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Old 10th January 2018, 08:19 PM   #1151
2 picoDumbs is offline 2 picoDumbs  Australia
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Quote:
Originally Posted by woofertester View Post
The capacitances of the hockey pucks or the capacitances of the front end transistors?
All of them, the hockeypucks are certainly the most offensive.
The cascoded front end stage is probably the least of your problems though.

Any reason you can't use 15V rails? Then you have 10V of PktoPk signal without issue
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Old 10th January 2018, 08:26 PM   #1152
woofertester is offline woofertester  United States
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Attached are the C-V curves for the ouput P-FET that I have initially chosen.
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File Type: jpg IPI120P04L3_CV.jpg (58.5 KB, 427 views)
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Old 10th January 2018, 08:56 PM   #1153
woofertester is offline woofertester  United States
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The front end and Vas is +/- 24v. The outputs are set a +/- 5V because the load will be a nearly short circuit compared to any ordinary speaker. The worst-case is what is shown in the schematic at 30 milli-ohms and that is a 24" skinny ribbon. A shorter skinny ribbon will be lower resistance.

Any voltage above 5V will be wasting power on the output devices.
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Old 10th January 2018, 09:15 PM   #1154
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That one looks quite nice.
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Old 10th January 2018, 10:00 PM   #1155
woofertester is offline woofertester  United States
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Originally Posted by 2 picoDumbs View Post
That one looks quite nice.
That is the IPB120P04P4L. Its a TO220. I am not certain how good it will be at getting the heat out to the heatsink. They were $2.31 each from Digikey. I bought 10 each of the P and N. If I blow up a few, no big deal.

If you look at the IdVd curve, Vds is 1V at 400A. Thats about 2.5 milliohms RDSon. I cannot find as good of a P part in the hockey puck package.
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Old 10th January 2018, 10:33 PM   #1156
2 picoDumbs is offline 2 picoDumbs  Australia
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In terms of Class A dissipation, I wouldn't probably exceed 10W.
When it comes to high frequency AC dissipation, I don't know.
I have no experience pushing the limits on to220 devices.
At $2 each it's worth a try.
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Old 11th January 2018, 07:16 PM   #1157
indra1 is online now indra1  Indonesia
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In trying to help woofertester I look for ways to stabilize bias using very low current sensing resistor and came across Hall effect current sensor. The ACS723 attached only have 0.65 mΩ sensing resistance. I posted some sim in Auto-Biasing Circuits for Complementary Followers thread. Could be useful to you guys. I have not tested it yet, but linuxguru in tubes forum have mentioned using similar device to sense cathode current.
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File Type: pdf ACS723-Datasheet.pdf (678.7 KB, 67 views)
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Old 18th January 2018, 02:19 PM   #1158
woofertester is offline woofertester  United States
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Default Ids vs Vgs

Today I ran some tests on the IPP120N04S4. Attached is the curve for Ids vs Vgs.

The test rig can only go to 10A and the cables from the rig are ridiculously resistive compared the RDSon of the MOSFET.

I just wanted to see where the bias wants to be to get a good running start at the linear portion of the curve. Looks like between 1.5A and 2A bias.

I am currently gathering heatsinks and parts to throw together a prototype.
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File Type: png 01Plot.png (48.0 KB, 299 views)
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Old 18th January 2018, 03:48 PM   #1159
indra1 is online now indra1  Indonesia
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Please be aware that Vds-Id measurement should be done at constant temperature. The curve is temperature dependent.
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Old 18th January 2018, 04:01 PM   #1160
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Originally Posted by indra1 View Post
Please be aware that Vds-Id measurement should be done at constant temperature. The curve is temperature dependent.
I agree. I have used two approaches:
  1. Vgs ramp at a sufficiently high frequency, such as 200Hz, that temperature effects are minimal. This requires a way to capture Idrain, such as measuring the voltage across a sense resistor.
  2. Low duty cycle Vgs pulses of varying pulse height, with the "nominal" level to maintain the desired operating temperature of the FET.
In my implementations, I use a Rigol DG1022 programmable waveform generator and a Rigol DS1054Z digital sampling scope. The measurements from the two approaches agree quite well.
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