MOSFET Rds ON ... does it change compared to the test conditions?

[doublec4]

Hi all,

If I am selecting a MOSFET (in this example, a P channel) and I look at the datasheet (IRF9Z24NPbF) I see a Rds ON value of 0.175 ohm. The test conditions show Vgs = -10V and Id = -7.2A

If my load that I am switching is only 0.3A @ 12V and my MOSFET gate is at 0V, source at 12V, therefore giving me Vgs = -12V, I can assume the MOSFET is fully "on". But what is the Rds value? Is it the 0.175 ohm even though my load is not the 7.2A from the test conditions? Does the Rds ON change with the load, or is it purely a function of the Vgs?

Thank you

[Benta]

MOSFET on-resistance is basically only controlled by the gate-source voltage. High drain-source currents can heat the die and shift the parameters a bit, but within the tolerances in the data sheet.

[doublec4]

I see, so if I were to use this component with said LED @0.3A I could use P = (I^2)*R to calculate the power dissipated by this MOSFET when ON.

P = (0.3^2)*0.175
P = 0.016W

Is that correct?

[Benta]

For DC: Yup.

[Peabody]

The datasheet for your mosfet doesn't have some of the graphs you often see in others.  Take a look at Figure 2 in the ON Semi datasheet for the NDP6020P.  RDSon does vary somewhat with GS voltage and with current.  And temperature of course.  But the .175-ohm figure is a maximum, so it will probably cover any situation.

[ledtester]

I see, so if I were to use this component with said LED @0.3A I could use P = (I^2)*R to calculate the power dissipated by this MOSFET when ON.

P = (0.3^2)*0.175
P = 0.016W

Is that correct?

Perhaps a more practical way of measuring power dissipation is using P =  V_DS * I, where V_DS is the voltage difference between the drain and source.

[Siwastaja]

Wrong advice.

0.175 Ohm "maximum" is not maximum over the environmental parameters, it's maximum due to the variation between sold units, and rated at junction temperature of +25 degC. Or as the datasheet says, "Electrical Characteristics @ Tj = 25 degC (unless otherwise specified)".

Rds_on is significantly dependent on die temperature! Fig.4 shows that at Tj = 100 degC, Rds_on is increased by 40% from the given +25degC value. This brings the "maximum" to 0.245 Ohms @ Tj = 100 degC. This is the value you should be using in your calculations. Once you actually calculate the die temperature, you can reiterate the calculation.

Note that Tj will be ambient temperature PLUS power dissipated times total Rth_Junction-to-Ambient.

Total Rth_Junction-to-Ambient will be Rth_J-C + Rth_thermal_interface_material + Rth_heatsink. Rth_J-C is given in the datasheet.

It's good to note, unless you severely overspecify your parts, that it's quite normal to run power MOSFETs roughly around Tj = 80 .. 110 degC.

This is why, for first order approximation, you should be routinely multiplying the datasheet front page Rds_on(@Tj=25) by 1.4 or 1.5.

And obviously, for design work, use the worst case numbers (maximum) instead of "typical". But that only covers the possible unit variation.

Using a bit higher gate voltage than what the Rds_on was characterised at, helps, but not by a lot, if at all; they have already done the characterisation at a high enough gate drive to get good numbers.

With such miniature load, Tj says pretty small and the FET easily takes the dissipation regardless of any small calculation error, but this is massively oversized part for the application. 0.016W dissipation heats up the junction by just 0.016W * 62 degC/W = 1 degC even without any heatsinking at all. Still, getting the process and calculation right helps when your next project involves more power.

[Zero999]

Why not use a BJT? The BC328 is a fraction of the cost of IRF9Z24 and much easier to solder, because it has thinner legs.

[Siwastaja]

Or, obviously, a smaller&cheaper MOSFET.

But if OP has this part lying around anyway, nothing wrong using such overspecified part. If nothing else, such massive part is less sensitive to ESD damage...

[Vovk_Z]

But what is the Rds value? Is it the 0.175 ohm even though my load is not the 7.2A from the test conditions? Does the Rds ON change with the load, or is it purely a function of the Vgs?

- It isn't hard to measure a source-drain voltage drop with a voltmeter. Of cause, a GS voltage must be constant but not changing (not pulsing).

[doublec4]

Yes, the reason for this post is that I had some of these laying around and wanted to breadboard them. Checked the datasheet and had some questions about how to interpret the datasheet for this application and future applications

Thanks guys!