Is their an electrolytic start up cap that has degraded in capacitance due to age?…try replacing it.
Sorry I cannot hear your video
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Was the power supply ever working?
What voltage do you have on the vdd pin of the uc3845 when it is not working?
Do you have 5v on the ref pin.?
Is the oscillator pin showing a sawtooth?....whats the frequency
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If you load the main output does it start working
If you unload the main output does it start working.
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https://www.ti.com/lit/ds/symlink/uc3845.pdf
According to datasheet the vcc of uc3845 nees to get up to 17.5v (14.5 to 17.5) before it starts working…….if your startup cap discharges below 11.5v (8.5 to 11.5) before the bias winding gets up to 12v or so then it will never get started.
It sounds like you may need to replace the startup cap, and possibly make it bigger.
I cannot tell from your video but it looks lke you are able to get it all up and running when you supply it from a separate psu?
What about the bias rail diode, does it need replacing?...was it of high enough voltage rating in the first place….. I presume it does have a bias rail?
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is it a flyback smps?
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also, Maybe your vref pin cap is too big
Would not be surprising that UVLO thresholds are a bit
different between vendors. TI/Unitrode says on at 8.4,
off at 7.6V (reality can be anywhere between). Another
vendor or even another lot could be a couple hundred
mV off from your first part. Also look at the VIN behavior
the first time you slug the line, does it keep sagging
below the turnon threshold and making a "hiccup" mode
or something?
But you need there to be a main regulated output.
Is it a flyback?
If you havent got a regulated output then how does the bias winding know what voltage to couple to?
Whatever is the voltage on the main secondary, then v(auxiliary) = V(main secondary) * Naux/Ns
It sounds to me that the uc3845 is trying always to draw its bias current through your startup resistor.
Do you have a schematic?
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The startup voltage for uc3845 is up to 17.5v...if you dont get up to that voltage then it will never properly start in the first place.
ok the datasheet i post on #2 above is not too clear whether the startup voltage for uc3845 is 16v or 8.4v.
But if you say you got it working with no more than 8.7v external psu then so it must be 8.7v to get it started.
That is likely to be your problem, because as you know, to start an smps, there has to be a bigger voltage difference between the start and stop voltage...hysteresis.
As you yourself describe, the startup capacitor must not discharge below the "off" threshold before the smps starts self-supplying via its bias winding.
If your start and stop voltages are really close, then you cannnot ever startup.
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i think you need to go for the uc3842 because that definetely has start = 16v and stop = 10v.
But then you must be sure your bias winding can stay above the stop voltage.
sometimes an smps needs a dummy load on the main winding or else the bias winding drops out.
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oh no hang on, the uc3845 has max duty of 50%...so you need to pick uc3844
...although it cannot be said for sure without schematic
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what voltage is coming out of your bias winding?
if the bias doesnt work then you are relying , as you know, on the startup resistor to supply the operating current, and 35k is too high ohms to do this.friend i am disconnesting the power mosfet out of the smps so the bais will never works
You should put a 20v zener on the vcc pin of the uc384x to ensure you don’t overvoltage it whilst you are testing.
(I am presuming your bias winding puts out less than 20V)
And i am assuming that you are using fet with vgs(max) of 30v
You should try replacing the uc3845 with a ucc38c45…this chip is pin for pin compatible but draws far less operating current.
As you know, If you are not using a bias winding (when it comes up) to power the uc3845, then it’s a waste of time increasing the startup capacitor.
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Is the vref of the uc3845 being used to power other circuitry?
Do you have a large resistor from gate to source of the fet, which could be making the uc3845 draw more current
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You could just put a 10k, 10 watt resistor as the startup resistor, and see if that does the trick...but you must add a 1w, 20v zener to the vcc pin of uc3845 if you do this....but i think you need to change to the ucc38c4x because it is far less operating current
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I presume you are using 220vac mains?...if using 110vac then you will need lower ohm startup resistor
Your resistor feed has to provide the worst case
standby / pre-UVLO current from the supply you've got
and still make > UVLO threshold. This is another place
that different vendor versions of the 3845 can vary -
even if the spec does not, the reality may very well
be different (within that box).
VIN-(Istby*Rbleed)>Vuvlo or you go nowhere. Might
want to measure Istby between parts that work and
parts that don't.
when you load the main secondary winding, the primary dc bus will dip down and if you are not then supplying the uc38xx by the bias winding then again your power resistor may be too high ohms, because the dc bus will dip down when you start loading the main secondary. Just a thought
when you load the main secondary winding, the primary dc bus will dip down and if you are not then supplying the uc38xx by the bias winding then again your power resistor may be too high ohms, because the dc bus will dip down when you start loading the main secondary. Just a thought
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