| Electronics > Repair |
| Help me understand this complex flyback PSU |
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| magic:
Are R4, R5 and Q1 alright? Those going open could be another reason for the Vcc rail being low under load. The most important thing about MOSFET is that it doesn't have a short between gate and the other terminals. Such short can easily blow up the PWM chip with HV. |
| F1GWR:
--- Quote from: magic on July 29, 2024, 04:49:58 pm ---Are R4, R5 and Q1 alright? Those going open could be another reason for the Vcc rail being low under load. The most important thing about MOSFET is that it doesn't have a short between gate and the other terminals. Such short can easily blow up the PWM chip with HV. --- End quote --- Yes all checked alright. And as written this morning at 9:48:47 CR6 cathode measured 19,6 V after I removed the controller U1 (before was less than 1 V). |
| magic:
This power supply can deliver up to 1mA of current coming from R2 through Q1 base-emitter junction, even if Q1 has zero β or R4,R5 are open. So the real test is to see if it can still maintain similar VCC voltage while delivering 5mA load current. Did you test for a short between VCC and GND pins of the suspected chip with a DMM? Are you sure that the rail wasn't pulled down by the Q1,Q2 circuit? These appear to implement some sort of emergency shutdown, shorting the VCC rail to ground when activated by VR1. |
| F1GWR:
--- Quote from: magic on July 29, 2024, 09:21:40 pm ---This power supply can deliver up to 1mA of current coming from R2 through Q1 base-emitter junction, even if Q1 has zero β or R4,R5 are open. So the real test is to see if it can still maintain similar VCC voltage while delivering 5mA load current. Did you test for a short between VCC and GND pins of the suspected chip with a DMM? Are you sure that the rail wasn't pulled down by the Q1,Q2 circuit? These appear to implement some sort of emergency shutdown, shorting the VCC rail to ground when activated by VR1. --- End quote --- Weird: no short between Vcc and ground of U1! A friend of mine explained that Q1-Q2 as a binistor. Found very little info about it. Therefore I come back with my initial question (sorry to quote myself): Please help me understand in the daughter (= fille) board around the UC3842A how work these weird structures: - the oscillator network with Q3 Q4 and what should I expect as a working frequency - the voltage reference with VR1 associated with Q1 and Q2 |
| wasedadoc:
I do not know what Q1, Q2 and VR1 are doing as there is no used output from that circuitry. Is your schematic complete and accurate? |
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