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My team and I are are in need of designing our own BLDC motor controller and need some help with the circuit design. We want to stick with an FOC setup and avoid a sensor solution as those can be far more complex; at least with our experience. The Allegro MicroSystems AMT49406 IC has a built-in FOC gate driver, however the supply range is 50v max. Our device runs from 48v-60v so we will use 100v rated MOSFETs (IRFI4110GPbF) to drive the BLDC. However, we are a bit confused as to whether or not we can use this IC with an isolated battery input and use a buck to supply the IC separately. Below is an image of our current design and we are wondering if this is a correct setup to isolate the MOSFETs using the battery input and use a buck to drive the IC/PWM signal.

Link to AMT49406 spec sheet and typical application diagram: https://www.allegromicro.com/%20/media/files/datasheets/amt49400-datasheet.pdf

Link to MOSFET https://www.mouser.com/datasheet/2/196/Infineon_IRFI4110G_DS_v01_02_EN-1227339.pdf

Our motor: 48v-60v BLDC at 23a continuous and 50a peak (1-2s)

Questions:

  1. Are we correct in using the power MOSFETs (IRFI4110GPbF) rated at 100v 51a continuous drain current by using the supply from the battery?
  2. The Lss (low-side source) pin is connected to the same ground as the MOSFET ground (battery ground). Is this correct or will it cause critical failure?
  3. We have 1k resistors for the MOSFET gate pins, do we need these?
  4. The IC has a built-in current sensor for proper FOC application. We used the Lss/MOSFET ground to feed the current, I assume this should be OK since the ground all goes to the same place?
  5. We have 1 electrolytic capacitor between the Vin from the battery and ground, is a 100uF/100V enough? Do we need 2? I am unsure how to size these.
  6. Are the decoupling capacitors sized properly for the DC buck?
  7. The IC spec sheet mentions a .1uf capacitor between Vbb and Vcp, and 1 between CP1 and CP2 which are all charge pumps. My understanding is this is some sort of voltage buck but I am unsure why we need to add capacitors here, is there a reason to enable these charge pumps?
  8. The IC has over-current protection where each power MOSFET are monitored and if Vds is higher than the threshold when that MOSFET is enabled, it will stop operation. This part is where we have some confusion, does it mean if the MOSFET is above 50v from our battery, it will trigger a stop? Or is this true if both our MOSFETs and the IC supply (Vbb) are connected to the battery, it will stop if above 50v? Basically the IC will be operating from 6v-12v from buck and trigger each MOSFET with its own supply so we assume there is isolation here.

In all, we are looking for a simple BLDC controller to power our motor which is a bit hefty. We can supply a PWM using a generator or MCU. If there are better circuits or IC available we can use, we are open to that as well. I have found some like the DRV8311 from TI, however that has amperage limitations. We also want to avoid making our own current sensing, or back-emf detection. This is where this IC is nice to us where it can handle the location of the stator internally and we basically supply a PWN signal.

Thank you so much! enter image description here

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  • \$\begingroup\$ Few comments, I have no experience with this chip, but I doubt you can use it at the voltage you want. 1) 1kohm gate resistors = failed board, 10-20 ohm please. 2) your sense resistor is in completely the wrong place, how did you conclude you should put it there? Or have you just forgotten to tie your DCDC to ground? In any case, tidy up your schematic... C5 is definitely in the wrong place, it needs to be tied to the high side VBB since it supplies the gates, and in any case, for these FETs, I think it is somewhat inadequate in size... They have 10nF Ciss, and you're feeding them from 100nF \$\endgroup\$ Dec 6, 2022 at 10:18
  • \$\begingroup\$ Bear in mind, these things are targeted at large fans, you don't state your application, but if you are hoping to do traction applications with them, you will probably be disappointed. Also, why use ancient MOSFETs? The datasheet date is 2017, but 4110 MOS are a 20 year old product re released with a different number. Modern ones switch much cleaner. \$\endgroup\$ Dec 6, 2022 at 10:25

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Unfortunately this IC will not work in the configuration you’ve specified in your schematic. I’ve worked with a number of these types of chips and while allegro doesn’t detail the inner structure we can make some guesses.

allegro data sheet snip

The capacitor across cp1 and cp2 is used by a charge pump inside the IC to charge the capacitor across vcp and vbb. Note here that vbb will be the low side of this cap - vcp will be up to 8V above vbb according to the absolute maximums. Vcp is the supply to the gate drivers for the HIGH SIDE nchannel mosfets - being high side n channel fets they will require a gate voltage in excess of vbb to switch efficiently.

So now the requirements are that vcp can’t be more than 8V above vbb and vcp must be higher than the high side mosfet drains - so much higher to drive them into saturation. This is not possible when vbb is 12V and the mosfets are on your battery bus, and I don’t see a way to rig this so that it works without exceeding the limits.

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  • \$\begingroup\$ Thank you for the detail. I am very new to this type of IC and we really want a controller that is simple. It is to drive a 6" fan at about 10K RPM at 23amps. Our battery is a 50v lithium but it can reach up to 59v when fully charged...why I am hesitant about the maximum supply voltage. I assume this IC is then designed for more of a 36v application to account for charge voltages? Basically there is no way to use this unless if I lower our battery voltage? The DC buck is flexible for us, it can be any voltage I just put 6-12 based on my understanding of the IC. \$\endgroup\$
    – vm1988
    Dec 6, 2022 at 18:38
  • \$\begingroup\$ @vm1988 the only way that I see you can leverage this IC is if you regulate the voltage to the bridge as well as the IC. If you find yourself a 50V out, 23A buck converter then it would be fine but that’s a nontrivial amount of buck converter. \$\endgroup\$
    – Bryan
    Dec 6, 2022 at 21:22
  • \$\begingroup\$ Ok thank you appreciate the honesty. I will look into the DRV8300 as that is a 100v supply. I am just a bit nervous about the current sense portion to detect the position of the rotor. I can use an RP2040 and maybe Python for all the logic. Seems TI has some good guides for this chip so I will have a go at it. If you have any recommendations or guidance I would appreciate it. Otherwise, I can maybe do another pass with the DRV8300 and post here for comments. \$\endgroup\$
    – vm1988
    Dec 7, 2022 at 2:53
  • \$\begingroup\$ There is also the Qorvo PAC5556 which has integrated sensorless application up to 600vdc. So I will look into this as well. \$\endgroup\$
    – vm1988
    Dec 7, 2022 at 3:05

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