How can the performance improvement of the latest available isolation components help the alternative architecture improve the system without compromising safety?
Langrui Energy (Shenzhen) Co.,Ltd , https://www.langruibattery.com
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 circuitry caused by high-voltage surges and, more importantly, to protect users from potential electrical hazards. To ensure safety, systems must comply with 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 provide essential guidelines for safety, they also allow engineers flexibility in choosing the right architecture, circuits, and components that meet both system requirements and regulatory demands. The success of a design depends on its ability to deliver efficiency, bandwidth, and accuracy while maintaining proper isolation. As new technologies emerge, traditional design rules may no longer apply, requiring engineers to carefully evaluate the performance of new components against EMC and safety standards. In some cases, engineers may even be personally liable if a safety function fails and causes harm. This article explores how different system architectures affect power and control circuit design, and how advancements in isolation technology can enhance system performance without compromising safety.
**Isolation Architecture**
The key concern in designing a safe system is ensuring that energy flows from the AC source to the load based on user commands, while maintaining electrical isolation between the control and power circuits. This is illustrated in the high-level motor drive system diagram (Figure 1), which shows three power domains: reference, control, and power. The main safety requirement is that the user interface must be electrically isolated from the high-voltage power circuit. The architectural choice determines whether the isolation barrier is placed between the command and control circuits or between the control and power circuits. Adding an isolation barrier can impact signal integrity and increase costs. Isolating analog feedback signals is particularly challenging because traditional transformers 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, often consuming significant power. In systems with 3-phase inverters, power isolation is especially difficult due to multiple power domains connected to the same circuit. The power supply must isolate four different domains, including the high-side gate drive and current signals, even when they are located close to the power section.
**Inverter Feedback Using Isolated Converter**
One effective 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. This approach often involves using a series ADC combined with a digital isolator. For motor current feedback, which requires high-frequency response and fast protection, a Σ-Δ ADC is a good choice. It uses a linear modulator to convert the analog signal into a bit stream, followed by a digital filter that reconstructs it into a high-resolution digital word. This method allows two different filters: one for high-fidelity feedback and another for fast fault detection. In Figure 2, a shunt resistor 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 control algorithm, while a lower-resolution filter detects overcurrent conditions and triggers a protective response. The Sinc filter's frequency response curve demonstrates how parameter selection can suppress PWM switching noise during current sampling.
**Power Output Isolation**
A common challenge in both control architectures is managing multiple isolated power domains. When each domain requires separate offset rails, the complexity increases significantly. 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 up to four different power domains, simplifying the overall design and improving reliability.