FULL BRIDGE SMPS: With or without poly cap in primary coil? - diyAudio
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Old 5th June 2009, 10:31 PM   #1
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Default FULL BRIDGE SMPS: With or without poly cap in primary coil?

Hi Dears
Exatly! This is my question for you:
In the FULL BRIDGE SMPS: Do I need to use the poly cap (1uF, 2u2,....400V) in primary coil?
This question is here because I see various SMPS FB projects with and without this cap.. (?)

Thak you very much

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Old 5th June 2009, 11:40 PM   #2
TechGuy is offline TechGuy  United States
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Location: Connecticut
The cap is used in Full bridge to prevent run away current flow if the transformer saturates. but the cap also has ESR, which increases the switch losses. The cap limits the amount of energy that can be transferred on each switching cycle. As the charge saturates in the capacitor the current input into the transformer drops. You may have to buy a large value Cap (or use multiple caps in parallel), if your output load is high and your core material limits your switching speed. If your output is drawing a heavy load the cap can increase output noise, because of a voltage drop on the secondary when the cap begins to saturate and limits input current to the transformer.

If you're concerned that your design may have bugs, you can also use a cap as safety training wheels to prevent blowing MOSFETS\IGBTs\FUSES ,if your controller fails to operate in the safe operating range.
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Old 6th June 2009, 12:01 AM   #3
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Default Great TechGuy

Mr. TechGuy

Ok, now I undestand because my 115/230VAC - 14.2VDC SMPS blowed very, so fast when I connected 0.15 ohms power resistor in output ...( ! )
Working with a 0.18 ohms power resistor in DC output (14.2V) no problem without the series cap, the IRFP460 mosfets (x4) units stay no warm, the output schotky diodes (Mospec S30D40C x 4 UNITS) no warm too, the IR2110 (02 UNITS) and SG3525 no problem, but, the core (toroid 4cm2 Ae) running in 57,45Khz stay some warm.
I 'll try build with, maybe 3u/400V poly cap to see the results.

Thank you very much by your comments.

Sandro from Brazil.
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Old 7th June 2009, 12:11 AM   #4
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In a voltage mode supply fitting the capacitor is a very good idea.

In a peak current mode power supply on the other hand, a series capacitor must not be used.
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Old 9th June 2009, 01:55 PM   #5
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Location: Warsaw
From your describtion it looks like you use a simple out of phase H-Bridge, don't you?
I suggest you used this capacitor then, for even RDSon differences may result in long-term DC saturation of transformer core.
A very long dead time helps though.

The case is different for phase-shift H-bridge, where you don't need that capacitor at all, aside from other benefits.
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