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I am pretty much a non-expert. I accept that power MOSFETs have a lot of gate capacitance because the data sheet says so. The manufacturers wouldn't put so much capacitance in by adding capacitors just for fun, so it must be the physics of the MOSFET that causes the capacitance. If you are asking about why the physics dictate the capacitance, you are ahead of me.
as for the effect, the gate capacitance demands quite a lot of current, fast, to get the MOSFET to act as a high speed switch. It is good to be a high speed switch because the time between "on" and "off" is when power waste happens in the MOSFET. Charge the capacitor really fast, and the MOSFET changes from from (zero current and zero power) to (some current with some voltage causing some power), to (lots of voltage, no current, no power), quickly.
This is the viewpoint of me and my mule. If it helps, fine. If it doesn't, somebody else will be along and try another explanation in a little while.
Thank you for your fast reply and links, I have read through the an18.pdf and found it quite good but am still confused in regards to actually why large gate capacitance is required. From reading through this is how I interpret it;
I guess to start firstly is that the gate capacitance / capacitors are utilized in tripping the switch and amplify the input power. So basically once the capacitors have fully charged / reached there charge point it trips the switch. Meaning basically larger capacitors are used to handle the large power input / output.
and in regards to switching times, (the effect) basically very low current is required to charge the capacitors meaning they can chanrge very quickly if required to allow a much faster acting switch?
basically I am still confused and making no sense hahaha I'm sorry like i said I am just not understanding / grasping this toipc
sorry your posts didn't come up before i was posting that lol, basically yeah I need the understanding of why large capacitance is required / why it is used. and inregards to the other part of your post, wouldn't a power mosfet require maximum speed / instantaneous as it is dealing with a very important task which is why they are utilized in this application.
I hear I'm GMT-5...and I heard G is in England, but I don't know how many hours wide England is. I do know my friend from Wales is about the strangest person I know. Kinky blonde hair and about as stubborn as my mule.
I understand Miller effect, but I've never actually used a MOSFET, so I hadn't looked in to whether the gate has lots of capacitance or the Miller effect has a lot of (effective) capacitance. I just figured I'd use a gate driver if I ever needed one.
I was thinking a really primitive view of MOSFETs might simplify things, and part of that was in saying it is good to be a HIGH SPEED switch. I guess that didn't come across.
I puzzle at your phrasing about a capacitor being REQUIRED. I see the gate capaciance as a necessary evil inherent in the architecture of the MOSFET, not like the designers make an effort to put in lots of capacitance.
I do understand that HIGH SPEED switch is good as why it is utilized in power applications but the way that the problem is addressed to me is "Why does the MOSFET have large capacitance and why do they increase in different applications (ie, off top of my head I assumed larger capacitors for higher power applications).
Capacitance is something you don’t want because it take current, lots of it, to switch it fast... it’s not done on purpose, it’s a byproduct of the MOSFET’s physical construction. If you look at datasheets of low Rds on MOSFET's, you'll find that the lower the on resistance the higher the gate capacitance. To get lower on resistance you need a bigger die and a bigger die means a larger surface area and a larger surface area in turn means a higher capacitance. In some applications you will find that using a MOSFET with higher on resistance is actually more efficient than using one with a lower on resistance because your switching losses due to the capacitance are higher than the I²R losses.
Power MOSFETS contain a relationship between the gate capacitance and resistance. For a large gate capacitance a small resistance is required and vice versa. Having a large capacitance allows a faster trip time?
I read somewhwere that a Mosfet is made of many tiny Mosfet "cells" in parallel. The more cells there are then the turned on resistance is lower and the gate capacitance is higher.
The gate capacitance is determined by the area of the gate. The gate looks like a parallel plate capacitor above the transistor conduction channel. It's what attracts the carriers to allow conduction when you apply a gate voltage. The lower the "on" resistance and the higher the rated transistor current, the larger the required gate area and thus gate capacitance. The designers would like to minimize the gate capacitance, but it's intrinsic to the fundamental design of a MOSFET.
A larger gate capacitance does not improve the transistor response time, in fact it just makes it more difficult to rapidly switch. It's the required rapid charging and discharging of this gate capacitance that makes driving large MOSFETS at high switching speeds a problem. It often requires a high peak current push-pull driver.
So with everythign considered now why is the MOSFETS chosen over the regular bipolar transistors? Or is there actually a situationw here you would use one over the mosfet for a power switch?
Take the case of a "low-side-switch" and say you had a BJT with a Vce sat of 1.3V and you wanted to pass 10A... compare that to a MOSFET with a Rds on of .005 ohms.... pretty much a no-contest in terms of power dissipation.
...is there actually a situation here you would use one over the mosfet for a power switch?
Judging by everythign I have read and seen the only reason you would choose a bipolar is if you wanted the switch voltage driven and the speed? otherwise it is more of a personal preferance?
It's really more application driven, but nothing prevents you from using either... however, there is usually a price to be paid for all the choices that are made in a design.
You could say that engineering is an art where choices are made in balancing trade-off’s!!
the gate capacitance is just a fact of life the same as a conductor has inductance and capacitance although we would prefer it not to have, mosfets are great as switches and have very low on resistances that go down to less than a milli-ohm in some cases so they pass current with less losses than a bipolar transistor, the drain to source path also acts like a resistor as oposed to a diode to sinals can pass both ways, mosfets are I think more flexible on the whole and more eaily used as switches
I have no idea what the question is because it's been edited out for some crazy reason but I can tell you right now that it has a large gate capacitance so that it can be mutually coupled with the inductance of the circuit, therefore you will be able to "tune" the circuit using the correct mosfet capacitance and as larger power applications have larger inductances (due to transformers and coils in general) you need a larger gate capacitance to match, this is why the larger power mosfet have such incredibly large gate capacitances compared to control mosfets.
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