I am working on a controller for heated seats I am adding to my van. I have a trivial circuit made using thermistors, resistors, potentiometers and comparators that I was using to drive a FET, and was switching an LED to indicate the status of the seats and it is doing exactly what I want: when the temperature is below the set point the FET is "on" and when it is above the fet is "off". However, when I connected the seat heaters themselves I found that the FET heated up very very quickly. Thinking it was an error in driving the FET and I was spending too long in switching (as I verified I was providing enough gate voltage) I reduced the circuit to the following:
The lower resistor is just a low value (500 ohm) pull-down, and the top right resistor is the 55W heating element, I just didn't have a good symbol in my library for it.
Still, with this simple setup the FET will heat up VERY quickly. Googling the problem led me to believe the problem is related to a high Rds(on) value. This FET (IRF840) has an Rds(on) of about 1.0ohms, so when the heater elements are trying to pass ~4.5A of current through it that means 4.5W of power, and with a junction to ambient thermal resistance of 62*C/W that means in open air the junction would (theoretically) reach 279*C above ambient, not really an ideal temperature.
I started searching around and came across the Alpha & Omega AOx516, which is very affordable and claims to have an Rds(on) of just 5 milli-ohms (10V gate voltage 20A current) with a similar thermal resistance, at 4.5A it would only be producing 0.0225W of heat and should only reach about 1.4*C above ambient in open air (although I will be heat sinking it to the enclosure of the controller). Unfortunately this shows as being a logic level FET, but it does also state that it allows up to 20V gate-source voltage and I can guarantee that I never exceed 16V gate-source in my application.
My concerns are as follows:
Have I made a huge mistake somewhere here with my math?
Will switching to this FET from the one I was using allow me to run the FET without any heat sink at all in an enclosed case?
Is there a better FET I could be using for this purpose?
Am I being really stupid in selecting a logic level FET?
Is the data sheet wrong? Can the FET really have an Rds(on) of just 0.005 ohms?
Thanks in advance for any help, I am kind of a newbie when it comes to analog electronics, most of my experience is with microcontrollers.
The lower resistor is just a low value (500 ohm) pull-down, and the top right resistor is the 55W heating element, I just didn't have a good symbol in my library for it.
Still, with this simple setup the FET will heat up VERY quickly. Googling the problem led me to believe the problem is related to a high Rds(on) value. This FET (IRF840) has an Rds(on) of about 1.0ohms, so when the heater elements are trying to pass ~4.5A of current through it that means 4.5W of power, and with a junction to ambient thermal resistance of 62*C/W that means in open air the junction would (theoretically) reach 279*C above ambient, not really an ideal temperature.
I started searching around and came across the Alpha & Omega AOx516, which is very affordable and claims to have an Rds(on) of just 5 milli-ohms (10V gate voltage 20A current) with a similar thermal resistance, at 4.5A it would only be producing 0.0225W of heat and should only reach about 1.4*C above ambient in open air (although I will be heat sinking it to the enclosure of the controller). Unfortunately this shows as being a logic level FET, but it does also state that it allows up to 20V gate-source voltage and I can guarantee that I never exceed 16V gate-source in my application.
My concerns are as follows:
Have I made a huge mistake somewhere here with my math?
Will switching to this FET from the one I was using allow me to run the FET without any heat sink at all in an enclosed case?
Is there a better FET I could be using for this purpose?
Am I being really stupid in selecting a logic level FET?
Is the data sheet wrong? Can the FET really have an Rds(on) of just 0.005 ohms?
Thanks in advance for any help, I am kind of a newbie when it comes to analog electronics, most of my experience is with microcontrollers.