[SOLVED] uc3842/3843 Vcc problem in smps.

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off course its my mistake C1 & R2 are both across the rectifier at output side. this schematic is from

powerEsim software but i made a little changes according to my pcb layout above i made.

D5 is actually IN5818 mistakenly wrote IN5819 that acts like zener at the pulse to prevent pulse overshoot.

i see circuits whether 103pf/1kv is used in the snubber with 47k/2w resistor and uf4007 for making snubber. but i don't have

oscilloscope to see the voltage spikes that damage the mosfet.
 

I bet that powerEsim software this is also incapable to define MossFet resistance characteristics.

Therefore with out Oscilloscope and with out current measurement probe, you will never solve the puzzle.
 

Looking at data sheets and application notes, the snubber capacitor values range from 3.3nF up to 10nF, 103pF still seems unreasonably low to have any significant effect.

I see some designs using Zener diodes to clamp the drive to the MOSFET but neither the 1N5818 or 1N5819 are Zener types. Possibly they are being used in reverse breakdown mode but that would seem a poor and unreliable solution to clamping a voltage.

Brian.
 

Yes all the points above are valid, and especially from Klaus who has given me and many many others some fantastic electronics info in the past.
But anyway……its interesting that you have 80v on your vcc…..usually the bias current of UC3843 is so high that its volt drop through your 150k resistor would stop the uc3843 from being overvoltaged….in fact, the 150k would normally only be able to supply the startup current for charging up the 47uf cap………so I guess you have a dry joint somewhere and the uc3843 isnt able to function……is your 47uf cap ok?…maybe it is dried out……maybe you replace it…try something like 220Uf 35V on there.
Maybe you esd damaged the fet?
Ive attached a folder on startup of offline smps

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Also, did you interleave wind the transformer?......could you show the transformer spec?...you say you built this one yourself.....so i we can see the transformer design docs?

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here is a free smps course for you
​https://drive.google.com/open?id=0B7aRNbu3Fes4TU92Mkw3YlA3ams
 

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  • STARTUP of an Offline SMPS.zip
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i also used zener diode of 18v at the gate of the mosfet for this purpose in my pcb layout not mention in schematic.

i also used IN5818 to clamp the pulse high volt. may be it will be the dried Vcc 47uf/50vdc capacitor. however i'm researching the problem.

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**broken link removed**

**broken link removed**

i made this calculator by researching many documents about flyback transformer design like an-104 on https://www.infineon.com/dgdl/an-1024.pdf?fileId=5546d462533600a401535591115e0f6d and some other documents

from such sites.

the transformer works correctly with this frequency in working circuit, but not in this circuit. problem is within the circuit

not with flyback transformer. i don't have special equipments like oscilloscope and others to see waveform & voltage spikes. only a DMM & frequency measuring meter.

i check with meter for uc3843 the frequency is fixed at 56.6KHz.

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today i tested it again & now the Vcc goes to 270vdc. the problem posted in post # 11 is the problem but how to avoid this problem

i 'm thinking that's the reason the high voltage first blow up the ic making path way to the mosfet overcome the gate-soure threshold turn the mosfet on and short

the mosfet. because every time when i check with DMM the ic & mosfet both are shorted.
 

    A

    akbar2009

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esd damage to your uc3843?

If a 150k resistor is connected to a >47uF cap at the vcc pin, it shoudl simply not be possible for the vcc voltage to rise to anywhere near 270v...because the bias current draw of the uc3842 is something like 10mA.....

I think , for the mean time, you are going to have to put a zener on the vcc pin.
(thats if youve got any more uc3843 chips that arent blown?)

You can get scope for £100, maybe less....do you have access to internet orders?
 

    A

    akbar2009

    Points: 2
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by placing zener diode of zd/21v the Vcc reduce to 84vdc without ic in the circuit. by placing ic in socket the Vcc drop to 13.5vdc round about and for

uc3842 and 8.9vdc for uc3843. now i put the mosfet and ic but oscillator is not starting. the Vref pin#8 has 1.5vdc instead of 5vdc to start the oscillator.
 

So you put a 21V zener in the circuit, and the voltage at the cathode of the zener went to 84v?..........something is very wrong here....the zener has blown up for a start.....then you put the chip in, and exposed that to 84v....so that then would also have blown up.
 

the zener and ic both are ok. i place the ic when the bulk capacitor completely discharge after switching off.

i 'm using series light trick for checking because i have no other method to prevent any big damage to the circuit instead of

puting directly on ac 220vac. the zener & ic both are ok. i check it with multimeter after putting it in circuit. also i have another circuit

i made through which i check uc384x series ic by checking its pulse. the ic is ok but the circuit has some hidden problem.
 

the 84vdc at the Vcc is when the circuit without uc3843 or uc3842 at 21v zener diode.

then i switch off the power & completely discharge the main bulk capacitor through discharge resistor of 470k/1w.

then place the ic in socket and power the circuit in series with 200watt/220vac light bulb. the Vcc show about 8.9vdc for uc3843 & 13.7vdc for uc3842.
 

it sounds like the light bulb is maybe messing things up...not leaving enough voltage for the flyback startup circuit.

Can you just get rid of the light bulb, and power the uc3xxx chip with an isolated bench psu and do straightforward unisolated resistive divider feedback till you get it running?...then bring the startup circuit and the opto feedback back in afterwards...when you have that running.
....
 
Yes the 200W light bulb is only a few ohms when cold - so unless your soft start is ~ 30 seconds, you will never get it going ... even then you likely won't be able to supply the current to warm it up enough ... keeping Vout low, and Vflyback low, and Vaux low ...
 

it is possible, but i think the problem is not with the series light because i using this method before this with smps.

and the smps start working even by putting some low load upto 30watt. i also have two smps based on uc38x series.

that i made before this and still works when i put it in series for checking start with their soft start circuit. this is third time this problem arise

in a newly made circuit. making about 6 pcb board of this smps only changing the size of the board. some works and in some circuits

the high voltage at Vcc which also fail to regulate with load get short the ic and power mosfet. however i investigating the cause of the

problem why this problem occure with my smps using uc384x.
 

I wonder of the trxformer is wrongly phased, and the bias outputs into vcc when your fet is on, and the turns are higher than should be also, hence the high voltage on your vcc?

...i think remove the bias diode, supply vcc with isolaterd bench supply, and try and get it working like that first.

Also, The UC3843 is world famous....i'll bet if any ic has been copied, it'd be the uc3843...maybe some bad copies got made with poor esd protection....but thats just another theory
 

the problem solved, the problem was unexpectedly due to the smps xformer, the circuit has no other problem.

it works fine with another flyback smps xformer. but the bad xformer works with low load upto 2amp and short the power

mosfet & ic at high load > 2-3amp/12v
 

For future reference, do you want my guide to how to know if the windings of a flyback transformer are correctly phased?
It should be in my smps course..which is to be found somewhere in here...
https://massey276.wixsite.com/electronicsdegree

..ive go to get on the road now , so will give more details later....
 

i already download it from the link given in post#24.

thanks treez
 

if one halve of an smps xformer cracked completely what will be the its effect on flyback converter?
will it may cause increase leakage inductance that becomes the cause of ringing and cutout power switch mosfet?
 

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