Power supply output not stable
Hi, we have designed a Constant voltage output power supply with TNY278PN (12V/2A). Input is 195-265V AC. As we increase the load to >1A, the voltage drops to about 10V (at low load its about 11.89V) and current starts to increase exponentially, even crosses the 2A mark after few min.
For the trafo, we have used 34SWG wire for the primary, 24SWG for the secondary. The output 2 diodes (in parallel) are rated at 2A each.
The diodes dont seem to overheat. What could be wrong? Pl advise. Thanks.
Comments
Hi,
The output cable length is 30cm. Trafo design as per PI design Suite 8.
Attached is schematic. Pl advise.
Thanks.
Hi,
As requested, had posted schematic and trafo specs.
Any clues whats wrong?
Thanks.
The schematic looks fine at first glance.
Have you measured the voltage of the power supply directly at the terminals without using the output cable?
What do you mean that the output current goes exponential after a couple of minutes?
-Wesley
Hi,
We connected different loads to the circuit, measuring the DC current through an ammeter. Voltage was measured directly at the o/p terminals.
1]Load 500mA - Circuit works fine. O/p (V & I) stable over entire input voltage range. V=11.98V.
2]Load 1A - Circuit still works fine. O/p (V & I) stable over entire input voltage range. V=11.45V.
3] Load 1.5A - Circuit works fine for a couple of minutes. i/p is kept at 230V AC. Then the o/p current starts to shoot up slowly, in steps, 1.6A then 1.7 then 1.8.....crosses the 2A mark. V=10.45V. Why does this happen? the Load are LED strips, which have individual current regulators per strip.
In the normal condition whats expected of a power supply if overloaded? What if we connect a load of 2.5A? Would it just latch off? Should we add a few turns to the secondary of the trafo, to compensate for vtg drop?
Pl advise.
Thanks.
What do you mean by the LED load has "individual current regulators per strip". The LED load dynamics will change based upon temperature and that could cause the voltage to decrease of the LED string.
However I am surprised that it runs at 10.45 V as I would expect the supply to go into auto-restart as that is nearly 13% out of regulation. The part will go into auto-restart or thermal shutdown if overloaded too heavily. Adding turns the output will not change the voltage regulation as that is dictated by the feedback circuit.
Can you include a photo or more detailed information of your test setup?
Can you place a resistor across the output to load it to 2 A and measure the output voltage and current? I am not sure if the LED load is interacting with the output of the power supply.
-Wesley
The led strip's have chip current regulators on each strip, to regulate current at 330mA, irrespective of variation in DC voltage. These IC's have a negative temp coefficient, so they auto reduce the current slightly, if overheating starts in the strip. Ideally then this should reduce the total load on the power supply.
The IC's have been configured to give out 330mA. Practically u get a reading of about 318-320mA. We have connected 5 such strips in parallel, so 320mA*5=1.6A. Our problem is that, say due to temp variations, even if the load current changes slightly, thats ok. but here it starts to tick up, n does not stop.
Should we try to increase the gauge of the primary winding? Is the primary winding heating up at higher loads, causing change in its resistance?
Also we'll try to connect a resistance as u suggest and let u know the results.
Hi,
Could you please provide me with a schematic and transformer specification? I'm really not sure what is going on from your description. Are you measuring the actual terminal voltage of the power supply that does not include the potential cable drop from the terminals to the load?
The schematic will greatly help me troubleshoot the problem and help you. Thanks!
-Wesley