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Old 1st December 2011, 02:21 PM   #1
jrenkin is offline jrenkin  United States
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Default high voltage on heat sink

is there a reason the mosfet and CCs in the TSE are electrically in contact with the heat sink? Is it ok to insulate them from it or will that cause a problem. I am using a well over sized heat sink, so a little reduction in thermal transfer shouldn't be an issue. I just would rather not have the large heat sink at high voltage while I am adjusting bias and working inside the amp unless necessary.
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Old 1st December 2011, 02:33 PM   #2
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What P/N mosfets are you using?
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Old 1st December 2011, 02:36 PM   #3
jrenkin is offline jrenkin  United States
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High Voltage Mosfet Toshiba 2SK3563
IXCP10M45S CCS IC

as listed in the parts list

I bought them a while ago, it has taken time to get to this project
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Old 1st December 2011, 03:16 PM   #4
Aiace is offline Aiace  Italy
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You could buy an insulated SilPad to cunduct just the heat and not the current.
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Old 1st December 2011, 03:41 PM   #5
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Quote:
Originally Posted by jrenkin View Post
is there a reason the mosfet and CCs in the TSE are electrically in contact with the heat sink? Is it ok to insulate them from it or will that cause a problem. I am using a well over sized heat sink, so a little reduction in thermal transfer shouldn't be an issue. I just would rather not have the large heat sink at high voltage while I am adjusting bias and working inside the amp unless necessary.
As mentioned, you can insulate them with a pad. The 2SK3563 has an insulated case/mounting tab, although the 10M45S mounting tab is connected to the anode according to the data sheet.
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Old 1st December 2011, 04:13 PM   #6
jrenkin is offline jrenkin  United States
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So no downside to insulating? I was curious why George doesn't, in the interest of safety.
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Old 1st December 2011, 05:07 PM   #7
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How hot are they getting without pads ?

A Sil-Pad or better a Mica Pad will increase the temperature by about 0.5 Degrees C per Watt. If they are critically hot you might toast them. If they are warm to the touch then you will do no harm.

I assume that no crazy designer has used the heatsink as a current / signal path.
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Old 1st December 2011, 05:13 PM   #8
jrenkin is offline jrenkin  United States
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Ha, ha, but I can't touch them without the pads, they are at high voltage. I could after I turn it off, I suppose, but I haven't built it yet. I have very large heatsinks, pulled from a SS amp, so am not worried about it being too hot, just want to be sure that i am not missing something. I didn't think so, and I will insulate them just not to have to worry about the voltage on the heatsink. The thing is, with these large heatsinks, they are quite prominent and working on the operating amp seems fool hardy since just adjusting the bias might bring me uncomfortably close to a large high voltage chunk of aluminum.
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Old 1st December 2011, 05:29 PM   #9
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Quote:
Originally Posted by jrenkin View Post
Ha, ha, but I can't touch them without the pads, they are at high voltage. I could after I turn it off, I suppose, but I haven't built it yet. I have very large heatsinks, pulled from a SS amp, so am not worried about it being too hot, just want to be sure that i am not missing something. I didn't think so, and I will insulate them just not to have to worry about the voltage on the heatsink. The thing is, with these large heatsinks, they are quite prominent and working on the operating amp seems fool hardy since just adjusting the bias might bring me uncomfortably close to a large high voltage chunk of aluminum.
If the heatsink forms no part of the circuit then you can but try.

See how hot they get.

Any transistor that gets above 75 Degrees after a few minutes is operating at its limits even on a heatsink.
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Old 1st December 2011, 05:42 PM   #10
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Some designs use the heatsink to connect the Collectors or Drains of the High Current Output Devices.
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