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Old 23rd September 2010, 05:51 PM   #501
Brit01 is offline Brit01  United Kingdom
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Just got through this whole thread and very informative especially regarding the mosfets.
Planning to incorporate these into one of my current projects.

Thanks guys.

Nice work.
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Old 24th September 2010, 12:06 PM   #502
Brit01 is offline Brit01  United Kingdom
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Mosfets IRF 710/820/821 will be fine for this application? I've got 310, maybe a bit small.
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Old 24th September 2010, 12:20 PM   #503
chrish is offline chrish  Australia
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I believe the important parameter is a low reverse transfer capacitance and enough drain source voltage capacity. Not sure how the IRF310 compares to the others...
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Old 24th September 2010, 12:44 PM   #504
Brit01 is offline Brit01  United Kingdom
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Thxs.

Let me check out the data sheets and compare
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Old 24th September 2010, 01:01 PM   #505
SY is offline SY  United States
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Quote:
Originally Posted by Brit01 View Post
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Mosfets IRF 710/820/821 will be fine for this application? I've got 310, maybe a bit small.
I've been using IRF820s for at least 20 years in that application with no problems. If the reverse transfer capacitance is an issue (and with 6SN7s ahead of it, it hasn't been for me), you could cascode. But a plain-Jane IRF820 source follower is sufficient to drive the hell out of the grids- or in screen drive with higher perveance tubes like 6JN6, 6LF6, and their ilk. And they're dirt cheap.

One thing to note- the source followers don't have to run Class A. That can save you some thermal issues.
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Old 24th September 2010, 01:03 PM   #506
Brit01 is offline Brit01  United Kingdom
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310:

transfer capacitance: 6.3 pf
gate voltage 400v
gs voltage: 20v

820:

transfer capacitance: 37 pf
gate voltage 500v
gs voltage: 20v

Am I looking for a higher transfer capacitance?
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Old 24th September 2010, 01:07 PM   #507
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As Chris and I both found out, either choose a mosfet that has zener diodes internally from gate to source, or add them externally. Turn on - warm up transients will blow the gate out of an unprotected mosfet.

Quote:
I believe the important parameter is a low reverse transfer capacitance
Look for a spec called Crss. You want it to be low, preferably under 10 pF, and constant under the range of voltages that will be seen in actual use. There is uaually a graph of this and other capacitance related specs VS voltage. Most fets are quite non linear below a certain voltage, then go fairly constant. The transition voltage is usually 10 to 50 volts although some fets never flatten out.

This means that you are putting a capacitor that changes value with the applied signal voltage directly in the audio path. TIM and PIM distortions can result, so we want to keep the capacitance constant and low.
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Old 24th September 2010, 01:18 PM   #508
Brit01 is offline Brit01  United Kingdom
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Quote:
Look for a spec called Crss. You want it to be low, preferably under 10 pF,
thxs tubelab.

Yes I get it now.
So the 310 are more ideal as they only have a Crss of 6.3 pf.

I see that some have put 2 x 15v zeners back to back to avoid over voltages and smoked fets.

This is a good practice with the 310 also? These look identical, just slightly different voltage and capacitance values.
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Old 24th September 2010, 01:28 PM   #509
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Quote:
I've been using IRF820s for at least 20 years in that application with no problems.
I used that one for a while. It needs external diodes, and yes I have blown a few because they were not present.

I have used Toshiba parts for the last 5 years. Look at the Toshiba 2SK3563. It has protection diodes built in, and the Crss is ruler flat above 20 volts. The case is isolated so that the heat sink will not shock you.
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Old 24th September 2010, 01:34 PM   #510
TheGimp is offline TheGimp  United States
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I found an IRFR310, but not an IRF310.

Crss 6.3pF VDS=25V, f=1.0MHz

IRFR310 looks good above 30V for Crss flatening out.

MTD2N40/MTD1N50 are the only FETs so far that I have found data at high VDS values and it shows a change in Crss from 7pF to 3pF with a VDS change from 35V to 300V for the MTD2N40, and 7pF to 2.5pF for a VDS change from 25V to 500V for the MTD1N50.
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