-15V linear power supply design issue

Hello,

I am designing a dual linear power supply for my home made oscilloscope project. The circuit is designed with 2 stage ripple suppression, the 1st stage is capacitor multiplier, the second stage is LM317/LM337.

Both side 1st stage capacitor multipliers(Q5 and Q6) are working well and providing a <2mV(@50Hz) output ripple.

The +15V side is working well and the output ripple of LM317 is further reduced to <1mV.

The negative 15V side is designed to track voltage of +15V using an OpAmp(U10). Now the problem comes. I have noticed a 70mV(p-p) high frequency oscillation at around 10kHz. I first thought the oscillation is coming from Q5 and Q6 so I connected pin 4 and pin 7 of U10 to the output side of LM317/LM337. I got buffed the high frequency oscillation has jumped twice to 150mV(p-p).

Could anyone help me to understand why is this oscillation happening and how can I reduce it?

Thanks a bunch!




Ripple at +15V output


Ripple at -15V output

Hello vivalite,

the 337 is wired-up using a virtual ground and this does not help with stability. I would try a 10u electrolytic in parallel with D33, which of course will send the opamp off the roof if you dont isolate. I would insert a 470R damping resistor at the opamps' output, which combined with the above capacitor will further filter out broadband noise.
It would also not hurt to slow down the opamp by connecting a small ceramic cap across its feedback.:wink:

Thanks ucah. I finally found the issue is Q5 Q6 self-oscillating. By shorting all pins on Q5 Q6 output ripple dropped to sub 1mv. Problem solved.