There are two essential elements of brushless motor control.  They are current control, which is accomplished by high frequency pulse-width-modulation (PWM), and commutation, which is a sequence of rotating magnetic fields moving in step with the position of the permanent magnet rotor.  Both of these functions utilize power transistors, of which MOSFETs or IGBTs are most common.

In the vast majority of brushless motor controller designs, these two elements are merged into one device, which is known as a three-phase inverter.  Each of the six legs of the inverter perform both PWM and commutation.  In the case of two-stage power conversion, the two primary functions are separated, with a half-bridge performing the PWM based current control for both torque and regen and a non-PWMing inverter performing commutation.  We refer to the non-PWMing inverter as an electronic commutator or six-step.  The term six-step refers to the six positions of the rotor per 360 electrical degrees and the corresponding three-phase, trapezoidal waveform.

The half-bridge, which consists of a pair of power transistor switches functioning as a buck regulator, provides variable voltage and current and is basically a DC motor control with current regulation.  This is the first stage or front end in the configuration we use.  The second stage is the electronic commutator.  Between the two stages is an external inductor, which is optimized for low core and resistance losses at high frequency.  Inductors of this type use cores made from powdered metals or amorphous materials, which have much lower losses than the steel laminations found inside motors.

There are many reasons why two-stage power conversion is attractive for brushless motor control. Among them is the ability to operate a motor with very low inductance and high current capacity. Also, the use of a single external inductor favors higher PWM frequencies than are typically used in brushless and DC motor drives, while making it possible to minimize PWM in the motor, which is a source of losses, heat and inefficiency.  Further, the performance and efficiency of main bus capacitors are improved at higher frequency, allowing their size and cost to be minimized.  The capacitors associated with the electronic commutator filter the PWM, so its effect on the motor is minimized.

Other advantages of two-stage power conversion are the specialization of power transistor function, which includes the associated gate drive requirements, and the ability to employ simple current sensing and processing.  A simple current sense resistor and control feedback loop, which is relatively immune to EMI (electrical noise), in conjunction with a motor drive scheme that produces less EMI and more fully exploits the capabilities of power transistors, significantly improves reliability and reduces cost.

Due to the use of an additional switch in series with the motor windings and the resistance and cost of the external inductor, two-stage power conversion has often been avoided, in some cases for good reason.  However, though both the extra switch in series and the external inductor add voltage drop and therefore losses, the reduction or elimination of PWM in the motor more than offsets them. Bearing in mind that it is desirable to minimize the size and mass of the motor, both of which have a substantial impact on its thermal impedance and cost, moving losses from the motor to a robust device like an inductor is very attractive.