Seeking Help: BLDC Motor Control Board Development (FOC Technique)

[mxlemming]

Hello everyone,

I’ve been trying to understand this concept for quite some time, but I’m still a bit confused — I’d really appreciate any insights you can share.

How does an op-amp measure negative current when using low-side current sensing?
I’ve done a lot of reading and research, but I’m still not entirely sure how it works in practice.

Specifically, I’m trying to understand:

• In what situations does the low-side MOSFET conduct current [how opamp measure negative current?]?
• When and how does the body diode conduct current back to the phase or to upper MOSFET body diode through body diode?
• How is this reflected as negative current in the sense resistor and measured by the op-amp?

If anyone could help clarify this or point me to a good resource — like an article, application note, or video — that would be really helpful for my understanding. I’d be very grateful!

Thanks in advance!
BldcLover

The current though the motor is pseudo continuous. It changes slowly because of inductance.

The mos are sequentially conducting high side then low side, swapping very fast relative to the current change

So whether the current is flowing in or out of the motor, it will still be conducting through the low side shunt when the low side FET is conducting.

Hence you can measure it. You just bias the opamp so that 0A is mid rail... 1.65V usually.

It's that simple.

 

[BldcLover]

The current though the motor is pseudo continuous. It changes slowly because of inductance.

The mos are sequentially conducting high side then low side, swapping very fast relative to the current change

So whether the current is flowing in or out of the motor, it will still be conducting through the low side shunt when the low side FET is conducting.

Hence you can measure it. You just bias the opamp so that 0A is mid rail... 1.65V usually.

It's that simple.

Hi mxlemming,

Apologies for the delayed response — I’ve been feeling unwell recently, which led to a slight delay in my work.

In the meantime, I’ve been studying more, and based on my understanding, due to MOSFET switching, the motor doesn’t accept an instant change in current. As a result, reverse current may flow from ground to the motor phase — is that correct?
I’ve attached an image to illustrate what I mean; could you please let me know if my understanding is accurate?

I’m particularly confused about how the low-side MOSFET behaves during switching, and how exactly the shunt resistor measures current in this scenario.

Best and regards,
BldcLover

 

[BldcLover]

Hi stancecoke,

I'm planning to use your LishuiFOC firmware with my custom motor controller design.

Could you kindly take a look at my schematic and let me know if there are any corrections or improvements you’d suggest?

I've attached both the schematic and my gear motor for your reference.





Thank you in advance for your time and support!


Best regards,
BldcLover