Technical Tips: Strengthening Power Delivery Systems with CMOS Isolation Drivers

Green standards are driving power system designers to design more energy-efficient, cost-effective, smaller, and more reliable power delivery systems. Optocoupler-based solutions and gate drive transformers have been the mainstay of switch-mode power supply ( SMPS ) systems for many years, but fully integrated isolated gate driver products based on RF technology and mainstream CMOS offer more reliable, smaller, and more efficient solutions.

To meet market demand, Silicon Labs recently wrote a white paper titled " CMOS Isolation Drivers Strengthen Power Transmission System Safety", which details the application and design of isolated gate drivers, a key component in AC-DC and isolated DC-DC power supplies. These trends are driving the need for higher power efficiency and increased isolation device integration. "Read original text" View the highlights of this white paper.

Analysis of Isolated Power Converters

Isolated power converters require both power stage and signal isolation to comply with safety standards. The example below shows a typical ac-dc converter for a 500W to 5 kW power system, such as those used in high-efficiency data center power supplies. At a higher level, this two-stage system has a power factor correction ( PFC ) circuit that forces the power system ac line current draw to be sinusoidal and in phase with the ac line voltage ; therefore, it appears to act as a purely resistive load for greater input power efficiency.

The high-side switch driver input above is referenced to the primary-side ground, and its output is referenced to the high-side MOSFET source pin. During high-side drive, the high-side driver must be able to withstand the 400 VDC common-mode voltage present at the source pin , traditionally provided by a high-voltage driver ( HVIC ). The corresponding low-side driver operates from a low-voltage supply (e.g., 18V ) and is referenced to the primary-side ground. Two ac current sensors in the low-side leg of the bridge monitor the current in each leg to facilitate flux balancing when voltage-mode control is used. An isolation barrier is also provided to ensure that no current flows between the primary and secondary-side grounds ; therefore, the drivers for the synchronous MOSFETs Q5 and Q6 must be isolated. For the same reason, the secondary-side feedback path must also be isolated.

Gate Driver Solutions

Optocoupler

Although optocouplers are commonly used for feedback isolation, their propagation delay performance is insufficient to realize the full benefits of synchronous MOSFET gate drive isolation circuits. Optocouplers with faster delay time specifications are available, but they are more expensive while still exhibiting some of the same performance and reliability issues found in low-cost optocouplers. This includes unstable operating characteristics over temperature, device aging, and marginal common-mode transient currents ( CMTI ) generated by the single-ended architecture with high internal coupling capacitance. In addition, the gallium arsenide-based process technology commonly found in optocouplers creates an inherent wear-out mechanism that causes the LED to lose brightness over time ( “ light output ” or LOP ).

Gate Drive Transformer

With the above considerations in mind, gate drive transformers have become a more popular method of providing isolated gate drive. Gate drive transformers are miniature toroidal transformers that are superior to optocouplers because they have shorter delay times. They are faster than optocouplers but cannot propagate DC levels or low frequency AC signals.

They can only pass a limited voltage - time across the isolation boundary , limiting the on-time ( tON ) and duty cycle range. These transformers must also be reset after each on-cycle to prevent core saturation, requiring external circuitry. Finally, transformer-based designs are inefficient, have high EMI , and take up excessive board space.

based on CMOS Isolated Gate Drivers

Fortunately, there are now better alternatives to gate drive transformers and optocouplers. Advances in CMOS -based isolation technology have enabled isolated gate drive solutions to deliver superior performance, power efficiency, integration, and reliability. Isolated gate drivers, such as Silicon Labs ’ Si823x ISOdriver family, combine isolation technology with gate driver circuitry to provide an integrated, low-latency isolated driver solution 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 single PWM input

• Dual isolated drivers

The Si823x ISOdriver family supports 0.5A and 4.0A 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 (the dual- ISO versions do not include overlap protection or dead-time generators). Therefore, the dual- ISO drivers can be used as dual low-side, dual high-side, or high-side / low-side isolated drivers. These devices have a three-die structure that isolates each drive channel from the other drive channels and the input side. This allows polarity reversal of the high-side and low-side channels without latch-up or other damage.

To download the full white paper, please click on the link below: http://www.silabs.com/products/power/isodrivers/Pages/cmos-isolated-gate-drivers-enhance-power-delivery-systems.aspx

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