One of four MOSFET in parallel heating too much

Question

I am doing an electronic load project in which I need to work with four MOSFETs in parallel working in the ohmic region (not switching).

I have a good heat sink. Furthermore, only one of the four MOSFETs is heating to the point that is impossible to touch it, while the other MOSFETs are cold.

The schematic is the following:

While the load is drawing 200 mA from the DUT, this MOSFET is barely warm. But when I increase the current, I can hear a little snap from the MOSFET and it starts overheating.

Before it was diagnosed as a MOSFET defect, I have to say that I'd already substituted the MOSFET for an equal one. But the problem is the same, it persists, and (again) it's in a single MOSFET.

The power MOSFETs that I'm using are the IRF3205. The power resistors are 20W 0.5 Ohms. The voltage range I want to test with this load goes from 5v to 12v and the max current it needs to support is 20 A at any voltage.

Some hint?

Answer 1

You need to use an opamp for each MOSFET to ensure balancing. Each MOSFET should have an independent sense resistor feeding its own opamp for feedback.

The opamps would all be driven from the filtered PWM control voltage in parallel.

There is too much difference between the threshold voltages of individual devices to use your current approach. When driving them with a common gate voltage one device may be hardly conducting while at the same time another may be conducting heavily due to the device differences.

Here is an example with two control MOSFETs where each one has its own current sense resistor and feedback amplifier. This can be extended to more devices for higher currents by adding MOSFETs and opamps.

Note: R3 - R6 and C1 - C2 are typical values of compensation components that may be needed to avoid instability due to the large input capacitance of the MOSFETs. They may require adjustment for the specific design and components.

Be aware of the difficulties of running power MOSFETs in linear mode as described by Andy Aka in his answer. I recommend you read his answer and the linked articles.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

I am doing an electronic load project in which I need to work with four MOSFETs in parallel working in the ohmic region

No, they are operating in the saturation region and, there's every possibility that in this region, you will have thermal runaway; one MOSFET hogs all the load current and the remaining MOSFETs are barely conducting any current at all. This is a commonplace problem and is sometimes referred to as the Spirito effect.

• Link to a Q and A about the Spirito effect.

Ideally, each MOSFET should be controlled by its own op-amp AND, all the MOSFETs need to be specifically chosen to operate in linear-current applications (saturation). Most "switching" MOSFETs will usually fail.

Answer 3

Since there are already a lot of feedback regarding the problem and the ideal solution (one control loop for each MOSFET), I'd like to mention only that, considering the components you already have, there is a solution that provides an intermediate behavior:

The Vg nodes are all connected (to avoid all the crossing wires in the diagram) and the circled resistors are connected to the source pins to provide some negative feedback to minimize the mismatch between the MOSFETs and possible thermal runway.

Answer 4

Is the problem down do bad drafting of the circiout diagram? It's not clear which crossing wires are meant to be connected and which aren't.

As others have mentioned, if you connect all the sources together, then whichever MOSFET has the lowest resistance will deliver most current. It will get hotter, which lowers its resistance more, and thermal runaway occurs.

If you disconnect all the sources from each other, then each MOSFET powers one pair of resistors, and the current gets shared fairly between them.

Answer 5

LM358 running from 5V is in no way, shape nor form usable as a gate driver for IRF3205. These mosfets have gate threshold voltage anywhere between 2V and 4V. An LM358 won't drive 4V, especially not with a pull-down.

As far as we know, you're probably not even turning some of those mosfets on. The one you got has the lowest gate threshold voltage.

You could try injecting permanent charge into the gate to equalize the threshold voltages. This requires a custom piece of gear, but not too complex. Any such manipulations require extensive device characterization to ensure they are reliable long-term. It's much simpler to just control the current on each mosfet individually, or even to use an analog multiplier to keep the dissipation on each mosfet equal. A fast PWM driven by one variable (e.g. V_DS) can be used to chop another variable (e.g. I_DS) to obtain multiplication without expensive precision multipliers.