Sound isolation is key to digital motor control
Guest columnist Baoxing Chen, senior staff engineer at Analog Devices, says that designing a motor-control system presents two main challenges.
First, the design requires sending accurate motor current information across an isolation barrier back to the controller. Second, it requires providing isolated gate-drive signals for high-side switches.
Shunt-based current sensing provides a low-cost, highly accurate and noise-immune solution. However, digital isolation is needed to provide communication between high-performance ADCs that interface with the shunt and the motor drive controllers.
Optocouplers pose significant constraints on system performance, cost and reliability. They degrade over time, have limited bandwidth and are difficult to integrate digital interfaces.
Pulse transformers or optocouplers can provide isolated gate-drive signals for the high-side switches. Unfortunately, optocoupler approaches not only lead to long propagation delay and inaccurate gate drive timing, but also need a floating supply.
On the other hand, pulse transformers do not provide dc correctness and need additional discrete components for ac-coupled drives, posing significant constraints on duty-cycle variations. However, gate drivers based on microtransformer isolators, with an integrated high-side supply, impose no constraints on duty-cycle variation.
In motor-drive applications, two main parts of the circuit require isolation: the gate drive for IGBTs of bridge inverters and motor-phase-current sensing. Phase-current sensing provides IGBT protection and linear current feedback information for the controller to maintain closed-loop current control. Series shunt resistors, together with high-precision ADCs at the inverter output, are typically used to sense the phase current.
Isolated power supplies are needed to provide the bias for the current-sensing ADC and gate-drive circuit, and separate supplies are needed for each phase. Microtransformer isolators can simplify this complicated signal and the power isolation needs of ac motor drives.
Figure 1 is an example of an implementation for a low-power motor drive using microtransformers. The half-bridge gate driver with an integrated 300mW high-side, 15V supply provides isolated 15V gate-drive output for the high-side IGBT and non-isolated 15V gate-drive output for the low-side IGBT.
The 15V high-side supply, generated through an integrated dc-dc converter, provides power for the buffer circuit to drive the IGBT, and also can be used with a zener diode to generate a 3V to 5V supply to power the current-sensing ADC. An isolated second-order sigma-delta modulator converts the analogue input to a high-speed single-bit data stream that can interface directly with the controller. It receives a clock signal from the controller and sends the clocked data stream back to the controller.
Integrated ADC alternatives
Without an integrated ADC, multiple optocouplers would be needed; slow optocouplers are usually not suitable for transmitting this high-speed data stream. High-voltage level shifters can be faster than optocouplers, but do not provide galvanic isolation, which is important to prevent latchup with negative transients.
Both high-side gate drivers and the current-sensing ADC have grounds referred to inverter outputs that can be switching very fast. Coupler isolation with high common-mode transient immunity is important to maintain data integrity for high-side switching and current sensing.
The red-dashed lines in figure 1 show the extent of the isolation barrier. The circuit components shown in the blue box can be replicated in bridge inverters for additional phases. The inverter outputs need to be isolated from each other and multiple half-bridge gate drivers will achieve that. Each of the half-bridge gate drivers will generate its own gate-drive signal and high-side supply.
For medium- to high-power motor-drive applications, isolation is also typically required for low-side gate drive, as shown in figure 2. Low-side isolation protects the controller from being damaged by IGBT inductive switching transients. The six gate-drive signals are usually isolated through logic isolators. They provide inputs to a gate-drive module, which provides further level shifting or isolation for the high-side IGBTs.
Logic isolation facilitates communication between controller and dc-link ground, such as passing the dc-link voltage or current-sensing information to the controller. The four-channel isolator isolates four of the six gate-drive signals from the controller. The other four-channel isolator provides reinforced isolation for the other two gate-drive signals.
Two unused isolation channels can be used for serial communication between the controller, and a non-isolated ADC can be used for high-voltage direct-current voltage sensing. The 500-mW isolated power from the ADuM5400 can be used to power any logic circuits referenced to the low-side ground, such as the output side of the ADuM2401 and the ADC used for voltage sensing.