Safe isolation and Low Power consumption in High Voltage Digital Control Applications

­Introduction

Effective galvanic isolation is crucial in high-voltage applications to prevent unwanted leakage currents from flowing between two parts of a system with different ground potentials (GPD) [1]. As depicted in Figure 1 (left), the DC return current from the input to the output can cause a potential difference between the two grounds, leading to low signal integrity and degraded quality. This is where isolators (i.e., isolated gate driver ICs [2] or digital isolators) come into play, as illustrated in Figure 1 (right). By stopping the DC and uncontrolled AC current between the two parts of the circuit, isolators allow only communication signals and power to pass over the isolation barrier. Additionally, they provide the necessary safety protection for humans interacting with high-voltage systems, as well as level-shift functionality, enabling interaction between systems powered by different supply voltage levels.

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In general, the isolators could be broadly classified into two different types: optical isolators and digital isolators.

Optical isolators, commonly called optocouplers, are analog isolation products that use light to transmit signals over the isolation barrier. They have been widely used in the industry for decades due to their early market presence and competitive price position. However, the switching speeds of the LED limit the achievable data rate to only a few megabits per second. Moreover, optical isolators require additional onboard circuit elements to provide proper biasing and drive the integrated LED. This requires additional PCB area and increases the bill of materials (BOM) of an application. While there are techniques to integrate the external biasing and drive circuitry in a single package for minimizing the PCB size and achieving high data rates, this significantly increases the cost of the solution.

On the other hand, digital isolators use either capacitive or magnetic isolation technologies to transfer signals across the isolation barrier. These technologies allow significant size reduction of the isolation element while achieving a long lifetime [3]. Recently they have been the preferred choice in many applications due to the following advantages compared to optical isolators:

›      Optimized system BOM and reduced PCB area

›      Accurate timing characteristics with lower power consumption

›      Enhanced common-mode transient immunity (CMTI)

›      Solid insulation lifetime certified by component level standard IEC 60747-17

›      More integrated features like input filters, transceivers (e.g., CAN, RS-485), and output enable options.

The key differences between optical and digital isolators are summarized in Table 1.

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Infineon’s first generation ISOFACE™ digital isolators 2DIBx4xxF

To meet the continually growing isolation requirements in industrial applications, Infineon has released the first generation of dual-channel digital isolators that combine high robustness, precise timing performance, and low power consumption.

The new products utilize Infineon’s patented coreless transformer (CT) technology. This innovative technology uses semiconductor manufacturing processes to integrate an on-chip transformer consisting of metal spirals separated by a silicon dioxide (SiO₂) insulation barrier, as illustrated in Figure 2. This approach transmits signals between the input and output through the transformer across the isolation barrier.

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In addition, functions like glitch filter, communication modulation, watchdog, and under-voltage lockout (UVLO) are integrated to ensure a robust and fail-safe data transmission even in the critical industrial environment where high voltages and noises are present.

Product variants with different channel configurations, fail-safe default output states, and variable or fixed input thresholds are available, as listed in Table 2.

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The dual-channel digital isolator family is certified according to the UL-1577 and IEC 60747-17 (VDE 0884-17) standards on the component level and also have system certification, such as IEC 62368-1 for telecom and server applications. Featuring a creepage and clearance distance of 4 mm, they are well-suited for applications requiring basic isolation, such as low voltage DC-DC brick, high-side floating drive, and isolated UART/CAN communication.

Table 3 gives an overview of Infineon’s ISOFACE™ dual-channel digital isolators’ features and benefits for the customers:

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Application examples of ISOFACE™ dual-channel digital isolators

Isolated low-voltage DC-DC brick

Low-voltage DC-DC bricks are widely used in telecom and server switched-mode power supplies (SMPS) to achieve a stable 12-V_DC output [4]. In order to fulfill the growing demand for higher power density, enhanced safety requirements, and communication capability, isolated DC-DC bricks above 800 W are predominantly designed with full-bridge to full-bridge (FB-FB) topology, which is controlled by a digital controller placed on the secondary side of the main power transformer. To ensure input-to-output safety isolation, a digital isolator with basic isolation is often utilized for transferring the PWM gate control signals over the isolation barrier.

As an example, Figure 3 illustrates Infineon’s solution for an isolated 1-kW DC-DC brick, which employs the XDP™ XDPP1100 digital power controller on the secondary side to control the primary side full-bridge topology. The PWM signals are transmitted over the isolation barrier through ISOFACE™ dual-channel digital isolator 2DIB0410F.

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Both sides of the converter use level-shift EiceDRIVER™ 2EDL802x gate driver ICs. The two channels of the digital isolator transfer the complementary PWM signals, which control the two OptiMOS™ power MOSFETs on different arms of the full bridge diagonally. Furthermore, the ISOFACE™ 2DIB0410F provides a fixed TTL input threshold that is immune to noise on the V_DD power supply line in SMPS applications. The default low output state ensures a safe turn-off of all MOSFETs in case the input-side supply of the digital isolator is below the UVLO.

Isolated CAN and UART communication

Controller Area Network (CAN) and Universal Asynchronous Receiver/Transmitter (UART) communication have been widely used in industrial and automotive applications. They have the common advantage that only a single pair of cables (two communication lines) is needed at the physical layer for data transmission.

When it comes to ensuring safety or preventing noise interference in an isolated CAN or UART interface, the ISOFACE™ dual-channel digital isolator 2DIB1401F stands out as a top choice for providing galvanic isolation.

This reliable isolator offers high CMTI and very low pulse width distortion (PWD), which are crucial features for achieving reliable communication. Additionally, the isolator’s default high output state ensures that the communication line (typically in a logic high during the idle state) will remain unblocked even in the event of a failure, preventing potential power supply loss on the input side.

Together with Infineon’s CAN transceiver TLE9251, an example of an isolated CAN interface is illustrated in Figure 4. ISOFACE™ 2DIB1401F is placed between the controller and the transceiver to provide galvanic isolation.

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Functional isolation in GaN IPS half-bridge

The importance of 650 V gallium nitride (GaN) HEMTs in SMPS designs has been increasing lately, owing to their ability to operate at far higher switching frequencies while significantly minimizing energy loss. Also, GaN integrated power stages (IPS) that combine a GaN switch with an integrated gate driver within a single package are becoming the preferred option to achieve designs with extreme power density, fewer components, reduced component count and minimum parasitics.

To ensure proper functioning of the high-side GaN IPS, a digital isolator is necessary to perform a level-shift function for the gate driver, since the gate driver refers to the switching node. Due to the GaN HEMT’s ability to generate fast dv/dt commutations of up to 100 V/ns, high common-mode transients can occur, requiring a robust solution. The ISOFACE™ digital isolators are suitable in this case as they can provide a minimum CMTI of 100 V/ns and ensure reliable transfer of PWM signals.

Figure 5 shows an example using ISOFACE™ dual-channel digital isolator 2DIB1410F for a high-side floating drive. One forward data channel is used to transfer the gate signal, while the reverse channel communicates the fault feedback signal to the controller.

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Using another ISOFACE™ 2DIB1410F for the low-side GaN IPS is optional. However, it is highly recommended in order to equalize the propagation delays between the high-side and low-side signal paths and to provide isolation for the different grounds of the GaN IPS (power ground) and the controller (digital ground).

Conclusion

In conclusion, Infineon’s cutting-edge ISOFACE™ digital isolators offer an exceptional combination of low current consumption, improved common-mode transient immunity performance, and high propagation delay accuracy, making them a robust and reliable choice for high-voltage digital control applications. When coupled with Infineon’s comprehensive portfolio of XDP™ digital controllers, EiceDRIVER™ gate driver ICs, and power switches such as OptiMOS™, CoolMOS™, CoolSiC™, and CoolGaN™, they create a complete system solution that meets the growing isolation requirements in modern power electronics applications.

By leveraging these cutting-edge technologies, Infineon’s customers can benefit from high supply security, better system performance, and increased reliability to meet the increasing isolation requirements in high-voltage digital control applications.

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