TNY285 power with 2 outputs

Posted by: Dalsgard on 05/14/2016

Hi,

We designed an AC/DC power with an TNY285. It has 2 outputs, 5V/0.5A and an isolated 5V/1.1A. It uses an EE16 size transformer.

It works OK when both outpus are loaded. But if the 0.5A output has only a light load, the output voltage increses to about 7V when the isolated output is loaded with 1A. Simulary, if there is no load, or only light load on the isolated output, the output on the other 5V output drops when loaded. At 400mA the voltage is only 3.5V. The voltage on the isolated output is stable all the time. What can be done to keep both outputs stable? Is it possible in the PI-desiner to simulate a design with varius loads?

Files

PI expert designfile (371 KB)

Schematic (201.2 KB)

Comments

Is there no suggestion from anybody to what can be done to have both otlets stable with various load?

Hi Dalsgard,

Thank you for considering the TNY285.

The voltage on the isolated output is stable all the time because it is where the feedback through the optocoupler CR200 is sensed from. Is it possible for you to use a pre-load (resistor across the output of the +5V0_2) or to have a minimum load requirement?

I will have to get back to you on the idea of simulation with differerent loads.

Regards,

PI-Picard

Hi Picard,

I have tried the preload and it does not help. The problem is that the isolated output either has no load

or it has full load. As you can see from the table atached in my last comment, whith no load the +5V_2 output can not take any load or the output drops as low as 3V.

Does it mean that this type of arcitecure is not suitable when you have a load that varies a lot?

Regards

Dalsgard

Hi Dalsgard,

I see from the PIExpert that the both outputs have post filters the 1.1A output has the larger output capacitance. Your main output only has 470uF. Have you tried increasing the output capacitance to match the recommended value?

The circuit's cross regulation is not tight. If you can still change your transformer, you can use a single secondary winding but with bigger wire size to combine the current requirement of both outputs. With the upper secondary winding open, connect pin 6 to pin 9 and pin 7 to pin 10. Electrically, the secondary outputs are sharing the same winding. Aside from this, you may try also at the same time taking the feedback from the 0.5A output by connecting a resistor to +5V0_2 output. You will have to tweak the values of the feedback resistors to your preferred cross regulation between the two outputs. Doing this will of course make the 1.1A output have less control with regards to its regulation as it will be sharing the feedback with the other output.

I hope this helps.

Regards,

PI-Picard