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Old 16th October 2018, 03:24 AM   #1421
vdi_nenna is offline vdi_nenna  United States
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You try testing with an 8 ohm power resistor on the output?
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Old 16th October 2018, 03:25 AM   #1422
e_fortier is offline e_fortier  Canada
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Is your bias still 2.5A after 1h ?
How about your Dell smps, it’s not overheating?
Short the INPUT and add a 8ohm load.
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Old 16th October 2018, 03:34 AM   #1423
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Yep - bias holds pretty steady at 2.5A once things are warmed up. The power brick is rated to deliver 12.3A at 19.5v (total of 240w) and is mildly warm after an hour.

No inputs or outputs are connected. Is this causing it to oscillating it's way into oblivion? I've burned two transistors now...
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Old 16th October 2018, 03:42 AM   #1424
e_fortier is offline e_fortier  Canada
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Looks like you didn’t install the 20v zener for Q1
That would help protecting it.
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Old 16th October 2018, 03:48 AM   #1425
Eric is offline Eric  United States
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No, I didn't install the 20v zener. I have them, but the BoM indicates they are optional and it's not shown in the schematic in Figure 1. Since I didn't find a description of it in the article, I left it out. Mike's image at the start of the thread shows the zener is missing as well.

I don't understand what role the zener serves...
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Old 16th October 2018, 04:02 AM   #1426
e_fortier is offline e_fortier  Canada
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The 20v Zener ensures that the max Vgs of the Mosfet (20v)is never above its limit.
See here:

what's the purpose of the zener, mosfet | All About Circuits
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Old 16th October 2018, 04:13 AM   #1427
Eric is offline Eric  United States
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I have a single IRFP250 left. I'll install the zener and test with my 8R dummy load and see what happens... I still don't understand how voltage can rise above 20v with a 19.5v PSU and no signal present.

I'll pick it up again tomorrow night and see how things go.
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Old 16th October 2018, 04:18 AM   #1428
e_fortier is offline e_fortier  Canada
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Hopefully by tomorrow someone will come with an explanation

I would test with the input shorted.
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Old 16th October 2018, 05:35 AM   #1429
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Quote:
Originally Posted by Eric View Post
....don't understand how voltage can rise above 20v with a 19.5v PSU and no signal.....
You have AMPS flowing in an inductor the size of a melon.

If current is cut-off, the inductor KICKS, "trying to keep Amps flowing".

Ideal inductor, instant cut-off, voltage kicks to infinity.

Iron-core inductor won't kick to infinity, but 10:1 kick is commonly seen in "working" systems (car spark coil), and some higher if NO load.

Here's a sim. The values are wrong but the trend is clear. If current is broken quickly, inductor kicks to -600V. If Gate has a cap, most of this appears across the Gate-Source oxide, which is rated 20V and dies easy at 30V-50V.

It is situational. If current is not cut-off but fades-away, the kick may be tiny. If choke is loaded to about 19V/2.5A (8r) the kick is <19V.

The Zener is not really optional.

Many MOSFETs have an internal Zener. This may be a very small diffusion not able to absorb the energy in a melon-choke. Get a BIG junction, 1W or more.

No-load is worst-case. Have a load. 10r 10W is good for the bench.

Don't yank the DC connector. If you yank the wall-plug of the Dell-wart, it *may* come-down slowly on stored charge.
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Old 16th October 2018, 05:46 AM   #1430
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Put >100uFd on the rails ON the card. If DC power is yanked, this gives a slow let-down and greatly reduces the kick.

(Side-note: you'd think you'd want a rail bypass "in the design". As implemented, audio current goes all the way out to the Dell-wart and back to complete the path.)
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