TNY279 dosen't start unless we touch pin #1 with a probe pin

Dear Mohamed.


Can you let me know if the part is in auto-restart (meaning that it will switch for around 50ms, and then stop for around 2 seconds)? If you're not measuring the drain to source voltage it may indeed look like no switching is happening.


I wonder if this is not a layout issue. Does it help if you place a 1nF capacitor on the place where you connect the scope probe to make it work?


By the way I noticed you placed a P6KE100 across the transformer, however you have a reflected output voltage of 105V. This will probably make the P6KE conduct and dissipate a lot of power. I advise a P6KE150.

Dear Wesley;

Yes it's auto restarting but there is no under voltage, no over voltage or output short circuit.

regards

Mohamed

Hi.


Ok, if we are in auto-restart there are a couple of things I would like you to try out:


1 - You designed for increased current limit, but you only use 100nF on BP the pin (which corresponds to standard current limit). This may be a reason for your issues starting up, and in addition it is possible that you won't be able to deliver full power. Add a 10uF capacitor in parallel to your 100nF BP cap (you can use electrolytic if you wish, or ceramic). Alternatively you may keep the 100nF and use a TNY280 instead (the standard current limit of TNY280 is the same as the increased current limit of TNY279; more info on the datasheet).


2 - You may need a soft finish circuit. Because your 12V output is relatively high current, and you want to start up at low input voltage, it may be that during startup you output voltage takes a while to go up to 12V, and therefore you get no feedback info. Try and add 10uF in parallel to see if you get a smooth start. If this works you may want to add a discharge path for this capacitor (otherwise if you quickly turn on and off the PSU may have the same problem again). You can use the configuration of this example. Components R16, D9 and C20

Hi,

the problem comes up again

attached the Drain scope capture of the TNY279PN there is no switching at all
no restart

the BP/M volt is 5.8
The EN/UV volt is 1.5 volt

input voltage is AC 120 rms

the drain signal is attched it seems no switching at all, do you have an explanation what might be the cause.

it's really urgent for us

Thanks

Hi.

I am about to send you a private message.

Hi;

Any updates, I have replied to your private message

regards

Mohamed

hi
i have same problem
please help me
thanks

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