Designing an ac power adapter can be a daunting task because of the need to minimize its cost and size. Meeting this need is a new monolithic switchmode IC, the TOPSwitch-GX, intended for power adapters used in LCD monitors, laptops, set top boxes, etc. The Figure shows a flyback configuration using the 250W TOPSwitch-GX, the fourth generation version of the original TOPSwitch that was rated at 150W.
The CMOS "GX" integrates a high voltage power MOSFET, PWM control and fault protectioncircuits. TOPSwitch-GX extends the performance of the original TOPSwitch while integrating new functions to reduce system cost and improve design flexibility and energy efficiency. The "GX" provides the following characteristics.
Internal soft-start reduces the peak primary current during power supply start-up by limiting the duty cycle and current limit during the first 10 msec of operation. As well as reducing the output overshoot, this minimizes current and voltage stresses on the MOSFET, primary clamp components and output diodes. It improves reliability because this condition is a major cause of power supply failures.
Programmable current limit tolerance is 510%, including temperature variations. This allows use of a larger MOSFET with a lower R subscript DS(ON) or a more heavily continuous design needing a larger transformer core size. This lowers device dissipation, improving efficiency and reducing heat sink size - an important consideration for a sealed power adapter enclosure.
Power limiting reduces the size and cost of the secondary and primary clamp components needed to survive an overload condition. A power supply is designed to provide full power at the lowest input voltage. However, as the input voltage increases, so does the maximum overload power. Overload power can be as high as 250% of rated power at high line. Power limiting reduces this to 130%.
Wide duty cycle (75%) provides a longer on time of the internal MOSFET, allowing voltage across the input capacitor to fall lower while keeping the output in regulation. Thus, you can use a smaller size, lower cost input capacitor.
Line feed forward adjusts the duty cycle based on the ripple across the input capacitor. This lowers the output ripple and together with the wider duty cycle allows a smaller capacitor value.
Line over and under voltage sense stop the power supply if the input voltage is too low or too high. Line under voltage prevents the power supply from trying to start if the voltage is too low to reach regulation and prevents output glitches when the ac line is removed. Line over voltage increases reliability by shutting down the internal MOSFET when the line voltage is too high. When shut down, the MOSFET can withstand line surges of up to 495Vac.
Frequency jittering modulates the switching frequency 54 kHz, which spreads EMI harmonics over a wider frequency range and lowers the maximum value. This improves the quasi-peak value up to 5 dB and average measurements 10 dB, lowering the EMI filter's cost and size.
A 132 kHz switching frequency provides the optimum switching frequency for transformer size. Including all tolerances and temperature variations, this guarantees that the second harmonic is below the 150 kHz start of most typical conducted EMC standards. By maximizing the switching frequency, the transformer core size is minimized - lowering core cost and reducing board size.
Very tight tolerances help the design and lower system cost. Tolerancing is straightforward, speeding up the design process, and is vital for high volume manufacturing.
Although a given design may not need all the above features, just using one or two can have a dramatic effect on component choices and system cost.