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Power Integrations Engineer Power Integrations ModeratorTNY279 dosen't start unless we touch pin #1 with a probe pin
Dear PI;
I'm using your TNY products for our energy meter power supply (www.sewedy.com.eg).
for the last 5 years we have used around 500 K pcs of your TNY chips
But from time to time I face a problem with TNY which i have faced with TNY 256Y and now I'm facing with TNY279PN the problem is that
- even the input is stable and ok the TNY dosen't start switching unless I touch pin #1 with the voltammeter or oscilloscope probe!!
BY the way input voltage is 120 line to line
The PI expert design file is attached
the real design file is also attached in PDF
would you please try to help how to fix this problem.
Regards
Mohamed
mabdeltawab@sewedy.com.eg
www.sewedy.com.eg
| Attachment | Size |
|---|---|
| 3Phase_Power.pdf | 29.96 KB |
| TinySwitch-III_PIDesign4_3phase_ST.uds | 161.5 KB |
mabdeltawab
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
| Attachment | Size |
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| print_002.png | 23.9 KB |
| print_001.png | 27.87 KB |
mabdeltawab | 7 posts 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.
PI-Wesley | 91 posts Posted: Dec 14, 2011 | 14:27 PM In response to Dear Mohamed. Can you let
Dear Wesley;
Yes it's auto restarting but there is no under voltage, no over voltage or output short circuit.
regards
Mohamed
mabdeltawab | 7 posts Posted: Dec 14, 2011 | 14:54 PM In response to TNY279 dosen't start unless we touch pin #1 with a probe pin
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
PI-Wesley | 91 posts 
