I am studying this types of gate drivers:
- High-side driver
- Low-side driver
- Opto-side driver
(particularly the first two)

I can't figure out which of them is best to drive an n-type enhancement buck converter.
In my opinion if I have to choose between High-side and Low-side ... I choose High-side because the source of the mosfet does not have to be connected to a fixed potential (like GND) .. but in the Buck Converter it's connected to the inductor and cathode of the diode (i.e. floating potential).



Which would you choose?

 

I am studying this types of gate drivers:
- High-side driver
- Low-side driver
- Opto-side driver
(particularly the first two)

I can't figure out which of them is best to drive an n-type enhancement buck converter.
In my opinion if I have to choose between High-side and Low-side ... I choose High-side because the source of the mosfet does not have to be connected to a fixed potential (like GND) .. but in the Buck Converter it's connected to the inductor and cathode of the diode (i.e. floating potential).

View attachment 295211

Which would you choose?

Hi,

If stuck with N type then probably low side but do you see why?
Any other specs we need to know?
Also, this could change depending on the output grounding requirement.
Does this make sense to you?
There are 4 different topologies here you should be aware of.

 

Hi,

If stuck with N type then probably low side but do you see why?
Any other specs we need to know?
Also, this could change depending on the output grounding requirement.
Does this make sense to you?
There are 4 different topologies here you should be aware of.

It's not a project and I don't have to implement anything, it's just a theoretical question to see if I understand some concepts well. So I don't have any other parameters/specifications to provide ... we can assume them.

If stuck with N type then probably low side but do you see why?

At the moment I don't know why .. isn't "high side gate driver" fine as I said?

 

It using an N channel MOSFET as a high side switch (as in the circuit you posted), you need a high side driver.

To understand why, ask yourself this: When the MOSFET is on, what voltage is on the source?

What voltage is needed in the gate then, to turn the MOSFET on?

Do you understand the term “high side switch?”

Do you know what a high side driver does?

 

If you are using a charge pump to power the gate diver, then think about how the MOSFET will turn on the first time you switch it.

 

Per capire perché, chiediti questo: quando il MOSFET è acceso, quale tensione c'è sulla sorgente?

Assuming the mosfet is a short circuit when it is closed (on), the source voltage will be the same as that of the input generator (Vs in the photo in my initial post) .. assuming CCM volt-second balance of inductor and capacitor

What voltage is needed in the gate then, to turn the MOSFET on?

Vg>Vgs,th

 

It using an N channel MOSFET as a high side switch (as in the circuit you posted), you need a high side driver.

To understand why, ask yourself this: When the MOSFET is on, what voltage is on the source?

What voltage is needed in the gate then, to turn the MOSFET on?

Do you understand the term “high side switch?”

Do you know what a high side driver does?

I'm not English so I might have misunderstood, you support high side while @MrAl supports low side?

 

It's not a project and I don't have to implement anything, it's just a theoretical question to see if I understand some concepts well. So I don't have any other parameters/specifications to provide ... we can assume them.


At the moment I don't know why .. isn't "high side gate driver" fine as I said?

Hello again,

Ok, that's good, but then we have to assume different scenarios because different ones will have more optimal solutions.
This will require a list of scenarios due to the different methods and why you might choose one over the other.

If you have to have a common ground with the input and output, then i think a high side driver is the only way to go because you can't be breaking the ground line just to obtain switch mode operation.
If you don't have to have a common ground, then you can avoid a special purpose high side driver because you probably have enough supply voltage to drive the gate.
In either case, if you need very fast MOSFET switching then you will need a special driver of some sort anyway. The usual complementary pair driver may not be fast enough due to the limited drive current.
A high side driver can be tricky anyway though because it may depend on bootstrapping to get the higher drive voltage. Another idea is to use a small boost circuit to power the driver. A more practical idea would probably be to use a P channel device for the high side driver, if possible.

Back in the 1980's when MOSFETs were just coming into their own, we used a separate power supply to power the drive circuits for ALL the MOSFETs in a MOSFET H bridge. That was the best way. In some converter models we also used fast opto-isolators, although they may limit your speed too. We used specially designed bipolar driver circuits sometimes with asymmetrical drive.

 

Assuming the mosfet is a short circuit when it is closed (on), the source voltage will be the same as that of the input generator (Vs in the photo in my initial post) .. assuming CCM volt-second balance of inductor and capacitor


Vg>Vgs,th

So source is at supply voltage, and gate needs to be higher (I’ll get to that layer.). Where is this higher voltage coming from? Does a low-side driver supply such a voltage? Does a high-side? You have to understand the difference.

Now, you seem to imply that the gate needs to be higher by the threshold voltage. No, that is wrong. The threshold voltage just barely starts turning the MOSFET on. Usually, it will conduct 250uA at the threshold voltage. The voltage needed to turn it on fully on is much higher. Typically it is 10V for a standard MOSFET and 4.5V for a logic level one.

 

Does a high-side?

I'll try to tell you my thoughts:

1) LOW SIDE:
Considering the UCC27528 gate driver (simplified schematic below), the output voltage at the gate is [0 .. Vcc] ... I can choose any Vcc.
So your "higher voltage" is coming from this Vcc.
Am I wrong?


2) HIGH SIDE:
For the High side, I could copy and paste what I just wrote for the LOW SIDE ... i.e. again I set the Vcc as I want:

 

I can't figure out which of them is best to drive an n-type enhancement buck converter.

Schematic shows a P channel mosfet.

 

Schematic shows a P channel mosfet.
View attachment 295320

You're right! I hadn't paid attention to that.