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Anatomy of an Isolated Power Converter

by:JINCHU     2020-07-09
As emerging green standards challenge designers to deliver more energy-efficient, cost-effective and reliable power delivery systems in smaller form factors, the necessity greater power and isolation device integration becomes increasingly fundamental. A critical building block within ac-dc and isolated dc-dc power supplies is the isolated gate golf driver. While optocoupler-based solutions and gate-drive transformers have been the mainstay for switch-mode power supply (SMPS) systems for a lot of 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 keeping the vehicle safe standards. From a high-level perspective, this two-stage system provides 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, together with its outputs are referenced to the high-side MOSFET source hooks. The high-side drivers must be able to stand up to 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 lv 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 online marketing in each leg to facilitate flux balancing when voltage mode control works extremely well. The isolation barrier shown is for ensure that there's really no current flow within primary- and secondary-side grounds; consequently, the drivers for synchronous MOSFETs Q5 and Q6 must be isolated. The secondary-side feedback path also have to be isolated for similar reason. Gate Drive Solution Options Although optocouplers are generally used for feedback isolation, they aren't fast enough specifically in the synchronous MOSFET gate-drive isolation circuit. While faster optocouplers are available, they tend to become 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 higher internal coupling capacitance. In addition, Gallium-Arsenide-based process technologies common in optocouplers create an intrinsic wear-out mechanism ('Light Output' or LOP) leads to the LED to get rid of brightness over period. Given the above considerations, gate drive transformers have are a more popular method of providing isolated gate drive. Gate drive transformers are miniature torroidal transformers that preferred over optocouplers because of their shorter delay moments. 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 all over the isolation boundary, thereby restricting ON time (tON) and duty cycle ranges. These transformers must be also reset after each ON cycle cease core saturation, necessitating external circuitry. Finally, transformer-based designs are inefficient, have high EMI and occupy excessive board room or 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 include exceptional performance, power efficiency, integration and reliability. These highly-integrated CMOS devices are very well positioned to supersede both optocouplers and gate drive transformers in SMPS courses. Isolated gate drivers, with regard to example 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.
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