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Dual switch flyback solution provides alternative to LLC, single QR flyback topologies

August 02, 2011 // Paul Buckley

Dual switch flyback solution provides alternative to LLC, single QR flyback topologies

Fairchild Semiconductor is offering a dual switch flyback solution incorporating mWSaver technology to meet these challenges faced by designers of power supplies for All-In-One(AIO) applications.


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Traditional power topology solutions for AIO applications include LLC and single switch Quasi-resonant (QR) flyback. When balancing ease of design, power loss at different loading, and efficiency, each of these topologies has advantages and disadvantages. For AIO power range, LLC offers the best overall efficiency, compared to dual switch QR and single switch QR flyback converters. The LLC transformer is difficult to design and results in the highest light load power loss among the three topologies. Single switch QR flyback converters are the least efficient, but they are easy to design and offer low power loss at light load.

Fairchild's dual switch QR flyback topology and secondary synchronous rectification is the ideal solution for AIO applications - providing good overall efficiency, managing low power loss at light load, while providing ease of design.

The AIO solution from Fairchild is perfectly suited for applications from 75 W~230 W and consists of the FAN6920MR integrated critical mode PFC and Quasi-resonant Current Mode PWM controller, the FAN7382 gate driver, in tandem with the FAN6204 secondary synchronous rectifier controller for flyback topology and forward freewheeling rectification. Combined, these devices provide best-in-class power consumption at no/light load, enabling designs to meet 2013 ErP standard without the additional circuitry LLC solutions require.

Incorporating mWSaver technology, these devices additionally offer the lowest standby power input for AIO solutions. Quasi-resonant control reduces snubber and leakage inductance losses, while improving thermal issues. The device also lowers the Vds of the SR MOSFET to provide improved efficiency, and allows for smaller board size. Built-in two-level PFC output improves low line efficiency.

Adding the FAN7382, a high and low side gate drive IC, rounds out Fairchild's solution by providing the ability to drive Fairchild's High Voltage MOSFETs up to 600 V. This device provides stable operation of the high-side driver under high-dv/dt noise circumstances.

The FAN6920MR combines a power factor correction (PFC) controller and a quasi-resonant PWM controller for a cost effective design that uses fewer external components. For PFC, the device uses a controlled on-time technique to provide a regulated DC output voltage and to perform natural power factor correction. With an innovative total harmonic distortion (THD) optimizer, the FAN6920MR can reduce input current distortion at zero-crossing duration in order to improve THD performance. For PWM control, the FAN6920MR provides several functions to enhance the power system performance, including extended valley detect up to the 12th valley cycle for reduced switching frequency, improved light load efficiency, green-mode operation, an internal 10ms soft start, and high/low line over-power compensation.

Using a proprietary linear prediction timing-control technique, not limited to the RDS(ON) of the external Fairchild power-trench SR MOSFET, the FAN6204 is suitable for fixed frequency or quasi-resonant (QR) flyback converters operating in discontinuous or continuous conduction mode (DCM and CCM) control. This technique includes a simple control method without current-sense circuitry, which provides better noise immunity. With PWM frequency tracking and secondary-side winding voltage detection, the device can operate in both fixed-and variable-frequency systems, making it adaptable to flyback and forward free-wheeling topologies. The FAN6204 also features an ultra-low green mode operating current (1.1 mA typical). This internal green mode provides lower no-load power consumption and higher light-load efficiency.

The FAN7382 can drive Fairchild's super-junction SupreMOS and SuperFET MOSFETs. Fairchild's high-voltage (up to 600 V) process and common-mode noise canceling technique provides stable operation under high dv/dt noise circumstances. An advanced level shift circuit allows high-side gate driver operation up to VS = -9.8 V (typ.) for VBS = 15 V. The input logic level is compatible with standard TTL-series logic gates. UVLO circuits for both channels prevent malfunction when VCC and VBS are lower than the specified threshold voltage.

Samples of the FAN6920MR, FAN6204 and FAN7382 devices are available now.

Prices for 1,000 quantity pieces are $1.20 for the FAN6920MR, $0.57 for the FAN6204 and $0.83 for the FAN7382.

More information about the FAN6920MR, FAN6204 and FAN7382 gate driver

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