How to best choose a method of driving a MOSFET

Question

I want to be able to control 12V as well as 24V LED strips that will be used in a video recording environment (so high switching frequency). In a few bullet points I will explain the planned circuit:

• Power supply will be either 12V or 24V
• LED strip will be controlled with a MOSFET ('logic-level' IRLR7843 for example)
• MCU controls the MOSFET with a 3.3V PWM signal
• To ensure that 3.3v is available to the MCU regardless of the input voltage LM2596 is used

So the question is how to drive the MOSFET? My first thought was to drive the MOSFET directly form the MCU but the problem with that is that the switching frequency would be limited due to the gate capacitance. So I have to use a MOSFET driver. Since I have only 24V/12V and 3.3V available on the board I would have to do one of the following:

1. Use a driver that has a minimum supply voltage of at least 3.3V to drive the MOSFET (not easy to find, on ebay or aliexpress at least)
2. Use a secondary Step-Down Voltage Regulator that steps down 24V/12V to a more usable voltage (like 10V or something) that a more readily available drivers support.
3. Use a different MOSFET that allows for a larger Gate-to-Source Voltage that I can drive with 24V/12V. And find a driver that supports 24V supply voltage.

Any other option?

Answer 1

There are two types of FETs as far as switching is concerned; logic level and standard level. Logic level devices claim to switch on fully at logic level voltages. In reality however, they tend to still have a high Rds ON even with 5 Vgs. Standard level FETs only claim to be ON with a higher voltage, but they tend to have a lot lower Rds ON when properly switched. The Rds ON only really matters when dealing with higher current loads, so will depend on how much your LEDs are drawing.

The second consideration, which you appear to be concentrating on, is the switching speed. If you are using an MCU, you can drive a logic level FET at around 1kHz without any real issue. MCUs can often provide 10-20mA or more out of a GPIO pin, so can switch your cheaper FETs close to their operating limits. you tend to use FET dirvers when using higher power FETs (FETs designed for 100Amps for instance).

I would say test it with the MCU driving logic level FETs at around 1kHz, as you haven't given what the "high frequency" is, I would suggest that this is fast enough for most video events.

Answer 2

Go about it the other way around - considering how FETs work you want the gate voltage as high as possible. There are drivers which allow high supply voltages while accepting logic level inputs - that's what you want, regardless of FETs used.

Also, as a tip on frequency - at the very least you need it to be the least common multiple of the FPS of all the cameras on set, or an integral multiple of that, to ensure constant light between frames. I once read a white paper that states that for humans the perfect PWM dimming is at 3 kHz or above.

As MOSFET switching has large peak currents and neglible constant draw if you can't find a 24V driver (seems unlikely) there are for sure 18V ones. Get one, get a large LDO, set it to, say, 17V and add a lot of good low ESR caps close to the drivers' supply