# p-channel mosfet heat dissipation

#### crookedspoon

Joined Dec 30, 2020
14
Hi all,

I'm testing a few P-Channel Mosfets (IRF4905) I bought.
It's just the most basic setup - see sketch; battery is 15V, Incandescent light is pulling about 1.5A.
When the gate is connected to (-), the light goes on and the Mosfet reaches +60deg C. in about a dozen or so second.

Rated RDS(on) for this mosfet os 0.02Ohm.

If I understood correctly, the heat coming from that Mosfet should be 1.5 * 1.5 * 0.02 = 0.045Watts.
That wattage shouldn't be enough for the mosfet to reach this temperature so fast.

Am I doing / seeing something wrong? Or did Aliexpress sent me different speced Mosfets?

thanks for you insights!
shiny

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#### ericgibbs

Joined Jan 29, 2010
15,378
hi crooked,
This is what LTspice shows, close to your maths,
Are you sure you have the MOS FET connected correctly.
E

#### ronsimpson

Joined Oct 7, 2019
2,017
Am I doing / seeing something wrong?
Data sheet; 62C/watt. No heat sink.
So 60-25=35C delta. It should take about 0.5W to get to 60C.

#### cmartinez

Joined Jan 17, 2007
7,633
You have drawn your circuit in an unconventional way. But other than that, it seems to be ok.

#### crookedspoon

Joined Dec 30, 2020
14
I bought a batch of 10 of these IRF4905s. Tested them all, all behaved the same.
Can I assume that these Mosfets are fakes?

#### crookedspoon

Joined Dec 30, 2020
14
This is what LTspice shows, close to your maths,
Are you sure you have the MOS FET connected correctly.
I'm pretty sure it's connected correctly. Mosfet with printed side facing me; left is gate, middle is drain, right is source. The circuit works, I think if the Mosfet was connected wrongly, the circuit wouldn't work. Just the heat is extreme!

#### dl324

Joined Mar 30, 2015
14,350
Or did Aliexpress sent me different speced Mosfets?
That's possible. What is the current or bulb type in your circuit? What is the drain voltage?

BTW, you've drawn the MOSFET upside down.

#### crookedspoon

Joined Dec 30, 2020
14
That's possible. What is the current or bulb type in your circuit? What is the drain voltage?

BTW, you've drawn the MOSFET upside down.
It's a 21W, 1.5A incandescent bulb (an automotive blinker bulb)
Voltage drop between source and drain is 0.5V

Sorry about the drawing - oblivious of the standards... I'm learning

#### ericgibbs

Joined Jan 29, 2010
15,378
hi cooker,
You appear to have the Lamp in the Source line, your drawing a little confusing.
The Lamp should be in the Drain, which not allow the MOS to fully turn On if in the Source...
E

#### dl324

Joined Mar 30, 2015
14,350
It's a 21W, 1.5A incandescent bulb (an automotive blinker bulb)
Voltage drop between source and drain is 0.5V
$$R = \frac{V}{I} = \frac{0.5V}{1.5A} = 0.33\Omega$$
Sorry about the drawing - oblivious of the standards... I'm learning
That's why we point it out; so you can learn if you're so inclined.

#### Audioguru again

Joined Oct 21, 2019
4,539
Of course AliExpress sent you fake Mosfets. What manufacturer's logo are on them? Any spelling errors?
Oh, I hope your wiring is soldered and not on a breadboard.

#### crookedspoon

Joined Dec 30, 2020
14
Of course AliExpress sent you fake Mosfets. What manufacturer's logo are on them? Any spelling errors?
Oh, I hope your wiring is soldered and not on a breadboard.

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#### dl324

Joined Mar 30, 2015
14,350
The apparent texture of the package and leads makes me believe they're counterfeit. The leads usually get pitted when they sandblast the original markings. They're stamped metal, so shouldn't have any pitting. Does the texture of the back and sides of the package look like the front?

#### cormih

Joined Mar 26, 2017
2
I think that you didn't take accont on using the PMOS in dc current. All values in the data sheet are given for switching signals. If you want to use a MOS transistor in dc current you should use a linear one. Btw, if voltage drop is 0.5V and current is 1.5A, than the dissipated power is 0.75W.

#### jojoAI

Joined Oct 8, 2021
5
Your voltage drop of 0.5V is to high for your given current (with your given Rds-on that would mean you have a current around 25A). Try to change the connection of the Mosfet pins. Maybe you connected this wrongly after all. Also measure the Gate-Source Voltage, to be sure that it is high enough to turn the mosfet on. Your Gate Source Voltage should be greater than the threshold voltage.
You could also an different load and measure again Gate-Source and Drain-Source Voltage.

#### crutschow

Joined Mar 14, 2008
29,531
All values in the data sheet are given for switching signals. If you want to use a MOS transistor in dc current you should use a linear one.
Where did you get that odd idea?
There's no such thing as a "linear one".
All MOSFETs can (and are) used as DC switches.

#### LowQCab

Joined Nov 6, 2012
1,931
Where did you get that odd idea?
There's no such thing as a "linear one".
All MOSFETs can (and are) used as DC switches.
Some MOSFETs that are designed specifically for High-Speed-Switching
may not "like" continuous operation in the Linear-Region, and may need to be "de-rated" some-what.
They can usually be identified by a generally lower Heat-Dissipation-Rating,
what type of application they were "idealized" for.
.
.
.

#### GaryW2

Joined Apr 28, 2019
2
Hi all,

I'm testing a few P-Channel Mosfets (IRF4905) I bought.
It's just the most basic setup - see sketch; battery is 15V, Incandescent light is pulling about 1.5A.
When the gate is connected to (-), the light goes on and the Mosfet reaches +60deg C. in about a dozen or so second.

Rated RDS(on) for this mosfet os 0.02Ohm.

If I understood correctly, the heat coming from that Mosfet should be 1.5 * 1.5 * 0.02 = 0.045Watts.
That wattage shouldn't be enough for the mosfet to reach this temperature so fast.

Am I doing / seeing something wrong? Or did Aliexpress sent me different speced Mosfets?

thanks for you insights!
shiny
Hello Crookedspoon,

I downloaded the Spice macro for the IRF4905 and ran the circuit in TINA-TI [a free simulation prog’ from TEXAS ind] and ran the DC Analysis, what you find is correct, and should be around 0.04 of a Watt.
To prove this wire the circuit as shown, I would also suggest replacing the bulb with a 10 Ohm resistor, about 10 to 15 watts [ don’t forget an incandescent lamp has a Positive temperature coefficient ]. Now check the voltage across the resistor, this should be almost 15v and across the device [ ds ] less than 0.5v, more than this and the device is probably faulty. As a matter of interest,
the gate ‘turn-on’ voltage only needs to be -4v, you don’t need to tie it to the -Ve supply.
I’ve included the circuit with detail, hope it helps. A final thought, if you’ve got a lot of these devices, knock up the circuit shown so so you can just plug & test !
Regards Gary W.

#### Audioguru again

Joined Oct 21, 2019
4,539
The gate ‘turn-on’ voltage only needs to be -4V.
No.
The datasheet says that its threshold voltage is from -2V to -4V when it is almost turned off, it needs more Vgs to turn on.

Last edited:

#### sparky 1

Joined Nov 3, 2018
706
I saw Watts above so I know the question was answered. What a setup might look like.