Hello Guys,

I am trying to design MOSFET switching circuit with 1MHZ Drive frequency.
I am really new to this driving circuit design so i am still at the starting stage

could any one help me how to select MOSFET for High switching frequency?
means How to estimate that a particular MOSFET is capable to handle 1 MHZ of switching frequency?

 

Do you have any circuit?
How is the input signal (V/I) and how is the output load (V/I/R)?

 

Scott is asking for a schematic because the driver and the mosfet are both involved in achieving a MHz.
Theoretically, you can slam any mosfet that fast with the right driver, but in reality, it's much better to consider the mosfet and the driver to be married. Right now, we don't even know if you have enough voltage to slam a mosfet completely on.

 

Hello Guys,

I am trying to design MOSFET switching circuit with 1MHZ Drive frequency.
I am really new to this driving circuit design so i am still at the starting stage

could any one help me how to select MOSFET for High switching frequency?
means How to estimate that a particular MOSFET is capable to handle 1 MHZ of switching frequency?

Sometimes MOSFETs like the IRF540 etc turn up in ham radio "top-band" transmitters, which is a few MHz.

That could be the easiest source of reference designs.

 

Hello Guys,

Thanks for your response,

I am sorry for my late reply.

I would like to share what i have done so far.

below are the schematic file.

Short description: I am trying to switch high power infrared LED for 1MHZ.
So i have selected "UCC2742" MOSFET driver (Because to convert 3.3V to 12 V) to drive the MOSFET "IRF840LC" @ 1MHZ.
Of course i got the square wave of 1MHZ and the magnitude of 12V at the output of Driver IC.
But at the drain w.r.t GND I am not getting a square wave (I got the waveform like the below image output voltage).



So, I read some articles from internet and understood the following things,
And I think to drive the MOSFET at high frequency we need to charge the gate capacitance fast during ON time and discharge the same during OFF time. To achieve this we need to calculate gate current (Ig) So,

Ig = Qg * fsw [ Qg = gate charge ,fsw = switching frequency]

Qg - I selected from Vgs to Qg wave form and i considered approx 24nc
Vgs - i considered 10V because @10v Rdson value = 0.84 ohm
fsw - 1MHZ

So I got,
Vgs = 10V (To achieve this, implemented voltage divider at the output of Driver IC
Ig - Gate current value 24mA (For this i have put a series gate resistance between Voltage divider and MOSFET)

So i have implemented the changes like below w.r.t the above consideration.

But still i am not able to achieve the square wave at 1MHZ output @ drain.



Kindly please review the above steps, Please correct me if i am wrong in any of the above steps.

Thank you !!!

Best Regards,
Lokesh Sampath

 

That's how I worried about the frequency can't be reach too high, because the Cgs will be pulls the Vgs down.

The first thing is that you may reducing the Vgs to about 7V, and adding a pnp bjt to discharging the Igs to ground when the input of Vgs is going to low.

Using the circuit on the right top of the page 14.
Design And Application Guide For High Speed MOSFET Gate Drive Circuits.

 

Hello Guys,

Thanks for your response,

I am sorry for my late reply.

I would like to share what i have done so far.

below are the schematic file.

Short description: I am trying to switch high power infrared LED for 1MHZ.
So i have selected "UCC2742" MOSFET driver (Because to convert 3.3V to 12 V) to drive the MOSFET "IRF840LC" @ 1MHZ.
Of course i got the square wave of 1MHZ and the magnitude of 12V at the output of Driver IC.
But at the drain w.r.t GND I am not getting a square wave (I got the waveform like the below image output voltage).

View attachment 126835

Use a FET with a lower gate capacitance.
Here is a simulation with one with a gate charge of 63Nc - like yours and one with a gate charge of 8 like this one:
http://www.mouser.com/ds/2/196/irf8707pbf-936201.pdf

 

That's how I worried about the frequency can't be reach too high, because the Cgs will be pulls the Vgs down.

The first thing is that you may reducing the Vgs to about 7V, and adding a pnp bjt to discharging the Igs to ground when the input of Vgs is going to low.

Using the circuit on the right top of the page 14.
Design And Application Guide For High Speed MOSFET Gate Drive Circuits.

As I mentioned elsewhere - common types of MOSFETs sometimes turn up in SW transmitters running a few MHz, but they're usually transformer driven. The MOSFET driver chip could be the bottleneck.

 

Also, what you should be interested in is not the voltage at the mosfet, but the current through the led, which at this frequency could have a very different waveform due to parasitics.

 

As I mentioned elsewhere - common types of MOSFETs sometimes turn up in SW transmitters running a few MHz, but they're usually transformer driven. The MOSFET driver chip could be the bottleneck.

I want to ask you but not yet since you posted the post #4, do you have a links about the application, because I thought that application could be working in a sine wave, but not a square wave.

 

Here's the IRF datasheet. I was thinkng about bypass caps on the mosfet driver chip and, "How much are R1 and R2 affecting the charge on the gate capacitance?" or adding a capacitor in the second drawing of post #5 from the driver chip output to the gate, but you guys are way ahead of me.
Carry on.