Problems with Transistor heat
hello,
I have an issue with driver transistor ckts. I am trying to build a simple ckt where the NMOS will act as a gate. I have a 12 Volt source and a 3 ohm load. The source and is connected to the load, the load to the drain, the source of the transistor is connected to ground. Everytime i build this ckt the transistor that I am using heats up and dies. I also need 4 amps of current to drive the load.
I have an issue with driver transistor ckts. I am trying to build a simple ckt where the NMOS will act as a gate. I have a 12 Volt source and a 3 ohm load. The source and is connected to the load, the load to the drain, the source of the transistor is connected to ground. Everytime i build this ckt the transistor that I am using heats up and dies. I also need 4 amps of current to drive the load.
![1ClBq.png](http://i.imgur.com/1ClBq.png)
Comments
- Actual part # of the transistor/FET/whatever you are burning up
- What is the 12V power source? Battery, wall wart, etc
- What is the 3ohm load? Is it just a large resistor? Heating element? Motor? or some device that uses 12V and draws 4 amps?
- What is controlling the switch and how is it wired?
The more details you provide, the more likely you'll get useful answers and some help.
Robert
enercell lead acid battery
3ohm is a SMA wire
haven't hooked it up to the board yet but i am planning on wiring to a Javelin stamp with a BOE, 500k resistor parallel to the gate and ground
is it overheating because the gate is floating?
I calculated wattage given off by the transistor on is only around 1.3 Watts, it is rated for 60V and around 40W so I don't see why it should be heating up without any voltage on the gate pin
What is an SMA wire?
I build most of this circuit but stop because the transistor used gets really hot.
So the question is, are you really driving the gate at 5 volts?
Now, I'm not sure about SMA, but most similar conductors have a temperature sensitive resistance where the resistance when cold is much lower than the resistance when hot. Are you measuring the resistance or is the 3 Ohm value the specification? If the cold resistance is much lower, the current through the MOSFET will be much higher and the heat generated will be a lot higher. I suggest you check out the cold resistance and the initial on current. You may need a heatsink.
Also, if you're trying to PWM the gate, your upstream circuit may not have enough instantaneous drive capacity to overcome the gate capacitance on each pulse and drive the transistor to full saturation. If that's the case, the gate will be seeing a lower effective voltage and the drain-source resistance will be even higher. This can be remedied with a MOSFET driver IC or with a lower PWM frequency.
Another thing to look at is your wiring. If there are IR losses between logic ground and the transistor's source, it will see a lower effective gate voltage. It's always best to connect logic ground and the load's return supply at one point and one point only: right at the transistor's source.
-Phil
The maximum power dissipation rating of 24W is at room temperature, i.e. with a heatsink. Above a die temp of 25C, you have to derate that 24W figure. Also, the on-resistance increases rapidly as the temperature increases, further exacerbating the dissipation issue.
-Phil
-Phil
-Phil
-Phil