Part 4: Power-Driver Design
Motor Drive Design
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.|
Power Driver Circuits
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 Vgh, a separate supply higher than Vs by the gate-drive voltage, typically 15 V. (Use of p-channel power MOSFETs eliminates the need for Vgh 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 Vgh. A diode in series with D2 would prevent such conduction if Q7 or Vgh 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 Vgh 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 Driver Design Considerations
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.