Current limiting using MOSFET
Current limiting using MOSFET
(OP)
Hello. I am designing a circuit to perform a discharge test on a battery. The load profile consists of several discrete resistance values that I will switch in and out of the load path at set times using an analog output controller and some power MOSFETs. The load bank design is almost complete and has worked well so far in testing. I have just one problem left. During the initial voltage rise time period of the battery, I am required to limit the current of the battery to a set value, for a set amount of time. After that time, I need to let the battery continue to finish its voltage rise naturally. Is there a simple way to perform this current limit function? Here is the circuit that I have tried so far:
_________________
| |
| |
+ |
VBatt ----
- |
| |----- MOSFET Gate Voltage
| ----
| |
| |
| \
| /-------- Load Resistor
| \
| /
|________________
This circuit "almost" works. I can vary the amount of current passed through the MOSFET by adjusting the gate voltage. However, it is not working as a "current clamp" like I would like. The current passed through the MOSFET is varying as a function of the battery voltage, which is a problem for me. What I really need is a way to limit the current no matter what the battery voltage is. Any ideas? I should mention that I know the MOSFET will get hot using this method. However, the time that I have to limit the current is very short, so this will not be a problem. Thanks in advance.
_________________
| |
| |
+ |
VBatt ----
- |
| |----- MOSFET Gate Voltage
| ----
| |
| |
| \
| /-------- Load Resistor
| \
| /
|________________
This circuit "almost" works. I can vary the amount of current passed through the MOSFET by adjusting the gate voltage. However, it is not working as a "current clamp" like I would like. The current passed through the MOSFET is varying as a function of the battery voltage, which is a problem for me. What I really need is a way to limit the current no matter what the battery voltage is. Any ideas? I should mention that I know the MOSFET will get hot using this method. However, the time that I have to limit the current is very short, so this will not be a problem. Thanks in advance.






RE: Current limiting using MOSFET
Near constant because there will be a variation in gate-source voltage depending on current and, to some extent, battery voltage.
An increasing current will then reduce the gate-source voltage and reduce current. Or, rather, keep it constant.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current limiting using MOSFET
RE: Current limiting using MOSFET
What are you controlling all this with? Why not just monitor the current with your controller and PWM the FET accordingly?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current limiting using MOSFET
I would use a voltage regulator like the National Semiconductor LM338. Run its input voltage from the battery's positive terminal. Put the load resistor on the output: since the output voltage is (nearly) constant, the current will be too. Making it stable is just a matter of putting in capacitors the way the datasheet says. (And just throw more capacitors at it, if that isn't enough.) The National LM-series regulators are generally pretty forgiving when it comes to oscillation, and protect themselves from burning up too. If one resistor/regulator can't deliver enough current, just put several in parallel.
The LM338 assumes you need a lot of current (5 amps or more). If your battery is small and you need more accuracy, a smaller regulator like the LM317 might be better.
Try to arrange so that the regulator is running close to its drop-out voltage (minimum voltage drop from input to output) when the battery is at its lowest. That way most of the voltage, and heat, goes to the resistor.
RE: Current limiting using MOSFET
Current limit is 6 A for the first 125ms after battery connection.
Load resistance is 0.2 ohms.
After 125ms, the battery current can rise naturally on the 0.2 ohm load until the other steps of the load profile kick in.
So if you multiply 6 x 0.2, you get 1.2 V, plus the MOSFET turn on voltage of approx. 3.3 V means I should set the gate to approx. 4.5 V. The MOSFET certainly does restrict current. This is easily proven because without the MOSFET at a 10V test voltage I would be pulling 50 A (!), but with the MOSFET it's right around 6 A like I want. The problem is that if I change the test voltage to 15V, 20V, 30V, etc, the current goes up (7A, 8A, etc.).
I like the idea of controlling the voltage using a regulator set at 1.2 V to limit the output current to 6 A. It would require a bit more circuitry than just adjusting the gate voltage on the FET though.
RE: Current limiting using MOSFET
a hard time finding one that can handle 50A.
I would suggest a better way but not as simple:
Switch the MOSFET gate with pwm. Put a current sensor in line with the MOSFET, feed this into a PI with a setpoint (current level) control. Feed the PI output into a duty cycle generator (compare voltage with a sawtooth waveform)and drive the MOSFET gate with that. Actually, you can probably buy switching power IC controllers that are used for switching power supplies to replace the PI and duty cycle generator.
RE: Current limiting using MOSFET