Hi all,

I have a 12vdc proportional coil which is controlled using a logic level N-channel enhancement MOSFET (FQP30N06L). Please see attached spec sheet for both and my circuit diagram.

In order to obtain proportionality I vary the current between 100mA and 1200mA using PWM control of the Mosfet Gate. At 100% PWM duty cycle the coil draws about 4A. I would like to limit the maximum current the coil is able to draw @ 1.2amps. I know I can do this via PWM, but then I wouldn't be driving the MOSFET gate as hard as it requires and as such heat dissipation is a problem and reliability is shocking.

So, I am thinking of using something like an LM317, LM338, or LM2930T configured as a current controller to achieve this. Given my voltage and current requirements can anyone recommend a suitable IC with accompanying circuitry / configuration? I want to keep heat to a minimum is possible.

Thanks in advance.
Jim

Petkan:
Using switching mode control is the most appropriate method of controlling inductor currents. Linear regulators will have very poor efficiency and will overheat. Typically a Buck regulators is used with current rather than voltage feedback. A typical Buck regulator maintains voltage. It is achieved by comparing a fraction of the output voltage (defined by resistor divider) with internal reference voltage. This voltage feedback makes the circuit maintain the output voltage (despite load changes). In your case your inductor will be the Buck inductor. The only "load" will be a small current sensing resistor in series with your inductor in the GND path. The voltage across this resistor will be proportional to current (Ohms law) and could be used as curent feedback. Connect it to the ADJ of FB pin of the Buck (instead of the voltage divider). Select a Buck with low Vref. The current will be defined by Vref/Rcs (where Rcs is the current sense resistor). This is reasonable for relatively small currents as most Buck have reference voltages at least 0.5V. If you multiply 4A to 0.5V you get 2W losses in the current sense resistor. It is better to use current sense amplifier like INA 193 (gain of 20). Now your current sense resistor could be 20 times lower value and could be not only in the return wire but even in the high side as INA 193 suppresses common mode voltage. Current mode is typically used to drive LEDs, solenoids, solenoid based proportionally controlled valves etc. The approach outlined so far is suitable for constant current. If you need variable current, one of the easiest ways is to use INA 139 as current sense amplifier. It is Trans-conductance amp (producing current proportional to input voltage). A variable resistor to GND in its output will make the current adjustable. Finally you may need the current controlled not manually by automatically (say by micro-controller generated command). It this case you will need an external error amplifier to compare the command with the current feedback and drive the ADJ or FB pin of the Buck. send me an email to plamen_petkov5@hotmail.com if you need further assistance

 

Thanks for the help chaps!

Fitting the Schottky diode across the coil made a massive difference. FET is now so cool I can hold it. Tested the circuitry on the scope and the difference is immense. After each pulse of the PWM the voltage was spiking up to 50vdc, thus causing the heating and eventually failure.

Before diode:
View attachment 104631

With diode:
View attachment 104630

Problem solved.
Thanks again

Petkan:
The schottky diode from the switching node to GND (if your switch is high side) should have been provided from day one. Otherwise the inductive kick of the inductor could kill the MOSFET. The voltage produced by an inductor kick is proportional to the rate of change the current. As you abruptly break the current (when switching the MOSFET off) the current has no way to go. The diode (often called fly-wheeling or free wheeling diode) provides a current path for the inductor during off state of the switch. A must have not nice to have.