Hi guys this is the second conversation I am starting on this forum,

I previously asked questions about why my mosfets kept blowing up when driving my outboard motor. Now I think I understood better I am now wondering how to properly choose your mosfet driver IC.

Here is the motor I want to drive : E30-150-24 https://www.ampflow.com/motors/highPerformance/threeInch/#economy
Here are the mosfets I'll be using : P75NF75 https://pdf1.alldatasheet.com/datasheet-pdf/download/999561/THINKISEMI/P75NF75.html

I want to drive my motor with a maximum current of around 80A and I am planning on paralleling 4 mosfets on the same heat sink to reduce conduction losses even though the the switching losses might increase a bit.

I am using a homemade 7S13P battery pack so Vbat [25,2-29,4V] and a 80A BMS, for my motor controller I planned on using a Buck converter to convert my battery voltage to a lower stable voltage lets say 15V, this 15V will be used to power my 555 timer for the PWM output and the mosfet driver ic or directly from Vbat ( I don't know which is best).

For my PWM signal input I am using a 555 timer with a frequency of around 1kHz (maybe it is too low ? we will see...)

For the driver I was not decided between these 2:

https://www.mouser.fr/datasheet/2/268/MCHPS05700_1-2520587.pdf

https://www.infineon.com/dgdl/Infin...5ddCNnXxUBZj6dHT8N0aAvBTEALw_wcB&gclsrc=aw.ds

I know the first one can source up to 12A so I think I will be ok to drive my 4 mosfets fast enough with a low gate resistance (around 5-10 ohms)

The second one can source/sink (+4A/-8A) which I don't think is high enough source current in order to drive my mosfet fast enough?

The problem I had before was that I used the 555timer to directly drive my 4 IRFZ44 mosfets which all had 100ohm gate resistant (I know really badly scaled but this is my first electronics project I'm a chemical engineer ahahso I'm still learning a lot thanks to youtube and you guys ) my mosfets kept blowing up and i didn't understand why. Now I know that I need to switch them on/off faster in order to lower the Resistance when startup and falldown so I have less switching losses and less heating up.

Now my real question is, what is an optimal turn on time + rise time (and same for the fall time ) because I have no order of magnitude.
For reference these are some of the mosfets specs :



So if let's say I have 1A driver current into each mosfet with a Qg=160nC (max) that means my turn on delay time+rise time= Qg/1A= 160ns. But in order to have 1A flowing out of my driver and for Vgs=10V(arbitrary and potentially the maximum) that means that Rg=10V/1A=10ohm right ?

But also how do I know what Vgs I need in order to have 20Amps flowing through one mosfet ?

If you need more information please tell me.
And thank you all in advance
Cheers
Timothee

 

Hi Timothee,

I remember the post about your outboard motor from a few months ago.

I'm no expert, unlike a lot of the people on here but I do have quite a few years of hobby electronics experience. So until the experts arrive, here are my thoughts!...

Based on your battery voltage, I'm assuming you will be using the 24v motor, with the most powerful one being E30-400-24 having a peak power of 2.1HP. This translates to approx 1500w, or 62.5A @24v.

Your MOSFETs are rated for 75A continuous, so I would suggest using two of them in parallel if well heatsynced, I'm not sure 4 are necessary considering your motor is driving a propeller in water where a stall event is unlikely.

1000Hz is a low frequency for pwm, which is good from an overheating perspective. Lower frequencies will tend to create more motor noise especially in the lower duty cycle range, but this is not really an issue unless you find the noise offensive.

I think either gate driver will be ok for your needs. They can both sink/source a lot more current than 200mA the 555 can manage.

Edit: I just had a look at the datasheets. The first gate driver is more suitable for your application.

Thanks

Dale

 

But in order to have 1A flowing out of my driver and for Vgs=10V(arbitrary and potentially the maximum) that means that Rg=10V/1A=10ohm right ?

What resistance are you referring to, intrinsic or added?

what is an optimal turn on time + rise time (and same for the fall time )

There is no optimum.
You just make it fast enough so the switching losses are acceptable.
A rough estimate for that power is to multiply 1/2 the current times the load voltage times the rise/fall time times the switching frequency.

what Vgs I need in order to have 20Amps flowing through one mosfet ?

For switching purposes you always use the Vgs in the MOSFET data sheet for the Rds(on) value (for the one you referenced that's 10V) to give the minimum ON voltage drop and thus power loss.

 

What you need for gate drive is strongly dependent on the switching frequency. If you are switching at 1Hz, a few milliamps is sufficient.

So what frequency are you switching at?

 

Hi Timothee,

I remember the post about your outboard motor from a few months ago.

I'm no expert, unlike a lot of the people on here but I do have quite a few years of hobby electronics experience. So until the experts arrive, here are my thoughts!...

Based on your battery voltage, I'm assuming you will be using the 24v motor, with the most powerful one being E30-400-24 having a peak power of 2.1HP. This translates to approx 1500w, or 62.5A @24v.

Your MOSFETs are rated for 75A continuous, so I would suggest using two of them in parallel if well heatsynced, I'm not sure 4 are necessary considering your motor is driving a propeller in water where a stall event is unlikely.

1000Hz is a low frequency for pwm, which is good from an overheating perspective. Lower frequencies will tend to create more motor noise especially in the lower duty cycle range, but this is not really an issue unless you find the noise offensive.

I think either gate driver will be ok for your needs. They can both sink/source a lot more current than 200mA the 555 can manage.

Edit: I just had a look at the datasheets. The first gate driver is more suitable for your application.

Thanks

Dale

Hi dale,

Thank you for taking the time to answer me, I was very busy at work and now i'm fully focused
No I'm actually using the E30-150-24 peak power 1HP. Alright but by going for 4 mosfets I will draw less current for each one and thus reduce conduction losses right ? I also plan on water cooling everything if needed. But i want to test out this first

Right. yes i don't mind the noise at all i don't think it will be problematic too !
Yes I think the first gate driver is good enough but maybe overkill ? This is what im trying to understand better in order to not overkill everything in futur projects !
Thanks again. Helps a lot.
Have a good day
Tim

 

What resistance are you referring to, intrinsic or added?
There is no optimum.
You just make it fast enough so the switching losses are acceptable.
A rough estimate for that power is to multiply 1/2 the current times the load voltage times the rise/fall time times the switching frequency.
For switching purposes you always use the Vgs in the MOSFET data sheet for the Rds(on) value (for the one you referenced that's 10V) to give the minimum ON voltage drop and thus power loss.

Hello Crutschwo,

I was referring to the added gate resistance that I am planning on using.
Alright great i'll use this formula to have a rough estimate of my switching losses.
Oh right so in the datasheets the minimal Rds on is given at that Vgs ? So if my Vgs in lower or above i should expect higher switching losses due to higher turnon on/off time right ?
Thanks again ! see you around.
Tim

 

What you need for gate drive is strongly dependent on the switching frequency. If you are switching at 1Hz, a few milliamps is sufficient.

So what frequency are you switching at?

Hi BobTPH, I remember you

I am switching at around 1kHz, 1Hz would be way too low for my motor I think. But yes i understand that switching losses are proportional to the switching frequency. But still if I was switching at 1 Hz it would be better to have high gate current to reduce turn on/off time right ?
See you around.
Tim

 

so in the datasheets the minimal Rds on is given at that Vgs ? So if my Vgs in lower or above i should expect higher switching losses due to higher turnon on/off time right ?

Only lower, not above.