| Electronics > Repair |
| HP E3615A power supply slow ramp up |
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| pqass:
Okay, so far we been assuming the problem was somewhere before the U9B inputs. I think we've exhausted all possibilities (bad U9, +10V ref., change in sense elements R82/R62, interference from CC amp, protection, and bandwidth limiting elements, trace resistance). However, maybe the transistors following U9B aren't reacting fast enough (cyan highlighting). The symptom of a gradual ramp up smells like a capacitor charging (I believe are C1 and C3). Add a parallel 2k2 resistor across R65 to reduce its resistance and see if there is a change in ramp speed. This shouldn't stress Q9 by much. FYI: I have an E3611A and is almost identical to yours (after U9B, although it uses BJTs; not MOSFETs) and I'd managed to cure an issue with it (I don't remember the symptom) by increasing the base current. |
| gaminn:
--- Quote from: pqass on September 13, 2024, 02:38:53 am ---Add a parallel 2k2 resistor across R65 to reduce its resistance and see if there is a change in ramp speed. This shouldn't stress Q9 by much. --- End quote --- Adding 2k2 accross R65 has no effect, the ramp speed is the same. If there is an issue with Series pass transistor and current balancing circuit causing the set and real output voltage to differ during the ramp up then I would expect the output of U9B.7 would try to compensate for that (saturated output ?). But U9B.7 never saturates during the ramp up. FYI If i connect external higher voltage to the power supply output, then U9B.7 saturates (approx -9V) trying to compensate the error it sees. |
| pqass:
--- Quote from: gaminn on September 18, 2024, 10:24:13 am ---Adding 2k2 accross R65 has no effect, the ramp speed is the same. --- End quote --- Maybe then the issue is higher up; how B is generated. Or the Shut Down Circuit. Or even the bulk power source (TP1,TP2)! (a) Lift one leg of R19 to eliminate the Shut Down Circuit from interfering. (b) Short Q10 collector to emitter to remove the Turn-on Overshoot Circuit from delaying power to the MOSFET gate pull ups. (c) Confirm how long the bulk source of power takes to ramp up. There could be an issue with the SCRs or bad bulk capacitors C8, C7. Whichever the above causes a different ramp up, we'll work back from there to identify the offending component. --- Quote ---If there is an issue with Series pass transistor and current balancing circuit causing the set and real output voltage to differ during the ramp up then I would expect the output of U9B.7 would try to compensate for that (saturated output ?). But U9B.7 never saturates during the ramp up. FYI If i connect external higher voltage to the power supply output, then U9B.7 saturates (approx -9V) trying to compensate the error it sees. --- End quote --- An important distinction needs to be made... just after turn-on while the output is ramping up, U9B.7 doesn't directly drive Q9. All U9B.7 actually does during this time is go fully positive (close to +15V rail) which doesn't go past CR21. So the ramp up drive voltage comes from R67 (shutting down Q9) allowing R65 to pull up the MOSFET gates hard. Only when the output overshoots the set voltage does U9B.7 go negative to turn on Q9 enough to pull back on the MOSFET gates. If the B voltage takes a long time to ramp up, or maybe even the bulk source (TP1,TP2), then U9B.7 will just wait at positive until the output overshoots the set voltage. |
| gaminn:
--- Quote from: pqass on September 18, 2024, 02:25:20 pm ---Maybe then the issue is higher up; how B is generated. Or the Shut Down Circuit. Or even the bulk power source (TP1,TP2)! (a) Lift one leg of R19 to eliminate the Shut Down Circuit from interfering. (b) Short Q10 collector to emitter to remove the Turn-on Overshoot Circuit from delaying power to the MOSFET gate pull ups. (c) Confirm how long the bulk source of power takes to ramp up. There could be an issue with the SCRs or bad bulk capacitors C8, C7. Whichever the above causes a different ramp up, we'll work back from there to identify the offending component. --- End quote --- (a) I have already disconnected R19. CR22 as well. No effect. (b) No effect (c) With approx 12V output voltage set, C8, C7, R11 voltage is stable immediatelly at 16.9V after power on. With max output voltage set, C8, C7, R11 is stable immediatelly at 22.2V after power on. |
| pqass:
I apologize for not responding sooner.... We've gone from the end (+/-OUTPUT) to source (before the MOSFETs) without identifying the problem so I'm sure we've missed it. Unfortunately, a few hundred mV on the output are probably micro-volts on the MOSFET gates, Q9 base, or U9B.7. This would make it hard to detect just monitoring with a multimeter (hard to distinguish from noise). One way to narrow-down the problem is to divide the path into two to determine which half has the issue. For example, if you disconnect Q9's emitter (or remove Q9 entirely), and you still see a ramp up issue on the output posts, then you know the problem is downstream of Q9; if not, the problem is upstream. Of course, since you've disconnected the voltage setting parts, you'd expect the output voltage to be close to the source voltage with the difference being does it still have the ramp up of few hundred mV over a minute. Eg. use two multimeters with one red lead on drain (TP1), another red on source (right side of R1,R2), with blacks on -OUTPUT, to determine if there's narrowing voltage difference (TP1=23.0V and R1 right=22.2V increasing to 22.6V) or stable difference? |
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