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Logic level vs "normal" MOSFETS...

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smanches

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I've been looking at the datasheets for logic level vs "normal" MOSFETs. I can't really tell if there are any disadvantages to using logic level ones. Can anyone possibly shed some light on the advantages and disadvantages of either, besides the obvious one of gate voltage? :p
 
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edeca

Active Member
Some FETs need a higher than 5v input to their gate before they will "turn on". This generally makes them unsuitable for use straight off of a PIC or other micro.
 

dknguyen

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THe advantage of logic level MOSFETs is that their source-drain saturates with at a low gate voltage. THe disadvantages are that they tend to have higher gate capacitance/gate charge (take longer to turn on for a given amount of drive current), have higher on-resistance, have lower maximum tolerable gate voltages, and cannot be made to have source-drain breakdown voltages as high as standard MOSFETs. You are basically trading the advantage of lower gate drive voltage for performance hits in every other area to varying degrees. Sometimes though the ability to use a logic-level gate voltage simplifies things enough to justify this.

Basically, if you can easily provide the gate voltage necessary for a standard level MOSFET choose a standard MOSFET over a logic level MOSFET (all things being equal). If you cannot provide such a voltage easily, then you have to start thinking about the complexity and performance tradeoffs of supplying that gate drive voltage or using a logic-level FET.
 
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ronsimpson

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"Normal MOSFETs can handle +/-20 volts on the gate.
Some of the logic level FETs can not withstand 20 volts on the gate.
 

smanches

New Member
THe advantage of logic level MOSFETs is that their source-drain saturates with at a low gate voltage. THe disadvantages are that they tend to have higher gate capacitance/gate charge (take longer to turn on for a given amount of drive current), have lower maximum tolerable gate voltages, and cannot be made to have source-drain breakdown voltages as high as standard MOSFETs.
I had noticed the gate capacitance values seemed to be higher, but couldn't find any two mosfets that I could compare directly. I also noticed it was harder to find high voltage (>200V) logic level mosfets as well. Didn't know if that was typical or not.
 

Sceadwian

Banned
If you want the best of both worlds you just get yourself a gate driver IC, or use a push/pull transistor config to drive the gate from the mosfets Drain/source voltage.
 

dknguyen

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I had noticed the gate capacitance values seemed to be higher, but couldn't find any two mosfets that I could compare directly. I also noticed it was harder to find high voltage (>200V) logic level mosfets as well. Didn't know if that was typical or not.
THere are also these differences that I forgot to mention. One probably not so important, but the other is pretty important:
-transconductance of standard MOSFETs is much higher than that of logic-level MOSFETs...if that means anything to you.
-Although a logic level MOSFET has lower resistance than a standard MOSFET with a logic level gate voltage (ie. turning on vs not turning on), if you drive a logic level MOSFET and a standard MOSFET with a standard gate voltage (>10V), the standard MOSFET will have a LOWER on resistance. So there is actually a clear disadvantage when using logic level MOSFETs with standard gate drive voltages rather than standard MOSFETs.

THere was a document that described some differences from International Rectifier but I can't find it anymore. It is hard to find MOSFETs that have both a standard and logic level version. But here are some for you to compare...

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https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRLS3034-7P
https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRFS3004-7P
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https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRFS3006-7P
https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRLS3036-7P
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https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRLS4030-7P
https://ec.irf.com/v6/en/US/adirect...omSendTo=byID&domProductQueryName=IRFS4010-7P
 
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smanches

New Member
Well, the entire question is spawned by me trying to isolate the power section (drivers and mosfets) using optoisolators. I'm using 12v to drive the drivers/mosfets, but the optocouplers want 5v. I can't seem to find optocouplers that can handle a 12v supply, so I was thinking to maybe try logic level MOSFETs.

It may be that optimum is to have two bias supplies, 5v and 12v. :(
 

dknguyen

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Most Helpful Member
Do you actually need galvanic isolation? It's a major pain in the ass. Also, remember an optocoupler actually has really crappy drive current. Optocoupler gate drivers usually need a power supply on the other side of the optocoupler to provide enough drive current to switch the MOSFET fast enough for high frequency operations. THe energy transmitted by the LED and received by the photodetector is enough to transmit the signal that the MOSFET should be switched, but does not actually have enough energy to do so.

http://www.electro-tech-online.com/custompdfs/2009/04/ADUM1234.pdf
and other gate drivers from the Analog Device's iCoupler series might help you out.
 
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smanches

New Member
I don't need it to be optical, but it's working great actually. I just don't like having to have two bias voltages just for the driver circuits.

I do need isolation though, from mains.

I'm using driver ICs for the actual driving of the MOSFETs. There is a total of 1us delay between the PIC firing and the MOSFET firing. I found that to be acceptable.
 
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smanches

New Member
Ok, I should say rectified/boosted mains. It's a ACIM motor control with a PFC boost circuit as the first stage.

BTW, that optical driver is exactly the kind of thing I'm looking for. Thanks! :D

Right now I'm doing Pic33F -> 6n137 -> TC4431 -> STP20NF20.
 
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dknguyen

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Both of the isolated drivers I linked still requires a power supply on both sides, however.
 
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