Help with part choice

[Chrishelvey]

Hello, This is my first post here. I am not an engineer, just a long time hack. I'm designing a "proof of concept" product and am having some difficulty with choosing the right IC.
In short, I am using a Microchip PIC running at 20Mhz to do one job only and that is generate a square wave pulse. I use this setup because I like to use software to quickly change my frequencies accurately while experimenting to get the end result I want. I will use another PIC to run sensor input and I/O for other things, including turning the output of the PWM PIC on/off to the load (power transistors.)
My problem is that generally MOSFET transistors are not driven high-enough to turn on all the way by a logic level signal, so I looked in to logic level chips such as the MIC5014 . At first glance of the datasheet, it looks to serve my purpose well, until I look at the rise times - 90us . If my math is correct (which it may not be) 90us without any "time off" time is only about 11Khz . The PIC can create a square wave much faster than that, and I would like to push frequencies of up to 200Khz.
Does anyone have some advice about this? I am looking for simplicity in the circuit. Essentially it is: PWM-->1-chip solution to --> drive power transistor(s) at up to 200Khz.
Thank you to all those who contribute.

Chris

 

[hjames]

The MIC5014 isn't a great idea if you want to PWM something. This chip is used when you want a mosfet in a "high-side" configuration. A "high-side" configuration is usually only used under special circumstances - typically safety related. A "low-side" switch is much simpler/faster and easier to deal with.

In any case, there are much faster chips - but no one will be able to make any reasonable guesses without more info:

1) do you really need a high-side switch
2) what voltages do you need to drive
3) or to cut to the chase - what are you planning on doing with this?

 

[Analog]

Hello, This is my first post here. I am not an engineer, just a long time hack. I'm designing a "proof of concept" product and am having some difficulty with choosing the right IC.
In short, I am using a Microchip PIC running at 20Mhz to do one job only and that is generate a square wave pulse. I use this setup because I like to use software to quickly change my frequencies accurately while experimenting to get the end result I want. I will use another PIC to run sensor input and I/O for other things, including turning the output of the PWM PIC on/off to the load (power transistors.)
My problem is that generally MOSFET transistors are not driven high-enough to turn on all the way by a logic level signal, so I looked in to logic level chips such as the MIC5014 . At first glance of the datasheet, it looks to serve my purpose well, until I look at the rise times - 90us . If my math is correct (which it may not be) 90us without any "time off" time is only about 11Khz . The PIC can create a square wave much faster than that, and I would like to push frequencies of up to 200Khz.
Does anyone have some advice about this? I am looking for simplicity in the circuit. Essentially it is: PWM-->1-chip solution to --> drive power transistor(s) at up to 200Khz.
Thank you to all those who contribute.

Chris

Chris, nice to have your question.

First off, the MIC5014 does low side drive as well (to answer jame's question). However, it is not meant for high speed. Now I'm not sure why you think you need this chip at all. For instance, the IRF-630, a part I use quite a bit, has a VGS (on) of 2 to 4 VDC, which is perfect for logic signals, so why not drive it from the PIC directly? What MOSFET are you using?

 

[Chrishelvey]

Thanks, I appreciate the replies.
The only reason I chose the MIC5014 is because I didn't know any better. It described a way (from a search on Google) to switch logic signals that I couldn't seem to get done experimentally.
More info:
I am developing an on-demand Hydrogen generation system from water as a catalyst for combustion in an automobile.
So, I want to switch 12-15VDC at rates of 200 to 200Khz with amperage draws upward of 75A (LOTS of current here.)
However, a land based unit may switch, perhaps, 150A at 12VDC (I can do this already at 120VAC rectified,) but I'm looking at it from a household, solar, wind, hydro array as an offshoot.
I am using an (or paralleled) RFP50N06 to drive this. (50Amp)
Basically, it is a very short pulse (low duty cycle) PWM. Water (electrolysis) reacts differently to different waveforms and I can experiment using software until I get the waveform on the current side that works the best. So far the results are very promising. I just want to use the right parts for the application , don't have the money to hire an EE, and don't have enough experience myself to know which part to choose.
Please forgive my ignorance.
The IRF-630 may suit the task perfectly. I'll look at the datasheet.
Thanks,
Chris

 

[Chrishelvey]

Awesome. In looking at the datasheet on the IRF-630 it looks like it may be more prudent (and even a simpler circuit) to parallel enough of these together to get the current I need and drive the gate directly from the PIC.
Is my logic sound here?
I'm not attached to the RFP50N06, it just worked for my first experiments and prototypes.
Not to mention that every time something is added to the chain (to the MOSFET gate,) it introduces some delay which affects the waveform. Simpler is always better...especially for me.
What do you think?
Chris

 

[Analog]

I'm a little short on time right now, but here is a good discussion:

https://www.electro-tech-online.com/threads/back-emf-protection.21519/

 

[picasm]

You could have a look at using a Microchip TC4420/TC4429 (non-inverted or inverted) to act as a buffer between the PIC and your chosen mosfets.
It may cope better with the high capacitance of paralleled mosfet gates and also has matched rise/fall times.
See: https://www.electro-tech-online.com/custompdfs/2006/10/21419c.pdf

 

[Chrishelvey]

Great input. I read the thread suggested - much about paralleling MOSFETs and the problems that can be incurred.
That somewhat brings me back full circle to using something to drive the MOSFET(s.)
The TC4420 looks like a good choice to drive the RFP50N06 or multiples.
It appears then that the IRF-630 mentioned earlier could do the same job.

I also looked at the datasheet for the STE180NE10 , which is a 180A MOSFET used in welding equip. That would solve issues of the troubles of paralleling MOSFETS to get the current I want. Cost of the part is not really an issue.

I could drive the Gate with one of the forementioned devices. The 5V from the PIC would drive TC4420/IRF-630 to supply 12-14V to the gate of the BIG MOSFET.
Sound reasonable?

Thanks for everyone's input. I would not know which of the gazillions of devices to choose from to accomplish the task.