I copy a letter to messtechnik-collberg.de I have just sent.
Can you or anybody else explain this riddle?
Dear Herr Bernd Collberg,
I am not even sure I am asking the right person, but your previous helpful reply prompts me to ask anyhow.
I have got the PS going after some repair, and want to use it regularly. Essentially, the two pass mosfets and the meters.Now, IRFP250 and a digital combo.
I don't see the heat sinks going beyond 75W or so, if that, even with the blower fan. The unregulated dc is 77.5 or 56.0V at no load, depending on the tap changing relay, and I expect these to drop to no less than 56 and 40V on 5A load. For regulated output below 41 and 25V respectively, the pass transistors will exceed the 75W limit continuously. How do R&S justify the claimed ratings?
To calibrate my new digital ammeter, I set the current limit at up to 8A; the load was essentially a short circuit. It took only seconds to overheat to 80C on the heatsink.
No, I did not set the voltage selector at 110V and operate at 230.
Is it common to rate linear PS's in some peculiar (American) way to maximize the apparent horse power?
I thank you in advance for a cogent explanation.
J.
Edit:
There was no reply from Messtechnik.
After lots of experimentation, I had to plump for simply augmenting the cooling of the pass transistors. Any preregulator scheme to charge a 15mF capacitor to 78V from a stiff source, and sustain loads up to 16A from this dc bus, will involve peak currents to 150A, and loss to 75W. It just is not worth it. The best of the lot was trailing edge phase control using a 100V 47A 290W mosfet, as this involved the least complication in control; I gave up when it called for two such in parallel.
So, I simply transferred just under half the pass transistor heat to a heat sink outside the case, on which are fitted two more identical pass mosfets. Split source resistors, one on each, provide a semblance of current sharing equality. R&S have provided a thermostatically controlled two speed 220V blower, and my new heatsink also stands to get lots of air. See photo.
Test showed that at rated load (10A, 17.5 to 0V) into a short circuit in the CC mode, the blower stays at low speed for at least ten minutes. 215W of heat! I declare success!
There is a lot more detail to say, if there is going to be any interest.
Sent from my Galaxy TabĀ® S2