So I am trying to build a serious discharger for testing batteries and battery packs from 2.0V to 80V and currents up to 250 amps (limited by wattage). Sticking with the design of many commercial dischargers I wanted to start by trying to do MOSFET driven in its linear region.
Through some other forums I was given a circuit diagram put together for a magazine article a while back for a simple constant current discharger. From that circuit I came up with this circuit:
The circuit is run by 10V. In the non inverting input I have a 0-5V that controls the level of current. On the inverting input I have the current sense from the 0.02ohm resistor. This all dumps out into a FET that will burn off as much as 100 watts.
So this circuit during testing didn't have the .1uf capacitor to begin and seemed to be working great at 3.7, 13v all the way up to about 30V but as soon as I hit around 40 volts POP! the fet fails. I hooked up a scope to find a nasty oscillation that seemed to be causing quite a bit of inductive voltage:
13.3V battery at 7 amps:
So I added that 0.1uf capacitor and everything went perfectly flat and back to DC
So I thought I had fixed the problem and tried a 60V source: .2 amps, .4 amps, .6 amps POOOOFFFFF!!!! Darn
So of course I didn't have the scope recording during the failure so I really have no idea what happened. Even DC mode the fets should be good to 5 amps @ 60V peak. My current current sensors never see any peaks and seems to be very steady.
FET:
Alpha & Omega Semiconductor Inc
100V 150A AOT410L
.45 C/W
http://aosmd.com/res/data_sheets/AOT410L.pdf
OP AMP:
Texas Instruments TLV2371IP
Rail to Rail input and output
http://www.ti.com/lit/ds/symlink/tlv2370.pdf
So I know there isn't a lot of information here but do you think this is the FET not up to the task? Or is my circuit not reacting fast enough as the voltage gets higher and just start pulses high current? Other possibilities?
Through some other forums I was given a circuit diagram put together for a magazine article a while back for a simple constant current discharger. From that circuit I came up with this circuit:
The circuit is run by 10V. In the non inverting input I have a 0-5V that controls the level of current. On the inverting input I have the current sense from the 0.02ohm resistor. This all dumps out into a FET that will burn off as much as 100 watts.
So this circuit during testing didn't have the .1uf capacitor to begin and seemed to be working great at 3.7, 13v all the way up to about 30V but as soon as I hit around 40 volts POP! the fet fails. I hooked up a scope to find a nasty oscillation that seemed to be causing quite a bit of inductive voltage:
13.3V battery at 7 amps:
So I added that 0.1uf capacitor and everything went perfectly flat and back to DC
So I thought I had fixed the problem and tried a 60V source: .2 amps, .4 amps, .6 amps POOOOFFFFF!!!! Darn
So of course I didn't have the scope recording during the failure so I really have no idea what happened. Even DC mode the fets should be good to 5 amps @ 60V peak. My current current sensors never see any peaks and seems to be very steady.
FET:
Alpha & Omega Semiconductor Inc
100V 150A AOT410L
.45 C/W
http://aosmd.com/res/data_sheets/AOT410L.pdf
OP AMP:
Texas Instruments TLV2371IP
Rail to Rail input and output
http://www.ti.com/lit/ds/symlink/tlv2370.pdf
So I know there isn't a lot of information here but do you think this is the FET not up to the task? Or is my circuit not reacting fast enough as the voltage gets higher and just start pulses high current? Other possibilities?