Motor applications are on the rise, and the dramatic reduction in the cost of power MOSFET devices over the last decade, particularly MOSFET gate-drive ICs for low voltage (less than 100 volts) motor-drive applications as well as microcontrollers, simplify implementation over discrete designs. Here's the basics you need to know for putting cost-effective, high-performance brushed DC, brushless DC, switched reluctance, and stepper motor designs to work for you.
Required gate-drive functions
As shown in the typical motor drive system (Figure 1), the controller takes the input from various sensors to generate the appropriate gating pulses for the motor.
(Click on Image to Enlarge)
Figure 1: Motor-control block diagram
The gate-drive circuit needs to:
• Generate a level-shifted output from 5 to 15 volts between the gate and source of the output switches.
• Have sufficient current capability during turn-on and turn-off to charge and discharge switch capacitances.
Figure 2 shows the typical gate-drive circuit. Generating a gate voltage for the low-side switch is straightforward, as it is ground referenced. But the high-side switch presents a problem. The gate-control voltage, which goes from rail-to-rail, must be referenced to the MOSFET's source terminal, which in this case is not at ground.
Figure 2: Gate drive
We can use a floating gate supply, a pulse transformer, or a charge pump to overcome this problem. The bootstrap technique is the cheapest and most commonly used one.
Additional desired features
Providing a continuous gate drive: In typical operation, the bootstrap capacitor ultimately discharges to turn the high-side switch off. Sometimes the high-side device must be kept on for an indefinite time, as for instance during stalled operation. This design consideration is of particular concern when using brushless DC and switched reluctance motors. Under such conditions, the charge in the bootstrap capacitor may not be adequate to keep the high-side switch on. A low-current top-up charge pump, which becomes active with the high-side switch, is an inexpensive way to maintain the bootstrap capacitor voltage.
Dead time adjustment: Motor-drive circuits commonly include a high-value gate resistor to drive high-current MOSFETs in order to reduce EMI, noise, and slew rates. As a result, switching times are longer. If one switch in an inverter leg is turned on before the other has completely turned off, a shoot-through occurs. To avoid this, you need to build dead-time logic into the driver circuitry.
Brushless DC motors (BLDCs) are replacing brushed dc motors throughout the application spectrum because they provide better torque-speed characteristics, higher power density, low maintenance (no mechanical contact brushes) and higher efficiency. Three-phase BLDCs have higher power density and produce much smoother torque compared to two-phase BLDCs. A three-phase BLDC requires an inverter (Fig. 3).
Figure 3: Inverter for BLDC motor
Switched reluctance motors (SRMs) need a double forward-converter bridge, as shown in Fig. 4. One phase is energized when both switches in a leg are turned on. One difference between the BLDC and SRM topologies is that converters used for BLDCs need to have dead-time protection, while there's no dead-time requirement for the double forward-converter used for switched reluctance motors.
Figure 4: Double forward inverter
Protection: Designers favor an output-disable feature to turn off all the power switches in case of a fault. Safety is important when there are moving parts and it's no different with motors. Under-voltage and over-temperature shutdown protection are also necessary in some applications.
Low propagation delay: Propagation delay of less than 100 ns is generally preferred.
Choosing the right driver IC
Designers have a wide choice of cost-effective ICs for simplifying the design of a gate drive. But select the chips judiciously to avoid the major pitfalls. One common error is choosing over-rated components. For example, a driver IC rated to work with a 600-volt load is about an order slower than a driver for an 80-volt application. Choosing the former for a low-voltage application increases switching losses, reduces efficiency, and reduces the controllability of the system.
For low-voltage applications, gate-drive ICs such as Intersil's HIP4086 provide compatibility to TTL and CMOS logic inputs, which extends its usability with different processors. Figure 5 shows how to apply it for three-phase operation.
(Click on Image to Enlarge) Figure 5: Applying the HIP4086
A resistor connected between the chip's RDEL and VDD pins controls the dead-time between the top and bottom switches. Capacitor CRFSH controls the duration of the pulse that initializes charge on the bootstrap capacitors when power is first applied to the IC. The high-side inputs are active-low, whereas the low-side inputs are active-high. Thus we can tie them together, which reduces the number of external components in such applications where, for instance, space vector modulation is used.
Using three half-bridge drivers for three-phase applications is another option. This however increases component count and board complexity.
For applications where the motor provides only a few watts, a brushed DC motor is perhaps the most economical and practical option. We can employ discrete circuitry, but using an integrated motor driver IC will reduce costs dramatically. For example, Intersil's HIP4020 is a driver-plus-converter IC that offers a single step solution for low-power brushed DC or stepper motor control. It offers four-quadrant operation, and there are shut-down options for fault and over-temperature conditions. For higher-power brushed DC motors, a full-bridge or two half-bridge driver ICs with external MOSFETs can be used.