How can the performance improvement of the latest available isolation components help the alternative architecture improve the system without compromising safety?

Author: Aengus Murray and Robert Zwicker, ADI company

Both motor and power control inverter designers have the same problem of isolating control and user interface circuits from dangerous power line voltages. The most important requirement for isolation is the way power line voltage damage control circuit, and more importantly, protect users from dangerous voltage damage. The system must comply with the safety requirements of the corresponding international standards, such as IEC 61800 and IEC 62109 covering motor drives and solar inverters. These standards focus on compliance testing. How does standard compliance testing give engineers freedom? Standards provide guidance to engineers on safety, but how do they give engineers the freedom to choose the appropriate architecture, circuits, and components that meet the target system specifications and standards? These are determined by the circuit's ability to provide the required performance in terms of efficiency, bandwidth, and accuracy while meeting safety isolation requirements. The challenge in designing an innovative system is that design rules 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. Engineers in some areas have greater responsibilities, and engineers may be personally liable once the safety functions of the designed system fail and cause injury. This article explores the impact of system architecture choices on power and control circuit design and system performance. This article also shows how the performance improvements of the latest available isolation components can help alternative architectures improve system performance without compromising security.

The concern we have is that you need to safely control the flow of energy from the AC source to the load based on commands provided by the user. This problem is illustrated in the high-level motor drive system diagram shown in Figure 1 for the following three power domains: reference, control, and power. The safety requirement is that the user-given circuit must be electrically isolated from the hazardous voltages on the power circuit. The architectural decision depends on whether the isolation barrier is placed between the given and control circuits or between the control and power circuits. Introducing an isolation barrier between circuits can affect signal integrity and increase cost. Isolation of analog feedback signals is particularly difficult because conventional transformer methods suppress DC signal components and introduce nonlinearities. Digital signal isolation at low speeds is fairly straightforward, but it is very difficult at high speeds or when low latency is required, and consumes a lot of power. Power isolation in systems with 3-phase inverters is particularly difficult because multiple power domains are connected to the power circuit. The power supply 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 circuitry, even though both may be co-located with power.

Figure 1. Isolation architecture in a motor control system

The non-isolated control architecture has a common ground connection between the control and power circuits. This allows the motor control ADC to capture all the signals in the power circuit. When the motor winding current flows into the low-side inverter arm, the ADC samples at the midpoint of the center-based PWM signal. The driver for the low-side IGBT gate can be simple non-isolated, but the PWM signal must be isolated from the three high-side IGBT gates by functional isolation or level shifting. The complexity caused by the isolation between the command and control circuitry depends on the end application, but typically involves the use of a standalone system and a communications processor. The architecture of a simple processor that manages the front panel interface and sends speed commands over the 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 drives for robotics and automation applications.

The isolated control architecture has 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 problem goes to the power inverter signal, which presents a range of different challenges. The gate drive signal requires relatively high speed digital isolation to meet the timing requirements of the inverter. Due to the very high voltages, magnetic or optically coupled drivers perform well in inverter applications where isolation is critical. The requirements for the DC bus voltage isolation circuit are modest because of the lower 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 (CT) are a good choice because they provide isolated signals that can be easily measured. CT is non-linear at low currents and does not transmit DC levels, but is widely used in low-end inverters. CT is also used in high-power inverters with non-isolated control architectures, as sampling with shunt resistors in these cases can result in excessive losses. Open-loop and closed-loop Hall-effect current sensors measure AC signals and are therefore more suitable for high-end drivers, but are affected by offsets. Resistive shunts provide high bandwidth, linear signals with low offset but need to match high bandwidth, low offset isolation amplifiers. Typically, motor-controlled ADCs can directly sample isolated current signals, but the alternative measurement architecture described in the next section shifts isolation problems to the digital domain and can dramatically improve performance.

A common way 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 the use of a series ADC in combination with a digital signal isolator. The Σ-Δ ADC can be selected due to the special requirements for high frequency of motor current feedback and the need to respond quickly to drive protection. The Σ-Δ ADC is equipped with a linear modulator that converts the analog signal into a 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: slower for high fidelity feedback and another low fidelity fast filter for protecting the inverter. 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 on the isolation barrier. The Sinc filter can submit high resolution current data to the motor control algorithm, which calculates the inverter duty cycle required to apply the required inverter voltage. Another low-resolution filter detects the current overload and sends a hopping signal to the PWM modulator in the event of a fault. The sinc filter frequency response curve explains how proper parameter selection enables the filter to suppress PWM switching ripple in current sampling.

Figure 2. Isolated current feedback

Figure 3. Sinc filter frequency response

A common problem with both control architectures is the need to support multiple isolated power domains. If each domain requires multiple offset rails, it is even more difficult to implement. The circuit in Figure 4 produces +15 V and –7.5 V for gate drive and +5 V for powering the ADC, all in one domain, with only one transformer winding and two pins per domain. . Use a transformer core and bobbin to create dual or triple power for four different power domains.

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