Motor drives consist of two, and optionally three, subsystems:

	Motor controller: controls excitation vector phase and magnitude that is applied to the power driver.
	Power driver: a power amplifier that provides power drive to the motor windings.
	Power supply: optionally a part of the motor drive, as such, or part of the larger system.

The simplest and least-efficient approach to power drivers is to use a linear amplifier. For very small motors, a power op-amp may be reasonable. For better efficiency, a switched amplifier is used. Usually, a single supply is available, and bipolar drive from a single supply is achieved by using a 3-phase H-bridge circuit, shown below.

A detailed driver is shown only for one of the three half-bridges, each consisting of a high-side and low-side power switch and driver circuitry (such as Q₅ and Q₆). The low-side MOSFETs (Q₂, Q₄, Q₆) are driven by IC gate drivers. High-side drivers require voltage translation to the upper MOSFETs. Q₈ is driven by the 6-step XU logic (see Part 3) and turns Q₇ on, which pulls the gate of Q₅ up to V_gh, a separate supply higher than V_s by the gate-drive voltage, typically 15 V. (Use of p-channel power MOSFETs eliminates the need for V_gh but at the price of larger, more expensive MOSFETs.)

Zener diode D₂ protects the gate-source terminals from over-voltage. Capacitors C₁ and C₂ speed up switching of Q₇ and Q₈ respectively. If D₂ conducts from anode to cathode (due to induced voltage at X), the collector junction of Q₇ forward biases, allowing a conduction path through R₄ to V_gh. A diode in series with D₂ would prevent such conduction if Q₇ or V_gh is adversely affected by it.

What is wrong with this driver? Its major failing is slow high-side turn-off. There is no active drive to turn off Q₅, only R₇. This resistor cannot be made too small without excessive V_gh and Q₇ current during the on time. MOSFET gate-source (or IGBT gate-emitter) capacitance is significant (typically 1 nF) and without an active turn-off mechanism, an upper switch can still be on when the lower switch is (quickly) turned on, causing conduction through Q₅ and Q₆, called shoot-through. This momentary short across the power rails is usually a fatal fault for a transistor, and can be avoided by extending the dead time between when one switch is turned off and the other turned on. But this approach, of itself, does not provide a high-side turn-off mechanism.

High-side drivers are not trivial to design, as this design example hopefully illustrates. Some design items to be considered are:

	High- and low-side turn-on and turn-off mechanisms.
	What happens during power-on and power-off? Consider supply sequencing and faulty current paths.
	DC versus AC (transformer) coupling and the effects at the gate for duty-ratios near zero and one.
	Effects due to fast changes in duty ratio (especially in transformer-coupled gate drivers).
	The effects of MOSFET body-drain diode conduction on transistor turn-on and turn-off.
	What happens when an active or reactive load (such as a motor) drives an off-state half-bridge?
	Conduction versus switching loss in power switches: MOSFETs versus IGBTs.

This cursory introduction to motors from a circuit-design standpoint is only a sparse beginning to the story of motor-drive design. For instance, it is possible to drive either Y or D -configured motors with a terminal open (as we have done), which is called discontinuous-current mode (DCM) drive. Alternatively, all terminals can always be driven, with two connected to either the + or – power rail and the other to the opposing rail, called continuous-current mode (CCM) drive. The drive waveforms generated by DCM are different than for CCM, and choice of mode can be tailored to the shape of the induced-voltage waveform of the motor.

Switch sequencing can be "phased" to reduce the magnitude of voltage transients across motor terminals, by switching only one power switch at a time instead of two. A sequencing scheme of recent years, known as "space vector modulation" (SVM) continuously interpolates between the six phase-steps of the six-step scheme, offering an alternative (and advantages) to three-phase PWM sine-wave drive.