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Switched mode constant current supply, huge spikes

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allanw:
ah crud, accidentally hit submit. I'm reposting better images.

I'm designing a buck converter using the HV9918 IC to drive some high current LED's.

I'm seeing some huge spikes at high bandwidth in the current waveform. Does anyone know where they're coming from?

These waveforms are across a 1.7 ohm current sense resistor. So the current in A is approximately 60% of whatever amplitude is shown in the pictures. The average current is about 250mA. I'm driving a 6.8V zener diode.

In all these pictures the bottom flat trace shows the 0V level for the top trace.


The switching frequency is about 1MHz. There are huge spikes that are 700mA peak-to-peak!


Zoomed in. Those sine waves are about 300MHz. My scope is a 2465B 400MHz and I'm using a 400MHz probe measuring across the sense resistor with the ground lead.


With bandwidth limit of 20MHz on the scope, the current ripple looks much more reasonable. Only 30mA ish.

Should I even worry about it? Will the LED's "see" this high frequency current spike if a 20MHz scope doesn't?

jahonen:
It might be a measurement artifact. Did you use the long ground clip during measurement, if yes, then throw it away, connect the ground from the tip sleeve of the scope probe and measure again? Big spikes voltage spikes are usually result of (relatively) large inductance in current path.

Regards,
Janne

allanw:
Sorry, I must've accidentally hit submit before finishing up my post.

My test setup has long alligator clips going from the board to the zener diode, then another set of alligator clips connecting a current sense resistor in series with it. Could that be the cause of the inductance? In my final application I intend for the LED's to be connected by a cable to this driver board anyway.

Could it be because the output capacitor for smoothing the current is located at the driver board, not at the load? Would putting it at the load fix it?

allanw:
I still see it even when probing the sense resistor and using touching the ground sleeve against the other end of the resistor (through hole).

I'm going to try to connect the sense resistor closer to the load and see what happens.

allanw:
I just verified this using a 600MHz 5GS/s digital scope: I used the ground sleeve directly touching one end of the through hole current sense resistor and the probe tip attached to the other end.

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