Hi

This article says "an interface is needed between the logic/control circuitry and the high power device. This can be implemented by driving a logic level n-channel MOSFET, which, in turn, can drive a power MOSFET"

Does this mean there's no such thing as a logic-level power mosfet? Ie, best of both worlds.

THX

 

Does this mean there's no such thing as a logic-level power mosfet?

NO. There are plenty of logic-level power MOSFETs.

 

As OBW says, there are lots of logic level high power devices. There is a caveat. "Logic level" now very often means 3 volts or less for HIGH. The number of FETs that are driven adequately for currents over a few amperes with 3 volts gate to source is much more limited than those perform well with 5 Vgs. New FETs that do perform well at 3 Vgs are being introduced and some are very impressive, though many are available in surface mount packages only.

Where logic level FETs are still pretty scarce is in the higher voltage (500 V and up) and high current types. There isn't a great deal of demand for logic level gate compatibility because FETs operating at those conditions usually require gate drive currents in the amperes range for fast switching, which means a special gate driver is required.

 

thx for great answers.

is a logic-level power MOSFET equivalent to a gate-driver + power mosfet?

is there a special term for a logic-level power mosfet? Or just "logic-level power MOSFET"?

Can anyone mention some common, inexpensive model numbers available today? How about these?

IRLZ44N
RFP15N05
IRLB3034PBF
BUK101
IRLR014
FDS6680A
FDS6670A

THX

 

No, they're just called logic level power MOSFETs or often just logic level FETs. There are "signal" type MOSFETs, but they are typically used for linear amplifiers, RF oscillators and the like and not as switches so "logic level" wouldn't be a concern. Something that can handle only 200 mA, such as the 2N7000, is often regarded as being a power MOSFET.

Here's a selector page at Mouser:
https://www.mouser.ca/Semiconductor...ET/_/N-awhng?keyword=logic+level+mosfet&No=25

 

is a logic-level power MOSFET equivalent to a gate-driver + power mosfet?

No. "Logic level" simply denotes a MOSFET with a very low gate-to-source threshold voltage making it suitable for driving with logic-level signals (as from, for example, a microcontroller).

A gate driver IC may sometimes still be needed to get adequate switching speed.

 

A gate driver IC may sometimes still be needed to get adequate switching speed.

Ok, are there logic-level mosfets with integrated gate drivers?

Trying to reduce parts-count...

THX

 

Ok, are there logic-level mosfets with integrated gate drivers?

Not that I know of.

Or, logic-level gate-drivers?

I've used the MCP1407.

 

Not that I know of.

That would be nice though.

 

Q: Do most gate-driver IC's have logic-level control-pins?

A gate driver IC may sometimes still be needed to get adequate switching speed.

Q: Why? If the mosfet already accommodates logic-level gate-input, then how does a gate driver help? Doesn't the logic-level mosfet-gate want a low-current, low-voltage input? Wouldn't a gate-driver overload it?

 

Q: Do most gate-driver IC's have logic-level control-pins?

I've only used the one I mentioned, which does, but I expect most of the others do, too.

Q: Why? If the mosfet already accommodates logic-level gate-input, then how does a gate driver help?

By providing higher drive current which allows for greater switching speed and lower switching transition losses.

Doesn't the logic-level mosfet-gate want a low-current, low-voltage input? Wouldn't a gate-driver overload it?

No, and no.

A logic-level MOSFET is merely usable with lower gate drive voltages; it doesn't require them.

 

This discusses dynamic characteristics and the all important
Cg-d and the miller effect.

https://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f


Regards, Dana.

 

This discusses dynamic characteristics and the all important
Cg-d and the miller effect.
https://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f

Fantastic!
i prolly won't understand it, but i'll try

 

The gate input of a FET or MOSFET is essentially a very small capacitor. The only reason you would need higher voltages or currents on the gate are to charge this capacitor faster. That would result in a faster switching speed if you really need it. Most applications don't.
I have used the IRLZ44N quite a bit and it works perfectly on a 5V logic input. It will switch up to something like 60 Amps according to the specs.

 

higher drive current which allows for greater switching speed and lower switching transition losses.

Trying to understand "transition losses". I got stuck at the beginning of this article (see my comments at the bottom).

only reason you would need higher voltages or currents on the gate are to charge this capacitor faster.

You can charge a capacitor faster with higher currents or higher voltages?

That would result in a faster switching speed if you really need it.

How fast? I'm trying to switch 30 amps at around 40 kHz.

Thx

 

Trying to understand "transition losses". I got stuck at the beginning of this article (see my comments at the bottom).

By "transition losses" I'm referring to the fact that a MOSFET cannot instantaneously start or stop conducting when turned ON or OFF; there is always a period in between when the MOSFET is conducting substantial current while at the same time having substantial drain-to-source voltage across it, and thus is dissipating considerable power.

Therefore, the faster the MOSFET can be made to switch between ON and OFF states, the briefer this period will be and as a result, the less energy will be lost.

 

Trying to understand "transition losses". I got stuck at the beginning of this article (see my comments at the bottom).

You can charge a capacitor faster with higher currents or higher voltages?
--- Higher voltages are required to push a higher current to charge a capacitor faster, (I think).

How fast? I'm trying to switch 30 amps at around 40 kHz.
--- You have to consult the FET spec sheet to determine switching speeds. There is usually a graph.

Thx

 

MOSFET cannot instantaneously start or stop conducting when turned ON or OFF; there is always a period in between when the MOSFET is conducting substantial current

To clarify, you mean before it starts conducting substantial current, correct?

 

To clarify, you mean before it starts conducting substantial current, correct?

I thought what I sad was pretty clear. But I'll try again.

When a MOSFET is ON, it has drain-source current flowing through it, but little drain-source voltage across it; hence, it is dissipating little power. When it is OFF, it has drain-source voltage across it, but little or no current through it; again, it is dissipating little power.

But in between, while in the process of turning OFF or turning ON, there is a brief period when it has both drain-source current flowing through it and drain-source voltage across it, and therefore is dissipating power. This can be minimized by making turn-on and turn-off as quick as possible. This, in turn, requires the gate capacitance to be charged and discharged quickly, which requires a lot of drive current.

Hence we have MOSFET driver chips, such as the MCP1406/1407, which can source and sink large currents into/out of a MOSFET's gate.

 

@ObiWan, your explanation was definitely clear. The problem is my comprehension.

i understand all of your last post, except the Drain-Source voltage differential during switching.
i'll need to study what the voltage is doing at that instant.

thx