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General Board Current Monitoring on Scope

Hi all,

I am a power electronics engineer and work for a company that manufacturers multphase synchronous buck converter control ICs (VRM) for core power applications (among other things).

For an analysis that I am doing I am trying to capture on the scope multiple phases inductor currents to see controller's current sharing scheme. The controller uses a DCR current sense network and the IC is voltage mode control. I use a standard Textronic 4 channel oscilloscope (TDS 3032).

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I have tried the following ways

  1. Using a current probe and a current loop near the inductor. I want to avoid such a measurement since the loop adds inductances to my current path and will interfere with DCR time constants and my current sense network

  1. Using a differential probe and measure the voltage across across the Capacitor (Cb) of the DCR sensing network. The signal levels are in the order of 30-50mV at full load. The offsets in the probe cause erroneous readings. If there is a good differential probes that any of you recommend, it would be helpful.

  2. Floating Ground measurements: Float the scopes ground and measure the differentia voltage across Cb by removing the ground connections on a standard scope probe. One end of Cb is close to Vout and hence is a quiet point which I can use as a reference. This measurement technique gives me accurate results but I cannot observe more than one phase current at a time as there is a circulating current path through the VRM and the scope that messes up my scope capture.

If anybody has used any circuit, instrument or have ideas which you think can help me....do let me know. Looking for ideas....thanks in advance

With this...I complete my third part of my triology of some of my interesting problems....great to find a discussion forum like this. VA ps:

Reply to
VA
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On Jul 3, 1:59 pm, VA wrote:

I think you mean TDS3034, since TDS3032 is 2 channel, but it hardly matters. I generally use TCP202 current probe, having done the circuit layout with probing in mind. If not, I put some effort into modifying the circuit so that as little inductance as possible is added to the switching loop. (By the way, in cases where you're measuring the current in a sizable inductance, it may not matter much that you've added a few nH in series.) Note that Tek current probes have one jaw smaller than the other: put the smaller jaw through a smaller loop. If you end up observing voltage drops, HV differential probes such as P5205 can be quite useful, even at relatively low voltages. You can also use an isolation transformer to power your circuit, rather than lifting the scope ground. Note that the combination of a variable transformer and an isolation transformer, while very handy for mains power supply bench work, presents a rather different source impedance to your circuit compared to the mains. If you're then able to ground one end of a sense resistor, ordinary single-ended probes will do. However, most scopes, including TDS3034, will not have sufficient CMRR to prevent displaying lots of spikes that the control circuitry doesn't necessarily see. Check this by observing what you get with the probe tip grounded. Of course, it's absolutely essential to minimize the loop area at the scope probe end. You can also get improved CMRR by passing the scope probe coax through a large ferrite toroid or by clamping ferrite clamps over it. Paul Mathews

Reply to
Paul Mathews

VA a écrit :

If the needed voltage compliance isn't too high (under 10V) then there's a superb diff probe amplifier for you : AD8129-8130. And it'll provide way more BW than you'll ever dream. A small PCB will the usual surrounding protection stuff for all the channels you need and voilà.

--
Thanks,
Fred.
Reply to
Fred Bartoli

Dear Mr. Mathews, Thanks for your ideas.....I will try them out.

Just a note. Our applications typically use inductors of about

100-200nH.....and I have seen that even 10nH can screw up the already delicate current sharing between multiple phases especially while trying to meet Intel's tough VRM requirements. I will give you feedback as and when I try it. Thanks

Krishnan

Reply to
VA

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