switched mode power supply(SMPS) 12v.500ma

[ABEL DANIEL]

Im working on mini pro..for which i selectd SMPS(with feedback).OP is designd for 12v dc,500mA. . But im not getting the OP...i dont know exactly why?i was wondering how this crkt starts working initially without any feebak...

 

[BradtheRad]

Does a load need to be connected for it to start operating?

Does a light load, or a heavy load, create an extreme condition at startup, perhaps causing stagnation of the oscillator?

 

[ABEL DANIEL]

Does a load need to be connected for it to start operating?

Does a light load, or a heavy load, create an extreme condition at startup, perhaps causing stagnation of the oscillator?

Thanku for de concern that u hv for my prblm....i did tested it with 2 ordinary LEDS(20mA) as the load,but it didnt serve..... also tried with no load,but no op.
here is my crkt that i got from an electronics forum....but im using 3843 instead of 3842..

 

[amu_cpln]

cant view the circuit .. bad link
also have some problem understanding abbreviations you used ..
for clarity please avoid abbreviations

you selected SMPS - does that mean you purchased one .. or made one from an existing tested design ?

 

[ABEL DANIEL]

i did one test...i removed all the feedback and disconnected the 3843 as well,ie i tried to get the op with open-loop,with just mosfet(switch),F.G(for switching MOSFET),,transfmr and fast rectifier section...but didnt get the op...what can we infer from this..problem with mosfet...can u suggest some mosfet for SMPS.???

 

[ABEL DANIEL]

sorry i got wrong with de link..here it is

 

[ABEL DANIEL]

i didnt purchase....im makin from design that i got fom an electronics forum..

 

[amu_cpln]

this is from the UC3842/3 datasheet
"The output stage, suitable for driving N-Channel MOSFETs, is low in the off-state.
Differences between members of this family are the under-voltage lockout thresholds and maximum
duty cycle ranges. The UC3842 and UC3844 have UVLO thresholds of 16V (on) and 10V (off), ideally
suited off-line applications The corresponding thresholds for the UC3843 and UC3845 are 8.5 V and 7.9 V"

For your circuit this means a change in the biasing resistors of the Mosfet ..
you should calculate the resistors based on turn ON/OFF threshold voltage requirements

 

[ABEL DANIEL]

so you are saying that...oscillations are not starting because its trapped in UVLO condition??do i need to make some change in the current sensing network(pin 3)???can u help me in that??
can u help me in that calculations?

 

[ABEL DANIEL]

Thanku for de concern that u hv for my prblm....i did tested it with 2 ordinary LEDS(20mA) as the load,but it didnt serve..... also tried with no load,but no op.
here is my crkt that i got from an electronics forum....but im using 3843 instead of 3842..View attachment 72808

today i tested the crkt again..but no Output...but when
i probed at pin6..there was a switching waveform..but i wonder..its of very low frequency..why is that?