Difficulty repairing UC3842 power supply...

[dave_k]

What's the voltage on C2 ?

voltage is about 11.5V after increasing value of C2

[mzzj]

What's the voltage on C2 ?

voltage is about 11.5V after increasing value of C2

I'm guessing you are talking about C3 value, not C2?

[xavier60]

With C2 being too small, I would expect that the UC3842 would have been repeatedly making start attempts. There should have been some output voltage.
The 22 ohm gate drive resistor might have gone high causing sluggish switching of the MOSFET and higher dissipation.

[dave_k]

Quite strange that there was originally only a 1uF cap there to begin with!
The 22 ohm resistor is okay, and substituting it for a 15 ohm resistor doesn't improve anything.

[xavier60]

There is soft start circuitry on the Comp pin,1. So it is possible with  C2 being 1uf, the UC3842 might not have had a chance to produce any MOSFET drive before pin 7 drops to the UVLO voltage. Pin 7 might have been cycling between 16v and 10v and the multimeter reading some between average.

[xavier60]

Take note of the operating frequency. It doesn't happen very often, I have seen the oscillator timing capacitor go low in value causing the switching frequency to go too high resulting in poor efficiency.

[xavier60]

A shorted snubber might also explain all of the trouble.

[dave_k]

Given the values shown in the schematic the operating freq works out to be about 115kHz. Again, typical application circuits show a normal value for Ct of 4700pF .. hmm.

I did check the snubber diode and resistor, they are okay.

[CJay]

The value of C2 seems unusually small.

C2 should be a higher value, they're usually around 22uF-100uF

Turns xavier and CJay were on the right path. After reading through a few data sheets and application circuits for the 3842 it was became obvious that C2 was too small. Increasing the value to 100uF allows the supply to reliably start.

Now another problem needs attention. With the supply running (even with minimal load) the MOSFET heatsink becomes quite warm after a few minutes.

Thoughts are either: 1) low gate drive and MOSFET not turning fully on, or 2) problem with snubber circuit.

Will try and capture some scope traces tomorrow..

You may find that whatever is causing the MOSFET to heat up is also contributing to your starting problem because the aux winding on the transformer will also be affected by the same problem and of course that will affect startup/run.

But, the typical application of that chip sees C2 being powered via a high value series pair of resistors to a zener and capacitor, the capacitor is usually 22 or 47uF* and is almost always faulty.

(which is where my 'rule of thumb' law comes from, if it's got a 4 and a 7 in the value, it's faulty, yes, I've repaired a *LOT* of SMPSUs, hundreds a month for two or more years of my life)

[dave_k]

Couple of scope captures with no load on the PSU...

[dave_k]

.. and with load on the PSU

[tautech]

This is what worries me:



One can only assume the gate drive is sufficient to drive the gate to ON then apparently falls below the gate threshold. 

That needs further investigation.

[xavier60]

The gate and source are noisy likely because non ideal probe grounding which is difficult to do with SMPS circuits.
The MOSFET turn off seems sluggish. Does the replacement MOSFET have a lower gate threshold voltage than the original?

Extra:
Take note of the plateau duration on the falling edge of the gate waveform.

[xavier60]

On second thoughts it looks like there is a high capacitive load on the drain circuit. The drain voltage is still rising after the gate has gone down to zero.

[T3sl4co1l]

You have to take gate waveforms with a grain of salt.  It's not mentioned if that's at the 3842, or at the gate -- the gate resistor (if present), and trace length, makes a difference.  Probing technique is important.  Some transistors simply bounce, especially when ferrite beads are involved.

I measured a similar waveform on my LED lamp, which is the same basic circuit, when I recently fixed its EMI problem finally (it was buzzing in the shortwave band; I threw ferrite beads at the switching transistor and rectifier, and that did well enough for the improvement I was looking for).

Tim

[dave_k]

The waveform on pin 6 of the 3842 looks nice and square, so I'm happy the IC output is okay. Similarly the voltage on pin 7 is stable and free of ripple, so I'm fairly sure it's not being excessively loaded down either.

The datasheet for the original MOSFET states V_GS(th) is 2.0(min) to 4.0(max) - V_DS=V_GS, I_DS=250uA. Replacement MOSFET values are exactly the same.

Similarly for input capacitance, original is 1800pF (f=1.0MHz) replacement is 1300pF (f=1.0MHz).

I'm beginning to form the opinion that perhaps the AUX winding is damaged somehow, and can barely supply enough voltage to run the 3842 properly .. pin 7 is 11.4 volts, and factoring a 0.6v drop for the internal switching transistor doesn't leave much headroom to drive the MOSFET.

Datasheet for replacement MOSFET is attached...

[xavier60]

I'm starting to think that nothing it wrong. The 2 MOSFETs have comparable specs. The replacement has slightly higher Total Gate Charge which will increase switching losses by a bit.
The UC3842 does not do any Green tricks like burst mode, it drives the MOSFET at the same frequency regardless of load. So the switching losses stay high all of the time regardless of load. 

Check the waveform on the snubber capacitor anyway, it should have only DC on it.

[dave_k]

Oh, I thought the replacement had a slightly lower total gate charge than original?

Original = 77nC (max) V_GS=10V, I_D=6A, V_DS=480V
Replacement = 60nC (max) V_GS=10V, I_D=6A, V_DS=360V

[xavier60]

11.4v is low considering that the UC3842 stops working at 10v. The UC3843 works down to about 8v.
Maybe the AUX winding is corroded. A resistance reading might show the problem.

[xavier60]

Oh, I thought the replacement had a slightly lower total gate charge than original?

Original = 77nC (max) V_GS=10V, I_D=6A, V_DS=480V
Replacement = 60nC (max) V_GS=10V, I_D=6A, V_DS=360V

Yes,you are right, I was looking at the wrong part.

[tautech]

11.4v is low considering that the UC3842 stops working at 10v. The UC3843 works down to about 8v.
Maybe the AUX winding is corroded. A resistance reading might show the problem.

Or D5 is sick.

[IanMacdonald]

Usual reason would be R11 (820k) gone high. You did lift one leg to test this? It can't be measured in circuit.