Selecting Isolators for High-Voltage Control Applications: Digital vs Optical
Knowing the key characteristics can help you make the right decision between digital and optical isolators for high-voltage systems. This article reviews why isolators are necessary, explores whether optical or digital isolators are best for high-voltage applications, and discusses the major issues with digital isolators.
High-voltage digital control applications range from several hundred to thousands of volts. Specific applications include intelligent power grid infrastructure, motor control for electric vehicles (EVs), renewable energy systems, and industrial power supplies. As the demand for high-voltage digital control has grown, so has the need for safe and reliable low-power consumption digital isolators.
Figure 1. Electric grid infrastructure is an example of a high-voltage digital control application. Image used courtesy of Adobe
Why Are Isolators Important?
The purpose of an isolator is to prevent unwanted currents, serving three essential objectives:
- Safety—Isolators protect users and sensitive equipment from electrical hazards.
- Multiple voltage domains—Isolators transmit signals between circuits with different voltage domains, which is crucial when a system must be powered by various sources, as in EVs or renewable energy systems.
- Noise immunity—Isolators improve noise immunity within the system.
In a high-voltage application, there is a risk that a failed isolator could put operators in danger or damage control circuitry, which, in turn, can lead to unpredictable behavior and system malfunction. For these reasons, isolators must be reliable and robust in the presence of noise.
Accurate, precise timing is also necessary in high-voltage applications, as it impacts overall system performance and consistency, ensuring stable system operation and highly predictable data communication.
In addition, minimizing isolator power consumption increases system efficiency.
Types of Isolators
There are two basic types of isolators: optical and digital. But which one works best for high-voltage control applications? Let’s review each type.
Optical isolators, also known as optocouplers, are analog isolation products that use light to transmit signals over the isolation barrier. When a current is applied to the optocoupler, an infrared LED transmits light proportional to the current. A photosensitive receiver recognizes the presence of the light and switches on. It also measures the light's intensity and conducts the indicated current level.
Optical isolators have been popular for years due to their early market presence and cost-effectiveness. The problem, however, is that their switching speeds are limited to a few megabits per second. They also require additional circuit elements to function correctly, increasing the required PCB area and the bill of materials (BOM). Alternatives to these limitations exist, but they add to the overall solution cost.
Digital isolators use capacitive or magnetic isolation technologies to transfer signals across the isolation barrier.
Capacitive digital isolators use capacitive coupling to transmit data across the isolation barrier. In this approach, a dielectric material electrically isolates the isolator's two capacitive plates. When a signal is applied to one capacitor plate, the other plate picks up the output signal.
Magnetic or transformer-based digital isolators use a magnetic field to couple signals across the isolation barrier. In this approach, the input signal is applied to one winding of the transformer, and the output signal is picked up from the other winding.
Comparing Optical and Digital Isolators
Table 1 summarizes the differences between optical and digital isolators. Digital isolators offer many benefits, including a longer useful life and significant size reduction of the isolation element. And one of the critical benefits, compared to optical isolators, is their switching speed.
Table 1. A summary of the differences between optical and digital isolators.
In addition to optimizing system BOM and reducing PCB area, digital isolators provide:
- Accurate timing characteristics with lower power consumption.
- Solid insulation lifetime certified by component level standard IEC 60747-17.
- Enhanced common-mode transient immunity (CMTI).
- Additional integrated features such as input filters, transceivers (e.g., CAN, RS-485), and output enable options.
These features make digital isolators an excellent choice for high-voltage digital control applications.
Introduction to Infineon’s ISOFACE Dual-Channel Digital Isolators
Infineon’s ISOFACE dual-channel digital isolators are engineered to meet the challenges of high-voltage isolation for digital control applications. Their patented coreless transformer (CT) technology isolates the signals crossing different voltage domains. This magnetically coupled isolator uses semiconductor manufacturing processes to integrate an on-chip transformer. As shown in Figure 2, this transformer consists of metal spirals separated by a silicon dioxide (SiO2) insulation barrier.
Figure 2.Cross-sectional view of Infineon’s coreless transformer technology. Image used courtesy of Infineon
These dual-channel operators possess a wide-operating supply voltage of 2.7 to 6.5 V with an absolute maximum of 7.5 V to support robustness to the auxiliary power supply even with high ripple. They also exhibit low power consumption, with a maximum of only 1.65 mA per channel (combined maximum of 3.3 mA at 3.3 V and 1 Mbps).
The ISOFACE dual-channel digital isolators provide accurate timing performance (26 ns typical propagation delay and a 3 ns pulse width distortion), ensuring consistent performance while enabling high-density power system designs. Their high Common-Mode Transient Immunity (CMTI) of a minimum of 100 kV/µs provides high signal transfer robustness.
These digital isolators include key functions such as communication modulation, glitch filtering, and undervoltage lockout for effective and fail-safe data transmission, even in critical industrial environments where high voltages and noises usually go hand-in-hand.
Infineon ISOFACE dual-channel digital isolators have additional key features, including TTL or CMOS input thresholds for dual-channel digital isolators and the availability of high or low fail-safe default output options. They are also safety certified for simplified safety approval and provide pin-to-pin compatibility for easy replacement of devices.
The Infineon ISOFACE dual channel isolators are certified according to UL-1577 and IEC 60747-17 (VDE 0884-17) standards on the component level, along with system certifications (IEC 62368-1) for telecommunication and server applications.
Table 2 summarizes the various product variants of the ISOFACE isolators.
Table 2. ISOFACE digital isolator product selection guide.
Example Applications of Infineon ISOFACE Digital Isolators
There are several areas where the Infineon dual-channel ISOFACE isolators work extremely well, including low voltage (LV) DC-DC bricks, high-side floating driver gate control for GaN with integrated power stage (often abbreviated GaN-IPS), and isolated UART/CAN communication.
Isolated 1-kW DC-DC Brick Using an ISOFACE Digital Isolator
LV DC-DC bricks are commonly implemented in telecommunications and server switched-mode power supplies (SMPS) to achieve a stable 12 VDC output. Safety requirements and the need for high power density and communications capabilities have led many engineers to adopt a full-bridge to full-bridge topology for isolated DC-DC bricks above 800 W.
Digital isolators are commonly used to transfer pulse width modulated (PWM) gate control signals past the isolation barrier. Figure 3 shows Infineon’s solution for a 1 kW DC-DC brick.
Figure 3. Diagram of an isolated LV DC-DC brick using ISOFACE 2DIB0410F. Image used courtesy of Infineon
This solution has an Infineon XDPP1100 digital power controller on the secondary side to control the primary side full-bridge topology. Both sides of the converter use a level-shift Infineon EiceDRIVER 2EDL802x gate driver IC.
An ISOFACE dual-channel digital isolator 2DIB0410F transmits the PWM signals across the isolation barrier, where the signals control two Infineon OptiMOS power MOSFETs on different arms of the full bridge diagonally.
In addition to providing isolation, the ISOFACE digital isolator provides a fixed TTL input threshold that is immune to the noise on the VDD power supply line in SMPS applications. Finally, the default low output state ensures a safe turn-off of all MOSFETs if the input-side supply of the digital isolator is below the under-voltage lockout (UVLO).
Isolated CAN and UART Communication Using an ISOFACE Digital Isolator
Other applications for ISOFACE digital isolators are isolated controller area network (CAN) and universal asynchronous receiver/transmitter (UART) communication interfaces, both widely used in industrial applications.
Figure 4 demonstrates the placement of an ISOFACE digital isolator between the CAN controller and the Infineon TLE9251 CAN transceiver.
Figure 4. An isolated CAN communication using the Infineon ISOFACE 2DIB1411F. Image used courtesy of Infineon
These communication applications require the ability to ensure safety and prevent noise. The ISOFACE dual-channel digital isolator 2DIB1401F provides high CMTI and very low pulse width distortion (PWD), features that are critical for successful data communication.
Furthermore, the ISOFACE isolator has a default high output state to ensure that the communication line (typically in a logic high during the idle state) remains unblocked even in the event of a failure, thus preventing a potential power supply loss on the input side.
Getting Started With Digital Isolators
Digital isolators for high-voltage applications must, first and foremost, be safe for both the operators and the control circuitry involved. They must offer consistent performance, accurate timing, and low power consumption.
The Infineon ISOFACE dual channel isolators meet these needs, making them an excellent choice for high-voltage digital control applications. Infineon provides evaluation boards, technical documentation, and customer support to help you get started designing with digital isolators.
Featured image used courtesy of Infineon.