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Old 25th October 2010, 01:33 PM   #541
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Quote:
Originally Posted by AndrewT View Post
the thermal resistance between the device and the heatsink is preventing the heatsink from getting warm.
Look,

if you followed my build status you see, that i installed Kapton insulator washers between the Mosfetīs and the heatsink,
with an extreme low terminal resistance of 0,07K/W. This config. was untouched all the time.

With the addition of the copper blocks (with "Arctic Silver 5" thermal conductance paste in between) witch conduct the heat
from the sinks to the aluminum bottom, i lower the overall Rth.

Thatīs all.
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Old 25th October 2010, 01:52 PM   #542
AndrewT is offline AndrewT  Scotland
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Now in the case, with the copper blocks between the sinks and the aluminum bottom, they are cold.
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means the sinks are not dissipating any heat.
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Old 25th October 2010, 02:47 PM   #543
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Andrew, I think you are making an assumption that Oliver is an idiot. He isn't.
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Old 25th October 2010, 03:36 PM   #544
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Lucas,
do you understand what I have been saying?

I cannot tell whether it's a language problem or other, but DVB does not seem to understand that if the heatsink is cold it cannot be dissipating any heat.
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Old 25th October 2010, 03:49 PM   #545
AndrewT is offline AndrewT  Scotland
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Originally Posted by LucasAdamson View Post
what do you think about 31.4v to 15v DC regulation and 18.5v to 5v DC regulation? Is it a problem to reduce the voltage by this amount? Will this generate a lot of heat? Should I set a lower current than 200mA?

What about a bleeder resistor after the caps?
cascaded regulators are an option.
Bleeders increase PSU ripple and hardly change output voltage. They give you nothing back for the heat they generate.
Relay switched bleeders that conduct after the mains' power is turned off could be of some use.

If input power is high and output power is low then the difference is made up of losses. High Losses = High Heat.
It does not matter whether the losses are through inductors, or resistors, or transistors or a combination. They all give out heat that must equate to losses.
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Last edited by AndrewT; 25th October 2010 at 03:52 PM.
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Old 25th October 2010, 03:57 PM   #546
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Default Heat sink Insulation

Would someone please explain why the Mosfets need insulated from the Heat Sink or chassis?

If insulation is required, what insulating material is best to insulate and pass the heat to the sink?

Do the Diodes need insulated also?

Would someone please recommend part numbers for the insulators?
I don't know Laterals from Verticals

Last edited by ppcblaster; 25th October 2010 at 04:09 PM.
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Old 25th October 2010, 04:04 PM   #547
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The middle pin of power transistors is electrically identical to the backplate of the device.

Laterals have the backplate as the Source.
Verticals have the backplate as the Drain.
BJTs have the backplate as the Collector.

There are many thermally conductive and electrically insulating materials, of various prices, various thermal conductances and easy/difficult to handle.
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Old 25th October 2010, 04:04 PM   #548
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Andrew, I think he understands very well. Perhaps you are taking his words too literally when he says that the sinks are cold. Such terms are always relative. Relative to before he added the copper with thermal paste, it is now much "cooler" or "less warm" perhaps you would have preferred he say.

He has now got some quite massive and well connected heatsinks on his mosfets, and at the present 200mA draw, it is surely pulling much less current than it could.

I do understand what you're saying - it's clear, and I am sure that Oliver understands the principle of what you are saying, that cold heatsinks AND a hot mosfet, means the sinks aren't connected properly, but it is unlikely that the mosfets are effectively unsinked and burning hot, because Oliver is very careful, and I am sure would connect them to the sinks very tightly, and as he has said already, he is using quality insulating pads and thermal paste.

Perhaps the problem is that he said they were "cold", which can mean "stone cold" or else "relatively cooler than before" which I took it to mean.

Oliver is German, don't forget. In Oliver's own language, he would normally have more specific technical terms to chose from.
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Old 25th October 2010, 04:11 PM   #549
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Quote:
Originally Posted by AndrewT View Post
cascaded regulators are an option.
Bleeders increase PSU ripple and hardly change output voltage. They give you nothing back for the heat they generate.
Relay switched bleeders that conduct after the mains' power is turned off could be of some use.

If input power is high and output power is low then the difference is made up of losses. High Losses = High Heat.
It does not matter whether the losses are through inductors, or resistors, or transistors or a combination. They all give out heat that must equate to losses.
Thanks Andrew. That's what I thought. Such losses will generate in the mosfets, right? I am concerned that I may need bigger sinks or lower current to compensate for my high voltages.

So bleeder resistors are of no use then, other than to drain the caps at power off, in your experience? Draining the caps is hardly necessary either, is it? I may leave them out then.
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Old 25th October 2010, 04:33 PM   #550
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Quote:
Originally Posted by ppcblaster View Post
Would someone please explain why the Mosfets need insulated from the Heat Sink or chassis?

If insulation is required, what insulating material is best to insulate and pass the heat to the sink?

Do the Diodes need insulated also?

Would someone please recommend part numbers for the insulators?
I don't know Laterals from Verticals
Hi Gary,

The mosfets need insulating only if they are sharing a heatsink or two. If they are 4x independent sinks, no insulation is required or desirable. If sharing a sink, they need electrically insulating from each other, in such a way that the heat is still conducted. Oliver tends to buy the best, so copy him - Kapton insulator washers.

Diodes need no insulation from 2x56R up to and including a 10R current resistor, which is 99% of us.
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