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Old 9th November 2006, 05:03 AM   #1
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Default Will this ckt work fine?

Pls help me in finding out any problem if there is any? Im doing this to get about 600W of power. What all things should be considered??Im attaching the circuit below. Pls do reply...
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Old 9th November 2006, 08:22 AM   #2
Elvee is offline Elvee  Belgium
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Hi,

I see a number of problems: first you have to provide some start-up supply; maybe you didn't bother to include it in the diagram.
Secondly, you've to include a filter choke in the output, otherwise the PWM will not work: the output voltage will always be the peak voltage (more or less).
Thirdly, BR1 seems to be a standard bridge; you'll need fast diodes.
You'd also need to provide a current sensing/limitation on the switching transistors; simply relying on the soft-start seems risky.
As it is drawn, the 56K/100n snubber is totally useless: it will simply heat up the 56K without protecting in anyway the MOSFETs.
You have to put them in // and add a diode in series.
LV
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Old 9th November 2006, 09:06 AM   #3
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Hi Elvee,

Thanks for ur valuable reply. Im actually quite unfamiliar with push pull smps designs. I have a few questions. Here, is it safe to use a 700v MOSFET? How much power the ckt can deliver safely? What value should be adopted for main filter capactitors. The one chosen was actually a rough figure. Can an additional power supply of abt 12v (less than 1A) can be delivered with another winding? Does the circuit around optocoupler make sense.
Also of what power rating shuld be used with Mosfets.

Im attaching the modified circuit here. They still have possible errors...
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Old 9th November 2006, 11:45 AM   #4
Elvee is offline Elvee  Belgium
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700V is really too short: the theoretical voltage is 640V, but if you take into account a 10% tolerance on the mains, you're already over 700V; in addition, you must be aware that your clipper/snubber won't be 100% effective. In my opinion, 800V is an absolute minimum.
With suitable MOSFETs and other components, the circuit could deliver 600W; if the main filter cap is able to take the ripple current, it should be OK (provided you've sufficient regulation margins to take the ~35Vpp ripple). The current in the FETs would be around 3A.
The chokes must be placed directly after the diodes, before any capacitor. The two coils may share the same core (respect polarity off course).
If you intend to use this PSU for audio applications, you'll have have to seriously overdesign it in order to cope with high power bass.
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Old 9th November 2006, 12:54 PM   #5
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Old 9th November 2006, 03:51 PM   #6
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I'll second that.

Rej-

Essentially, you're taking a topology (center-tap push-pull) most often used for +12V inputs and adapting it for use of the 230VAC Line. While this can be done, it is not advised. The only real pro I can see for this is the simplified drive arrangement from the SG3525 to the MOSFETs, that's it. The cons for this are the high-voltage MOSFETs needed that Elvee referred to, and the higher Rds(on) they will have, resulting in lower efficiency. Also, you will be introducing very high voltages into the circuit (>700V). Very dangerous.

Re-arranging the two MOSFETs in a half-bridge will lower the minimum Vds rating to ~400-500V- much easier to find, and perhaps cheaper, too.

Also, the two inductors between C5&C6, and C8&C9, respectively should actually come before C5&C8, since this is ra egulated and PWM'ed supply. These coils store energy during the t(off) part of the cycle, much like the inductor in a classic buck regulator.

Better cross-regulation between the (+) and (-) output rails can be achieved by coupling these two inductors. A good source for a core to do this is the BIG yellow toroid found in amlost all AT & ATX pc supplies. Simply calculate your new windings for the new application, and drop it in the circuit. Your clock freq of 64Khz is OK enough, though you might want to kick that up a bit, just to really remove it from the sudio band.

Other than that, the basic ckt looks OK.
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Old 10th November 2006, 09:37 AM   #7
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hi frnds,

First of all thank u very much for ur replies. Let me tell you that the one reason that made me chose this circuit was the possible elimination of transformers as they r the hardest part to find. I had seen some half bridge topologies using many transformers at different places. I could already find a 900v, 7A MOSFET. Is there any reason for adopting another topology? Is main trasformer winding 'too' critical during the design? What about the wattage? Is half-bridge better than this one? waiting for ur replies...
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Old 10th November 2006, 09:52 AM   #8
sss is offline sss  Israel
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there are pretty simple transformers to drive the mosfets,if i remember correctly they called pulse transformers.
u can wind them yourself on some small ferrite cores that u can take from small inductors.
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Old 10th November 2006, 11:22 AM   #9
Elvee is offline Elvee  Belgium
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Default Re: reply

Quote:
Originally posted by rejithcv
hi frnds,

First of all thank u very much for ur replies. Let me tell you that the one reason that made me chose this circuit was the possible elimination of transformers as they r the hardest part to find. I had seen some half bridge topologies using many transformers at different places. I could already find a 900v, 7A MOSFET. Is there any reason for adopting another topology? Is main trasformer winding 'too' critical during the design? What about the wattage? Is half-bridge better than this one? waiting for ur replies...
Gate transformers aren't really a problem: they are small and cheap, and you can even wind them yourself on a base drive toroid salvaged from an ES lamp. There is also the option of dedicated high-side gate drivers.
The power transformer winding with your topology is quite critical: you want an as close coupling as possible between the two primaries to minimize stray inductances and associated overshoots, but with 700V between them it's no easy task.
And accessorily, the copper utilization is better in a half-bridge converter.
LV
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Old 10th November 2006, 11:26 AM   #10
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Is there any driver transformer readily available in market. If not pls let me know the widely available and almost cheap dedicated high-side gate driver ic
-Thnks
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