Using a Mosfet for 100KHz switching

Thread Starter

fmrPIC

Joined Nov 11, 2016
22
I want to use a MOSFET to strobe a bank of parallel wired LEDs each with series resistor at about 100KHz. My steady state current for the LEDs would be about 1 amp.
I want to use an N type MOSFET. My understanding is that it takes almost no base current to close the MOSFET. However, you are essentially charging a capacitor and at high frequencies or quick turn-on , you need a driver. There are MOSFETs with logic level gates, but are there MOSFETs that are designed to be driven directly at 3.3v with a built in driver? I saw something from onsemi but it is not designed to be soldered by hand. Also it has 30 pins and I am looking for about 3. I am not against using a separate driver and MOSFET if I can operate them both the same voltage 9-18 volts. I have used TINA and LTSpice and I would like to model the circuit. Yesterday, I spent hours with no success trying to find a driver I could import and model. I need help finding a MOSFET/DRIVER or combo family that cold be easily modeled and is not discontinued. Please note, I am not asking anyone to do my shopping. I am hoping that someone has experience with a family of components that meet my needs that they could share with me and possibly share certain parameters I should consider for the MOSFET design.

I tried this before using an npn transistor but my PCB light ring was for a c-mount camera and the camera was junk. I want to go with more LEDs and 1 or more MOSFET circuits if needed. My design plan is to get my 5 PCBs made from our friends at JLPCB. Purchase components and a photodiode. Assemble the circuit. Flash the LEDS about 5 times in a row at 10KHz. View the flash from the photodiode and my o-scope. Build a light shielding box about 6 x 6 x 6". Jet ink drops in the box using my DEO-Nano_SoC while triggering the LEDs. View the drops using multiple exposures with a Canon camera and an o-scope. Adjust timing and each LED series resistor as needed. Hopefully, I can measure speed and shape of the drops.
 

wayneh

Joined Sep 9, 2010
17,498
You're on the right path. Switching a 1A MOSFET at 100kHz is a good example of when you'd use a driver. Sort of. I mean it's perhaps a borderline case and you might get away without a driver IF your digital drive signal has enough oomph and your MOSFET has a low gate capacitance and is heat-sinked to accommodate the less-than-perfect switching. Since you have plenty of supply voltage, I wouldn't worry about choosing a logic level MOSFET.
 

BobTPH

Joined Jun 5, 2013
8,994
Where is the gate signal coming from? A bipolar 555, for example, can source and sink enough current to get by without an additional driver. A micro probably cannot.
 

DickCappels

Joined Aug 21, 2008
10,185
There might not be any MOSFETS that will pass an amp with only 3.3 volts. It would be good to cjoose a MOSFET and the design the gate drive circuit for that device.

100 kHz also makes the gate drive difficult. Consider whether you really need such a switching speed.
 
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Thread Starter

fmrPIC

Joined Nov 11, 2016
22
100KHz is as follows. The drops should move at about 8 m/s as per nozzle literature... call it 10m/s. If my math is correct 10KHz or 0.1ms would "trap" 1mm of distance. If I could get 0.01ms ON time(100KHz), I would trap 0.1mm of travel. It should be clear enough to see. I would strobe that at ~5KHz.
I am now looking at an Infineon 1ED7511 MOSFET driver that has a spice file. Does that look like a good choice? I want my 3.3V(15ma) signal to drive this driver and I would pick A MOSFET that can handle a few amps. I would power then both off a 12V rail. They have a picture of both the push and pull leads driving a MOSFET. That looks like something I can understand.
 

MisterBill2

Joined Jan 23, 2018
18,550
To capture images of motion with good resolution the flashes must be very fast rise time and fall time, and quite short. That is how they take pictures of armor piercing projectiles. But to get adequate exposures with that short a flash, a lot of power is required. The LED may be adequate for the slower droplet motion. The challenge being that the mosfet must switch fully on and fully off rapidly, which means that the gate capacitance must be charged and discharged very quickly. That usually requires a lot of current, and that iis not simple to do vary rapidly. So the driver circuit is very important.
 

MrAl

Joined Jun 17, 2014
11,489
Hello there,

I did this with a guitar string for use as a tuner. That's relatively low frequency though and can be as low as 220Hz or lower.
At 100kHz you have to have a GOOD driver and keep leads short, and also the power supply has to have good decoupling or else it will not be able to supply the fast rising gate current nor the fast rising LED current. Probably keep drive power and LED power separate.

100kHz power converters have been built even as long ago as the 1980's using MOSFETs, so this could be doable. You can look up design notes for those converters to get more hints.

There is also a chance you may need a snubber if you see any voltage spikes appearing across the MOSFETs when they turn off.
 
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