Can I add an additional stage of mosfets to increase current capacity of a speed controller? Whatwou

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#1 21681390 23 Dec 2018 23:30

Shepherd Ginzburg Shepherd Ginzburg

Anonymous
Post #1
21681390 23 Dec 2018 23:30

I build electric bicycles. they all use brushless motors, and all the controllers are fairly similar.

There are various current capacity controllers available, but I want to know if I can use the output of a small controller to drive an additional bank of mosfets, thereby making it a large current controller. Can this be done or do I need to use the signal that drives the original mosfets?

The goal is being able to increase the power of a bike with an add on box to the controller.
If this can be done, What other circuitry, besides the mosfets themselves would be required?
Here is a schematic of a speed controller for reference.

Attachments:

forums-mc33035-cr.jpg Download (55.32 KB)
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#2 21681391 25 Dec 2018 03:35

David Ashton David Ashton

Anonymous

Post #2
21681391 25 Dec 2018 03:35

Shepherd, that MC33035 looks like a nice little IC. Looking at the data sheet, the outputs are push/pull bipolar transistors and they give rise times in the ns for a load of 47pf and 1K. Elsewhere they specify sink and source currents of 25 or 50 mA.So given that this is for driving a bicycle motor (a) you wouldn't need a huge amount of power and (b) you wouldn't want very high motor speeds, I reckon you could just increase the beefiness of your output transistors. There is also a current limit sensor circuit at bottom right. This is a comparator with a threshold voltage of around 100 mV. You are using a 50 milliohm resistor which will switch the comparator on at 2A. You'd need to reduce this value to allow bigger motor currents (0.01 ohms would increase this to 10A).You could also approach ON Semiconductor support for assistance, especially for commercial applications where they will be selling ICs, they should be helpful.If your controllers are sealed and it is impossible or impractical to modify them then yes you should be able to use your controller as a driver. But if possible you should still use a smaller current sense resistor - you could just connect it to ground and TP2 on your diagram, if TP2 is accessible to you. If you are looking at high volumes then it might be worth your while getting some custom boards made.Good luck with it!
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#3 21681392 25 Dec 2018 03:45

Rick Curl Rick Curl

Anonymous

Post #3
21681392 25 Dec 2018 03:45

Hi Shepherd-Thanks for providing a schematic. That makes the answer a lot easier.Yes- you can do this, but probably not as an external box. You'll need to disable the existing output stage and attach wires to a new circuit board containing drivers and bigger MOSFETS.I would start by removing the 1K resistors and 470 ohm resistors going to the existing MOSFET gates. Now add a three dual channel non-inverting MOSFET drivers, like the Microchip MCP14E4. Connect the inputs to the point where you removed the resistors, and connect the outputs to the bigger MOSFETS. Remember that the top three MOSFETS must be P-channel, and the bottom ones must be N-channel. It would be good practice to connect a 10K ohm pull-down resistor between each of the driver inputs and ground. This will help insure that the upper and lower MOSFETS of each pair can never both be on at the same time (that would be BAD).This leaves one remaining issue- the original circuit has a current monitoring feature. To make sure this still works you will need to provide a new lower-value shunt resistor between the source pins of the lower three MOSFETS and ground. It will need to be connected back to the point labeled TP2 on the schematic after removing the original .05 ohm resistor.I know this may sound very intimidating, but if you want to pursue it we will be happy to help walk you through it.-Rick
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#4 21681393 25 Dec 2018 22:19

Shepherd Ginzburg Shepherd Ginzburg

Anonymous

Post #4
21681393 25 Dec 2018 22:19

David,Thank you for providing such a thorough and detailed answer to my question!I really appreciate the effort you put into this, and I think you answered this to the point where I have no more questions.
#5 21681394 25 Dec 2018 22:20

Shepherd Ginzburg Shepherd Ginzburg

Anonymous

Post #5
21681394 25 Dec 2018 22:20

Rick,I really have the same thing to say to you as I told David.Thank you for providing such a thorough and detailed answer to my question!I really appreciate the effort you put into this, and I think you answered this to the point where I have no more questions.
#6 21681395 03 Jan 2019 15:41

Cologne LED Cologne LED

Anonymous

Post #6
21681395 03 Jan 2019 15:41

Now add a three dual channel non-inverting MOSFET drivers, like the Microchip MCP14E4. Connect the inputs to the point where you removed the resistors, and connect the outputs to the bigger MOSFETS.
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#7 21681396 03 Jan 2019 19:10

Rick Curl Rick Curl

Anonymous

Post #7
21681396 03 Jan 2019 19:10

Cologne- Your post was copied in its entirety from my post above. Not good. Don't do that again.
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Topic summary

The discussion addresses the feasibility of increasing the current capacity of a brushless motor speed controller for electric bicycles by adding an additional stage of MOSFETs. It is confirmed that this can be done, but not simply as an external add-on box. Instead, the original output stage must be disabled, and the control signals driving the original MOSFET gates should be used to drive a new circuit board with larger MOSFETs. Key modifications include removing existing gate resistors (1kΩ and 470Ω) and inserting dedicated MOSFET driver ICs, such as the Microchip MCP14E4, which are non-inverting dual-channel drivers. The upper MOSFETs in the new stage must be P-channel, and the lower ones N-channel, with 10kΩ pull-down resistors on driver inputs to prevent shoot-through by ensuring complementary switching. Additionally, the current sensing resistor value should be adjusted (e.g., from 50 mΩ to 10 mΩ) to accommodate higher current limits, as the original comparator-based current limit circuit triggers at about 2A with the original resistor. The MC33035 controller IC is noted for its bipolar push-pull transistor outputs capable of sourcing/sinking 25-50 mA, suitable for moderate power levels typical in bicycle motors. Overall, the approach involves integrating larger MOSFETs driven by proper gate drivers and adjusting current sensing to safely increase power output.
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Can I add an additional stage of mosfets to increase current capacity of a speed controller? Whatwou

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