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General Board Current Monitoring on Scope

V

VA

Jan 1, 1970
0
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).

http://www.valuetronics.com/Details.aspx?ProdID=4238&Model=Tektronix_TDS3032
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

2. 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.

3. 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:
 
P

Paul Mathews

Jan 1, 1970
0
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).

http://www.valuetronics.com/Details.aspx?ProdID=4238&Model=Tektronix_...
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

2. 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.

3. 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:

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
 
F

Fred Bartoli

Jan 1, 1970
0
VA a écrit :
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).

http://www.valuetronics.com/Details.aspx?ProdID=4238&Model=Tektronix_TDS3032
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

2. 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.

3. 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:

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.
 
V

VA

Jan 1, 1970
0
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
 
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