Author Topic: TNY265 flyback issue  (Read 676 times)

Online AstRii

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TNY265 flyback issue
« on: February 09, 2024, 06:45:46 PM »
Hello,
Is there anybody here who has experiences with off-line flyback converters?

I've put together a circuit based on https://danyk.cz/impulz4.html

This circuit should be foolproof and running up to 13W of output power. However in my case the chip is just cycling when running more than 1W.
This is my schematic:


The circuit on a prototyping board:


The chip I'm using is TNY265PN. The transformer is taken out of some old computer power supply. I have not measured the dimensions but I've disassembled it to see if there is an air gap and there is.

This is the primary waveform when the chip is running at very light load <1W


The voltage on the bypass pin is stable 5.9V both when the chip is working properly and when it cycles.

This is the voltage on the EN/UV (feedback) pin when the chip is running well (<1W) (0.5V/div):


This is the voltage on the EN/UV (feedback) pin when the chip is cycling (>1W) (1V/div):


I'm not very sure with these waveforms..

Things I've tried:
1.) Reversing the polarity of primary coil (Wouldn't work at all)
2.) Adding 100k resistor between primary positive and EN/UV pin as recommended by the datasheet (didn't make a change)
3.) Adding 22nF capacitor across the output of the optocoupler (didn't make a change)
4.) Adding another 100nF capacitor from Bypass pin to ground (didn't make a change)
5.) Disconnecting the feedback (At light load the output voltage wasn't regulated at expected, at >1W load the chip started cycling again)
6.) 1K resistor in series with the Zener diode on the output instead of 100R (didn't make a change)
7.) 500R instead of 1K resistance parallel to the optocoupler's input (didn't make a change)
8.) Shorting the 5R6 resistor on Neutral line (didn't make a change)
9.) Using other windings of the transformer as primary N=117 and N=30 (chip cycles at even lower output power than 1W)
10.) Using other winding N=30 as a shield (connecting one end to the primary positive) (didn't make a change)
11.) Placing 2n2 capacitor between the primary positive and secondary negative (improved the EN/UV feedback pin waveform a lot, but the problem is still there)
12.) Powering the Bypass pin with lab bench power supply (would not switch at all)

Interesting is that once the chip cycles it gets stuck and cycles even after the load on the output is disconnected.

I would appreciate any advice.

« Last Edit: February 09, 2024, 07:28:23 PM by AstRii »
Bc. Marek Novotny
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www.uhvlab.org

Offline davekni

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Re: TNY265 flyback issue
« Reply #1 on: February 10, 2024, 03:55:35 AM »
Quote
The transformer is taken out of some old computer power supply.
Small transformers within a higher-power supply are likely to be for feedback rather than power.  I'd guess core is saturating.

BTW, I presume primary waveform is with scope ground (or - side of differential probe) to + line input rather than - (as used for waveforms in data sheet).

Quote
The chip I'm using is TNY265PN.
Look at data sheet for power capability.  Larger version is needed for 13W.
David Knierim

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Re: TNY265 flyback issue
« Reply #2 on: February 12, 2024, 07:30:42 PM »
Quote
I presume primary waveform is with scope ground (or - side of differential probe) to + line input rather than - (as used for waveforms in data sheet).

Yes the primary waveform was scoped relative to line +. I'm using a isolation transformer to power the circuit.

Quote
I'd guess core is saturating.
Well I have tried to change all the other components except the chip itself and transformer so it seems like it's either core saturation or faulty chip.

I thought these inductors with air gap are hard to saturate. I guess I'll have to try different transformer.


EDIT:
I've just tried replacing the transformer for a different one

Yet the results are completely the same, the chip starts cycling at the exact same load.
Thefore I think there is something bad with the chip, as I highly doubt 2 completely different transformers from 2 different circuit boards would have the exact same effect
« Last Edit: February 12, 2024, 08:51:45 PM by AstRii »
Bc. Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline davekni

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Re: TNY265 flyback issue
« Reply #3 on: February 13, 2024, 03:50:05 AM »
Quote
Thefore I think there is something bad with the chip
Unless the chip is counterfeit, a bad chip isn't too likely.  Possible that ESD partially damaged an input.  My second guess would be interconnect parasitic inductances, given breadboard construction without ground plane.  But core saturation is still possible, or excess leakage inductance.  Random unknown transformers aren't that likely to work.  If you have access to an impedance bridge or at least LRC meter, measure winding inductances and leakage inductances.  Then you can make a Spice model.  Measure saturation with a capacitor ring-down test.  (Charge capacitor, then discharge into transformer winding while scoping voltage.)  Gapped ferrite cores have a relatively sharp saturation knee.  Reasonably obvious on scope trace.  Easier to explain after you see traces.  If entire ring-down waveform is sine-wave, then increase voltage or capacitance.
David Knierim

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Re: TNY265 flyback issue
« Reply #3 on: February 13, 2024, 03:50:05 AM »

 


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