The advantages of coreless transformer-based isolators/drivers

Design options allow system designers to configure their system with the right performance, reliability, and safety considerations while meeting design cost and efficiency targets. The right design options can be even more important in high-voltage and/or high-current applications. In these high-power designs, an isolation technique with several integrated features can mean the difference between a product that meets and even exceeds customer expectations and one that generates numerous customer complaints.

For example, an integrated solid-state isolator (SSI) based on coreless transformer (CT) provides galvanic isolation with several design benefits. With integrated features such as a dynamic Miller clamp (DMC), overcurrent and overtemperature protection (OTP), under-voltage lockout protection, fast turn-on, and more, an integrated SSI driver can provide essential protection and ensure proper operation and extended life for high-power systems. These integrated protection features are not available in optical-based solid-state relays (SSRs).

Combined with the appropriate power switches, the highly integrated solid-state isolators allow designers to create custom solid-state relays capable of controlling loads in excess of 1,000 V and 100 A. The CT-based isolators enable energy transfer across the isolation barrier capable of driving large MOSFET or IGBT without the added circuitry of a power supply on the isolated side. SSRs designed with these innovative protection features can be highly reliable and extremely robust.

These coreless transformer-based isolators enable ON and OFF control, acting like a relay switch without requiring a secondary side, isolated power supply. Combined with MOSFETs and IGBTs, SSIs enable cost effective, reliable, and low power solid-state relays for a variety of applications. This includes battery management systems, power supplies, power transmission and distribution, programmable logic controllers (PLCs), industrial automation, and robotics as well as smart building applications such as heating, ventilation, and air conditioning (HVAC) controllers and smart thermostats.

Energy transfer through coreless transformer

The main design feature of an SSI device is a coreless transformer which enables power transfer across a galvanic isolation barrier of up to 10 mW. This eliminates the need for an isolated power supply for the switch reducing the bill of material (BOM) volume, count, and cost as well as providing a fast turn ON/OFF feature (≤ 1 µs) to ensure that the safe operating area (SOA) of the switch is adhered to.

Figure 1 Highly integrated solid-state isolators easily drive MOSFETs or IGBTs and do not require an isolated bias supply. Source: Infineon

Integrated protection

The integrated protection features of the CT-based isolators deserve further explanation. These include overcurrent and overtemperature protection (OTP), a dynamic Miller clamp, and under-voltage lockout (latch-off) protection as well as satisfying essential industry standards.

System and switch protection

Depending on the application’s need and product variant selected, SSIs offers overcurrent protection (OCP) as well as OTP either via an external positive temperature coefficient (PTC) thermistor/resistor or a MOSFET’s integrated direct temperature sensor.

In case of a failure event (overcurrent or overtemperature), SSI triggers a latch-off. Once triggered, the protection reacts quickly, turning off in less than 1 μs. Furthermore, it can support the AC-15 system tests, required for electromechanical relays according to the IEC 60947-5-1 under appropriate operating conditions.

Overcurrent protection

When operating solid-state relays, a common problem is the handling of fast overcurrent or short circuit events in the range of 20 A/μs up to 100 A/μs. Isolation issues often result in a short circuit with an extremely high current level that is defined by the power source’s impedance and cabling resistance.

Figure 2 shows a circuit for implementing the overcurrent protection. The shunt resistor (RSh) and its inherent stray inductance (LSh) generate a voltage drop that is monitored by the current sense comparator. Noise on the grid needs to be filtered out from the shunt signal, so an external filter (CF and RF) complements the integrated filter. When the comparator triggers, it activates the fast turn-off and latches the fault leaving the system in a safe state.

Figure 2 The above circuitry implements overcurrent protection using an isolator driver. Source: Infineon

Overtemperature protection

Another major known issue when operating solid-sate relays is the slow overload events that heat up the switches and the current sensor (shunt). Increased load current and insufficient thermal management can additionally shift the overall temperature above the thermal power transistor limits.

Figure 3 shows an example measurement of the overtemperature protection using an isolated driver. The SSI turns off two MOSFETs with integrated temperature sensors configured in a common-source mode. The sensing MOSFET heats up from the load current until the sensor voltage decreases below the comparator trigger threshold. As a result, the SSI’s output is turned off.

Figure 3 Isolated driver’s overtemperature protection triggers within 500 ns. Source: Infineon

The lower part of Figure 3 depicts a detailed zoom into the turn-off in this measurement with a time resolution of 500 ns per division. This reduced timeframe shows that the gate is turned off in much less than 500 ns. This means that the switched transistors do not violate their safe operating area.

Dynamic Miller clamping protection

Some SSIs also have an integrated dynamic Miller clamp to protect against spurious switching due to surge voltages and fast electric transients as well as the dv/dt of the line voltage. The dv/dt applied by the connected AC voltage creates capacitive displacement currents through the parasitic capacitances of a power transistor.

This can lead to parasitic turn-on of the power switch by increasing the voltage at its gate node during its “off” state. The dynamic Miller clamping feature ensures that the power switch remains safe in the “off” state.

When failure is not an option

When matched with the appropriate power switch, the isolator drivers enable switching designs with a much lower resistance compared to optically driven/isolated solid-state solutions. This translates to longer lifespans and lower cost of ownership in system designs. As with all solid-state isolators, the devices also offer superior performance compared to electromagnetic relays, including 40% lower turn-on power loss and increased reliability due to the elimination of moving or degrading parts.

When failure is not an option, the right choice of isolation can mean the difference between design success and failure.

Dan Callen Jr. is a senior manager at Power IC Group of Infineon Technologies.

Davide Giacomini is director of marketing at Power IC Group of Infineon Technologies.

Sameh Snene is a product applications engineer at Infineon Technologies.

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The advantages of coreless transformer-based isolators/drivers

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