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Old 20th March 2007, 10:53 AM   #1
Pierre is offline Pierre  France
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Default Selecting coupling cap for half-bridge offline SMPS

Hello all.
I am designing a half-bridge switching power supply, in the range of 800W-1Kw.

I would like to have your advice on what type of capacitor to use to couple one of the primary pins to the center point between the bus capacitors.
For 230VAC operation, where you have a bus voltage of up to around 350V, I guess I need half (one of the pins of the cap is connected to half bus, around 175V and the other one swings from 0 to 350V) of that plus a safety margin. Say, a 250Vdc cap would be a nice choose?

About capacitance, something around 1uF could be ok.

BUT what about the current rating and dV/dt capability? Must I go for any special kind of capacitor, or is a polyester type ok for this kind of power (peak current can reach 8A)?

Thanks!!!
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Old 20th March 2007, 11:32 AM   #2
Eva is offline Eva  Spain
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Consider average or peak current mode control in order to avoid coupling capacitors.
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Old 20th March 2007, 11:36 AM   #3
Pierre is offline Pierre  France
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Thanks, Eva. Your help is always appreciated.

I know what you mean, the coupling cap is there to avoid flux imbalance, and that can be avoided by precise current mode control, elliminating the need for the cap.
However, I really would like to stick to a simple topology by the moment (will probably go for current mode control afterwards) and for my first experiments I would like to use a coupling cap.
Can I use a simple Poly or ceramic cap or must I look for something special?
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Old 20th March 2007, 04:25 PM   #4
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Hi Pierre,

If you have access to Geo. Chryssis' book "High Frequency Switching Power Supplies" (1984 & 1989), check out the coupling cap design procedure. As for value of Capacitor, most 200-250W AT/ATX power supplies use 1mF rated at 200VDC. As for the type of cap, I can't remember the type used in pc supplies, but that type should do. For 800W - 1kW, this value needs to be beefed up a bit. I don't have the equation in front of me, but I will post it when I get home (at work now) .

I would guess that your value is going to be in the 3-6mF range, so you're better off going with 3 to 6- 1mF caps in parallel, since lower-value 200V caps will be much easier to locate, not to mention less expensive, too.

I saw this trick in an article in either QST or 73 a few years back about a half-bridge SMPS rated at 13.8V @ 40A, continuous duty-cycle. The coupling caps there were something like 8 to 10 0.47 mF, 200V caps in parallel.

Hope this helps,

Streve
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Old 20th March 2007, 07:20 PM   #5
Pierre is offline Pierre  France
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Thanks.
The thing is not simple, however.

Polypropilene MKT caps that are rated at 250V or 400V are are typically limited in dV/dT. And the max. AC allowable voltage decreases drastically with frequency: the 250 or 400V rating is usually for 50/60Hz. If you see the graphs that describe some of these caps, the useful voltage swing at 80KHz or so is below 30V!!!

However, as N-Channel said, 250V caps they are routinely used in PC PSUs, etc. Don't know why they are able to work for years, however, as they are, if I am right, severly underrated.

Perhaps Eva can put some light on this issue...

BTW: Did some tests with my PSU today and it runs perfect with no coupling cap (primary directly connected to half bus voltage). I have tested it at 700W output for an hour with no problems.

Pierre
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Old 20th March 2007, 07:52 PM   #6
Duo is offline Duo  Canada
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You must remember that as the AC current increases in frequency, the voltage drop occurring across the coupling caps decreases as their reactance decreases.

The idea here is to have very little voltage across the caps for most of the time.
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Old 21st March 2007, 03:08 AM   #7
luka is offline luka  Slovenia
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Hi Pierre

What core do you use? Do you have it regulated? And what Fsw?
I used 2uF and was semi good for 550w.
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Old 21st March 2007, 04:31 AM   #8
Eva is offline Eva  Spain
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Quote:
Originally posted by Pierre
Thanks.
The thing is not simple, however.

Polypropilene MKT caps that are rated at 250V or 400V are are typically limited in dV/dT. And the max. AC allowable voltage decreases drastically with frequency: the 250 or 400V rating is usually for 50/60Hz. If you see the graphs that describe some of these caps, the useful voltage swing at 80KHz or so is below 30V!!!

However, as N-Channel said, 250V caps they are routinely used in PC PSUs, etc. Don't know why they are able to work for years, however, as they are, if I am right, severly underrated.

Perhaps Eva can put some light on this issue...
It's reliable because the voltage swing across the capacitor during normal operation is in the 30Vp-p range...


Quote:

BTW: Did some tests with my PSU today and it runs perfect with no coupling cap (primary directly connected to half bus voltage). I have tested it at 700W output for an hour with no problems.

Pierre
You should also check it with low loads and with load steps...
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Old 21st March 2007, 10:25 AM   #9
Pierre is offline Pierre  France
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Is this capacitor, for example, adequate, or too slow?

http://catalog.digikey.com/scripts/p...name=BC1807-ND

Thanks!
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Old 21st March 2007, 10:40 AM   #10
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That link was for a 100mH adjustable inductor.
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