Audio power supply oscillation

Posted by: gtean on 09/29/2014

Hi!

I've designed a power supply for my amplifier, and though it works, i'd like to ask a little help to deal with a few issues.

My first problem was that the supply worked only with no load or with a resistive load. When i connected the amplifier (10000uF capacitors on the positive and negative supply, on both channel, 100mA quiescent current per channel), the output voltage was only about 13V, and the supply was in auto restart mode continously.

It seemed to me that the PI device was unable to supply enough energy to the bulk capacitors in soft start to close the control loop, so it was restarting all the time.

To deal with this problem, i found a solution in DI-203 design idea. R31, C26, and D11 part was implemented to my design in a similar way to help charge the bulk capacitors, i only changed the value of C26 from 47uF to 100uF.

This way my suplly started correctly, but now i have another issue.

After i switch the supply on (with the amplifier connected, but with no driving), the output voltage ramps quickly to 26V, then it climbs to 34V slowly, and starts to oscillate, which is a very disturbing sound from the speakers.

On the attached image you can see that there is an oscillation on the output voltage, repeating in 90ms intervals.

I think this is caused by the few components i placed in to prevent restarting.

Could you give me an idea how the get rid of this oscillation, or advice me another solution to prevent auto restart after powering up?

I also attached the PI Expert file of my design.

Thanks

Bálint Czimmerer

Files

JLH75W.uds (453.5 KB)

Comments

Hi,

That circuit is used for soft finish. But 100uF is too aggressive for C26.

DI-203 uses only 10uF. What happens when you use 10uF?

Regards,

PI-NANO

Hi!

I don't know why i wrote 47uF, you are right, it's only 10uF in DI-203.

When i tried with 10uF cap, there was no change, the supply stayed in auto restart.

With 47uF starting was unstable: sometimes it started, but sometimes it just stayed in auto restart.

With 100uF it always starts, but with the above mentioned issues.

Thanks

Bálint

Hi,

Did you try any values in between 47uf and 100uf? If yes What is the result.

Can you try using bigger TOPSwitch device (TOP250YN instead of TOP249YN)?

Is that amont of capacitance (10000uF+10000uF) is necessary on the output? Can you try with smaller value (Like 5000uF+5000uF) and test your transinet loading and see if it meets your performance requirements.

Regards,

PI-NANO

Regards,

PI-NANO

Hi!

I tried some of your advices with the following results:

I lowered the bulk capacitors value by disconnecting the right channel's amplifier board.

This way, the supply started without any soft finish circuit, but the low freq. oscillation is still there, so it seems that the soft finish circuit was not the cause of the noise.

I also tried other values for the soft finish capacitor, but the only difference was whether the circuit started or not, the noise appeared every time i switched on the supply.

The interesting part is that without the soft finish circuit there is no oscillation until i send input signal to the amplifier (i.e. i load the supply).

After listening for a few seconds, when i stop the music, the noise is there.

But if i switch the supply on without sending any input signal to the amp, it is quiet.

Any ideas?

Hi,

I am little lost. Is it a noise or an oscillation?

If it is a noise, is it coming from power supply or audio amplifier?

Are your magnetics are varnished?

If it is oscillation on the power supply, can you try with the resistive load and see if there is still oscillation in the power supply.

Regards,

PI-NANO