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SMPS common ilogical problem

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mostafa0020

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HI ALL,
A common problem repeated with me ,and with some other engineers,regarding the SMPS,is that it stop responding at all for a while,and when u open it ,and touch any thing inside,it starts to do.
I have no explaination for this other than accumulated static charge on one or more legs of PWM generator or so on,but,how to solve???!!!first of all,what is the cause of the problem!!!???
 

Do you think it's because of loosen connection, e.g. bad soldering?
 

I think soldering flux may be responsible for such a matter,but,i think it is not that direct cause,the problem persists
 

increase the gound planes and check for earth.
 

On the PWM you are using, is there an enable pin that is now open, not connected to anything? That can cause problems like this.
Also, check for a cold solder on the PCB.
 

An IC which is common is UC3842. The startup sequence for this (and for some other circuits powered from mains) is made with a power resistor, around 68kohm/2-3W which insert few miliamps in the circuit, from the DC high voltage. There is also a small capacitor after the resistor (around 47uF/25V). If the capacitor is dried, which can happen very often in hot places like power supplies (and especially with this one because it is placed near the power resistor), the startup sequence is not taking place, but it can be triggered by an ESD event.

/pisoiu
 

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