Solution Finder Get Tech Support

SMPS output voltages become double

Posted by: hemanth on 07/24/2022


we have design the SMPS using TNY287K . The voltage range is 85V to 265Vac. but we have following problems here

  1. SMPS is turning on 54Vac input itself
  2. Output voltges are desinged for +15V,0.075Amp but when i am increasing the input voltage the all output voltages going to >30V

Kindly let me know the problem. Here i am attaching the SMPS schematic & Traansformer design and in transformer we are using 10pin in the schematic we have conneted 14pin bobin where as we did got the 14pin bobin in market so we have 10 pin bobin in the circuit


Attachment Size
smps_5.pdf 369.99 KB
transformer_6.pdf 160.82 KB


Submitted by PI-Wrench on 07/26/2022

There are a couple of things that might be happening. Does the output voltage increase as you increase the input voltage? If so, the output windings might be reverse phased. In a conventional flyback design, the start of the primary should be connected to the TNY drain. In each secondary, for a positive (+) output voltage the secondary start should be connected to the anode of the output diode. When the primary switch is "on", there should be a negative voltage at each output rectifier anode. If the outputs are mis-phased, the output voltage will track the input voltage instead of remaining relatively constant.

Also, unfortunately, this sort of output arrangement is the worst case for  cross regulation, with only the 5V output in the control loop and the other 15V outputs floating free with no common ground.  The preload resistor value on each of the 15V outputs is too high (150k) to exert any significant control over the output voltage. 

The first thing to do is to make sure that the 5V output is adequately controlled by the AS431.If this is OK, you can then proceed to work with the value of the preload resistor on each of the 15V outputs for better control over the output voltage. I strongly suspect that the optimum value will be much smaller than the 150k currently used.

Also, keep in mind that in a system like this with only one controlled main output and four outputs with separate grounds, the cross-regulation will likely be a problem, with the voltage on the 15V outputs varying as a function of 5V load. This can be helped to a certain extent by extra preload on each output, but the regulation will never be perfect.


Submitted by PI-Wrench on 07/26/2022

On another note, the resistors needed for undervoltage should total to around 4.56 M instead of the value currently used. This should increase your startup voltage to ~85 VAC

Submitted by hemanth on 07/28/2022

Hi Wrench

Thanks now my SMPS turn on at 80Vac. But when i reduced the voltage below 80Vac input it is not turning off. it is turning off at 22Vac .

can you please let me know what is the problem.




Submitted by PI-Wrench on 07/29/2022

I think what you might be encountering is the extended on-time feature of the Tiny-4. This feature allows extended  low voltage operation for a given size of input bulk capacitor. You might want to try reducing the size of your input bulk capacitors to compensate. For reference, what is the total output power of your power supply as designed?

Submitted by hemanth on 08/09/2022

i have designed output power is 5.65W

Submitted by PI-Wrench on 08/09/2022

According to the schematic you presented at the beginning of this thread, you are using a total bulk capacitance of 44uF for a supply with an output power of 5.65W. This is very much in excess of what is necessary at this power level. The general rule of thumb is 2-3 uF/Watt for a universal input supply. This extra capacitance, combined with the extended  on-time feature of the TNY-4 may be the reason why your supply is turning off  at such a low input voltage. Can you try reducing the total capacitance to 10-15 uF and see if that changes the shutdown voltage?