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I'm currently in the process of selecting a MOSFET transistor for an 8x8x8 LED cube I'm building. Each layer of 64 RGB LEDs will have a transistor connected to a common cathode for the layer, when I want to turn on the layer I'll turn on the MOSFET via a pin on my microcontroller running at 3.3V. This is my first time selecting a transistor for this type of application and I just want to confirm I've selected the proper part. I've used this fantastic post as a guide to help me.

I've selected the AO3400A for my application. As I'm running at 3.3V I wanted a Vgs(th) max that would be below my 3.3V level. I do have a question about the minimum value for Vgs(th) which is 650mV, I'm planning on using the Nordic NRF52832 as my microcontroller but I can't seem to find the minimum and maximum values for the logic levels in the document. Based on general reading 650mV appears to be high enough so that the gate of the transistor won't turn on when my microcontroller pin is pulled low, but I can't confirm this.

Secondly, I need to sink a lot of current through the transistor, I've assumed each LED will use a maximum of 30mA, given that I'm using RGB LEDs I have 3 x 64 LEDS in each layer. 3 x 64 x 30mA gives me 3.84 Amps of current when all the LEDS are on at once. The maximum continuous drain current is 5.7A for the transistor at 25C and 4.7A at 70C so I think I'm ok there.

I'm powering the cube with a 5V 12A supply, so the Max Vds would be 5V in my circuit and the transistor is rated for a max of 30V so again I think I'm alright.

Finally I calculated the power dissipation. P = I^2*R. I used the Rds(on) at 2.5V and 3A as a worst case scenario and that specifies a max resistance of 48mOhm. so P = 3.84A^2*48mOhm which gives me .7 Watts. The transistor is rated 1.4W at 25C and 0.9W at 70C so I figure I'm ok here as well.

Have I missed anything? Do I have enough of a safety buffer for all of my design parameters or should I select a different transistor? Thanks for taking a look!

EDIT I made a mistake in the specification of the power supply, copied some details down incorrectly when writing this. It's a 5V 12A so 60W. EDIT 2 I mistakenly put down common anode LED's instead of common cathode.

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  • \$\begingroup\$ Yes you have missed something. What is the switching frequency of the LEDs? I.e. How many times per second will EACH of the MOSFETs (from your description it sounds like there will be 8 of them) switch on-to-off and off-to-on? Also, how much copper area are you allocating on your PCB for each MOSFET SOT-23 package? Also, what is the ambient operating temperature of your cube. And, what ambient air temperature will the MOSFETs experience? Are you incorporating any kind of PWM dimming? \$\endgroup\$ – FiddyOhm Aug 15 '16 at 19:00
  • \$\begingroup\$ Um. Powering the entire cube with a 5V 12W power supply?? About 2A max? Just asking. \$\endgroup\$ – jonk Aug 15 '16 at 19:01
  • \$\begingroup\$ Also, your chip has 32 I/O's, so unless you are using external latch chips, you cannot be drawing 64x3 LEDs on at the same time. You could use latches, or something the like, but I don't have the impression you thought about that one yet. \$\endgroup\$ – Asmyldof Aug 15 '16 at 19:04
  • \$\begingroup\$ Asmyldof- I'm planning on using LED Driver chips to control the individual RGB cathodes. \$\endgroup\$ – Fuzzy_Bunnys Aug 15 '16 at 20:03
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    \$\begingroup\$ Are you using a mosfet driver? The rise and fall times are dependent on the current driving the gate due to the gate capacitance. If you are driving it directly from the micro controller, you might have problems. Also, the turn on time is going to affect your heat equation: if the fet takes a while to turn on it will be in a higher resistance state for longer, and will thus heat up more. \$\endgroup\$ – Andrew Spott Aug 15 '16 at 22:31
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Thank you for all your help, I've confirmed that the transistor I selected should work for the circuit.

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A small comment on maximum current that you have calculated. If you want to sink such a large amount of current through a single transistor (3.84 Amps in your case) you should look into the input and output characteristics of the transistor and figure out what amount of Vgs and Vds you have to maintain to allow 3.84 Amps of current. The maximum continuous drain current of 5.7A is the maximum value the transistor can allow which is again provided taking into account the Vgs and Vds. If you do not follow the input and output characteristics of the transistor then you may end up either in complete non-functioning of the device or burning up the LEDs.

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