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12 watt SMPS using uc3845 & MJE13007

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Junior Member level 2
Jun 21, 2008
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i have made a 12V ,1 amp SMPS Using uc3845 and MJE13007 (NPN Transistor). for load current 100 mA the smps is stable but whe load current is 500 mA the transistor getting damage (Base collector junction short circuit). please help.



what kind of snubber did you use ? what is the leakeage of the primary winding of the transfomer ? did you measure with the oscilloscope the maximum amplitude of the pulses ?
another thing, did you buffered the gate (base in our case) drive circuit ? the bipoalar transistors might need larger base currents than the MOS gate current. also is a good ideea if you can provide negative pulse for switch-off. connect the base of the transistor to negative pin of an electrolytic cap and the positive pin to the base driver. in parallel with the cap should be a 22-47R resistor. the cap should be 10-47uF at min 25V.
for this power level will be more easy to use dedicated smps IC wich embed the control and switch. powerint (TOP, TNY, LinkSw) ST (Viper) Fairchild (FAN) are just few options. they have comprehensive literature and design examples and is very easy to use them.

smps design forum using uc3845

Thank you for your response.
Basically i don't have oscilloscope. Finaly i have replaced the BJT with Mosfet 2SK727 and fix the problem. I think Vceo of MJE13007 is not sufficient for this application. Now the problem related with the stability fo the system. The o/p voltage of the SMPS is very unstable.
I am new in SMPS system. I have only theoretical idea (no practical). I have made the same rating SMPS with ST Viper22 PWM controller and found excellent result. But i want to built SMPS only with discreet PWM current mode controller (UC3844/45) and power mosfet, which is readily available in our market. So I request you to give me circuit schematic with all details of the transformer (core, turns, wire size etc) and design equation of a practically built and tested SMPS, so that I can built the full proof system easily.

yes me too, any info will be helpful

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