Electronics > Beginners

Strange Osiloscope Anomaly

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orbanp:
Hi Jwillis,
The pass transistor balancing is done by the 0.1 Ohm emitter resistors, and not by the base resistors!Learn from the schematics of other linear regulators!
Your simulation is as good as your models! I did not find a 2SD2061 model, I used a TIP31 model. The 2SD2062 has an Ft of 8MHz, the TIP31 3MHz.
Still, experience has shown that at such low frequencies BJT circuit simulations are quite reliable.Light load stability is unacceptable at 150mA load and with the 100nF cap attached.Do read up about linear regulator compensation here: https://www.ti.com/lit/an/snva020b/snva020b.pdfI enclose the picture of the loop gain!
Regards, Peter
PS: Sorry for the jumbled format, my browser settings, or something else, cuts spaces and new lines arbitrarily.

Kleinstein:
The 100 nF parallel to the 56 K may be part of the loop compensation, though they look rather large.  To reduce the capacitive load on the OP, one could add some 100 Ohms to 1K in series to that cap.  This should not effect the regulation loop that much, but keep the OP more happy.

The usual SOA is for a perfect heat sink. So one should keep the actual power lower than the limit quite a bit. The 2SC2922 is in deed a rather large transistor, but still boarder line.
Current sharing is more efficient with emitter resistors than base resistors. So the base resistors may be OK, though the cost a little extra voltage. This circuit may like an additional fast acting current limit to avoid excessive spikes on transients (e.g. a short from a high voltage). The base- resistors alone are not very effective here. 

Still to get 40 V out the raw DC voltage may need to be in the 45-50 V range. This is one weakness of the circuit. Especially at high power, one should have separate filtering for the critical OP, so that the OPs supply does not see as much 120 Hz ripple. This way one could save quite a bit on the large filter caps and this way improve the power factor a little and save also on the transformer. An excessive amount of capacitance also needs a larger transformer.

Transformers with a symmetric split supply (e.g. 2x18 V) are relatively common. Those low power auxiliary supplies may have to use separate transformers.

Jwillis:
I'm scared of not having Base resistors coming from past experience. The  2SC2922 is an expensive transistor and hard to find without the complementary paired 2SA1216.Occasionally I find them for around $8 but most times they can be as high as $20 . So I'm being cautious by putting the base resistors in to limit their maximum to 3.5A each,
I need more Heat sink but really don't want to spend hundreds of dollars for a new one. I'm looking for another one that's more in my price range. I have 4 more transistors to bring things into a safer operating region.
I have a 40V 1000VA transformer for this project but I'm concerned  about the  56Vdc after rectification.
I would have to recalculate everything I've done .I guess I could set it for 30V instead of 40V output. That's easy to do.
I have the project working really well but will look into the small tweaks people have suggested.

Thank you everyone

orbanp:
Hi Jwillis,
Building a linear power supply for the above specs (40V, 20A) is the brute force approach. You need the "big" hardware for that. Maybe you need to reevaluate your specs or the regulator architecture. Commercial linear PSs with such specs usually have an SCR based pre-regulator so the linear part needs to drop only 5v to 10V, the power dissipation is limited.For inspiration look for the HP-625x or HP-626x series power supplies.Also, you could look at this thread: https://www.eevblog.com/forum/projects/thrysistor-voltage-regulator/ The Linear application note by Jim Williams (AN-32, also referenced in the above thread) is also a worthwhile read. As for expensive transistors, you could use the KSE44H series transistors, have a bit lesser specs, 80V, 10A, but a lot cheaper (C$12.53 for 10): https://www.digikey.ca/en/products/detail/on-semiconductor/kse44h11/1053454. The complementer pair is the KSE45H.
Regards, Peter

Jwillis:

--- Quote from: orbanp on February 16, 2020, 01:07:08 pm ---Hi Jwillis,
Building a linear power supply for the above specs (40V, 20A) is the brute force approach. You need the "big" hardware for that. Maybe you need to reevaluate your specs or the regulator architecture. Commercial linear PSs with such specs usually have an SCR based pre-regulator so the linear part needs to drop only 5v to 10V, the power dissipation is limited.For inspiration look for the HP-625x or HP-626x series power supplies.Also, you could look at this thread: https://www.eevblog.com/forum/projects/thrysistor-voltage-regulator/ The Linear application note by Jim Williams (AN-32, also referenced in the above thread) is also a worthwhile read. As for expensive transistors, you could use the KSE44H series transistors, have a bit lesser specs, 80V, 10A, but a lot cheaper (C$12.53 for 10): https://www.digikey.ca/en/products/detail/on-semiconductor/kse44h11/1053454. The complementer pair is the KSE45H.
Regards, Peter

--- End quote ---
 

I completely forgot about SCR regulators. Now you got the gears turning again. I haven't worked with many SCRs.
I have some of those D45H11 and D44H11 but I thought they were obsolete so I didn't bother looking for more .They're a pretty robust transistor . 

Thanks.

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