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Below is the circuit I used to implement an active load which use an op-amp to adjust the mosfets' resistance based on DAC voltage at the op-amp input.

VIN_NANO is about 10VDC.

Mosfet is IXYS IXFH28N60P3, Vds: 600V, On-resistance: 260mOhm @ 14A,10V Vgs, power dissipation: 695 Watt.

The scheme works well with Battery String voltage of 12VDC. When I increase it to 125 VDC, the mosfets die almost instantaneously. What would be the reason behind this?

enter image description here

Test load current looks like this:

enter image description here

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    \$\begingroup\$ Your mosfet's Vgs is 30-40v, which is less than 125v \$\endgroup\$ – dandavis Jun 1 '17 at 3:29
  • \$\begingroup\$ Not sure how you're getting 2A peak out of it even... The maximum current through R20 is: I = Vdac * R20/(R20+R12) (Golden op-amp rule, V+ terminal = V- terminal, then apply a voltage divider equation to R20) 10 * 1/(1001) ~ 10mA \$\endgroup\$ – rusty_old_jfet Jun 1 '17 at 3:36
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The devices you have do not have a suitable DC SOA. The device is rated only for pulse operation at high powers. Going beyond 1 ms on time puts you in a very dark place.

You need to use something like an IXTH30N60L2 ...these devices are rated for large linear load dissipation.

Read this on the L2 range from IXYS.

For the devices you chose, SOA is this: enter image description here

You actually need something like this: enter image description here

....for your application you could use just two devices with plenty of margin or select a larger one and use just a single device.

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  • \$\begingroup\$ Should I use RMS current or Peak current to select the appropriate Mosfet? \$\endgroup\$ – HacLe Jun 1 '17 at 11:05
  • \$\begingroup\$ You can only use RMS if you are providing a continuous waveform load. In the case of a load provider application I'd design for a DC load (no waveform). \$\endgroup\$ – Jack Creasey Jun 1 '17 at 15:25

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