Certainly the torque load is much greater with the prop in the water. That is how power to move the boat is delivered. 5700RPM is not a reasonable speed for a prop. The secondary benefit of providing a serous reduction ratio is the ability to get the motor farther away from the water.

I understand that the ratio between RPM and torque probably isnt optimized but I dont see why this would be the source of failure for the electronic controller, maybe I am missing something ?

 

If the load on the motor is such that it draws more current than the transistors can handle, they will fail. And ifthose devices are not driven fully into saturation, they are not able to handle as much current.

 

If the load on the motor is such that it draws more current than the transistors can handle, they will fail. And ifthose devices are not driven fully into saturation, they are not able to handle as much current.

Yes i understand but the BMS will only allow 80 amps to flow at max so the real problem is why can't my 4 transistors hold this 80amps while they are rated for 35amps at 100°C ?

 

The watts turning into heat are the product of current and voltage across the device,
See also "Safe Operating Area." in te data sheet.

 

Yes i understand but the BMS will only allow 80 amps to flow at max so the real problem is why can't my 4 transistors hold this 80amps while they are rated for 35amps at 100°C ?

A datasheet for, or at least a link to, the BMS would be very helpful.

 

A datasheet for, or at least a link to, the BMS would be very helpful.

I responded to Alec on the previous page with the link to the BMS and photos of the specifications !

 

The watts turning into heat are the product of current and voltage across the device,
See also "Safe Operating Area." in te data sheet.

Thanks for the response, yes i saw the SOA section of the mosfet but Im not sure I really understood what it really meant, would you be kind to explain it to me ?

 

Hi,

I had similar issue some time ago in a lower power application using just one MOSFET (go-kart speed controller handling approx 20A)
This was caused by the MOSFET spending to much time in the linear region which generates a lot more heat then when fully turned on/off.
I would look at using a dedicated MOSFET gate driver to improve your switching time, this solved the issue for me.

Thanks

Dale

 

The BMS is designed for continuous operation at 80A, but it‘s overcurrent protection isn’t specified to operate until 120A.

ignore the word “voltage”, it’s just an error
​

A BMS doesn’t limit current, it just dsconnects the battery if it detects excessive discharge. In this case it should still be within the capailities of the IRFZ44N—if the juntion tempertature is within parameters. As it heats, it becomes less able to handle current. If the motor starts in air, the stall current might be achieved for a very short time. If it is started in wate—which is effectively incompressible—the stall current of ≥225A might persist for a considerable time.

The BMS specifications say that short-circuit protection has a delay of as much as 500μs. This is more than enough time to overwhelm the IRF44ZNs. So, I still think the stalled motor is a possility.

How long does it take to blow up the MOSFETs?

 

And so you think this is too high of a current for the gates ?

On the contrary, it seems rather low for your 1kHz PWM frequency. A dedicated gate driver IC typically can source/sink at least 1 Amp to ensure the gate capacitance charges quickly. If a FET is not driven quickly on and off it generates considerable heat.

 

Hi,

I had similar issue some time ago in a lower power application using just one MOSFET (go-kart speed controller handling approx 20A)
This was caused by the MOSFET spending to much time in the linear region which generates a lot more heat then when fully turned on/off.
I would look at using a dedicated MOSFET gate driver to improve your switching time, this solved the issue for me.

Thanks

Dale

Hi,

I feel like your response is important but I'm not sure I quite get it. From what I understand if Vgs> Vgatethreshhold there is a region where for a certain Vdss my mosfet is in the linear region (saturation) and the current drawn is constant even if the Vdss changes a bit (see photo attached). But in my case when i am full throttle I want the mosfet to be fully on because having it fully on means more current flowing through right ?
Let's imagine I have a dedicated mosfet driver IC, what will it do differently then my 555 timer set up ? I will still have a PWM output to the gate of the mosfet which will fully turn the mosfet on at full throttle no ?

 

The BMS is designed for continuous operation at 80A, but it‘s overcurrent protection isn’t specified to operate until 120A.

View attachment 304300
ignore the word “voltage”, it’s just an error
​

A BMS doesn’t limit current, it just dsconnects the battery if it detects excessive discharge. In this case it should still be within the capailities of the IRFZ44N—if the juntion tempertature is within parameters. As it heats, it becomes less able to handle current. If the motor starts in air, the stall current might be achieved for a very short time. If it is started in wate—which is effectively incompressible—the stall current of ≥225A might persist for a considerable time.

The BMS specifications say that short-circuit protection has a delay of as much as 500μs. This is more than enough time to overwhelm the IRF44ZNs. So, I still think the stalled motor is a possility.

How long does it take to blow up the MOSFETs?

wow ok very good point I missed right there, the current can reach 120amps, but I dont think the stall current is the problem because the mosfets exploded about 2 minutes after the start, I slowly started turning the throttle so the boat was moving forward and at some point I arrived at full throttle and after like 5-10seconds the mosfets exploded which is probably due to overheating ?

 

On the contrary, it seems rather low for your 1kHz PWM frequency. A dedicated gate driver IC typically can source/sink at least 1 Amp to ensure the gate capacitance charges quickly. If a FET is not driven quickly on and off it generates considerable heat.

Ok from what I understand with a 14mA gate current which is considered low for you for a low frequency PWM it is going to take time to charge the gate capacitance and right when it is charged the PWM is off so its actually getting discharged and basically the mosfet isnt really properly being fully closed or open (thats what I understood from your comment)
So either I lower the gate resistance to obtain a higher current or I get a proper mosfet driver ? Do you have any recommandations for the driver for this application ?

 

Let's imagine I have a dedicated mosfet driver IC, what will it do differently then my 555 timer set up ?

It will source and sink a higher gate current, to ensure a FET drain-source path isn't only 'half-way on' for longer than necessary.

 

It will source and sink a higher gate current, to ensure a FET drain-source path isn't only 'half-way on' for longer than necessary.

Ok got it for the source sink current but for the only "half way on" in my head it means when i have a PWM with 50% duty cycle right ? And how do i know the maximum time it is allowed to be "half way on" or fully on ?

 

I have found this driver IC which seems suitable for high current inductive loads: https://www.mouser.fr/datasheet/2/609/11602fb-3124037.pdf

Can someone give me a feedback on it ? Thanks

 

wow ok very good point I missed right there, the current can reach 120amps, but I dont think the stall current is the problem because the mosfets exploded about 2 minutes after the start, I slowly started turning the throttle so the boat was moving forward and at some point I arrived at full throttle and after like 5-10seconds the mosfets exploded which is probably due to overheating ?

Yes, the two minutes suggests it is not stall current related but it does seem to be load related. The gate drive issues others have mentioned seems a very good candidate. Also, make sure your heatsinking is adequate. You might even want some active cooling (a fan, or even water cooling which is normal for outboards which makes parts avaialble).

 

Yes, the two minutes suggests it is not stall current related but it does seem to be load related. The gate drive issues others have mentioned seems a very good candidate. Also, make sure your heatsinking is adequate. You might even want some active cooling (a fan, or even water cooling which is normal for outboards which makes parts avaialble).

Yes it seems like it thanks a lot, for the gate driver IC im not competent enough to select the right one but I'll look into it, if you have any suggestions I'll be happy to hear them. And yes I am going to add active cooling as well as proper N channel mosfets suited for high current demands and brushed DC motor applications !

 

it means when i have a PWM with 50% duty cycle right ?

No. It's nothing to do with duty cycle. It means in the microseconds it takes for the FET to switch from fully off (Rds say 1 megohm) to fully on (Rds say 20 milliohms), or vice versa, at each PWM pulse edge. In the 'half-way' state Rds might be, e.g, an Ohm or so. Passing lots of Amps through 1 Ohm generates lots of Watts of heat, albeit for a brief period. The shorter that period the better.

 

I went back and checked and the motor specification sheet ONLY GIVES NO LOAD CURRENT! So the only other hints are the "K" factors shown farther down on the sheet, which can be used to eventually give a hint at full load current. That is less than stall current, and happens to be a useful piece of information that is not provided.
Of course we do not know what the load is with the prop installed and the motor in the water, and so we have no clue as to how much current the motor is drawing.
The relationship between the prop size, torque, and RPM is an area where I have no experience. But given that the motor HP rating is provided, although without any current or RPM conditions, But that HP information should help you select an optimum prop size. Based on the few trolling motors I have seen, the prop will be rather small.