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.

Anatomy of an Isolated Power Converter

Isolated power converters require power stage and signal isolation to comply with safety standards. The example below shows a typical ac-dc converter for 500 W to 5 kW power systems, such as those used in high- efficiency 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.

The high-side switch driver inputs above 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 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 Driver Solutions

Optocouplers

Although optocouplers are commonly used for feedback isolation, their propagation delay performance is not fast enough to achieve the full benefit of the synchronous MOSFET gate-drive isolation circuit.

Optocouplers with faster delay-time specifications are available, but they tend to be expensive while still exhibiting some of the same performance and reliability issues found in lower-cost optocouplers. This includes unstable operating characteristics over temperature, device aging, and marginal common mode transient current (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.

Gate Drive Transformers

Given the above considerations, gate drive transformers have become a more popular method of providing isolated gate drive. Gate drive transformers are miniature toroidal transformers that are preferred over optocouplers because of their shorter delay times. They are faster than optocouplers, but 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.

CMOS-based Isolated Gate Drivers

Fortunately, better alternatives to gate drive transformers and optocouplers are now available. Advancements in CMOS-based isolation technology have enabled isolated gate drive solutions that offer exceptional performance, power efficiency, integration, and reliability. Isolated gate drivers, such as Silicon Labs’ Si823x ISOdriver family, combine isolation technology with gate driver circuits, providing integrated, low-latency isolated driver solutions for MOSFET and insulated-gate bipolar transistor (IGBT) applications.

The Si823x ISOdriver products are available in three basic configurations (see Figure 2), including:

• high-side and low-side isolated drivers with separate control inputs for each output
• high-side and low-side isolated drivers with a single PWM input
• dual isolated driver

The Si823x ISOdriver family supports 0.5 A and 4.0 A peak output drive options and is available in 1 kV, 2.5 kV, and 5 kV isolation ratings. The high-side/low-side versions have built-in overlap protection and an adjustable dead time generator (dual ISOdriver versions contain no overlap protection or dead time generator). As such, the dual ISOdriver can be used as a dual low-side, dual high-side or high-side/low-side isolated driver.

These devices have a three-die architecture that causes each drive channel to be isolated from the others as well as from the input side. This allows the polarity of the high-side and low-side channel to reverse without latch-up or other damage.

Read the Whitepaper

To learn more about how isolated gate drivers can significantly increase the efficiency, performance, and reliability of switch-mode power supplies compared to legacy solutions, check out this whitepaper.