Half Bridge SMPS series Capacitor selection - diyAudio
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Old 13th May 2011, 11:56 AM   #1
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Default Half Bridge SMPS series Capacitor selection

I have made a 14V15Amp and 28V10A HALF Bridge SMPS, where I am using 2.2 microF in series with the main transformer.
Now I want to design 28V 20 amp and 28v 30 amp.
What series capacitor I should use ?
Any thumb rule or design parameters ?
Please guide.
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Old 11th September 2011, 09:23 AM   #2
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Their is one rule.

Use as enormous capacitor as you like...the bigger the better.

However, the bigger the more expensive.

So therefore, use as small as you can get away with.

As long as the capacitor can handle the ripple current in it, then you are fine.

So, get on the simulator eg ltspice, and keep reducing the cap value until you see the normal half bridge waveform "curling over"

when it start to curl over, that means that the capacitive reactance is offering a impedance to the power flow, at the switching frequency.

A bit of "curl" is ok, but not too much.....i usually just get it so that the waveforms just loose their edgyness, and then stop there....but often , i then have to increase the cap value from this in order to handle the ripple current.
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Old 11th September 2011, 09:55 AM   #3
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There is a formula for it... Coupling capacitor size in microfarads is given by primary peak current in amps times maximum on time in microseconds over permissible voltage drop.

C = Ipk * Ton / Vdrop

Remember that the voltage drop of the coupling cap affects the voltage going into the transformer, with the average voltage the transformer sees at full load being the mean of the input voltage and what's left of it after you subtract the capacitor drop. Voltage drops of 15 to 30 volts are permissible.
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Old 11th September 2011, 12:24 PM   #4
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the conclusion of the following thread was that a capacitor in sereis with the primary is not even needed for a half-bridge....

i dont personally agree with that but they are convincing me against myself

Circuit Design - Half-Bridge vs Full Bridge SMPS?
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Old 11th September 2011, 01:37 PM   #5
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It is needed when you're feeding it 230v AC.
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Old 11th September 2011, 05:00 PM   #6
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ok thanks.
-you mean because with 230vac you dont have the benfit of a hard centre tap.?
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Old 15th September 2011, 02:32 PM   #7
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The cap provides a safety factor with excessive loads or core saturation. The cap will limit the amount of current passed through the transformer with a fixed switching frequency. The cap will only permit a fixed amount of energy to pass through the transformer per switching cycle.

Typically when testing new designs its good practice to under-size the switching cap so you can evaluate your circuit without blowing up the switching transistors if there is a problem (ie core saturation, problem on the secondary side, etc). Once an initial assessment is made, the cap is replaced with the design cap size.

Also note that you need to use an AC cap, not a polarized cap. Cap size is also dependant on switching frequency. The higher the switching frequency the smaller the cap size is needed to meet output current capacity. Other cap parameters to consider is ESR (which can change depending on switching frequency. Most datasheets spec. ESR at 1Khz, which is likely below your switching frequency). Caps with high ESR will impact energy transfer and the efficiency of your Power supply.
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Old 16th September 2011, 06:01 PM   #8
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thanks, the point about ESR is a good one

most metal polyprop film caps dont state the esr at typical switchign frequencies, which leaves people in the dark, somewhat.

eg this series dont mention esr......

http://www.panasonic.com/industrial/...BD0000CE48.pdf

Last edited by eem2am; 16th September 2011 at 06:03 PM.
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