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!!!
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!!!
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?
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?
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
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
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
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
Is this capacitor, for example, adequate, or too slow?
http://catalog.digikey.com/scripts/partsearch.dll?Detail?name=BC1807-ND
Thanks!
http://catalog.digikey.com/scripts/partsearch.dll?Detail?name=BC1807-ND
Thanks!
¿¿??? Don't know what happened...
Check for BC1807-ND at www.digikey.com and have a look at the datasheet, please.
Thanks!
Check for BC1807-ND at www.digikey.com and have a look at the datasheet, please.
Thanks!
Maybe You Can Use a Smaller Coupling Capacitor
I think you may like to try using a smaller capacitor than commonly considered.
I have built many circuits for my own experimentation and testing which use capacitances like .22uf for a few hundred watts output off a 320v supply. A good quality higher voltage mylar capacitor apparently is rather durable.
Before I realized that I could shunt resonance peaks between the transformer inductance and such a coupling capacitor to the power supply rails during current limiting supplied by the capacitor, I used to work on bench test circuits which exceeded the rated breakdown voltage of the capacitors for a short time without them even failing. I have read that such capacitors have a certain amount of self-healing capability of the dielectric layer. The capacitors would eventually fail if I did not reduce the output power under such circumstances, but they were still functional so long as I did not leave them breaking down for more than a second, or so, at 50khz. I still have many of those capacitors with mangled, bent leads because of having continued using them in successive test circuits.
So now, that the capacitors which were rated at 250vdc were able to survive peaks over 320VDC at 50khz gives you good assurance that ones rated at 400VDC should survive with a good margin of safety at 340v peak when shunting diodes clip off the resonance peaks during maximum power output conditions from the power supply.
Right now on my test bench is a circuit with a good quality .22uF mylar capacitor coupling the primary winding of my half-bridge circuit to the MOSFET totem pole. I can short out the secondary output voltage which causes the voltage on the coupling capacitor to clip against the power supply rails at a frequency of about 25khz without any hint of circuit malfunction or stress.
Here is an LTspice circuit showing how my real circuit uses the .22uF coupling capacitor. The series inductor on the secondary side will help to limit the maximum current through your output rectifiers during a prolonged output short condition. "Your mileage may vary."
http://www.diyaudio.com/forums/showthread.php?postid=1156412#post1156412
I think you may like to try using a smaller capacitor than commonly considered.
I have built many circuits for my own experimentation and testing which use capacitances like .22uf for a few hundred watts output off a 320v supply. A good quality higher voltage mylar capacitor apparently is rather durable.
Before I realized that I could shunt resonance peaks between the transformer inductance and such a coupling capacitor to the power supply rails during current limiting supplied by the capacitor, I used to work on bench test circuits which exceeded the rated breakdown voltage of the capacitors for a short time without them even failing. I have read that such capacitors have a certain amount of self-healing capability of the dielectric layer. The capacitors would eventually fail if I did not reduce the output power under such circumstances, but they were still functional so long as I did not leave them breaking down for more than a second, or so, at 50khz. I still have many of those capacitors with mangled, bent leads because of having continued using them in successive test circuits.
So now, that the capacitors which were rated at 250vdc were able to survive peaks over 320VDC at 50khz gives you good assurance that ones rated at 400VDC should survive with a good margin of safety at 340v peak when shunting diodes clip off the resonance peaks during maximum power output conditions from the power supply.
Right now on my test bench is a circuit with a good quality .22uF mylar capacitor coupling the primary winding of my half-bridge circuit to the MOSFET totem pole. I can short out the secondary output voltage which causes the voltage on the coupling capacitor to clip against the power supply rails at a frequency of about 25khz without any hint of circuit malfunction or stress.
Here is an LTspice circuit showing how my real circuit uses the .22uF coupling capacitor. The series inductor on the secondary side will help to limit the maximum current through your output rectifiers during a prolonged output short condition. "Your mileage may vary."
http://www.diyaudio.com/forums/showthread.php?postid=1156412#post1156412
Rick,
Illegibility is in the eye of the beholder. This diagram is structured for easy simulation on LTspice without straining my eyes. You are welcome to get the LTspice circuits and then stretch out the diagrams by using the component drag function until the components sizes get as small as you like and there is as much empty space on the diagram as you feel comfortable with. I prefer to draw my diagrams in the compact style of the Korean Tatung television repair manuals.
However, if you ask politely, I can help you find something if you are really interested.
Illegibility is in the eye of the beholder. This diagram is structured for easy simulation on LTspice without straining my eyes. You are welcome to get the LTspice circuits and then stretch out the diagrams by using the component drag function until the components sizes get as small as you like and there is as much empty space on the diagram as you feel comfortable with. I prefer to draw my diagrams in the compact style of the Korean Tatung television repair manuals.
However, if you ask politely, I can help you find something if you are really interested.
Pierre said:
Pierre,
Here is the equation for your original question: to choose the value of the coupling capacitor C(c), C(c) = [I(max) x dt] / dV(C(c)),
Where I(max) is 1.2 x the drain current I(d) of the MOSFETs, dt = period of 1 cycle, or 1/F, and d(V(C(c)) is the maximum voltage across the cap, usually chosen at ~30V so as not to interfere with regulation at low AC line (180VAC or 90VAC).
So for a frequency of 60kHz, a drain current of 1.25A, & coupling cap voltage of 30V, we get:
C(c) = [ (1.5A) x (16.67 mS ] / 30V = 0.83mF, for a 120W half-bridge. Extrapolate these numbers for your power levels (say, 1kW) and frequency (80-kHz) to get:
C(c) = [11.78A x 12.5mS] / 30V = 4.91mF. Since the nearest standard value, 5.6mF is a BIG cap at 400V, you would choose 5 - 1mF Caps in parallel.
Steve
Hi
Have you bought those 50W resistors cos they aren't cheap.
I hope I will able to get resistor wire and make one big resistor out of it, wich willbe/can be use as power trimer. I have to test my supply on 4 of 1ohm 20w resistor each to give me 4 ohms to test 500w+ but if you do the math you can see that not for more than few seconds
Have you bought those 50W resistors cos they aren't cheap.
I hope I will able to get resistor wire and make one big resistor out of it, wich willbe/can be use as power trimer
. I have to test my supply on 4 of 1ohm 20w resistor each to give me 4 ohms to test 500w+ but if you do the math you can see that not for more than few seconds
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