Ringing from switch-mode PSU poluting power rails

"Daniel Kelly (AKA Jack)" schreef in bericht news:cko5qp$n34$ snipped-for-privacy@uns-a.ucl.ac.uk...

Most likely the ringing comes from radiation. Too long powerlines that are not properly decoupled or long and/or thin traces in the power circuit. The coil itself also may be a radiation source. You may have to redesign the layout. Keep the traces around chip and coil thick and short. The same for the other components. You may need some shielding or a ground plane. Keep the input lines away from the powersupply.

I see no use for a diode in the power supply input. A decoupling capacitor is absolutely necessary as shown in the example circuit. If that capacitor is good there is no use for a coil. A good decoupling requires a low ESR elco of some tens uF (or larger, depending on the current) parallel to a ceramic capacitor of let's say 10nF.

Main rule: Prevent the disturbing "signal" to escape from the circuit.

petrus bitbyter

Hi,

Thanks loads for your ideas. Yes, I originally thought it was radiation. But then I cut the PCB in half, separating the powersupply and the mic pre-amp. This didn't solve the problem. Then I noticed that if I powered the switchmode PSU from a separate battery then the intereference went away. I've also done some tests with an oscilloscope and the PSU is DEFINITELY injecting noise into the battery.

Here's a quick box diagram showing the circuits:

Battery---+---------+ | | PSU Amp | Camera

anyone

work?

The

away.

You might be getting ground/supply bounce on common return/suppy lines. Use a star point technique for both ground and supplies to minimise this. Make sure the prime supply ac impedance is low too. Put big C's at the SMPS ,Mic and star point. Use ground plane at the SMPS.

If that doesn't work try one of these

Brian

-- Brian Howie BAE SYSTEMS Avionics Limited Sensor Systems Division Crewe Toll Phase II, 1st Floor, Edinburgh EH5 2XS Phone +44 (0)131 343 8769 FAX +44 (0)131 343 8941 Email snipped-for-privacy@baesystems.com

This circuit makes noise when the internal switch disconnects the LXP,LXN terminal from ground. This allows the inductor to snap positive, forward bias the output diode and dump the inductor current into the output capacitor. So the ground terminal of the output capacitor has fast rising current coming out of it. If this capacitor is not connected very closely to the bottom of the input capacitor and to the ground terminal of the chip (where the internal switch suddenly turns off the current), this current path switch produces voltage drop between these three points on the ground line. This voltage tends to leak out into the rest of the system.

If you already have these three points closely connected, then you may have to add another small inductance in series with the input supply line. I would first try a bead on lead or other small inductance with a low DC resistance. It has to be rated to not saturate at the maximum current that will pass through it. Increasing the value or paralleling a couple caps at the input side may help. also.

"Daniel Kelly (AKA Jack)" schreef in bericht news:cko8vb$k8k$ snipped-for-privacy@uns-a.ucl.ac.uk...

So the battery and its lines are prime suspect. You have to decouple the battery and the amp the same way you have to decouple the PSU. Then you have to use separate power lines for PSU and Amp directly from the decoupling capacitors of the battery so the powerline parts used by both simultaniously are as short as possible. That should be enough. But, as you know, "should be" is not always enough. You can try a coil in the powerline to the PSU, as close to it as possible. You can also try ferrite rings in the power lines.

To rule out radiation further you can experiment packing camera, PSU and the powerlines to it in alufoil. (Pack it in a plastic bag first to prevent shorts.)

(As an aside: I'm not interested in discussions about top posting. IMHO it's a free choice for everyone. I just brought down your responce for my own conveniance.)

petrus bitbyter

I don't have tons of experience with this kind of problem, but I'll share my views anyway.

For this configuration, it seems to me that adding another inductor between the battery and the power supply might not be necessary, since the current is already being drawn through an inductor.

So, try a bigger (or lower ESR) capacitor to ground. Put the capacitor closer to the output inductor, and keep the ground lead short and fat. The chip itself undoubtedly produces current spikes when it turns its internal FET's on, so try putting a high frequency cap as close as possible to the chip (again, keep any leads as short as possible).

If all else fails, put an inductor in series with the power supply line. Experiment with different values. You could always start by putting in another one of the main inductors, since you probably have them lying around. If that works, and if you want, you can try lower values until you find the smallest value that will work.

Good luck.

--Mac

Thanks a lot, everyone!

All these ideas have helped... but I'm still getting noise reaching the amp. Urg! So now I'm using two separate batteries! Not the most elegant solution but it works.

Jack

anyone

work?