TNY278 low power design with non-isolated output and elimination of opto coupler

Posted by: freepiston on 09/19/2012

Hi,

I'm trying to get a PIExpert design working where I've replaced the opto couper with a BC848 driven from the output rail via a zenner and resistor. All else remains the same as advised by PIExpert8 software. From 90-220v ac in i'm looking for 15v, 1A out. Any help would be much appreciated.

Comments

Hi,

I am not sure what your concern is. Please send the schematic for better understanding of the design.

rgds..

Hi, I did not express myself well - cannot get start up - voltage drifts up to 3.4V under no load instead of 15V. Using UF3002 (higher junction capacity than the one recommended) - could this be the problem?

Thanks.

Hi,

What voltage are getting on BP pin of TNY278?

rgds

5.0 volt. Diode now as per recommended type - no change - still will not start.

rgds

Hi,

That seems low. BP voltage needs to be atleast 5.85V for the mOSFET to turn_on. I assume right now you don't see any MOSFET turn on. Can you please sne me the waveforms of BP voltage and drain node waveform.

rgds.

Hi,

Thanks for the suggestion. Have got an isolation transformer in the supply now so can cannect my scope. The voltage on pin 4 (D) is 151 peak to peak - wave is rounded square in shape - sorry have no means of taking a picture. The voltage on pin 2 (BP) is 5.76 - constant and the voltage on pin 1(EN/UV) is 2.31 with a 115kHz small amplitude wave superimposed.

rgds.

Hi,

Seems like you are switching but not sure without the waveforms.

Anyways, is T4 a common mode choke, seems like a differential? Also what voltage do you meaure at the Bulk cap C14? Please check your transformer T3 winding polarity. Are you regulating at 3V currently instead of 15V?

rgds

Sundar.S

Hi,

Found the problem - wrong value of limiting diode across the primary. Used 5.1v instead of 200v.

Thanks very much for you help.

rgds