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SMPS battery charger control IC UC3842

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imranahmed

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Please let me know that I am repairing (Chinese) smps based car battery charger I have changed 3 times Mosfet 10N60
and source resistor 1-ohm and uc3842.

But now I increase 1-ohm to 3-ohms but output current reduce to 1 Amp and before it was 6 Amps.
And now UC3842 slightly heat-up.

Can any one tell me reason behind heating-up UC3842.

Thanks.
 

betwixt

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We need a schematic to be certain but usually, the current through the MOSFET is monitored by reading the voltage across it's source resistor. If you increase the value, the voltage is also increased and it thinks more current is flowing than really is. Each Amp would previously read as 1 Volt but with 3 Ohms resistance it will now be 3V. The driver is probably struggling to compensate and the increased resistance will in any case reduce the available output power.

Brian.
 

imranahmed

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20180907_024922.jpg

Please find the attachment

- - - Updated - - -

20180907_025045.jpg

Now rectifier 2 diodes burn and I connect small wire piece as a fuse it also blown this occurs 4 times.
Now I am tired how to fix this issue.

20180907_024952.jpg
 

betwixt

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I can't see where it is closed and none of the moderators have taken any action about your posts other than to approve them.

I'm afraid it isn't possible to help with the fault by just seeing the photographs. Many things can cause the parts to be damaged but we would need a schematic to guide you through the process.

Obviously, something is making the input circuit draw far too much current but almost any of the components could do that. Your best bet is to measure the components in-circuit with a DVM and see if any capacitors or diodes appear shorted. You can do a limited test on resistors but try with the meter probes both ways around and use the higher reading, often other components across the resistor will result in a lower measurement though. I did a repair on a similar circuit (not the same though) and found the opto-isolator was open circuit but the only way to test it was to unsolder it and place it in a component tester.

Brian.
 

Easy peasy

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You have likely killed quite a few components, possibly the transformer has a shorted turn, causing successive failures of the mosfet, if the feedback loop is broken you may be getting excessive output volts which are killing your output diodes.

Time to recycle this unit and get a new one ..
 

Akanimo

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Resistor 1R is used to sense and limit the current flowing through the MOSFET. Interrupting this current (inductor current) causes a large voltage spike when the MOSFET is switched off. The higher the current, the higher the voltage spike. The idea is to have the voltage due to the current flowing through the inductor (measured across the resistor) iL(t)peak*R) = Vramp so that (iL(t)peak*R) - Vramp) = 0 . Increasing R from 1R to 3R results in an extra voltage of (iL(t)peak*3*R) - Vramp) = ((iL(t)peak*3*R) = (iL(t)peak*R))) = (iL(t)peak*2*R) = (2*V(1R)) during every MOSFET on-time. The controller would normally limit the peak of this current-representing voltage to Vramp. This means that the V(1R) = Vramp is attained sooner, about on-third the designed MOSFET on-time (duty cycle), so obviously, you have changed the duty cycle of the converter and as such the current flowing through your MOSFET after replacing the 1R-resistor with 3R is one-third the value that it was designed for and ,hence, the voltage spike is also reduced. Consequently, the current transferred to the secondary is reduced by the same factor (and, of course, the turns ratio). But the current you are measuring at the output is an average value of this peak current. It is very likely that your MOSFET isn't failing any more after this replacement and that is because the voltage spike is now probably within maximum Vds rating of the MOSFET.

The replacement, however, tend to fold back the output voltage for the same load reduce but the controller doesn't want to see that. As a matter of fact, duty cycle is a function of the input and output voltages of the converter and perturbations in these voltages are what should cause the controller to change the duty cycle. Now that the duty cycle is altered not by these voltages, the controller is having a nightmare. And moreso that this alteration may have changed the mode of operation of the converter all contribute in causing the controller to suffer and so heating up.


My recommendation:
1) Check the snubber components. You may find one that had gone faulty.
2) If you eventually find one, reinstate the 1R-resistor.
3) If none is faulty, then, I don't know; [I scratch my head] the transformer? The...?[I scratch my head again]. Maybe, be patient for more help to come...maybe, throw away...maybe give more information.

I think Vramp for UC3842 is 1V. I might be wrong, I haven't checked the datasheet though. If it is 1V, then that's 1A peak-current through 1R (and you've reduced it to 0.3333A+ with 3R.

What are the charger nameplate info?
 

treez

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Heres some tests you can do.

What is Vcc voltage on ucc3842?
What is miller voltage length on gate (in ns)
Is it switching smoothly when on constant load...or is it bursting on and off
What does the fet drain voltage go up to?...youll need at least a 25MHz scope to tell this.
Whats the voltage across the output diode when fet is on?

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Also, are the duty cycles, when on constant load, the same from pulse to pulse?
 

imranahmed

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I do not have oscilloscope but UNI-T 55 multimeter it has only option for 20khz.

When repair the charger it was working for 2 to 5mins then blast occurs mosfet,uc3842 and 0.5 ohms resistor burst and also optocoupler too.

- - - Updated - - -

I think there is anything is missing in designing circuit who prevent components from spike or malfunctioning.

Is it right or wrong?
 

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