High Side Driver IC not fully saturating mosfet

Thread Starter

thedrive

Joined Sep 23, 2016
32
Hi Friends,

I'm working with the LTC1155 high side driver IC. This IC has an internal charge-pump to boost your gate voltage well over your supply voltage to fully saturate your N-channel mosfet gate being used as a high side switch. No matter what I try I can't get the ic to provide gate voltage high enough to fully saturate the mosfet. This leads to the FET getting hot. I have included a pics to the circuit I'm using to test my setup and also a screen shot from my scope. I can't figure this out as this is my first time working with high side drivers and Mosfets being placed in the high side. Any help on what I'm missing would be appreciated.

The red line is my input squarewave at 12v 20KHZ into the IC. Yellow line is mosfet gate at 13.2V The internal charge pump in the IC should boost the gate to over 22V from 12v to fully saturate the MOSFET gate

IMG_4586_800x600.jpg
IMG_458_LTC1155_800x600.jpg
 

ronv

Joined Nov 12, 2008
3,770
Hi Friends,

I'm working with the LTC1155 high side driver IC. This IC has an internal charge-pump to boost your gate voltage well over your supply voltage to fully saturate your N-channel mosfet gate being used as a high side switch. No matter what I try I can't get the ic to provide gate voltage high enough to fully saturate the mosfet. This leads to the FET getting hot. I have included a pics to the circuit I'm using to test my setup and also a screen shot from my scope. I can't figure this out as this is my first time working with high side drivers and Mosfets being placed in the high side. Any help on what I'm missing would be appreciated.

The red line is my input squarewave at 12v 20KHZ into the IC. Yellow line is mosfet gate at 13.2V The internal charge pump in the IC should boost the gate to over 22V from 12v to fully saturate the MOSFET gateView attachment 120476
View attachment 120478
I think it is very slow. It's made to switch on and off things like disk drives.
Try running it at 2Khz.
 

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Thread Starter

thedrive

Joined Sep 23, 2016
32
I think it is very slow. It's made to switch on and off things like disk drives.
Try running it at 2Khz.
Wow Thanks! You are correct! Running around 1Khz produces a 24v gate voltage. Interestingly the fet still got rather hot (although not as hot as before) vs running the same setup except switching the low side. I wonder why?
I guess I'm going to have to look at another high side driver IC as I'm switching a rather large motor and the noise is terrible. I need to be up in the 20Khz range. Anyone have any experience with a good IC that will work? Also open to other suggestions. Thanks in advance
 

Thread Starter

thedrive

Joined Sep 23, 2016
32
The IR2103 is a personal favorite- used this chip in many designs with great results.
Thanks for the suggestion. I will take a look at them. What do you all think about switching to P-channel mosfets instead of a N-channel fet and a high side driver? Looks like the new P-fets are close in price and performance to N-fet. Anything I should be aware of?
 

Sensacell

Joined Jun 19, 2012
2,449
Stick with the N-FETS, they outperform the P's - and you still need a gate driver, right?

P-FET's are good when you need 100% duty cycle, (power switch applications) bootstrap high side drivers aren't very good at that.
 

Thread Starter

thedrive

Joined Sep 23, 2016
32
Stick with the N-FETS, they outperform the P's - and you still need a gate driver, right?

P-FET's are good when you need 100% duty cycle, (power switch applications) bootstrap high side drivers aren't very good at that.
Thanks for the input. Yes, I would like to keep a gate driver either way to maintain fast switching time. trying to keep my switching loss down to a minimum. I'm switching 3-4 fairly large motors.
 

ronv

Joined Nov 12, 2008
3,770
Thanks for the input. Yes, I would like to keep a gate driver either way to maintain fast switching time. trying to keep my switching loss down to a minimum. I'm switching 3-4 fairly large motors.
Just be aware that if you need 100% duty cycle (full on) for the motor you need the IC with the charge pump. You can use the bootstrap driver (2103) if you limit the on time to 90% or so.
 
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