What value is the voltage measured at the COMP pin?Dear All,
I have replaced the IC UC3842 and also rewinded the Transformer. The turns ratio is now (60:9:8(aux)). This time I am keeping Vcc 18V. The magnetizing inductance is 850uH. However, the IC is not generating any gating pulses. What could be reason for IC to not generate any gating voltage. As this board has experience few burnt outs (gate resistor, diode bridge ect), may be traces/pth may be damaged. I will try soldering items on a spare PCB and check again.
The waveform measured in the COMP is attached. The comp pin waveform has a square wave shape. The DC input is around 140V.What value is the voltage measured at the COMP pin?
I have attached the gerbers.perhaps we need to see your actual build ... ?
Dear KlausHi,
I don't have a Gerber viewer on my tablet..
Schematic, PCB --> PNG, PDF....
Klaus
May I know whats the unusual part. I have used TI appnote (Pg 22 Schematic) for calcualtions (did not use slope compensation) .the way you have hooked the opto to the feedback on the IC is a little unusual - have you ever had this working - ever - before ...?
It does work.the way you have hooked the opto to the feedback on the IC is a little unusual - have you ever had this working - ever - before ...?
You do not need slope compensation. Your duty cycle is less that 50%. It's around 35%.May I know whats the unusual part. I have used TI appnote (Pg 22 Schematic) for calcualtions (did not use slope compensation) .
The waveform measured in the COMP is attached. The comp pin waveform has a square wave shape. The DC input is around 140V.
When I am giving separate supply to Vcc (18V, from external PSU). One end of the differential probe is connected to Source Pin of Mosfet and if the other end is made to touch PIN 2 of UC3842, the system runs briefly for a second or so. Again If I repeat for a few times, it works then stops all together.
Thanks for the reply.It does work.
That's the CC configuration. I use it for DCM, with Ri and RF (his R15 and R16, respectively) having equal values of 47k to 100k so that the IC error amp contributes no gain (ie unity gain) but phase inversion. With this I set the gain using RLED, CTR and Re.
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You do not need slope compensation. Your duty cycle is less that 50%. It's around 35%.
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If everything is working fine, the voltage at the IC's COMP pin should be around 3.1V
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The voltage at the COMP pin should not be a square wave. Seems like it's is starting up and then shutting down intermittently. I'd advice you use UC3844 and also make provision for soft-starting.
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