Need help in understanding the voltage regulation setup with TNY278

Posted by: atanum on 02/11/2013

Hi,

I am trying to build an SMPS with TNY278 and using the schematic in RDR-91.

The document says that the output voltage is set by the combination of VR3, R6 and R4. As VR3 is a 11V zener, I assume for an output of 12V, the drop across R4 and R6 should be 1V, at a current of 5mA. However with the values of r4 and R6 at 2K and 390R, it is not the case.

Please help explain what I am missing.. Any help is greatly appreciated.

Rgds

-Atanu

Comments

The output is mainly decided by the voltage across the zenner and the voltage across the LED of the opto. The current through the 390ohm resistor is around 0.5mA, since the threshold current of EN/UV is 115uA, and 1V across the 2K resistor.

Thetolerance of the11V Zenner is very important to the setpoint of the board. If your board suffers from setpoint problem, double check the 11V zenner first, and make sure its tolerance is tight enogh.

regards

Thanks for the explanation. I think I get it now. I will try to get a zener with a better/tighter tolerance (1-2%).

The other issue I have with my power supply is that the moment I increase the current beyond 250mA, the device latches out. I have to turn the AC power off and restart a few minutes later to get the circuit to work again.

I have attached my schematic for reference. I noticed that the voltage on pin BP/M increases to 6.38V when the circuit is pushing current through the load, which is a high wattage 40R, 50W resistor. When the circuit output reduces to zero, the voltage on this pin falls to 5.78V. I am trying to understand what is happening here?

Secondly, which could be the culprits in this design that are causing the circuit to latch off. Another fact is that when delivering the circuit stays on for sometime (10-15mins) and then turns off. As I increase the current, it turns off much earlier.

Looking forward to assistance from this forum. Any other advice is welcome.

Rgds

-Atanu

For the bias voltage:

At no load, the bias voltage comes from the line input and thus it is around 5.8V. At higher load or full load, the output of the Bias winding is a lot higher than no load, and thus the BP is powered from the bias winding, and a zenner inside the BP pin limits ithe voltage to 6.4V.

The latch off is caused by too much current into the BP pin, so you have to increase the zenner voltage or increase the 40ohm resistor to solve this problem.

regrads