Arduino Forum

Using Arduino => LEDs and Multiplexing => Topic started by: davivid on Mar 17, 2016, 12:15 pm

Title: Mosfet VDSS requirement for 24V LED string
Post by: davivid on Mar 17, 2016, 12:15 pm
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 (http://www.farnell.com/datasheets/1915825.pdf)

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

 


Title: Re: Mosfet VDSS requirement for 24V LED string
Post by: Paul__B on Mar 17, 2016, 12:37 pm
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.
Title: Re: Mosfet VDSS requirement for 24V LED string
Post by: davivid on Mar 17, 2016, 12:47 pm
Great thanks.

How would you know whether to trust your power supply?
I am using higher end(?) PSUs: Mean Well HLG-320H-24A (http://www.meanwell.com/mw_search/hlg-320h/HLG-320H-SPEC.PDF)

Would I want to add any additional protection?
Title: Re: Mosfet VDSS requirement for 24V LED string
Post by: CrossRoads on Mar 17, 2016, 07:18 pm
Calculations are off:
Rds = 0.042 at VGS = 4.5 V

2x2x.042 = 168mW
Maybe not hot enough to need heatsinking.
Title: Re: Mosfet VDSS requirement for 24V LED string
Post by: Paul__B on Mar 17, 2016, 10:18 pm
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.
Title: Re: Mosfet VDSS requirement for 24V LED string
Post by: davivid on Mar 17, 2016, 10:37 pm
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.