uc3845 wont work after new replacment..seems new chip needs different start up!

[libyantiger]

You should have a diode from your startup supply to the bias winding, and it should get reverse biased by the bias power, so that power does not keep coming through the power resistor after full startup.
Your bias winding should be powering the uc3845, not the power resistor.
You have actually described the proper startup method before.....when you said, dribble current through startup resistor , charge up startup capacitor, then uc3845 starts and then bias windig takes over.

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try this
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What about changing to ucc38c4x series, its pin-for-pin the same and drinks far less current.

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What about using LR645
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...IT SOUNDS LIKE THOUGH YOUR STARTUP RESISTOR IS TOO LOW OHMS, AND (sorry for caps) it is supplying chip all time. Again it sounds like your startup capacitor is not big enough....or your bias winding is not working....or your bias diode has died.

the bias winding is working fine ...it produces 12vdc ...i test it ...lets calculate 300vdc /30000 = 10ma

10ma*300vdc=3 watt ........oh ...my....god


my simlple calculation shows that resistor acoress such dc power with have to be 3 watt so it can handle current that is passing through

it never exceed limits?


or maybe just diode will sove it all since low drop 0.3 shoutcky can be installed and the resistor will still supply ..

but stilll i think if the winding working and it is working sir should not that put the load off the startup?

 

[treez]

your startup resistor should only be big enough to charge the startup capacitor to the uc384x threshold voltage in your allotted time, whatever you choose. You definetely described the startup situation yourself previously

 

[kathmandu]

You should have a diode from your startup supply to the bias winding, and it should get reverse biased by the bias power, so that power does not keep coming through the power resistor after full startup.
Your bias winding should be powering the uc3845, not the power resistor.

You could not isolate the startup resistor using a diode or such. The voltage across that resistor will stay the same during the entire operation.

That's it, the startup resistor doesn't have to sustain the chip startup current - you have the supply capacitor for that.

At startup, the chip is just waiting for that capacitor to be charged at a voltage greater than the UL threshold then it starts the PWM operation. Next, the housekeeping voltage is in charge for supplying the whole energy demand.

So you just need to choose a supply capacitor bigger enough to sustain the startup operation then keep the startup resistor value high enough to minimize the power loss (by dissipation).

A higher resistor value just means more time (delay) before the whole circuit actually starts (waiting for the supply capacitor to reach the UL voltage threshold).

 

[treez]

You could not isolate the startup resistor using a diode or such. The voltage across that resistor will stay the same during the entire operation.

my apologies , i was inadvertently thinking of the power resistor being in the collector of a high voltage zener/bjt regulator, with a diode from that to the chip vcc, bias coil, etc etc