Many motion control designs now include an MCU or digital signal controller (DSC) to execute motion control algorithms. The cost of these has decreased dramatically in the last decade, bringing their benefits to low-cost applications.
This article shows some techniques used to interface the logic-level I/O of an MCU or DSC to a power electronics driver stage.
There are many trade-offs that can be made in the interface design, in addition to price vs. performance. Many of the interface component selections can be based on the following questions:
* What kind of motor has to be driven?
* What kind of algorithm is used to control the motor?
* What interface requirements can be reduced by the controller peripherals?
* What are the electrical safety requirements?
* Will the design be used for product development?
The gate drive interface
The half-bridge output topology is used for the control of many motor types, including brushed-DC, brushless-DC, AC induction and permanent magnet AC motors. The power stage requires a gate drive interface that, at a minimum, provides the following functions:
* Level-shift the logic-level outputs of the MCU to generate a 10-15 V potential between the gate and source of the transistor;
* Provide sufficient drive current during turn-on and turn-off to overcome Miller capacitance effects.
The high-side output device always presents a problem for the gate drive interface. It is desirable to use N-channel devices for both the high-side and low-side output devices in a power stage. For a given die size and breakdown voltage, a P-channel device will always have a higher on-resistance than the equivalent N-channel device. The use of a P-channel device can simplify the gate drive circuit, but will increase the cost of the design. Die size is money, and a P-channel device will always cost more than the matching N-channel device.
Generating a gate supply voltage for the low-side devices in the power stage is easy, since the low-side devices are referenced to the circuit ground. The gate control voltage must be referenced to the source voltage, which swings from rail to rail for the high-side transistor. Therefore, a gate voltage supply is required for the high-side devices in the power stage that can float on the source voltage.
Today, a selection of inexpensive ICs is available to simplify the job of designing the gate driver circuit. Some of these are relatively simple high-current drivers that do not have the level-shifting circuitry required for the high-side device. Others include the level-shifting circuitry and can directly interface to the logic and power devices. The gate driver selected may depend on the isolation requirements of the design. Opto-couplers are often used to satisfy the level-shifting requirements and allow a simpler gate driver IC to be used in the design.
The power bus for many motion control applications is derived from a full-wave rectifier circuit connected directly to the AC line and filtered. The low side of the rectifier becomes the 0 V reference for the entire application. However, this reference is not at a ground potential; there is an AC voltage on the low side that is fluctuating between approximately 0 V and the peak line voltage! In many low-cost applications, it makes sense to simply float the MCU or DSC on this low-side potential. However, it is wise to add signal isolation for safety reasons if the design will require testing or field service. At a minimum, the motor drive hardware used during the product development cycle should have signal isolation.
The use of isolation circuits also makes sense for 'damage containment' reasons. Even if the feedback signals are not isolated in a particular design, it may make sense to isolate the gate control signals. If not, the power devices could fail and short in a manner such that the DC bus voltage is coupled back through the driver circuits and into the logic level devices.
Gate driver ICs often offer additional features, including under-voltage lockout, dead-time insertion, cross-conduction protection and automatic current shutdown. The additional cost of these features should be considered.
The power for the gate drive circuit can be produced by a number of methods. Ultimately, the high-side driver circuit must generate a voltage that is 10-15 V higher than the DC bus voltage for the output stage. A bootstrapped power supply is one of the cheapest ways because a floating power supply is not required. Referring to Figure 1, the bootstrap circuit charges a capacitor that floats on the source voltage of the high-side output transistor. This circuit only charges the capacitor when the bottom transistor is turned on, pulling the source of the high-side transistor to 0 V. The capacitor must store enough charge to maintain the required gate drive voltage during the time the upper transistor is turned on. This is a limitation with the bootstrap power supply - the top transistor cannot be turned on indefinitely. The voltage on the bootstrap capacitor will decay and the high-side device will turn off.
The side effects of bootstrap supply limitations will depend on the type of motor. For motors that are driven with sinusoidal currents, the supply may limit the range of PWM duty cycles that can be applied to the inverter. The size of the bootstrap circuit components can be changed to increase the available range of duty cycles. However, the commutation requirements of BLDC and SR motors generally do not permit a bootstrap supply to be used.
If the gate drive for the high-side device must be continuous, a floating power supply must be implemented to generate a voltage 10-15 V above the DC bus voltage. One solution is to implement a charge pump circuit that is referenced to the source of the high-side transistor. Another technique is to modulate the gate-drive signal with a high frequency signal such that the high-frequency signal is present whenever the gate-drive signal is active. Figure 2 shows the modulating signal coupled to the gate and source of the transistor via a transformer and rectified on the secondary side to produce the gate-drive voltage. Both of these techniques increase the cost of the design.
Motor feedback signals
A variety of signals are required from the motor and power electronics, depending on the type of motor and the control algorithm. For a given feedback signal, there are numerous acquisition methods. For example, many motor control algorithms need to know the phase currents in the load. The simplest way to measure these is to use Hall effect current transducers. These are inherently isolated from the high-voltage circuitry that drives the motor, operate from the logic supply voltage, and require few components, if any, to interface to the A/D converter on the MCU or DSC. The downside is cost.
Another way to measure the phase currents is to use PWM current sensor ICs that measure the voltage drop across a current-sense resistor in series with the load. These devices are designed to float on the rail-to-rail voltage swing of the power stage output and to operate from a bootstrapped power supply. The output from this type of sensor is a PWM signal with a duty cycle proportional to the current in the sense resistor. This may be connected to the controller by one of two methods. First, the PWM output can be filtered using an RC filter network that converts the signal back to an analog voltage. The downside to this approach is that the control algorithm may not be able to tolerate any ripple or phase errors from the filter network. The filter components also increase the cost of the design. An alternative solution is to connect the PWM output directly to an input capture peripheral on the controller (Figure 3). This peripheral captures the count value of a free-running digital timebase on a rising edge of the input signal, falling edge, or both. The captured values can then be processed in the application software to determine the input signal period, frequency, or duty cycle.
Digital conversion provides a good alternative to using analog isolation amplifiers in a design when input capture pins are available on the controller device. The analog signal is converted to a digital one using V/F or voltage-to-PWM converters. The signals can then be passed across the isolation barrier with digital opto-coupler devices. The combined V/F converter and opto-isolator solution may cost less than the analog isolation solution.
A third way to measure phase currents uses sense resistors in series with the source of each low-side transistor in the power stage (Figure 4). The voltage across the resistor is amplified by a differential amplifier circuit and connected to an A/D input. When this technique is used, the A/D conversion must be synchronised to the PWM signal that controls the transistor. The measurement should be made when the low-side transistor in the output stage is turned on, to get an accurate current signal. A controller device with built-in A/D synchronisation logic is helpful when the shunt current measurement technique is used.
Software development
The development of motion control software with traditional emulation devices has always been tricky. An emulator allows the user to halt the application software at any time, to look at register values, code execution history, and so on. Unfortunately, stopping the software at some arbitrary point is the worst thing that could be done for the motor and power electronics! PWM control values will no longer be updated when the software is halted, resulting in high DC currents in the motor and power stages. To combat this problem, the emulator should put the PWM signals in a state that will not damage the load. For example, the dsPIC30F family PWM peripheral can place all PWM output pins in the inactive state when the emulator is halted. This will turn off all output devices and allow the motor to coast to a stop. During the product development cycle, it may be useful to incorporate extra hardware features for protection of the power stage. These features protect the hardware from software mistakes during product development and would be removed in the production version of the design to reduce cost. Examples of such features include hardware current limiting, bus over-voltage protection and cross-conduction lockout.
Controller selection
The choice of controller will affect the selection of components that connect it to the power electronic stages. A general purpose MCU may fit the algorithm processing requirements, but may not have peripherals that directly address a motion control application. Additional hardware may be required in the interface circuit to protect output devices or provide additional signal conditioning for feedback signals. Devices that are targeted at motion control applications, such as the new Microchip dsPIC30F 16-bit digital signal controller family, have specialised motion control peripherals that significantly reduce the complexity of the power stage interface.
For further information about Microchip Technology's products contact: Avnet Kopp, 011 444 2333; Azona, 012 665 2880; Memec SA, 021 674 4103, or Tempe Technologies, 011 452 0530
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