Coreless transformers (CTs) in solid state isolators (SSIs) rely on inductive coupling to transfer information and power across an isolation barrier. CTs employ near-field magnetic induction coupling, which is used in some wireless charging protocols.
Simple induction coupling relies on the proximity of the coils for optimal efficiency and is less efficient as the separation increases. Magnetic resonant induction coupling is often used in CTs for SSIs (Figure 1). The tuned resonance increases coupling, supports higher frequency operation, and enhances energy transfer efficiency.
CT structures
A CT uses spiral coils separated by silicon dioxide (SiO2) insulation. Their fabrication is compatible with conventional CMOS processes, enabling high levels of integration. That contrasts with opto-isolators, which require two elements: an LED and a phototransistor, which are physically separated.
Unlike transformers that use a magnetic core to enhance coupling, the primary winding in a CT directly induces current in the secondary. SiO2 can provide a very thin and effective isolation barrier while minimizing the spacing between the windings.
The dielectric strength of SiO2 is about 10 megavolts per centimeter (10 MV/cm). This is equivalent to 1000 volts per micrometer (1000 V/µm). As a result, the breakdown voltage is around 1kV for a 1µm-thick SiO2 dielectric layer (Figure 2).
CT advantages
CT technology supports faster switching speeds compared to optical SSRs. The lack of magnetic core materials eliminates the possibility of core saturation or hysteresis, providing more consistent and reliable performance compared to traditional magnetic isolation.
Unlike optical isolators, CTs can transfer sufficient energy for MOSFET or IGBT gate drive, eliminating the need for a separate power supply on the isolated side. The rapid switching of CTs can be crucial in various applications, such as solid-state relays (SSRs).
Compared to optical solutions, CTs offer lower losses, especially at higher switching frequencies. The elimination of core losses can result in higher efficiency compared to conventional transformers. Compared with either optical solutions or conventional transformers, the use of CTs improves overall system efficiency.
Compatibility with CMOS fabrication enables the easy integration of control, protection, filtering, and other functions with the CT. The high impedance of CT-based designs allows direct connection with microcontrollers, further simplifying system design and integration.
CTs are more immune to high-slew-rate (high-frequency) transients that can corrupt data transmission across the isolation barrier. The common-mode transient immunity (CMTI) of a typical CT-based design is 200 kV/μs, compared to 100 kV/μs for an optocoupler.
CTs can be fast
The low input capacitance, under 1 pF, compared with an input capacitance of approximately 25 pF for an opto-isolator, and the absence of core losses present in traditional transformers enable CTs to support fast switching speeds in SSIs. CTs can have input-to-output propagation delays of 100 ns (±7 ns), compared to 150 ns (±90 ns) for a typical opto-isolator.
Some designs can transmit pulse information at a rate of several MHz, making them suitable for high-frequency applications. In other cases, CTs are being integrated into high-side MOSFET drivers with a maximum frequency of about 500 kHz. That’s more than fast enough for use in applications like resonant half-bridges and digital power factor correction (PFC).
Summary
CTs offer several advantages in SSIs, including faster switching speeds, higher energy transfer capabilities, reduced component count, lower power dissipation, high CMTI immunity, and improved reliability. The reduced component count of integrated CT solutions enhances reliability and simplifies circuit design in SSRs, resonant half bridges, digital PFC, and other applications.
References
Advantages of coreless-transformer gate drivers over gate drive optocouplers, Infineon
Advantages of Solid State Relays over Electromechanical Relays, Dynamic
Coreless Power Transformer Technology in Isolated Gate Driver Circuits, Cadence
Coreless Transformer Provides Innovative Features, Bodo’s Power Systems
Coreless transformer a new technology for half bridge driver ICs, PCIM
Design of High-Power High-Frequency Coreless Transformer Systems, MIT
Examining Wireless Power Transfer, Texas Instruments
Integrated Coreless Transformer for High Temperatures Design and Evaluation, HAL
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