As emerging green standards challenge designers to deliver more energy-efficient, cost-effective and reliable power delivery systems in smaller form factors, the need for greater power and isolation device integration becomes increasingly important. A critical building block within ac-dc and isolated dc-dc power supplies is the isolated gate driver. While optocoupler-based solutions and gate-drive transformers have been the mainstay for switch-mode power supply (SMPS) systems for many years, fully-integrated isolated gate driver products based on RF technology and implemented in mainstream CMOS provide a more reliable and power-efficient solution.
Anatomy of an Isolated Power Converter
Isolated power converters require power stage and signal isolation to comply with safety standards. From a high-level perspective, this two-stage system has a power factor correction circuit (PFC) that forces power system ac line current draw to be sinusoidal and in-phase with the ac line voltage; thus, it appears to the line as a purely resistive load for greater input power efficiency.
The high-side switch driver inputs are referenced to the primary-side ground, and its outputs are referenced to the high-side MOSFET source pins. The high-side drivers must be able to withstand the 400 VDC common-mode voltage present at the source pin during high-side drive, a need traditionally served by high voltage drivers (HVIC). The corresponding low-side drivers operate from a low voltage supply (e.g.18 V) and are referenced to the primary-side ground. The two ac current sensors in the low-side legs of the bridge monitor the current in each leg to facilitate flux balancing when voltage mode control is used.
The isolation barrier shown is provided to ensure that there is no current flow between the primary- and secondary-side grounds; consequently, the drivers for synchronous MOSFETs Q5 and Q6 must be isolated. The secondary-side feedback path must also be isolated for the same reason.
Gate Drive Solution Options
Although optocouplers are commonly used for feedback isolation, they are not fast enough for use in the synchronous MOSFET gate-drive isolation circuit. While faster optocouplers are available, they tend to be expensive and exhibit the same performance and reliability issues typical of optocouplers, including unstable operating characteristics over temperature and device age and marginal CMTI resulting from a single-ended architecture with high internal coupling capacitance. In addition, Gallium-Arsenide-based process technologies common in optocouplers create an intrinsic wear-out mechanism ('Light Output' or LOP) that causes the LED to lose brightness over time.
Given the above considerations, gate drive transformers have become a more popular method of providing isolated gate drive. Gate drive transformers are miniature torroidal transformers that are preferred over optocouplers because of their shorter delay times. While faster than optocouplers, gate drive transformers cannot propagate a dc level or low-frequency ac signal; they can pass only a finite voltage-time product across the isolation boundary, thereby restricting ON time (tON) and duty cycle ranges. These transformers must also be reset after each ON cycle to prevent core saturation, necessitating external circuitry. Finally, transformer-based designs are inefficient, have high EMI and occupy excessive board space.
An Optimum Isolated Gate Drive Solution
Fortunately, better alternatives to gate drive transformers and optocouplers are now available. Advancements in CMOS-based RF isolation technology have enabled isolated gate drive solutions that offer exceptional performance, power efficiency, integration and reliability. These highly-integrated CMOS devices are well positioned to supersede both optocouplers and gate drive transformers in SMPS applications.
Isolated gate drivers, such as Silicon Labs' ISOdriver family, combine RF-based isolation technology with gate driver circuits, providing integrated, low-latency isolated driver solutions for MOSFET and IGBT applications.