you can have same power because half+half = Full, but pulse balancing is better in full bridge.

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Old 11th July 2008, 02:33 PM   #4
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Hi dears

I know what you tell. Why there is some FB projects with the series capacitor (1uF, 2,2uF) and anothers without this? I try built the FB topology and working no charge the mosfets is "ice" in heatsink and with some 15A charge they're very hot. In HB topology the mosfets don't worm in 30A, only after 40A charge.

Thank you very much.
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Old 11th July 2008, 03:29 PM   #5
areza is offline areza  Bangladesh
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show your schematics with correct part details, like supply voltage , mosfet type , etc,
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Old 11th July 2008, 03:42 PM   #6
Tahmid is offline Tahmid  Bangladesh
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Hi,
In Full bridge circuit, the series capacitor is not required. In Half bridge it is required to prevent DC current coming to the load from the MOSFETs/capacitors. This is very unlikely in Full bridge as the switching is COMPLETELY based on the MOSFETs.
MOSFETs in a full bridge configuration are not supposed to get that hot. A reason for heating up could be improper high side MOSFET gate drive.



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Old 17th July 2008, 07:03 PM   #7
golam is offline golam  Bangladesh
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Hi all,

Full Bridge circuit is the most complex one out of all the configurations. If settings are not proper, lot of problems have to be faced not only Mosfets becoming hot, even mosfets can be destroyed and if setting is correct, FB is most dependable circuit. So, one should be conversant with all modalities of a FB circuit before indulging in it. Otherwise, success rate will be zero.
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Old 17th July 2008, 09:00 PM   #8
TechGuy is offline TechGuy  United States
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Default Re: Thanks

Quote:
Originally posted by CHACALPOWERS
Hi dears

I know what you tell. Why there is some FB projects with the series capacitor (1uF, 2,2uF) and anothers without this? I try built the FB topology and working no charge the mosfets is "ice" in heatsink and with some 15A charge they're very hot. In HB topology the mosfets don't worm in 30A, only after 40A charge.

Thank you very much.

A wild guess is that there is something wrong with your high-side gate drivers, that preventing them from saturating or saturaturing fast enough. If you connect an Oscilliscope and probe to monitor the output switching voltage on the primary side. The voltage change should appear vertical and the waveform should be close to a perfect square. If the edges are rounded then you might not be applying enough gate charge, the High side gate voltage is too low or there is too much impedience in your gate drive. Another issue unrelated to FB/HB, is transformer saturation.


The Purpose of a series cap would be to limit current flow or to prevent a transformer from saturating. Its usefull in lower power circuits. but not recommend for high power use.

One issue with the HB is that its dependant on a electrolytic Cap. Over time these dry out resulting in increased ESR and they will eventually fail. FWIW: I don't like using electrolytic caps to transfer power or with High Ripple, as they have limited lifetimes before they fall or must be replaced. Over time the ESR will increase causing the cap. to run hotter causing faster electrolyte depletion and loss of efficiency.

Electrolyte Caps in a Car Power Supply are even worse because of the high tempertures in the summer heat as well as the frequent temperture swings between day and night and the seasons. These conditions reduce the lifetime of electrolyte caps.
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Old 18th July 2008, 04:45 PM   #9
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Default Re: Re: Thanks

Quote:
Originally posted by TechGuy



A wild guess is that there is something wrong with your high-side gate drivers, that preventing them from saturating or saturaturing fast enough. If you connect an Oscilliscope and probe to monitor the output switching voltage on the primary side. The voltage change should appear vertical and the waveform should be close to a perfect square. If the edges are rounded then you might not be applying enough gate charge, the High side gate voltage is too low or there is too much impedience in your gate drive. Another issue unrelated to FB/HB, is transformer saturation.


The Purpose of a series cap would be to limit current flow or to prevent a transformer from saturating. Its usefull in lower power circuits. but not recommend for high power use.

One issue with the HB is that its dependant on a electrolytic Cap. Over time these dry out resulting in increased ESR and they will eventually fail. FWIW: I don't like using electrolytic caps to transfer power or with High Ripple, as they have limited lifetimes before they fall or must be replaced. Over time the ESR will increase causing the cap. to run hotter causing faster electrolyte depletion and loss of efficiency.

Electrolyte Caps in a Car Power Supply are even worse because of the high tempertures in the summer heat as well as the frequent temperture swings between day and night and the seasons. These conditions reduce the lifetime of electrolyte caps.
I use film capacitors to couple the primary transformer winding to the half bridge MOSFETs.
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