Method and apparatus to drive two-phase motors from a three-phase bridge

A motor drive for a two phase motor having a first motor phase winding and a second motor phase winding, comprising:

• a phase current controller having at least one command input and a plurality of switch control signal outputs comprising:
• a phase voltage calculator having at least one input coupled to the at least one command input and a plurality of motor phase voltage outputs;
• a modulation converter comprising a plurality of inputs coupled to the plurality of motor phase voltage outputs of the phase voltage calculator, and a plurality of duty-cycle signal outputs;
• a pulse-width modulator having a plurality of inputs coupled to the plurality of duty-cycle signal outputs of the modulation converter and a plurality of outputs coupled to the switch control signal outputs,
• such that the phase voltage calculator produces motor phase voltages on the motor phase voltage outputs controlled by signals on at least one command input, the modulation converter converts the motor phase voltage outputs to duty-cycle signals on the duty-cycle signal outputs, and the pulse-width modulator produces pulse-width modulated switch drive signals on the plurality of switch control signal outputs having duty cycles controlled by the duty-cycle signals on the plurality of inputs of the pulse-width modulator;
• a first half-bridge output stage, a second half-bridge output stage and a third half-bridge output stage, each half bridge output stage comprising:
• an output for coupling to a motor phase winding of a motor;
• a first switch having an input coupled to a DC voltage source, an output coupled to the output of the half-bridge, and a control input coupled to a switch control signal output of the controller, such that a signal on the control input of the first switch couples the output of the first switch to the DC voltage source;
• and
• a second switch having an input coupled to a DC voltage source return, an output coupled to the output of the half-bridge, and a control input coupled to a switch control signal output of the controller, such that a signal on the control input of the second switch couples the output of the second switch to the DC voltage source return;
• wherein when the output of the first half-bridge is coupled to a first end of the first motor phase winding of the two phase motor, the output of the second half-bridge is coupled to a first end of the second motor phase winding of the two phase motor, and the output of the third half-bridge is coupled to a common point coupled to the second ends of the first motor phase winding and the second motor phase winding, the controller controls the outputs of the controller coupled to the half-bridges such that:
• an average voltage across the first motor phase winding of the motor is controlled by modulating the difference between a duty cycle of the output of the first half-bridge and the output of the third half-bridge, and
• an average voltage across the second motor phase winding of the motor is controlled by modulating a difference between the duty cycle of the output of the second half-bridge and the output of the third half-bridge.