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Using a 2602 for Switchmode Regulator Measurements

Models 2601, 2602, 2611, 2612, 2635, 2636
Models 2601A, 2602A, 2611A, 2612A, 2635A, 2636A
Models 2601B, 2602B, 2604B, 2611B, 2612B, 2614B, 2634B, 2635B, 2636B
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kcnicho
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Joined: March 28th, 2011, 4:29 pm
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Using a 2602 for Switchmode Regulator Measurements

Post by kcnicho » March 28th, 2011, 4:37 pm

I am attempting to use a Keithley model 2602 sourcemeter to make measurements on small switchmode regulators.

The basic idea is simple, connect channel “A” to the input of the regulator, as a stepped voltage source, with a maximum compliance current. For example, for a boost converter, this may be stepped from 1.0 volts to 1.6 volts, with a limit of 2 amps. Positive (red) line to the input terminal, negative (black) line to ground. This causes a predetermined input voltage to be provided to the regulator while measuring the input current being drawn.

Channel “B” is set up as the load, a swept current source of -0.001 A to -0.100 A, with a compliance of 3.3 volts (for a 3.3 volt regulator.) Again, positive (red) line to the output terminal, negative (black) line to ground. This causes a predetermined load current to be drawn from the regulator while measuring the output voltage.

Many useful curves of regulator operation can be determined, such as efficiency vs load current, output voltage vs load current, etc.

While I have successfully made measurements of several switchmode regulators with LabTracer 2.0 using this method, I have recently run into problems with some new regulators that I am testing.

I believe the problem has to do with the way LabTracer 2.0 / Sourcemeter handles the current source compliance for the switchmode regulators load. If the regulator cannot supply the current required, the sourcemeter is allowed to apply up to NEGATIVE 3.3 volts to the output of the switchmode regulator to try to get that current to flow. The problem is, I believe that applying this negative voltage is failing the regulators.

I think what I need is some way to limit the compliance voltage to some positive level that is lower than the output of the regulator (while still remaining positive.) For example, for the +3.3v regulator output, I might want to limit the compliance voltage to go from POSITIVE 3.3 volts down as far as POSITIVE 3.0 volts to get the current to flow, but no less. That way, the regulator will not be damaged by large negative voltages.

As far as I can tell, whenever a negative current is entered for a swept current channel, the compliance voltage you enter is interpreted as a NEGATIVE voltage at the output of the Sourcemeter. Also, I cannot simply swap the output leads and enter a positive current, because then the Sourcemeter uses the compliance voltage as a POSITIVE voltage and I have the same problem.

Does anyone know of any way, using LabTracer 2.0 to do this type of measurement? Have switchmode regulators been analyzed with Keithley sourcemeters in other ways? Is there a way to have the sourcemeter simulate a load resistor, while measuring the output voltage and current of the regulator under test?


Thanks,

- Kevin

Dale C
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Re: Using a 2602 for Switchmode Regulator Measurements

Post by Dale C » March 30th, 2011, 11:43 am

I don't believe that Labtracer would be able to help with this application. For that matter the 2602.
Compliance is an absolute number. There is no polarity to be set.
But it can change direction.
For example if you source 1.5V in to a 1V battery and the compliance is set to 100mA then everyone is happy. And the batter charges at 100mA rate.
If you source 0.5V into the 1V battery then the 2602A would sink the current at the 100mA. It would accept the current back into the unit. The compliance would be negative.
So the unit and parameters decide which quadrant the unit will operate in. Not the programming.

Qing S
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Re: Using a 2602 for Switchmode Regulator Measurements

Post by Qing S » July 6th, 2011, 9:08 am

Instead of sinking a current on the load source meter, try sourcing a voltage that is smaller than the output voltage of the regulator. Set the compliance current of the load source meter to the desired load current. I have tried this method with a couple of linear voltage regulators here. It is little more cumbersome, but at least the direction of the current is always correct and you will be able to monitor the regulator output.

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