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CMOS Isolated Gate Drivers (ISOdrivers) Enhance Power Delivery Systems

[ Edited ]

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Green standards are challenging power designers to deliver more energy-efficient, cost-effective, smaller, and more reliable power delivery systems. A critical building block within ac-dc and isolated dc-dc power supplies is the isolated gate driver. These trends push the need for greater power efficiency and increased isolation-device integration.

Optocoupler-based solutions and gate-drive transformers have been the mainstay for switch-mode power supply (SMPS) systems for many years, but fully integrated isolated gate driver products based on RF technology and mainstream CMOS provide more reliable, smaller, and power-efficient solutions.

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Anatomy of an Isolated Power Converter

Isolated power converters require power stage and signal isolation to comply with safety standards. Figure 1 shows an example of a typical ac-dc converter for 500 W to 5 kW power systems, such as those used in highefficiency data center power supplies.

From a high-level perspective, this two-stage system has a power factor correction (PFC) circuit 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.

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Figure 1: AC/DC Converter Based on Full Bridge Topology


The high-side switch driver inputs in Figure 1 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 highvoltage 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 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. 


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