?High Power IR LEDs and MOSFET mod?

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wagnerpj
 
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?High Power IR LEDs and MOSFET mod?

Post by wagnerpj »

Do you think the TV B Gone v1.2 could be modified to use MOSFETs to drive high-power IR LEDs (with a separate power supply to the high-power LEDS)? Could the current-design bias transistors drive the MOSFETs?

Could someone suggest a circuit?

Thanks!

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adafruit_support_mike
 
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Re: ?High Power IR LEDs and MOSFET mod?

Post by adafruit_support_mike »

wagnerpj wrote: Tue Apr 25, 2023 1:36 pm Do you think the TV B Gone v1.2 could be modified to use MOSFETs to drive high-power IR LEDs
Sure.. just replace the whole Darlington BJT subcircuit with a power mosfet.

The same high=on/low=off signal that controls an NPN transistor will control an N-mosfet. The main differences are that a mosfet's gate draws no curent (a BJT's base current is about 1% of its collector current), and the mosfet's gate voltage can swing from 0V to VCC (a BJT's base-emitter junction is a diode, so it stays near 0.65V).

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wagnerpj
 
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Re: ?High Power IR LEDs and MOSFET mod?

Post by wagnerpj »

Do you think the PNP driver transistor ("Q5" in some of the TV b Gone circuit diagrams) would be up to the task of driving a high-power MOSFET, or should I replace it with something higher spec? I'm looking to drive a 4 x 4 3W IR LED matrix (some LEDs with focus lenses) similar like that found illuminating night-vision security cameras- they can draw a lot of current (briefly...). There will be a power issue with the 5V microcontroller supply limit vs what's needed for the LEDs. ?DC-DC step up circuit?

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Re: ?High Power IR LEDs and MOSFET mod?

Post by adafruit_support_mike »

You don't need anything more than a microcontroller pin to drive a power mosfet.

The amount of current a mosfet can switch has absolutely no relation to the amount of power it takes to turn it on and off. The only thing gate power controls is the speed at which the mosfet turns on or off.

I refer to mosfets as 'capacitors running sideways': the channel between the drain and source is a region of doped silicon that, by default, has no free electrons or empty orbitals. There's a thin layer of insulation on top of the channel, and the contact for the gate is on the other side of the insulation.

When you apply positive voltage to the gate of an n-mosfet, it pulls free electrons from the silicon below the channel up to the surface just below the insulating layer. Those electrons form a conductive path between the drain and the source.

The gate and channel are two plates of a capacitor, and the insulating layer is the dielectric between them. Channel current flows from one side of the channel to the other (one plate of the capacitor), which is roughly perpendicular to the way we use normal capacitors.

The amount of current the channel can carry depends on the number of electrons pulled up to the surface below the insulation. That number of electrons is controlled by the difference in voltage between the gate and the silicon below the channel.

Once a mosfet is turned on, it takes zero power to keep it turned on. A common party trick is to hook up a mosfet so it controls power to an LED, touch a wire to the gate to turn the mosfet (and the LED) on, then let the mosfet sit with its gate unconnected to anything and see how long it takes for the LED to go out. Under good conditions (low humidity, no airflow), the mosfet can remain turned on for months.

The only time you care about the current to a mosfet's gate is when you turn it on or off. Just like any other capacitor, it takes time to charge or discharge the gate-channel capacitor. That matters when you want to turn the mosfet on and off at high frequency (like in a computer's microprocessor core). If you have 4ns per clock cycle (a 250MHz processor), you need enough current to make the mosfet turn on or off in that time. And since you have to spend that much current 250 million times per second, it adds up.

For your application -- turning the LED arrays on and off at human speeds -- switching speed and gate current are mostly irrelevant. A power mosfet like the IRLB8721 can switch 60A:

https://www.adafruit.com/product/355

and its gate capacitance is about 1nF. A 1mA drive signal from a microcontroller pin can turn it on in about 1ms. The 1A signal from a high-current gate driver can turn it on in about 1us. The design question is, "do you care about the difference between 1ms and 1us enough to accept the additional complexity of a high-current gate driver?"

For some applications the answer to that question is "yes". For this one, the answer is "no".

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