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Greetings!
I have designed and prototyped the following 5V to 13V boost regulator (max 600mA) using the LM3478 and need a bit of help troubleshooting a noise problem. Here's the schematic (all caps are SMT ceramic unless otherwise noted):
The circuit pretty much works, except that I see noise spikes of around 300mV pk-pk on the output under a relatively light 10mA load measured directly across C9 using an oscilloscope probe with a short ground:
Here's a close-up of one of those pulses:
It seems to me that these noise spikes are way too big and that we will fail EMI, but I have not been able to eliminate them. Can you please help?
An astute observer will note that the period of these noise spikes is equal to the period of the LM3478 switching frequency, about 800kHz. The largest spikes occur (shown above) when the MOSFET switches on and the voltage on the switching node drops to zero.
Speaking of the switching node (on the drain of Q2), the waveform looks pretty decent. I don't see the large underdamped oscillation that would indicate the presence of significant stray L and C in the PCB design. There is, however, a bit of undershoot prior to the rising edge and negative overshoot after the falling edge.
I also measured voltage at the Source of the MOSFET, directly across the current sense resistor R13 and saw the following noise. I expect this node to be fairly noisy, but these noise spikes coincide very closely with the ringing on the output voltage. Correlation or causation?
I have tried capacitors of various values across R13. Each one changes the characteristics of the ringing in image above (mostly different natural resonant frequencies) but doesn't significantly reduce or eliminate it. I also observed that the ringing seems to get worse when I use a shorter ground lead! When I use the usual 3 inch ground lead on the scope probe, the ringing appears to be much lower amplitude than when I use the short piece of copper wire looped around the ground tab of the scope probe. This is the opposite of what I expected! In my experience, the stray inductance in a long ground lead tends to form a resonant tank circuit with stray capacitance on the PCB leading to phantom noise on the scope that isn't really there, but that's the opposite of what's happening here!
In conclusion, I have the following questions:
Any help is much appreciated.
Cheers,
Scott
Wouldn't an RC snubber from drain to ground only be used to reduce ringing on the switching node (the drain of the MOSFET)? As I showed above, the SW node is clean. The noise is present across the output cap.
BTW, a no-load problem would appear as a low-frequency ripple--4 or 5 kHz. I've seen that issue before and the problem above is something different. Thank you very much for the reply.
You are right . THe HF noise is not because of SW node ringing as shown in yur waveform .
THe internal LDO output is typical 7.2V (which in your lot may be closer to 5V) when Vin is higher and is equal to Vin voltage when voltage is lower than 7.2V.
When when the transition happens , there is a glitch in reference voltage which can be the cause of HF noise you are seeing at the output .
I think in your lot this transition is happening around 5V and because of which your seeing HF noise as output will replicated the reference voltage glitch .
Under light load , you would be in transition and at higher load , input voltage would drop a bit and hence you wnt be seeing the HF noise .
Refer to page 13 of the datasheet .
You can simply verify this by changing the input voltage to 4.5V or 6V and see if the HF goes away . If this is the case you may add a series resistance from input of the supply to VIN to drop the voltage a little .
Let me know if you have any questions .
---Ambreesh