My Multiple Outputs SMPS problem(It's OK now)

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tny266 trouble

I made a multiple outputs SMPS using TNY266G, with 10VDC and 24VDC output. I found that when no current drawed from 10VDC output, the other output is around 25VDC. But when it draws 300mA from the 10VDC output, the voltage on the other output will turn to be up to 40VDC!! By the way, there is always no load on the other output when I got this result, what's wrong? Is it normal?
 

Re: My Multiple Outputs SMPS problem

It is normal. It is necessary to draw a current from the 24V output using a resistor or other load.
The minimum optimal value of this current depends of the transformer windings (inductance of the 24V winding).
 

My Multiple Outputs SMPS problem

Thanks. But if I get the inductance of the 24V winding, then how to calculate the optimal current value?
 

Re: My Multiple Outputs SMPS problem

Working in continuous mode with proper RC snubber across the secondary windings should reduce this effect.
But I don't know if this particular chip can be used in continuous mode SMPS.
 

My Multiple Outputs SMPS problem

It's working in discontinuous mode.
Another question. My SMPS work abnormal when drawing 400mA from 10VDC output. The 10VDC output is fine at first, but after a while (about 2 minutes or so), it turns out to be a pulse as illustrated below, and may turn back to be fine output later for a while and then return back. When I design using the PIExpert software, My SMPS is rated to output 7.1W max. So, why only 400mA will cause this. How to solve this problem? pls, help me out if you can. thank you very much!
 

Re: My Multiple Outputs SMPS problem

It looks like an overcurrent(overvoltage) protection in action.Try reducing the current sensing resistor a bit to see if the problem remains.
 

My Multiple Outputs SMPS problem

Maybe my primary turns wire gauge is a bit too large or the primary indutance is a bit small.
 

Re: My Multiple Outputs SMPS problem

Still can't get it work properly. Below is my switcher circuit. Please help me check problem out. Thank you very much.
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

I use a single output (max. 15V / 10W) SMPS with TNY268 using TL431 for feedback.
The core is E20/6 with 1.36mH primary inductance and 24uH for secondary.
I think that the primary inductance of your transformer is too small.
In the past I tried to design a transformer using PI Expert, but the transformer losses were too big because the primary inductance (offered by the software) was to small. When I increased the primary inductance at the same transformer ratio, everything was normal.
 

Re: My Multiple Outputs SMPS problem(help, see the last to p


Thank you! Will verify with the transformer manufacturer. I sent them the data with the primary inductance about 1.33mH.
 

Re: My Multiple Outputs SMPS problem(help, see the last to p


By the way, your output is 15V/10W, what is the output capaciator you are using. I use 470uF/16V 10uH and 220uF/16V for 10V/4W output. The 470uF cap got very hot during working. Is it normal? How about yours? May this hot be the reason which cause the problem described upstairs?
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

I am using 1000uF/16V 10uH 100uF/25V. A hot capacitor is a death capacitor. You must change it with another capacitor (watch the polarity!).
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

MADURAN said:
I am using 1000uF/16V 10uH 100uF/25V. A hot capacitor is a death capacitor. You must change it with another capacitor (watch the polarity!).
Click to expand...

but no problem with the polarity. what 's the trouble???
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

If a polarized capacitor is mounted by error with positive terminal to common the effect is the over heating. I made this kind of error, it is always possible to do it in the experimentation phase.

The overheating can appear if the quality of the capacitor is bad, or if the voltage on the capacitor was for a short time too high.

So you must change this capacitor.
 

My Multiple Outputs SMPS problem(help, see the last to post)

maybe the output diode generate too much heat?

I found the 24VDC output capacitor 220uF/35V also can feel hot even no load connected, but a bit less. As I know, a power capacitor always hot. wrong?
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

The expression "Power capacitor" is valid only for medium or high power inverters (few kW to hundreds kW). In this equipments the designer must maintain a normal temperature even for the capacitors.
But in your situation it is abnormal to have a hot capacitor.
An exception can appear if the load current is high and the rectifier diode is hot and there is no heatsink for it. In this case, the heat of the rectifier will be dissipated thru the terminals to the capacitor and the capacitor can be heated too. But this is only theory, because in your case the load currents are small.
I think that you must use a good capacitor and after that try to verify the output rectifier diodes. Do not forget to check the transformer. Try to increase the primary inductance if it is too small (the transformer ratio must remain unchanged).
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

Several people above have given good advice about the capacitor in a qualitative way. To be more precise, capacitors have a "ripple current" rating. You need to check the data sheet for this value. Also calculate the current in and out of the capacitor on each cycle of switching to see if it is less than the rating. Heat is the big reliability problem on electronic components. You should in general never run parts near their maximum temperature rating if you want a circuit that will last.
 

My Multiple Outputs SMPS problem(help, see the last to post)

I think the heat on the capacitor is caused by the output diode. When it has been working for several minutes, the TNY266, output diode are quite hot. And then it turns to be abnormal. So this heat would be the problem.
 

Re: My Multiple Outputs SMPS problem(help, see the last to p

You must use low ESR (Equivalent Series Resistance) capacitors at the output stages. There are special grade capacitors for this kind of application. Check some capacitor vendor catalogs.
 

My Multiple Outputs SMPS problem(help, see the last to post)

Thanks , I found that be working for a few minutes , the TNY266 temperature turns up to 90 degree C. and the output diode is more hot up to 125 degree C. But I have followed the layout guide in TNY266 datasheet. Make the copper area maximum for them to sink heat.
 

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