Solving SMPS design challenges with Quasi-Resonant Converter (QRC) power switches

Today’s electronic manufacturers continue to look for green and highly-reliable solutions for SPMS design. Reliability is pivotal in these designs as power supplies employ protection schemes against various fault situations, such as over-load, over-voltage, output short, output diode short and over-temperature. However, such protection can require many additional circuits, resulting in increased system cost.

Fairchild’s FSCQ series eliminates the need for extra components by incorporating selfprotective functions in its design. These functions include Over-Load Protection (OLP), Abnormal Over-Current Protection (AOCP), Over-Voltage Protection (OVP) and Thermal Shutdown (TSD). By fully integrating these circuits into the IC, the FSCQ series eliminates the need for external components, therefore increasing reliability while decreasing cost.

 

Standby

Another compelling issue that SMPS designers must grapple with is the energy wasted by electrical appliances when in standby mode. Although electrical appliances require electricity for standby functions, most standby power is consumed by an inefficient power supply and unnecessarily energised components. Typically, a conventional SMPS offers inefficient power conversion, mainly because the losses become dominant as the output load decreases, especially at light-load conditions. This creates a problem when SMPSs must meet power efficiency or green standards that require low standby power.

The FSCQ series features burst-mode operation for standby mode and satisfies the International Energy Agency’s (IEA) 1W initiative aimed at reducing standby power losses to below 1W. In burst mode, the power switch can alternately enable and disable the switching operation, which reduces the effective switching frequency. This method reduces the switching loss in the MOSFET and the hysteresis loss in the transformer. The functional blocks in the PWM controller are also disabled in standby mode, thus the operating current can be reduced, which minimises power consumption in the PWM IC.

 

QRC power switches

Compared to conventional hard-switched converters with fixed switching frequencies, the QRC topology has become a very attractive alternative for power-supply designers. The increasing popularity of the QRC approach is based on its ability to reduce EMI while increasing power-conversion efficiency.

This design note describes the basic operation principle behind a QRC and introduces an integrated QRC power switch that offers distinct advantages over typical discrete-MOSFET and PWM-controller solutions. It also explains how the power switch, designed especially for quasi-resonant off-line SMPS, is able to reduce total design costs, component count, size and weight, while simultaneously increasing efficiency, productivity and system reliability.

 

An integrated power switch designed for QRC

One technology that specifically addresses QRC designs is the the FSCQ series of integrated Green FPS™ power switches introduced by Fairchild Semiconductor. This technology requires minimal external components by integrating a PWM controller and a SenseFET into one device. Figure 1 shows the internal block diagram of FSCQ series power switches.

 

 

A common nuisance when implementing an SMPS is the difficulty of sensing the MOSFET current. The typical method is to use a sensing resistor. However, this approach causes severe heat dissipation and has other limitations, especially for high-power applications. To solve this problem, FSCQ series employs a fully avalanche-rated SenseFET. While a conventional MOSFET has three pins, a SenseFET offers one additional pin to sense the MOSFET current. This ability to sense the MOSFET results in virtually zero power dissipation and very low noise.

The major drawback of applying a QRC topology is that it causes switching frequency to increase during light-load conditions. As the load decreases, the peak drain current diminishes and, therefore, the switching frequency increases (Figure 2). This results in severe switching losses during light-load conditions, as well as intermittent switching and audible noise. Because of these problems, the QRC topology has limitations in a wide range of applications.

 

 

To overcome switching-frequency problems, the FSCQ series employs extended quasi-resonant switching operation (Figure 2). When the switching frequency exceeds 90kHz as the load decreases, the FSCQ series device ignores the first minimum value and turns on the MOSFET when the drain-to-source voltage reaches its second minimum value, therefore reducing switching frequency. The switch then goes back to its normal quasiresonant operation when the switching frequency reaches 45kHz as the load increases.

 

Conclusion

Due to the EMI and efficiency benefits of using a QRC topology in SMPS, designers increasingly use this approach in their power-supply designs. Traditionally, they have used discrete-pluscontroller solutions to enable basic quasi-resonant flyback converter approaches. But, today, a power switch integrating a SenseFET with a PWM IC, provides a basic platform that is well suited to cost-effective designs of quasiresonant switching flyback converters. By integrating protection features and employing burst-mode operation, this power-switch technology greatly increases reliability while offering low power consumption to meet stringent energy-efficiency standards.

 

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