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Old 17th April 2013, 08:57 PM   #3571
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Thanks.

I am afraid I have another dumb question.

I picked this diode.

http://www.onsemi.com/pub_link/Colla...MBR40250-D.PDF

Do I need a heat sink? I couldn't make sense of the thermal graph.
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Old 21st April 2013, 04:40 AM   #3572
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Is there a specific reason that the design calls for 2 rectifier bridges? Why not just use a single bridge to create a bipolar supply? Or is it just for the increased current handling capability?
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Old 21st April 2013, 09:21 AM   #3573
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cleaner final gnd point ;

also - when using CT secondary , current imbalance between windings can induce sort of xformer buzz ; this way that's avoided

you can take that as Pa's words ( so granted ) even if I just recreate it by memory
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Old 21st April 2013, 11:11 AM   #3574
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Thanks.

I intend to use 2 transformers, each having 2 x 18V secondary, to create a single power supply. So there are a few ways that I can hook them up.

The simplest seems to be each transformer in full wave rectification configuration, with one doing the positive and the other the negative. But I don't understand about the current imbalance thing . Would it cause the aforementioned problem? Or according to the oracle , I need to have one bridge on each transformer?
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Old 21st April 2013, 11:25 AM   #3575
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yup - one Graetz per xformer/winding ...... no headache
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Old 13th May 2013, 10:15 PM   #3576
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Just when I thought I am inches away from the glorious finish...

I have not adjusted the bias. On one of the channel, the output resistor measures about 120mV across. There isn't any measurable DC at output. Seems good.

However things aren't so good on the other channel. Most output resistor measures about 150mV, except 2 of the IRFP240. One measures about 70mV and the other about 220mV. There is about 1V DC at output. Questions:

1. I suppose those 2 IRFP240 are fried? Or only 1 of them?
2. Is it likely that other MOSFETs are fried too?
3. Is it possible that the MOSFETs are alright and there is merely a bad connection somewhere that cause these symptoms?
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Old 13th May 2013, 10:25 PM   #3577
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are those mosfets matched ?
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Old 13th May 2013, 11:46 PM   #3578
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If you match to .1V (which I recommend) then this is within range.

Your offset is off. Try adjusting the resistors from the supply to the bias circuit.

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Old 14th May 2013, 02:15 AM   #3579
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I totally forgot to post this link -

- Building the Pass/Firstwatt F4 -
A guide to building the Pass F4 amplifier.
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Old 14th May 2013, 06:03 AM   #3580
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Thanks guys.

The MOSFETs are supposed to be matched. However I can only take the seller's word for it.

Although I have not adjusted the offset too, I matched the trimpot resistance to the resistor on the negative rail. I don't know if this would ensure no DC offset, but this work for the other channel. My feeling is the MOSFET, no?

Edit:

I asked the seller to matched Vgs to 10mV for all 12 MOSFETs (2 channels). The seller provided the Vgs reading of each individual MOSFET, the numbers are indeed matched to under 10mV, but at unknown condition/parameters. From the pool of 12 I split them into 2 groups and each group has under 5mV variation. But as said I can only take the seller's word for it.

Last edited by Navyblue; 14th May 2013 at 06:12 AM.
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