# Selecting a MOSFET for an Application

Discussion in 'The Projects Forum' started by oKCfGYhmJQi, May 13, 2019.

1. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
As a few of the regulars have noticed, I've been posting questions about MOSFETs. And I'd like to thank you for all the help: Thanks!

Now it's time to put to use what I've learned and, along with asking some extra questions, purchase some components.

I'm going to be using the MOSFET to switch a device that will draw a constant 5 amps from 14 volts, dissipating 65 watts. This circuit will potentially stay active for hours at a time. The actuation speed is slow, responding to a hand-actuated switch whose signal will go through some basic logic before hitting the Gate terminal on the transistor. The MOSFET won't have any heat sink. The Gate will be fed 14 volts, as will the drain.

My question is whether this part is suitable for this application.

Here are the "maximums:"
Vgs(max) = 20v <<corrected. thank you
Rds(on) = 1.3 mOhms @ Tj=25°c; 1.8 mOhms @ Tj=100°c
Vds(max) = 30v <<corrected. thank you
Ptot = 338w
Id(max) and Is(max) = 120a

Here's what I anticipate:
Vgs = 14 v
Rds = 1.8 mOhms (using the higher figure for tolerance)
Vds = 9 mVolts <<corrected. thank you
P = 45 mWatts <<corrected. thank you
Id and Is = 5 amps (how are these two ever different?)

But I'd like someone to confirm that, given the description above, it seems like it would work. In particular, I'm concerned with the 100% duty cycle, as I don't know how to account for that.

The part is the Nexperia PSMN1R1-30PL
Product page on Mouser
Datasheet

Opinions?

Last edited: May 14, 2019
2. ### OBW0549 Distinguished Member

Mar 2, 2015
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I don't know how you got Vds = 6.5 millivolts and P = 23.5 mW; if Rds = 1.8 mΩ and Id = 5 amps, Vds would therefore be 1.8 mΩ * 5A = 9 mV, and P would be 5A * 5A * 0.0018Ω, or 45 mW.

But in any case, the heating due to the load current flowing through Rds will be so small as to be nearly unmeasurable, much less large enough to be of any concern.

3. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
You are correct, sir. Wow I'm so sloppy. I copied those figures from some previous calculations, which were in no way based on the same voltage or Rds!

I've redone the calcs and updated them. Thank you for pointing it out!

4. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
Great. I just wanted to make sure that with the datasheet in mind, the 100% duty cycle isn't going to throw everything off.

5. ### OBW0549 Distinguished Member

Mar 2, 2015
2,820
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To put your situation in perspective, Wikipedia gives the thermal resistance from case to ambient of a TO-220 package without heatsink as 70 °C/Watt (typical). When dissipating 0.045 Watts with 100% duty cycle, the temperature of the your MOSFET's case will therefore rise roughly 0.045 Watt * 70 °C/Watt ≈ 3 °C.

Not exactly "sizzling hot."

6. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
Awesome, that's great info. I saw several articles describing how this calculation was performed, but I didn't see an "Rtheta(ja)" spec (junction to ambient air) in the datasheet…

7. ### OBW0549 Distinguished Member

Mar 2, 2015
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That particular datasheet doesn't list it. Some do, some don't; but the rough approximation of 70 °C/Watt is applicable to just about any semiconductor device housed in a TO-220 package.

8. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
I was wrong about this… the datasheet does have a graph further in that shows it as 60 K/W. Thank you for the guidance!

9. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
So let me see if I understand this… If we're planning for operating temps that may include the desert, we'd say that ambient-max would be 120°F, which is about 50°C. With the degree on both the kelvin and celsius scales representing the same unit of energy, we can say that with a specified Tj(max) of 175°C and an Rtheta(ja) of 60°K/W, we should be able to dissipate in open air:

P = (175 - 50) / (60) = 2.08 watts

And since I'm dissipating like 1/463 of that, I'm well within limits

Mar 2, 2015
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Correct.

11. ### shortbus AAC Fanatic!

Sep 30, 2009
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Hi, didn't you say this was going in a bike, in one of the other threads? If so you also need to account for "load dumps" that spike in and automotive or in this case bike total circuit. They do or did make certain mosfets just for vehicle applications due to this.

12. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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You don't say what package your FET is in, but that number is consistent with an old rule of thumb from way back - a TO-220 package can handle 2 W max. in free air.

ak

13. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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I would choose a FET witn a minimum Vds of 30 V, not 20 V. A good rule for long term reliability is to overrate semiconductors by 100%. 14 V circuit = 30 V part; 5 A output current = 10 A pass transistor; etc.

In round numbers, as Vds increases so does Rds, but you've got plenty of margin to spare.

ak

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14. ### OBW0549 Distinguished Member

Mar 2, 2015
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According to the datasheet it's maximum Vds is 30V, not 20V. Ids(max) is 120 Amps.

15. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
Yes it is. Would the 30V Vds(max) be enough to cover this?

Wow, I am so sloppy sometimes… I had Vds(max) and Vgs(max) reversed. It is 30v and 20v, respectively. Corrected above.

16. ### shortbus AAC Fanatic!

Sep 30, 2009
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Don't know. Is there a reason not to use a SSR (Solid State Relay)?

17. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
Are they better guarded against spikes and whatnot? What type of relay would you have in mind?

18. ### shortbus AAC Fanatic!

Sep 30, 2009
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Better yet tell us what your trying/wanting to do. Most cars and motorcycles use Bosch style relays for control of loads. They are small, rugged and proven. And available in many different configurations.

19. ### oKCfGYhmJQi Thread Starter Member

May 3, 2019
60
2
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