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| Diy bench power supply trouble |
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| andrejsstrumfs:
Hi all! I'm trying to build the power supply based on this circuit. It kind of works, but the problem is when the power supply switches between voltage limit mode and current limit mode, the opamp output can swing to negative rail and the power transistors get reverse biased and burn out. I actually do not see any protection against reverse biasing the transistors in this circuit. Hovewer my understanding could be quite limited :) Anyone got any ideas how to improve this? |
| Yansi:
The most negative they can go against the emitter is -5V. Realistically speaking, about -4V at best. You are not going to damage a NPN with 4V reverse on BE junction. I think the issue you are describing is non existent. There are other issues instead, that are not caught in the design: Like when you force external voltage on the output terminals, especially with the mains powered off. |
| andrejsstrumfs:
I've got a bunch of burnt out transistors already :) The voltage regulation works and the current regulation works but as soon as the modes change - the transistors burns out. i just can't image what else could be wrong. About the external voltage on output - TIP142 have got internal diode between emitter and collector. Shouldn't that protect the transistors? |
| Kleinstein:
For this circuit there is a chance the it can start to oscillate. When the circuit was designed and build first they likely also observed this and added those 4.7 pF and 100 pF caps - but just at poor positions (C21 is at a good position but to small). So they can help with external RF coupling in but for oscillation of the regulator it makes things even worse. The current regulator can take quite some time to activate as the OP has to come from it's positive side down to about -1 V. So a short might already damage the output transistors from too much current before the regulators sets in. With oscillations or when turning on the circuit the same might happen. To reduce the turn on problem it might help to change R3: instead of going toward GND an additional 5.6 V Zener in series and than connect to the -5 V would delay the turn on until the -5 V supply has at least about -3 V. Normally one should have a second fast current limit, e.g. with an extra NPN transistor driven directly from the shunts to limit the current to a fixed about 2-3 A per transistor. |
| Yansi:
Or probably be VERY aware of the origin of those TIP142. I would be suspicious if they burn out just that quickly. It takes some effort to knock out a decent power transistor, let alone by the microsecond or two when the CV/CC changeover happens. H. |
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