Need some help for understanding electronic loads

Greetings,
I was trying to understand electronic loads. Then I decided to build my own very simple circuit. First of all I connected MOSFET’s source to ground; drain to power supply 1 (aka my load) and gate to another power supply (aka power supply to in schematic) but I keep destroying my MOSFET’s one by one. Every recommendations are welcome.
(Note: Grounds are connected one to another and I used heatsink for the MOSFET.)

Thanks.
Ekran Alıntısı

There is no “load” in your circuit. The MOSFET essentially connects the power supply to ground, which will usually destroy either the MOSFET, the power supply, or both.

Typical MOSFET control circuit for a DC motor:

You got me wrong.I was trying to build an electronic load or programmable load. Here is an example of an electronic load
download

I missed that the naked MOSFET is supposed to be the “load”.

At the very least you will need a current sense resistor, a feedback loop and power dissipation control circuit, plus a big heat sink on the MOSFET.

Hi,
Depending on the mode of loading most loads will have a resistive load inside in series with the control MOSFET.

Even though its an “electronic” load, if it is loading up to 100W, that energy has to go somewhere.

The gate of the MOSFET is PWM controlled for switching efficiency.
OR
If its gate is controlled by a variable voltage, then most of the ENERGY will be dissipated in the MOSFET and it will be mounted on a heatsink to keep it cool.

What voltage were you using on the gate of the MOSFET and did you have a 10K resistor between Gate and Source to make sure the MOSFET is not left with a charge on it if the control voltage is switched OFF.
This may help.

Tom… :grinning: :+1: :coffee: :australia:

Thanks for the answer ,

“If its gate is controlled by a variable voltage, then most of the ENERGY will be dissipated in the MOSFET and it will be mounted on a heatsink to keep it cool.”
I was using a big heatsink (really big).

“What voltage were you using on the gate of the MOSFET”
5 to 12 volts

“did you have a 10K resistor between Gate and Source to make sure the MOSFET is not left with a charge on it if the control voltage is switched OFF.”
No i did’t have a resistor.

I will try your recomendations.

Hi,

That sort of voltage would have instantly turned the MOSFET almost full ON.
In fact that is a logic level MOSFET so it would be ON and at its lowest ON resistance with 5V.

Tom… :grinning: :+1: :coffee: :australia:

Take a look at this IRLZ44 transfer curve, which shows the drain current as a function of the gate to source voltage for a typical device. At Vgs=2.5V and Vds=25V, the device will conduct around 7 Amperes (over 50 A at Vgs = 5V). Note also the strong temperature dependence of the transfer characteristics.

This is why, for a constant current electronic load, you need a current sense resistor, voltage reference and error amplifier, as shown in the link Tom posted.

Capture

Thanks I tried again and did’t destroy my MOSFET.

“Take a look at this IRLZ44 transfer curve, which shows the drain current as a function of the gate to source voltage for a typical device. At Vgs=2.5V and Vds=25V, the device will conduct around 7 Amperes (over 50 A at Vgs = 5V). Note also the strong temperature dependence of the transfer characteristics.”

Can u explain a bit more please?

I didnt undestand. Why should I use a current sense ,voltage reference and error amplifier. I just want to see how does a MOSFET work as a current source. Of course i will use current sense resistor, voltage reference and error amplifier in my finalized schematic but for now I just want to keep things simple.

Thanks.

You need to become familiar with reading device data sheets, and the fact that semiconductor devices have very large tolerances in their published characteristics.

For the example I posted above, locate Vgs = 2.5V on the horizontal axis of the graph.

Read up along the vertical axis to see the drain current curve. At 25 degrees Celsius, the drain current is typically 7 Amperes. At 175 degrees Celsius, it is maybe 12 Amperes.

What that tells you is that you cannot safely control the drain current by simply varying the gate voltage, without additional sensors and information, like a current sense resistor, a voltage reference or control voltage and an error amplifier in a feedback loop. Like this:

I got it. Thanks for your help.

Thanks again. I really appericiate it.

Would probably be easier to use multiple FETs and use them to switch resistor loads in combination.

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