# Gas station without pumps

## 2016 July 27

### Measuring a high-voltage nFET

Filed under: Circuits course — gasstationwithoutpumps @ 21:07
Tags: , , ,

I got the nFETs today that we’re planning to use for the LED theater lights, and decided to characterize them, to make sure that they would work as we expected.  These are IPU50R950CEAKMA1 500V 4.3A nFETs from Infineon.

The gate voltage at which these nFETs turn on is a fairly high one (traditional, rather than low-threshold for logic-level input), so I had to use a voltage divider to measure Vgs.  I tried measuring with the 3.3V power from the Teensy board and with a 9V power supply—I needed to use voltage dividers to measure Vds and Vdd with the 9V power supply.  (Note: the power supply was a Mean Well GS60A09-P1J power supply—it delivered 9.24V without load and 9.14V with a 790mA load.)

The gate voltage at which the nFET turns on drops as the nFET gets warm:

The flat current at higher voltages is an artifact of the load resistor being used—the load resistor acts as a current-limiting resistor.

The specs give a Vgs for a current of 0.1mA of 2.5V to 3.5V, and so this device seems to be in spec (I have to guesstimate the shift for the higher currents).

The current is limited by the load resistor, but the maximum current here is close to what we expect to use for the LEDs, and the gate voltage is supposed to be at least 7.25V for the driver we are using (HV9910B), so the characteristics here are probably a bit conservative.

I tried using a smaller resistor (1.8Ω) with the 9V power supply to try to characterize at higher currents. The power supply and the 50W resistor had no problems with this, but the nFET got very hot very fast, so I terminated the test before I could damage the nFET. I think that the nFET was trying to dissipate about 8W of power, and at 63°C/W that would exceed the 150°C max junction temperature long before reaching equilibrium.

I also tried running with the 10Ω resistor for a few minutes, to get closer to an equilibrium condition for the nFET, and then recorded for about 87 seconds. I used this recording to plot power dissipated in the nFET versus Vgs, in order to figure out what the steady-state power dissipation would be at the currents we expect to use.

The power peaks at 2.11W when the effective drain-to-source resistance matches the load resistance (around Vgs=4.4V), but even with the transistor fully on, we’re dissipating about 1.26W.

The 1.26W at 63°C/W would give a junction temperature around 105°C, which is well below the 150°C limit. Of course, that is assuming an ambient temperature of 25°C, and the interior of the lighting can will be warmer than that. We’ll have to run fans to keep things cool enough.

I would not want to use this nFET for class-D amplifier lab in the circuits course, because it has such a high threshold voltage, though 35¢ price in hundreds is not bad.  It does look like it will work well for the theater lights, though.

## 1 Comment »

1. […] in Possible new FET lab for electronics course, , More thoughts on measuring FETs, and Measuring a high-voltage nFET were not coming out the way I expected. In particular, I was measuring much higher on-resistances […]

Pingback by Why my nFET measurements were bad | Gas station without pumps — 2016 July 31 @ 22:53

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