High Side vs Low Side Mosfet Drivers?

[paragon37]

Hello All,

This is my first post to this forum...

Could someone explain the benifits and drawbacks of using either high side / low side mosfet drivers?

In the past I have used lowside drivers such as the ir2121 / ir ir2120 connected to pulse issolators ( Ferite corred) to drive mosfets in a half bridge and H-bridge design.

I was looking at the ir2184 highside driver as an alternative so I could do away with the pulse issolators.

Could someone please explain the benifits / drawbacks and issues of using the highside method as opposed to the lowside method?

Kind Regards,
SRG

 

[smanches]

High-side drivers are meant to run with a floating ground. Low side drivers make the assumption that VSS is ground.

You should use the correct driver for the correct application. It's not a matter of advantages or disadvantages, but using the proper chip for the proper application.

 

[paragon37]

Thank you for your reply!

Could someone please highlight in what applications one would select one method over the other?

Kind Regards,
SRG

 

[dknguyen]

As smanches already said, you do not get to choose whether to use a high-side or low-side gate because you do not get to choose whether you get to use a low or high side MOSFET. If you are forced to use a low-side MOSFET, you use a low-side gate driver. If you are forced to use a high-side MOSFET, you use a high-side gate driver. Simple as that.

 

[RCinFLA]

A high side driver is a boot-strapped supply driver of an output N-ch MOSFET with a level shifter on the driver's input.

One typical useage is for an H-bridge MOSFET output driver. It could also be a simple push-pull output MOSFET output (half an H-bridge).

When the bottom output N-ch MOSFET is 'ON' it also charges a capacitor that become the high side driver power supply when bottom N-ch releases and upper (highside) N-channel MOSFET conducts. The capacitor is 'boot-strapped' up in the air (to higher voltage) to become the supply for the high side gate drive. There is an external diode to the regular supply that is connected to this highside supply capacitor.

The are chips that have both the low side and high side drivers within a single I.C. They also include a small delay between the two drivers activation to ensure both output MOSFET do not have any portion of time where they are both conducting, which would momentary short circuit the output supply.

Drivers are rated on their amperage output capability. It may sound strange but if you want to switch on or off a power MOSFET in a couple of hundred nanoseconds the current spike to do this can be quite high. Large MOSFET can have an input capacitance of 0.01 uF. This capacitance must be charged or discharge in the hundred nanoseconds.

 

[Arkham00]

In industrial electronics, in PLC environments, where loads such as relays or electro-valves are far in the plant, high-side driver are preferred because a short between a conductor and "protection ground" (usually connected to 0V) will not fire the load. In PLC technical terms, this kind of outputs is called "PNP output".

For the same reason, inputs (pushbuttons, limit switches, ...) are usually connected to "high side" (+24V) and the input. This kind of inputs is also called "PNP input" even if it has no PNP transistor in the circuit.

 

[smanches]

A high side MOSFET is when the source connects to the load. That is, the switch is BEFORE the load, or looking at most schematics, it's on the top side or high side of the circuit near VDD.

A low side MOSFET is when the source connects to ground. That is, the switch is AFTER the load, or it's on the bottom side or low side near GND.

Of course, this assumes NPN. If it's PNP, then it's all backwards.

 

[Russ Hensel]

The nice thing about a low side switch is that it is normally easy to drive, and the drive voltage need not be inverted ( that is + turns it on {I am also assuming a positive supply } ) and the drive voltage is usually less than or equal to the power supply voltage. If you use the right driver IC all this complication pretty much goes away.

 

[anz]

can we use only one side at a time? in most of circuits i have seen that output of both the high side and low side are tied togather.what should i do if i want to use only one of them at a time?

 

[dr pepper]

Sounds like your talking about a half bridge, which can be used to reverse the polarity of the load by using only 2 switches, but you need 2 voltages to do it.

And as mentioned allready the high side driver needs a higher voltage to drive the gate, the gate volatge needs to be higher than the source by the fets threshold voltage, usually around 3 to 5 volts, and is often accomplished with a pulse transformer also mentioned further up.

 

[shortbus=]

Sounds like your talking about a half bridge, which can be used to reverse the polarity of the load by using only 2 switches, but you need 2 voltages to do it.

And as mentioned allready the high side driver needs a higher voltage to drive the gate, the gate volatge needs to be higher than the source by the fets threshold voltage, usually around 3 to 5 volts, and is often accomplished with a pulse transformer also mentioned further up.

If using a half-bridge driver, there is no need for a second higher voltage or a pulse transformer. That is what the boot strap circuit is for. All that is needed is the ~10V to turn the gate fully on.

The threshold voltage has nothing to do with it in a switch situation. The threshold voltage is only a concern when a mosfet is used as a amplifier, Threshold voltage is the point when the mosfet is almost on or off.

 

[dr pepper]

Agreed with your first sentence, that was my bad.
I still dont get the second though, you want the fet to be fully on in a switch application, dont you need a minimum voltage at the gate to make the fet conduct fully?

 

[shortbus=]

Agreed with your first sentence, that was my bad.
I still dont get the second though, you want the fet to be fully on in a switch application, dont you need a minimum voltage at the gate to make the fet conduct fully?

On most non "logic" level mosfets the gate voltage for "fully on" is listed as 10V. The information for gate on voltage is almost always on the first page of a data sheet.

The threshold voltage is for when you want to use the mosfet as a amplifier, like in a audio amplifier. threshold voltage is where the mosfet will conduct but with almost no current passing. Threshold voltage tells a designer how low the gate voltage needs to go, saves the need to go clear to "0" volts and makes the audio signal sound more "natural" than digital(on-off).

 

[dr pepper]

Yes I think I explained that bad.
I wasnt meaning just apply enough to make the device conduct I meant you'd need enough volatge to create a full switch on.
My head hurts.