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Topic: Mosfet VDSS requirement for 24V LED string (Read 1 time) previous topic - next topic

davivid

Hi,

I'm planning on using a PCA9685 to PWM N-Channel Mosfets to switch 24V 2A strings.

When picking a Mosfet would a VDSS of 30V be sufficient to insure reliable operation?

I have been looking at:

VISHAY SI2366

Transistor Polarity N Channel
Continuous Drain Current Id 5.8A
Drain Source Voltage Vds 30V
On Resistance Rds(on) 0.03ohm
Rds(on) Test Voltage Vgs 10V
Threshold Voltage Vgs 1.2V
Power Dissipation Pd 2.1W
Transistor Case Style SOT-23
No. of Pins 3
Operating Temperature Max 150°C


I have calculated the temperature rise for this would be: 14.4°C
I x I x R = W
2 x 2 x 0.036 = 0.144W
Junction-to-Ambient thermal resistance = 100°C/W
0.144 * 100 = 14.4°C

 



Paul__B

Yes.

LEDs are non-inductive, so cannot generate excess voltages in themselves.  As long as you can trust your 24 V power supply to be just that and no more, it will be fine.

davivid

Great thanks.

How would you know whether to trust your power supply?
I am using higher end(?) PSUs: Mean Well HLG-320H-24A

Would I want to add any additional protection?

CrossRoads

Calculations are off:
Rds = 0.042 at VGS = 4.5 V

2x2x.042 = 168mW
Maybe not hot enough to need heatsinking.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Paul__B

How would you know whether to trust your power supply?  I am using higher end(?) PSUs
Presumably just fine.  Merely pointing out that you actually want to be using a regulated power supply.

Would I want to add any additional protection?
Proper switchmode supplies include a "crowbar" circuit to short out and shut down the supply with a thyristor if there is an internal failure and the output voltage rises above a reasonable level.  Not all however are properly engineered.

davivid

Calculations are off:
Rds = 0.042 at VGS = 4.5 V
Thanks for pointing that out... I have been making that mistake in all of my calcs.

Proper switchmode supplies include a "crowbar" circuit to short out and shut down the supply with a thyristor if there is an internal failure and the output voltage rises above a reasonable level.  Not all however are properly engineered.
Great - lets hope mine is.

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