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Old 25th July 2005, 04:29 AM   #1
Loial is offline Loial  Sweden
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Default Recommended BJT/MOSFET:s

Hi
I'm in the beginning of constructing a very lightweight, slim amplifier. The amp itself is a Tripath-circuit, already up & running.
But now I'm starting the construction of the PSU, an SMPS PSU capable of approx. 2*30V@200W.
I've built a couple 12V DC/DC converters, with no major problems at all. Now this amp is running on the mains ( 230V )
And my question is - what topology do you think is suitable? and what transistor/s would you recommend for primary switching?
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Old 25th July 2005, 04:31 AM   #2
Loial is offline Loial  Sweden
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I forgot - Is a PFC necessary, or is it considered overkill for 200W?
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Old 25th July 2005, 05:09 AM   #3
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Loial
I forgot - Is a PFC necessary, or is it considered overkill for 200W?
Definetely waste of time in DIY. Leave PFC to be your power companys problem. Nowadays PFC is required in europe for power supplies over 70W but for DIY projects building a PFC for anything else than fun doesnt make sense. You just end up with lower effiency when running with PFC.
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Old 25th July 2005, 06:15 AM   #4
Eva is offline Eva  Spain
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I have designed and built a 2Kw PFC for fun

Actually PFC provides some advantages since it outputs a high voltage and it's almost regulated, thus a simple unregulated full bridge with low-cost TO-220 IGBTs rated at 10A and a single transformer (with no additional inductors) is enough to step-down the output of the PFC and provide as much isolated and coupled outputs as you want. Note that a single ampere of primary-side current at 400V already produces 400W output.

When a PFC is not used, you either have to mess with regulation or accept huge fluctuations in output voltages. I agree that PFC increases cost, size and complexity but I like it anyway.


Concerning switching devices, as most people will probably recommend MOSFET, I'm going to recommend SKP10N60 IGBTs from Infineon and MJE13009 bipolars from On-Semi. In the past I had bad experiences with high-voltage MOSFETs so I decided to no longer use them.

Concerning topologies, the obvious recommendation for your application is half-bridge, and full-bridge when you want to double power output without using more expensive switches.
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Old 25th July 2005, 07:17 AM   #5
Loial is offline Loial  Sweden
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How come the use of a PFC makes the regulation better?
If the output of the PFC is almost regulated, doesn't the SMPS need to have a regulation of some kind? thinking of the difference in current consumption in idle vs. full load.
What makes the PFC 400V different from the 325V rectified mains voltage?

I'm not really into what the PFC does, more than making the PSU produce less EMI?
A PFC doesn't seem too complicated though, maybe ill try building one.
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Old 25th July 2005, 07:58 AM   #6
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Loial
How come the use of a PFC makes the regulation better?
If the output of the PFC is almost regulated, doesn't the SMPS need to have a regulation of some kind? thinking of the difference in current consumption in idle vs. full load.
What makes the PFC 400V different from the 325V rectified mains voltage?

I'm not really into what the PFC does, more than making the PSU produce less EMI?
PFC 400V is already regulated vs. 325v rectified mains is something like +-10% to +-30% depending wether you live in scandinavia or nepal
There is still some ripple voltage left on PFC output caps, maybe 5-10% typically, but this ripple voltage can be reached as well in direct mains rectification. Universal-input pre-regulator is another benefit of PFC, but i doubt that you need to desing your power supply as 80-250Vac input for DIY.

EMI can easily be even worse with PFC than without. All PFC does it that it makes your power supply look like resistive load instead of drawing huge peaks of current for 5ms and no current at all for rest of half-cycle. your electricity company will be more happy for easier load and bigger consuption

Why PFC is nonsense for DIY is that there is maybe one handfull of SMPS hobbyist in Sweden and still 5 million older-made computer and other smps power supplies in service without any kind of PFC. Lots of load with bad power factor causes unneccessary heating of transmission lines, requires bigger fuses and so on.
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Old 25th July 2005, 08:40 AM   #7
Loial is offline Loial  Sweden
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Anyone have any experience in how much an unregulated PSU fluctuates in voltage, between idle and full load, if the mains voltage is at a stable 230V?
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Old 25th July 2005, 09:52 AM   #8
Eva is offline Eva  Spain
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With 220AC and 940uF of storage I remember measuring 320DC with no load and 280V with 30Vp-p ripple witrh 1Kw load. Neglecting SMPS loses you can scale down these numbers, so for example you would have 60V rails with no load and 52.5V rails witn 5.6Vp-p ripple under 1Kw load.

Having 940uF with 320V is like having more than 13.000uF per rail with +-60V (use E=.5*C*V^2 to compare energy storage).

(Obvious disclaimer: High voltages are dangerous, particularly when they are not isolated from earth, so use them at your own risk).
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Old 25th July 2005, 10:59 AM   #9
Loial is offline Loial  Sweden
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That sounds promosing, 10-15% voltage difference shouldn't be a problem. The amp-circuit itself can handle +/- 60Vdc.

with half-bridge topology, what is the voltage stress on the primary switchers? I've got an excellent page on my favorites at home - cant remember the name now, though...

One last question ( hehe, i doubt THAT... )
- Would a UCC3810 circuit do the job as a timer/oscillator?
i know it is a little bit of an overkill that to - but they grow on trees here were i work :-D
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Old 25th July 2005, 12:40 PM   #10
Loial is offline Loial  Sweden
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Oh, concerning the half-bridge SMPS input caps, is there any formulae or thumb rule to use when dimensioning these?
The caps is the reason why i've been avoiding Half bridge - capacitors seem so... fragile, if you know what i mean :-)
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