Designers of motor and power control inverters face the same challenge: isolating control and user interface circuitry from hazardous power line voltages. The primary requirement for isolation is to prevent power line voltages from damaging control circuitry; more importantly, it is to protect users from dangerous voltages. Systems must comply with the safety requirements of relevant international standards, such as IEC 61800 and IEC 62109, which cover motor drives and solar inverters. These standards primarily focus on compliance testing. How does compliance testing grant engineers freedom? Standards provide guidance on safety, but how do they grant engineers the freedom to choose architectures, circuits, and components that conform to the target system specifications and standards? This depends on whether the circuits provide the required system performance in terms of efficiency, bandwidth, and accuracy, while also meeting safety isolation requirements. A challenge in designing innovative systems is that design rules established for existing architectures, circuits, and components may no longer apply. Therefore, engineers need to spend time carefully evaluating the ability of new circuits or components to comply with EMC and safety standards. In some regions, engineers bear greater responsibility; they may be held personally liable if the safety features of a designed system fail and cause injury.
Isolation architecture
Our concern is the need for you to safely control the flow of energy from AC power to the load according to user-provided commands. This issue is illustrated in Figure 1, a high-level motor drive system diagram, for the following three power domains: setpoint, control, and power. The safety requirement is that the user-setpoint circuit must be potential-isolated from hazardous voltages on the power circuit. The architectural decision depends on whether the isolation barrier is placed between the setpoint and control circuits or between the control and power circuits. Introducing an isolation barrier between circuits affects signal integrity and increases cost. Isolation of analog feedback signals is particularly difficult because traditional transformer methods suppress DC signal components and introduce nonlinearity. Digital signal isolation is relatively simple at low speeds, but becomes very difficult and power-intensive at high speeds or when low latency is required. Power isolation in systems with 3-phase inverters is particularly challenging because multiple power domains are connected to the power circuit. The power circuit has four distinct domains that require functional isolation from each other; therefore, the high-side gate drive and winding current signals need to be functionally isolated from the control circuit, even if both may share a power ground.
Figure 1. Isolation architecture in motor control system
Non-isolated control architectures share a common ground connection between the control and power supply circuits. This allows the motor control ADC to acquire all signals from the power supply circuitry. The ADC samples the center-based PWM signal at the midpoint as motor winding current flows into the low-side inverter arm. The driver for the low-side IGBT gates can be simple and non-isolated, but the PWM signal must be isolated from the three high-side IGBT gates via functional isolation or level shifting. The complexity introduced by isolation between the command and control circuitry depends on the end application but typically involves the use of a separate system and communication processor. An architecture where a simple processor manages the front panel interface and sends speed commands over a slow serial interface is acceptable in home appliances or low-end industrial applications. Due to the high bandwidth requirements of the command interface, non-isolated architectures are less common in high-performance drivers for robotics and automation applications.
Isolated control architectures share a common ground connection between the control and command circuits. This allows for very tight coupling between the control and command interfaces and the use of a single processor. Isolation issues then shift to the power inverter signals, presenting a range of different challenges. Gate drive signals require relatively high-speed digital isolation to meet the inverter's timing requirements. Due to the very high voltages involved, magnetically or optically coupled drivers perform well in inverter applications with extremely high isolation requirements. DC bus voltage isolation circuitry has moderate requirements due to its lower dynamic range and bandwidth. Motor current feedback is the biggest challenge in high-performance drivers because it requires high bandwidth and linear isolation. Current transformers (CTs) are a good choice because they provide isolated signals that can be easily measured. CTs are nonlinear at low currents and do not transmit DC levels, but are widely used in low-end inverters. CTs are also used in high-power inverters with non-isolated control architectures because shunt resistor sampling in these applications would result in excessive losses. Open-loop and closed-loop Hall effect current sensors can measure AC signals and are therefore more suitable for high-end drivers, but are affected by offset. Resistive shunts offer high bandwidth, linear signals with low offset, but require matching with high bandwidth, low offset isolation amplifiers. Typically, motor control ADCs can directly sample isolated current signals, but the alternative measurement architecture described in the next section shifts the isolation problem to the digital domain and significantly improves performance.
Inverter feedback using isolated converters
A common method to improve the linearity of an isolation system is to move the ADC to the other side of the isolation barrier and isolate the digital signal. In many cases, this requires combining a series ADC with a digital signal isolator. Due to the specific high-frequency requirements for motor current feedback and the need for fast response to drive protection, a Σ-Δ ADC is chosen. A Σ-Δ ADC is equipped with a linear modulator that converts an analog signal into a one-bit code stream, followed by a digital filter that reconstructs the signal into a high-resolution digital word. The advantage of this approach is that two different digital filters can be used: a slower one for high-fidelity feedback and another low-fidelity fast filter for inverter protection. In Figure 2, a winding shunt is used to measure the motor winding current, and an isolated ADC is used to transmit a 10MHz data stream across the isolation barrier. A Sinc filter submits the high-resolution current data to the motor control algorithm, which calculates the inverter duty cycle required to apply the desired inverter voltage. Another low-resolution filter detects current overload and sends a jump signal to the PWM modulator in the event of a fault. The Sinc filter frequency response curve illustrates how appropriate parameter selection can enable the filter to suppress PWM switching ripple in the current sampling.
Figure 2. Isolated current feedback
Figure 3. Frequency response of the Sinc filter
Power output isolation
A common problem with both control architectures is the need to support multiple isolated power supplies.
If each domain requires multiple bias rails, it becomes even more difficult to implement. The circuit in Figure 4 generates +15V and -7.5V voltages for gate drive and +5V voltage for ADC power, all within one domain, while each domain uses only one transformer winding and two pins. A single transformer core and frame can be used to create dual or triple power supplies for four different power domains.
Figure 4. Isolated power supply circuit domain of gate drive and current feedback converter
