Apparatus for detecting the rotor position of an electric motor and related method

1. A device comprising:

a first half-bridge having a first supply terminal configured to be coupled to a power supply, a second supply terminal configured to be coupled to a reference node, and an output terminal configured to be coupled to a first terminal of an electric motor having a rotor and a plurality of windings;

a second half-bridge having a first supply terminal configured to be coupled to the power supply, a second supply terminal configured to be coupled to the reference node, and an output terminal configured to be coupled to a second terminal of the electric motor;

a first current sense element coupled between the second supply terminal of the first half-bridge and the reference node;

a second current sense element coupled between the second supply terminal of the second half-bridge and the reference node;

a comparator having a first input configured to receive a value corresponding to a reference current;

a multiplexer having a plurality of inputs comprising a first input coupled to the first current sense element, and a second input coupled to the second current sense element, the multiplexer further comprising an output coupled to a second input of the comparator; and

a controller coupled to an output of the comparator, the controller configured to:

drive the first half-bridge and the second half-bridge using pulse width modulation during charging of a first current that passes through two windings of the plurality of windings via a first current path,

discharge the first current stored in the two windings via a second current path,

determine a time constant associated with two of the plurality of windings by measuring a time between a peak value of the first current and a second value of the first current, and

detect a rotor position of the rotor based on the determined time constant.

2. The device of claim 1 , wherein the first current sense element comprises a Hall sensor.

3. The device of claim 1 , wherein the first current sense element comprises a resistor.

4. The device of claim 1 , wherein the first current sense element comprises a metal-oxide-semiconductor field-effect transistor (MOSFET).

5. The device of claim 1 , wherein the first current sense element has a unitary gain.

6. The device of claim 1 , wherein the first current sense element has a first gain, the second current sense element has a second gain, and the first gain is different from the second gain.

7. The device of claim 1 , wherein the controller is configured cause the multiplexer to select an input from the plurality of inputs by controlling a select input of the multiplexer.

8. The device of claim 1 , wherein the first supply terminal of the first half-bridge is configured to be coupled to the power supply via a first resistance, and to the reference node via a first capacitor, wherein the first resistance and the first capacitor form an RC filter.

9. The device of claim 1 , further comprising a digital-to-analog converter (DAC) coupled between the controller and the first input of the comparator.

10. The device of claim 1 , further comprising the power supply and the electric motor.

11. The device of claim 1 , further comprising:

a third half-bridge having a first supply terminal configured to be coupled to the power supply, a second supply terminal coupled to the reference node, and an output terminal configured to be coupled to a third terminal of the electric motor; and

a third current sense element coupled between the second supply terminal of the third half-bridge and the reference node, wherein the plurality of inputs of the multiplexer further comprises a third input coupled to the third current sense element.

12. The device of claim 11 , wherein the controller is configured to control a select input of the multiplexer to sequentially iterate between the first, second, and third inputs of the multiplexer.

13. The device of claim 1 , wherein the controller is configured to begin charging the first current when a BEMF of the electric motor is zero.

14. The device of claim 1 , wherein the second supply terminal of the first half-bridge is configured to receive a ground reference and the first supply terminal of the first half-bridge is configured to receive a voltage higher than the ground reference.

15. The device of claim 1 , wherein the first half-bridge and the second half-bridge are comprised in a printed circuit board (PCB).

16. The device of claim 1 , wherein the controller is configured to drive the first half-bridge and the second half-bridge using pulse width modulation having a duty cycle lower than 100%.

17. The device of claim 1 , wherein the reference current is half the peak value of the first current.

18. A method for detecting a rotor position of a rotor of an electric motor having a plurality of windings, the method comprising:

charging a current passing through two windings of the plurality of windings via a first current path by driving a first half-bridge coupled to the electric motor and a second half-bridge coupled to the electric motor using pulse width modulation, wherein the first half-bridge is coupled between a supply node and a reference node, and wherein the second half-bridge is coupled between the supply node and the reference node;

discharging the current stored in the two windings via a second current path;

determining a time constant of the two of the plurality of windings by measuring a time between a peak value of the current and a second value of the current using a current sense element coupled between the second half-bridge and the reference node; and

detecting the rotor position of the rotor based on the determined time constant.

19. The method of claim 18 , wherein a voltage of the supply node varies during driving of the electric motor.

20. The method of claim 18 , wherein discharging the current comprises turning on a low-side transistor of the first half-bridge and turning on a low-side transistor of the second half-bridge.

21. The method of claim 18 , wherein charging the current begins when a BEMF of the electric motor is zero.

22. A device comprising:

a first half-bridge having a first supply terminal configured to be coupled to a power supply, a second supply terminal configured to be coupled to a reference node, and an output terminal configured to be coupled to a first terminal of an electric motor having a rotor and a plurality of windings;

a second half-bridge having a first supply terminal configured to be coupled to the power supply, a second supply terminal configured to be coupled to the reference node, and an output terminal configured to be coupled to a second terminal of the electric motor;

a third half-bridge having a first supply terminal configured to be coupled to the power supply, a second supply terminal coupled to the reference node, and an output terminal configured to be coupled to a third terminal of the electric motor;

a first current sense element coupled between the second supply terminal of the first half-bridge and the reference node, the first current sense element comprising a first transistor;

a second current sense element coupled between the second supply terminal of the second half-bridge and the reference node, the second current sense element comprising a second transistor;

a third current sense element coupled between the second supply terminal of the third half-bridge and the reference node, the third current sense element comprising a third transistor;

a comparator having a first input configured to receive a value corresponding to a reference current;

a multiplexer having a first input coupled to the first current sense element, a second input coupled to the second current sense element, a third input coupled to the third current sense element, and an output coupled to a second input of the comparator; and

a controller coupled to an output of the comparator, the controller configured to:

drive the first half-bridge and the second half-bridge using pulse width modulation during charging of a first current that passes through two windings of the plurality of windings via a first current path,

discharge the first current stored in the two windings via a second current path,

determine a time constant associated with two of the plurality of windings by measuring a time between a peak value of the first current and a second value of the first current, and

detect a rotor position of the rotor based on the determined time constant.