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Old 16th October 2018, 05:55 PM   #1451
Eric is offline Eric  United States
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OK - I have enough parts on hand to play around tonight without needing to wait for another order, then.

I actually started this project before completing others so I could measure the differences among various thermal interface materials. My progress ground to a halt when the mosfets started dying. I should screw stuff up more often - I'll learn more
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Old 16th October 2018, 05:59 PM   #1452
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oh yeah ...... so I should know 42 until now ........

which obviously isn't the case

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Old 16th October 2018, 07:51 PM   #1453
Eric is offline Eric  United States
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Is there any other answer to the meaning of life?!?
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Old 16th October 2018, 07:56 PM   #1454
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Buddha is everywhere ........ but 42 is easiest to pronounce

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Old 16th October 2018, 08:55 PM   #1455
PRR is online now PRR  United States
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Quote:
Originally Posted by Eric View Post
...will a 9v1 zener will limit the output capability...
No.

The circuit is a Source Follower. Source "follows" Gate very closely.

Assume the working gain is 0.95. The other 0.05 appears across Gate-Source. Taking 20V peak swings, Gate wobbles +/-1V. Assuming the 2.5A static bias is 2V, voltage Gate-Source goes 1V to 3V. Assuming a lesser MOSFET with 4V bias and 2V swing, 2V to 8V. In audio we NEVER go to the gigantic currents shown on datasheets for 10Vgs, and (for enhancement mode) we don't get that close to zero.

So yeah: maybe 6V, certainly 9V, anything to not-quite 20V.

A redneck option is a large BJT transistor. The Base-Emitter is usually doped for 7V reverse breakdown. The Base is not a hi-current terminal so you want a big part like TIP41 or even TIP3055. NPN or PNP. Wire the emitter arrow pointing like a Zener is drawn. If you get it wrong you can't get the bias past 0.6V and the MOSFET is hardly tickled.
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Old 16th October 2018, 09:13 PM   #1456
PRR is online now PRR  United States
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Quote:
Originally Posted by Eric View Post
...the bias current creeps up as the mosfet and sink heat up, so I kept adjusting it down.... The choke warms up a little...
The choke DCR rises as it warms. The same current makes a higher voltage.

The warmth you describe is probably not a significant "error"; just something to keep fussy fiddlers busy.

The Temperature Coefficient of Copper (near room temperature) is +0.393 percent per degree C. This means if the temperature increases 1C, the resistance will increase 0.393%.
"Significant" heat might be 10C which is a 4% difference. At the same observed voltage, 2.5A cold would be 2.4A warm.
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Old 16th October 2018, 10:39 PM   #1457
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Awesome information, PRR - thank you! Little by little, the pieces are coming together for me. Many thanks to everyone for their input. I think I'll be back up and running this evening...
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Old 17th October 2018, 12:33 PM   #1458
DIYBras is offline DIYBras  Brazil
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Quote:
Originally Posted by PRR View Post
No.

The circuit is a Source Follower. Source "follows" Gate very closely.

Assume the working gain is 0.95. The other 0.05 appears across Gate-Source. Taking 20V peak swings, Gate wobbles +/-1V. Assuming the 2.5A static bias is 2V, voltage Gate-Source goes 1V to 3V. Assuming a lesser MOSFET with 4V bias and 2V swing, 2V to 8V. In audio we NEVER go to the gigantic currents shown on datasheets for 10Vgs, and (for enhancement mode) we don't get that close to zero.

So yeah: maybe 6V, certainly 9V, anything to not-quite 20V.

A redneck option is a large BJT transistor. The Base-Emitter is usually doped for 7V reverse breakdown. The Base is not a hi-current terminal so you want a big part like TIP41 or even TIP3055. NPN or PNP. Wire the emitter arrow pointing like a Zener is drawn. If you get it wrong you can't get the bias past 0.6V and the MOSFET is hardly tickled.
I have some curiosity info and musings about this subject:

About current limiting, the only exception for this rule is when we use a lateral MOSFET in PP amps, with it's typical low transconductance (exception being some RF ones), and we can limit maximum current delivered per device simply limiting Vgs with a zener. Of course, within device variations.
For this MoFo assembly, if one opts for a Lateral device can relax about current limitation since this is a SE amp with limited positive current for the sake of class A purpose (it don't need more than 2x iq for maximum power at rated load). In fact, a current limiting will be interesting for not throwing a lot of current at positive half cicle at overload, but only for an academic point of view with our civilized use.

About inductor kickback, I wondered that we expect very large kickback if one removes instantly the PSU, perhaps so high that is clamped by the drain-source intrinsic zener of MOSFET and destroying it. Well, this is only a "musing", since probably the gate will be destroyed far earlier, and impulse probably is not so fast to provoke kV on output...

I concur with that several people said here: is a good thing to inductors projects to maintain a load or be gentle with PSU on/off cycle, or even with extraneous signals from preamp if output is unloaded.
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Old 17th October 2018, 12:42 PM   #1459
Eric is offline Eric  United States
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I did some experimenting last night to see what I needed to do to prevent blowing things up by cutting off the DC power supply. My ultimate plan is to have two channels connected to the PSU and switch the DC on/off at the amp.

I replaced the dead mosfet from the night before and added a 9v1 zener to the board. Plugged it in and adjusted the bias to just about 0.5A and unplugged / replugged the DC power jack. Both the zener and the mosfet died...

I replaced both and repeated, this time using the AC power cord at the wall. Both zener and mosfet survived.

At this point, I added a 470uF 50v cap to the DC input at the board and raised the bias to 1.4A. I unplugged and reconnected the DC power jack. Both zener and mosfet survived.

Raised the bias to 1.8A and repeated, using DC power plug. Both zener and mosfet survived again.

Raised bias to 2.5A and repeated, using DC power plug. Both zener and mosfet survived again.

I found no difference in behavior with or without an 8R dummy load connected to the output for any of these trials.

I did not try just using an extra cap without the zener in place, but based on this behavior, I'm surmising that having a cap at the DC input plays the largest role in preserving the mosfet.
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Old 17th October 2018, 01:18 PM   #1460
Michael Rothacher is offline Michael Rothacher  United States
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Breaking at the AC end (which is what I do) solves most of the issue (it also tames the turn off pop you get if you disconnect on the DC side). I use an IEC cord with an inline switch I bought on Amazon. If you're going to disconnect the DC, then the add the cap, and certainly doing both can't hurt.

The 8 ohms load wasn't suggested for the protection of the MOSFET. IMHO it's just good general practice.
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