System Architecture Selection for Motor and Power Control Inverter Design

Motor and power control inverter designers all face the same problem of isolating the control and user interface circuits from dangerous power line voltages. The primary requirement for isolation is to prevent power line voltages from damaging the control circuits and, more importantly, to protect the user from dangerous voltages. The system must meet the safety requirements set forth in the appropriate international standards, such as IEC 61800 and IEC 62109, which cover motor drives and solar inverters, and which focus primarily on compliance testing.

How does compliance testing for standards give engineers the freedom to choose appropriate architectures, circuits, and components that meet target system specifications and standards? These are determined by the circuit's ability to provide the required system performance in terms of efficiency, bandwidth, and accuracy while meeting safety isolation requirements. The challenge in designing innovative systems is that the design rules established for existing architectures, circuits, and components may no longer apply. Therefore, engineers need to take the time to carefully evaluate the ability of new circuits or components to meet EMC and safety standards. This article explores the impact of system architecture choices on power and control circuit design and system performance. It will also explain how the performance improvements of the latest available isolation components can help alternative architectures improve system performance without compromising safety.

Isolation Architecture

The concern is that customers need to safely control the flow of energy from the AC source to the load based on user-supplied commands. This problem is illustrated in the high-level motor drive system diagram shown in Figure 1 for the three power domains: reference, control, and power. The safety requirement is that the user reference circuit must be galvanically isolated from the hazardous voltages on the power circuit. The architectural decision depends on whether the isolation barrier is placed between the reference 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 approaches suppress DC signal components and introduce nonlinearities. Digital signal isolation is fairly simple at low speeds, but very difficult and consumes a lot of power at high speeds or when low latency is required. Power isolation in systems with 3-phase inverters is particularly difficult because there are multiple power domains connected to the power circuit. The power circuit has four different domains that need to be functionally isolated from each other; so the high-side gate drive and winding current signals need to be functionally isolated from the control circuit, even though both may share the power ground.

Figure 1. Isolation architecture in a motor control system.

Non-isolated control architectures have a common ground connection between the control and power circuits. This allows the motor control ADC to access all signals in the power circuit. The ADC samples the motor winding current at the midpoint of the center-based PWM signal as it flows into the low-side inverter arm. The drivers for the low-side IGBT gates can be simple non-isolated, but the PWM signals must be isolated from the three high-side IGBT gates by either functional isolation or level-shifting. The complexity caused by isolation between the command and control circuits depends on the end application, but typically involves the use of separate system and communication processors. An architecture where a simple processor can manage the front panel interface and send speed commands over a slow serial interface is acceptable in home appliances or low-end industrial applications. Non-isolated architectures are less common in high-performance drives for robotics and automation applications due to the high bandwidth requirements of the command interface.

Isolated control architectures have a common ground connection between the control and command circuits. This allows very tight coupling between the control and command interfaces and the use of a single processor. The isolation issue moves to the power inverter signals, which present a different set of challenges. The gate drive signals require relatively high-speed digital isolation to meet the inverter timing requirements. Magnetic or optically coupled drives work well in inverter applications where isolation requirements are very high due to the very high voltages present. The requirements for DC bus voltage isolation circuits are more moderate due to the low dynamic range and bandwidth required. Motor current feedback is the biggest challenge in high-performance drives 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 where shunt resistor sampling would be too lossy. Open-loop and closed-loop Hall effect current sensors measure AC signals and are therefore more suitable for high-end drives, but suffer from offsets. Resistive shunts provide high bandwidth, linear signals with low offset, but need to be matched with high bandwidth, low offset isolation amplifiers. Typically, the motor control ADC samples the isolated current signal directly, but the alternative measurement architecture described in the next section moves the isolation problem to the digital domain and can significantly improve performance.

Inverter feedback using isolated converters

A common approach to improving the linearity of an isolated system is to move the ADC to the other side of the isolation barrier and isolate the digital signal. In many cases, this requires the use of a serial ADC in conjunction with a digital signal isolator. Due to the special high frequency requirements for motor current feedback and the need for fast response for drive protection, a Σ-Δ ADC is the choice. The Σ-Δ ADC has a linear modulator that converts the analog signal into a single-bit stream, followed by a digital filter that reconstructs the signal into a high-resolution digital word. The benefit of this approach is that two different digital filters can be used: a slower one for high-fidelity feedback and a lower-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 10 MHz data stream across the isolation barrier. The Sinc filter delivers 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 overloads and sends a trip signal to the PWM modulator in the event of a fault. The Sinc filter frequency response curve illustrates how proper parameter selection can enable the filter to suppress the PWM switching ripple in the current sampling.

Figure 2. Isolated current feedback.

Figure 3. Sinc filter frequency response.

Power output isolation

A common problem for both control architectures is the need to support multiple isolated power domains. This becomes even more difficult if each domain requires multiple bias rails. The circuit in Figure 4 generates +15 V and –7.5 V for gate drive and +5 V for powering the ADC, all in one domain, while using only one transformer winding and two pins per domain. Using one transformer core and bobbin creates dual or triple supplies for four different power domains.

Figure 4. Isolated power supply circuit for gate drive and current feedback converter.