Author: Aengus Murray and Robert Zwicker, ADI Company
Both motor and power control inverter designers face a common challenge: isolating control and user interface circuits from dangerous power line voltages. The primary goal of isolation is to prevent damage to the control circuit caused by high voltage and, more importantly, to protect users from electrical hazards. To ensure safety, systems must meet international standards such as IEC 61800 for motor drives and IEC 62109 for solar inverters. These standards emphasize compliance testing, but how do they actually empower engineers? While standards offer guidance on safety, they also give engineers the freedom to choose the most suitable architecture, circuits, and components that align with both system specifications and safety requirements. This decision depends on the circuit’s ability to deliver the necessary performance in terms of efficiency, bandwidth, and accuracy while maintaining isolation. Designing an innovative system can be challenging, as traditional design rules may no longer apply. Therefore, engineers must take the time to carefully evaluate new components and circuits to ensure they meet EMC and safety standards. In some cases, engineers may even face personal liability if a safety function fails and causes harm. This article explores how system architecture choices affect power and control circuit design, as well as overall system performance. It also highlights how advancements in isolation components can enable alternative architectures to achieve better performance without compromising safety.
**Isolation Architecture**
The main concern is ensuring safe energy transfer from the AC source to the load based on user commands. This is illustrated in the high-level motor drive system shown in Figure 1, which includes three power domains: reference, control, and power. The key requirement is that the user interface must be electrically isolated from the hazardous voltages on the power circuit. The architectural choice involves deciding whether to place the isolation barrier between the command and control circuits or between the control and power circuits. Introducing an isolation barrier can impact signal integrity and increase costs. Isolating analog feedback signals is especially difficult because traditional transformer methods suppress DC components and introduce nonlinearities. Digital signal isolation at low speeds is manageable, but becomes complex at high speeds or when low latency is required, as it consumes more power. In systems with 3-phase inverters, power isolation is particularly challenging due to multiple connected power domains. For example, the high-side gate drive and winding current signals need to be functionally isolated from the control circuitry, even though they may be located nearby.
In a non-isolated control architecture, the control and power circuits share a common ground. This allows the motor control ADC to capture all signals from the power circuit. When the motor winding current flows into the low-side inverter arm, the ADC samples the midpoint of the PWM signal. The low-side IGBT gate driver can be simple and non-isolated, but the PWM signal must be isolated from the three high-side IGBT gates using functional isolation or level shifting. The complexity introduced by this isolation depends on the application, often involving a standalone system and a communications processor. A simple processor managing the front panel interface and sending speed commands via a slow serial interface works well for home appliances or low-end industrial applications. However, non-isolated architectures are less common in high-performance robotics and automation systems due to the high bandwidth demands of the command interface.
In an isolated control architecture, the control and command circuits share a common ground, enabling tight coupling between the interfaces and allowing the use of a single processor. The isolation challenge then shifts to the power inverter signals, which present various difficulties. The gate drive signal requires high-speed digital isolation to meet inverter timing requirements. Magnetic or optically coupled drivers perform well in critical isolation applications due to the high voltages involved. The DC bus voltage isolation circuit has more modest requirements due to lower dynamic range and bandwidth. Motor current feedback, however, is the biggest challenge in high-performance drives, as it requires high bandwidth and linear isolation. Current transformers (CTs) are commonly used for their ability to provide isolated signals, although they are nonlinear at low currents and cannot transmit DC levels. They are widely used in low-end inverters and even in high-power systems where shunt resistors would cause excessive losses. Open-loop and closed-loop Hall-effect sensors are suitable for high-end drives but are affected by offset errors. Resistive shunts offer high bandwidth and low offset but require isolation amplifiers with similar characteristics. Motor-controlled ADCs can directly sample isolated current signals, but an alternative approach described in the next section moves the isolation problem into the digital domain, significantly improving performance.
**Inverter Feedback Using an Isolated Converter**
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. This typically involves using a series ADC combined with a digital signal isolator. The Σ-Δ ADC is ideal for motor current feedback due to its high-frequency response and fast protection capabilities. This type of ADC uses a linear modulator to convert the analog signal into a bit stream, followed by a digital filter that reconstructs the signal into a high-resolution digital word. This approach allows for two different digital filters: one slower for high-fidelity feedback and another faster for protection. In Figure 2, a winding shunt measures the motor current, and an isolated ADC transmits a 10 MHz data stream across the isolation barrier. The Sinc filter provides high-resolution current data to the motor control algorithm, which calculates the required inverter duty cycle. Another low-resolution filter detects overcurrent conditions and sends a fault signal to the PWM modulator. The Sinc filter frequency response curve demonstrates how proper parameter selection can suppress PWM switching ripple during current sampling.
**Power Output Isolation**
A major challenge in both control architectures is supporting multiple isolated power domains. If each domain requires multiple offset rails, the implementation becomes even more complex. The circuit in Figure 4 generates +15 V and –7.5 V for gate drive and +5 V for the ADC, all within a single domain, using just one transformer winding and two pins per domain. By using a transformer core and bobbin, it's possible to create dual or triple power supplies for four different power domains. This approach simplifies the design and improves reliability.
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