Author Topic: When to use a MOSFET driver?  (Read 10716 times)

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Offline shadewindTopic starter

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When to use a MOSFET driver?
« on: October 15, 2011, 06:41:55 pm »
Hi,

I'm planning to dim lots of LEDs with MOSFETs and I've been looking at these:
http://www.irf.com/product-info/datasheets/data/irfh7914pbf.pdf

I've designed a board which uses a Microchip MOSFET driver but I see now that I could probably drive it directly with the MCU by looking at Fig 7 on page 4. What are the disadvantages of doing so in this case? From what I've learned, this would lead to a slower switching time since the MCU cannot deliver as much current and also higher on resistance since the MCU only gives out 3.3V as opposed to the MOSFET driver which I drive with 12V. Would you use a driver in this case?
 

Offline jackbob

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Re: When to use a MOSFET driver?
« Reply #1 on: October 15, 2011, 09:20:09 pm »
What you linked is just a volatage converter it won't do the job your looking to do. I believe you are trying to drive mosfets with microcontroller using pwm. In this case yes use a tc4422 fet driver they work great.

Jacob.
 

Offline shadewindTopic starter

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Re: When to use a MOSFET driver?
« Reply #2 on: October 15, 2011, 09:41:07 pm »
What you linked is just a volatage converter it won't do the job your looking to do. I believe you are trying to drive mosfets with microcontroller using pwm. In this case yes use a tc4422 fet driver they work great.

Jacob.
No I'm pretty sure I linked to IRFH7914PbF which is a power MOSFET from International Rectifier. And yes, I'm sure the Microchip FET drivers work great. That was not my question though.
 

Offline mikeselectricstuff

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Re: When to use a MOSFET driver?
« Reply #3 on: October 15, 2011, 10:43:12 pm »
Bottom line is you need a driver when you want to switch big mosfets fast, as the gates have significant capacitance.
The driver (a)  supplies the high pulse current to charge & discharge  the gate quickly to keep it out of the partly-conducting state for any length of time to maximise switching efficiency, and (b) increases the drive voltage from logic level to a higher voltage (typically 5-15v) to turn the fet on as hard as possible to minimise static losses. 
If you're just turning LEDs on, or PWMing at a few hundred Hz, chances are you don't need a seperate driver unless you're up in the tens of amps range.
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Offline shadewindTopic starter

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Re: When to use a MOSFET driver?
« Reply #4 on: October 15, 2011, 10:48:14 pm »
Bottom line is you need a driver when you want to switch big mosfets fast, as the gates have significant capacitance.
The driver (a)  supplies the high pulse current to charge & discharge  the gate quickly to keep it out of the partly-conducting state for any length of time to maximise switching efficiency, and (b) increases the drive voltage from logic level to a higher voltage (typically 5-15v) to turn the fet on as hard as possible to minimise static losses. 
If you're just turning LEDs on, or PWMing at a few hundred Hz, chances are you don't need a seperate driver unless you're up in the tens of amps range.
Right, so in this case it probably won't matter? I'm assuming this MOSFET doesn't qualify as "big"? But I'm thinking I might as well use a driver anyway to learn new things. It also means that the board can be driven at higher frequencies/currents and by arbitrary sources if I should ever want to.
 

Offline amspire

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Re: When to use a MOSFET driver?
« Reply #5 on: October 15, 2011, 11:00:48 pm »
You can drive the mosfet directly, as long as you use a mosfet spec'ed for a 2.7V gate drive, such as this:

http://www.irf.com/product-info/datasheets/data/irf7463pbf.pdf

If you go to the IR selector, you can search by 2.7V on resistance:

https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672

You will see it has much more stored charge, so it will switch even slower, but to dim the leds, you probably only have to switch at 100Hz, so the switching losses may not be bad.

If you need more current, you can just parallel more mosfets.

Where you might get into a problem is if there is any chance of fast transient conditions - like the LED string shorting out, then the micro is not capable of sinking the drain source charge, and you might end up blowing up both the mosfet and the micro.  The driver chip will definitely be more robust, and if you do have a 12v supply available, you can choose mosfets with a better on resistance then the 2,7V gate drive mosfets.

Richard.
 

Offline jackbob

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Re: When to use a MOSFET driver?
« Reply #6 on: October 15, 2011, 11:14:37 pm »
It depends how many led's you are driving. A mcu alone isn't the best to directly drive the fet especially if you have fets in parallel, it won't have the power. A great MOSFET I use all the time is the irf3205. It is a small TO-220 that will put out serious amps for it's size, and it allows very small gate voltages. You can drive them with a microcontroller but only so many. Always remember if you are driving fets directly with a mcu put a resistor in because they can suck up too much power from the mcu.
 

Offline jackbob

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Re: When to use a MOSFET driver?
« Reply #7 on: October 15, 2011, 11:40:00 pm »
The MOSFET you linked will work I was in a hurry and didn't look at it for long. Yes so your best bet is to use a micro and the tc4422 to drive the fets you listed or a different one. I am building a pwm motor control now and just started using the tc4422 (amspire told me about them) and I love them. They are very easy to use and require almost no external components accept maybe one input and output resistor but that's it and they do the job very well. How many led's are we talking about dimming?
 

Offline shadewindTopic starter

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Re: When to use a MOSFET driver?
« Reply #8 on: October 15, 2011, 11:54:09 pm »
I am using the TC4468 which is a 4 channel version of the TC4422 I believe. I don't know exactly how many LEDs there are. I'm planning on using those cheap RGB LED strips you can get on eBay and they're usually 0.6 A per meter and I want to build it so that it can drive at least 10 m to have some margin. So that's 6 A though I doubt I will use that many. Still, you never know and I might as well design for that target.
 

Offline mikeselectricstuff

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Re: When to use a MOSFET driver?
« Reply #9 on: October 16, 2011, 12:05:24 am »
One important thing to remember is that the drivers need good decoupling due to the high pulse currents - at least 1uf ceramic very close to the pins. 
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Offline jackbob

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Re: When to use a MOSFET driver?
« Reply #10 on: October 16, 2011, 12:12:48 am »
I would assume you are driving the colors individually so if it's .6 amps per meter divided by 3 colors that's .2 amps per meter per color. It's always good to have headroom on your mosfets as far as current goes you can't oversize them really. You can use three outputs on the mcu and 3 channels on the driver to drive on MOSFET for each color. That's if you are doing the colors individually. If not just do it the same with a little bigger MOSFET to handle the current. 6amps is not big for mosfets all you need is one good MOSFET or three smaller ones if you do each color.
 


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