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Evening all,

I'm planning to use MOSFET drivers in a design I'm working on, to minimize switching time - and hence losses. However, whilst I have read plenty of application notes regarding selecting driver current requirements etc, I'm still lost as to their actual type.

On the Farnell MOSFET driver page (http://uk.farnell.com/jsp/search/browse.jsp?N=2031+203340&Ntk=gensearch&Ntt=mosfet+driver&Ntx=mode+matchallpartial) the types listed as extensive.

Would I be correct in thinking a full bridge driver, is for driving four FETs as used in an H bridge, two high and two low. (Question 2, low side drivers for N channel? High side for P channel?) Therefore, for general purpose driving of N channel FETs, use a low side driver?

But then, MOSFET is actually listed as a driver type too? What does that mean then, it can do N and P? ...and power? Well - pretty much all FETs these days are power FETS, so a power FET driver? Hmm

So, my summarizing question really is - to drive general purpose N channel MOSFETs, do I need a 'standard' low side driver?
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So, my summarizing question really is - to drive general purpose N channel MOSFETs, do I need a 'standard' low side driver?

It really depends on the specific mosfet you are using, it's gate capacitance specifically and also the switching frequency you will be using and of course the actual drain voltage/current amount that will be switching. It's all related to how to eliminate switching delays for the mosfet to keep it's device heat dissipation as low as possible to prevent device damage. The AVR output pins are pretty robust and are active sinking and sourcing so many pwm switching applications using 'logic level' n-channel mosfets in a low side switching configuration don't require external driver help at all, just drive the gate directly with the arduino output pin. But again it's all about your specific device, applications, and how much load power you are trying to switch.

But to answer your question, if your using a n channel mosfet in a low side switching application then yes a standard low side driver would be the proper choice.

Lefty
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Most mosfet drivers are designed for N-channels.

If you are driving a h-bridge, you need a h-bridge driver (both high side and low side).
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The FET I am using is an IRLR024N - http://www.farnell.com/datasheets/68804.pdf

I am using the SMD D Pak so want to keep heat to a minimum.

Two of them need only be switched slowly, around 2-4kHz - whilst the other 4 will switch around 25kHz, I need to look at the base and divisors available for PWM. The slow switched FETS will be carrying about 4A peak, whilst the much faster FETS will mostly be switching up to 1A - but I would like the option of switching higher currents too.

Hence, I was not planning to use a driver for the low speed switchers, but only for the higher speed FETS. I have read a good document by IRF about selecting drivers based on gate capacitance etc, I just wanted to know the high vs low - now sorted, many thanks!
« Last Edit: November 30, 2012, 03:27:17 pm by jtw11 » Logged

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I am using the SMD D Pak so want to keep heat to a minimum.

The Ron spec for that device is not very impressive, as there should be devices available with a much lower Ron resistance which when you come down to it is a most important spec if keeping device dissipation to a minimum.

Lefty
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I thought so too - however, narrowing down parts using the following parameters it seems most FETs have this sort of value, or those with smaller are in the smaller SOT223 packages that will get hot anyway.

N channel.
10-20A cont
55-75A
Typical threshold 3V
Rds test max 5V
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The AVR output pins are pretty robust and are active sinking and sourcing so many pwm switching applications using 'logic level' n-channel mosfets in a low side switching configuration don't require external driver help at all, just drive the gate directly with the arduino output pin. But again it's all about your specific device, applications, and how much load power you are trying to switch.

For PWM power switching, general advice would be to always use a driver (learning how to calculate switching losses and avoiding issues with heat-spots is mandatory).
The driving force towards logic-level MOSFET’s is not CMOS logic level switching, but the possibility of using a single (shared controller and driver) supply.

A logic-level MOSFET has a higher total gate charge and so requires more switching power than standard drive.  High power means high-current, and low voltage means higher current still (peak current drive requirements are in excess of 1 amp for popular logic-level power MOSFET’s and may be as high as 10 amps).  A standard drive MOSFET reduces current requirements two-fold in comparison to logic-level drive.
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Hmm - that's what I've always heard around too, just good practice to use a driver.

Do you have any recommendations for very good resources re calculating heat generated during switching?

EDIT: I've just widened up my search options a bit, with larger currents - and I've come up with this device. Much lower on resistance :-) all the super low resistance devices appear to be in all the weird and wonderful packages, most of which I've never heard of.

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002155.pdf
« Last Edit: November 30, 2012, 05:40:08 pm by jtw11 » Logged

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I thought so too - however, narrowing down parts using the following parameters it seems most FETs have this sort of value, or those with smaller are in the smaller SOT223 packages that will get hot anyway.

N channel.
10-20A cont
55-75A
Typical threshold 3V
Rds test max 5V

Rds test voltage is not a maximum, its the Vgs for a given value of Rds.  There is a separate spec for max Vgs values.

The max current ratings are best ignored unless you have water cooling.

If threshold is 3V it won't be logic level, thresholds are about 1V for logic level. The way to choose a MOSFET is to ignore max I and threshold ratings, choose the Rds(on) for the Vgs you are using (and for switching 4A I'd aim for 0.03 ohms or less, not 0.08, since you don't want to bother with a heatsink)

Pick a device with a voltage rating about twice the power supply voltage - but not much more as
typical Rds(on) values increase with device voltage rating.

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EDIT: I've just widened up my search options a bit, with larger currents - and I've come up with this device. Much lower on resistance :-) all the super low resistance devices appear to be in all the weird and wonderful packages, most of which I've never heard of.

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002155.pdf
No, that's not low Rds(on), this one is more like it: http://www.irf.com/product-info/datasheets/data/irls3036-7ppbf.pdf  - I use one to test lead acid batteries and it handles upto 20A nicely without heatsink
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A logic-level MOSFET has a higher total gate charge and so requires more switching power than standard drive.  High power means high-current, and low voltage means higher current still (peak current drive requirements are in excess of 1 amp for popular logic-level power MOSFET’s and may be as high as 10 amps).  A standard drive MOSFET reduces current requirements two-fold in comparison to logic-level drive.

So, I've just recieved a bunch of Texas UCC27524 driver ICs as samples - and looking through the datasheet (http://www.farnell.com/datasheets/1634148.pdf), it seems the supply can go up to 20V. Having read that, I now understand the above quoted statement... at least I think I do, please correct me if not.

Logic level FETs typically have both a higher gate charge, and a larger RDSON value - therefore, instead of using a logic level FET one can use a 'normal' level FET and connect the power supply of the driver to the 12V supply, this way - the driver forces high peak current 12V signals into the gate of the FET? Correct?

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instead of using a logic level FET one can use a 'normal' level FET and connect the power supply of the driver to the 12V supply, this way - the driver forces high peak current 12V signals into the gate of the FET? Correct?

Correct.
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