LCR Meter DC Bias For Inductor Measurements

[mawyatt]

Following the DC Bias Adapter for Voltage Effects LCR Measurements, we are looking into DC Bias for Current LCR Measurements.

https://www.eevblog.com/forum/projects/bias-network-for-lcr-meter/

Quality Bench Type LCR Meters employ a 4 terminal Kelvin like technique where the test signal stimulus is from the Hcur terminal and the DUT Potential is measured with Hpot terminal. Likewise the test current thru the DUT is measured with Lcur and Lpot. From these the meter evaluates the DUT characteristics to display the various parameters.

If one could "Couple" a DC Bias current thru the DUT without interfering with the LCR Meter AC measurements then the effects of the DC Bias Current could be studied in a similar fashion as the DC Bias Voltage effects mentioned in the link above.

In the concept shown, a pair of isolation inductors created from Common Mode Chokes allows isolating the High and Low DUT Terminals from the DC Bias Source while allowing the DC Current to flow thru the DUT. DC Isolation Capacitors are shown to isolate and DC voltage developed across the DUT due to the DC Current flow.

The key to getting good results is to understand how the LCR Meter works and realize how the meter measurements are created. Since the Hcur terminal voltage is measured directly with the Hpot terminal, impedance effects at this DUT high side end aren't as sensitive as the DUT low side. Here one wants to create a situation where the DUT AC current seeks the return path to the Lcur terminal and the DUT DC current seeks at path back to the Power Supply. Any AC current that "leaks" back to the Power Supply won't be measured by the LCR Meter an induce a measurement error. A good low leakage (and Z) Isolation capacitor forces the DC DUT Current to return to the power supply while allowing the AC current to pass, whereas a good Isolation Inductor forces the DUT AC current to return the the Lcur LCR Meter terminal, while allowing the DC Current to pass.

The Isolation Inductors must carry the DC Bias Current and need to capable of such without significant reactance degradation. Adding a series resistor in the Low Side helps improve the impedance as "Seen" by the DUT low side and helps imped the DUT AC current from flowing thur the Power Supply. This also provides a positive DC bias across the DUT for the polarity sensitive Isolation capacitors. One could include a resistor on the high side as well, however we found this not necessary as it also adds additional voltage drop and both resistors must dissipate I^R heating, which is why we used a 10 watt 2 ohm resistor as shown.

Also one needs to pay attention to DUT heating as well, this causes the inductive value to slowly change and if too much DC Bias is applied can damage the DUT, we've already smoked a small leaded inductor

Anyway, this is an interesting concept that works well with our equipment, hope some folks find this useful. We'll be updating as time permits, including writing a routine to plot Inductance vs DC Bias similar to the routine that plots Capacitance vs DC Bias Voltage.

https://www.eevblog.com/forum/testgear/lcr-meter-plot-software/

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[mawyatt]

Here's a few plots of some inductors we had laying around, evaluated at 1KHz. This was done quickly with a modified version of the routines mentioned above utilizing the Tonghui TH2830 and Siglent SPD3303X.

It's interesting to note how the ESR behaves, believe this is due to self heating which is significant at higher DC Bias Currents.

Best

[trobbins]

I'd anticipate the applied DUT AC voltage and frequency would need to be reported, and perhaps made pre-settable, to identify the inductor operating flux condition for the test.  That is what would be relevant some other inductor applications, like power supply chokes, given that reported parameters would vary with Vac and frequency.

[mawyatt]

Yes of course the inductance will vary with frequency, the tests above were at 1KHz (Thx, we added this note). A LCR Meter "Spot Frequency" Open/Short Calibration was used at 1KHz, this helps remove the effects of the Isolation Inductors (Chokes) and Capacitors. For a more involved and thorough test one could perform a "Spot Frequency" Calibration at each DC Bias Current and different evaluation Frequencies, or even "Sweep" the DC Bias and Frequency.

The software routine we quickly developed just allows the user to input LCR Meter AC amplitude and Frequency, and the DC Bias Current Start, Stop and Increment setting, also the Power Supply DC Voltage. The Power Supply CC or Current Limit is utilized to control the DUT DC Bias Current at the user input fixed frequency (1KHz in our case above).

For larger DC Bias Current ranges the Isolation Inductors (Chokes) need to be replaced with ones that can handle higher DC Bias currents and will be physically (much) larger. The Isolation Caps (DC Blocks) won't need changing since they only handle the AC test current from the LCR Meter.

Another concept we are looking into is an "Active" DC Isolation technique where the Isolation Inductors are replaced with active circuitry, such as a Current Source with a High AC and DC Impedance. The idea here is to KISS rather than a complex SMU type implementation (we don't have a SMU).

Best