The two 0.33r resistors simulate a bar laser load. The power supply is
+48 at X10 and the laser drive (current sink at X7) can be up to 150 amps.
The pushbutton discharges the caps in the laser driver, so we can unload the DUT quickly, and the LED switch lets us turn off the HV indicator LED to measure standby power. Q1 and Q2 are depletion fets.
See what's wrong?
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John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
We use a floating-input scope with this fixture, a TPS2024, 1M Zin. The "hot" connectors have a warning screened on the test board next to them, the triangle-exclamation thing, just as a reminder.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
Shouldn't X7 be grounded? Or is that done externally>
Not sure that's kosher with the FETs, seems that will overvoltage that gates unless you provide a current path (like R2) on Q2 that keeps Vgs from rising to 48V, and the abs max spec is probably 10 or 20V. They may not die at 48V, but..
Best regards, Spehro Pefhany
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X10 is +48, from a capacitor bank on the DUT, charged by the notorious boost-doubler supply. X7 goes to the drain of a mosfet on the DUT, a biggish pulsed closed-loop current sink. The two power resistors simulate a bar laser load.
Yeah, that's the problem. D-S leakage can pull up the source and blow the gates.
Most mosfets can tolerate numbers like 75 volts on the gate, but these guys can't. I can add an R2, 50K or something, and kluge a resistor across S1, to save the fets. That messes up my standby current measurement a little, but I can allow for the small extra current in my test procedure.
I should have put the switches in the drains. The depletion fets are cool here, though. Cap discharge is linear, not exponential, and the LED stays on almost to ground.
This has implications for certain cascodes. I've got to remember this.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
The pushbutton discharges the capacitors in the laser driver board. You do that before you remove it from the test fixture. The switch lets you turn off the HV warning LED, so that we can measure the standby power consumption of the DUT. Maybe that should have been a pushbutton too.
The fets are both depletion types, normally on at zero gate voltage.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
Why not just place a zener diode from source to drain? That way you can prevent the gate going more negative that 10V or whatever versus source and since there is no need to enhance in this scenario you don't even need a regular diode in series with the zener.
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