Power Electronics

Power-Supply ICs Boost Efficiency Over Load Range

Power Integrations' TOPSwitch-HX family of ac-dc power-conversion ICs help power supplies achieve high efficiency across the load range. In doing so, these chips enable a variety of electronic products to meet proposed and existing energy standards such as ENERGY STAR without resorting to the use of a separate standby supply. This improvement over existing power-conversion chips is intended to reduce power-supply costs in audio and video products, as well as appliances.

These latest additions to the TOPSwitch product line are monolithic ICs that integrate a 700-V power MOSFET with a controller and supervisory functions. The HX devices implement the primary-side functions of switching power supplies that can deliver up to 48 W of output without a heatsink, or up to 150 W when mounted to a heatsink (Fig. 1).

The HX family contains several different models in different package styles and with different sized MOSFETs and drivers. So, power ratings will vary from device to device and with power-supply packaging. The datasheet quotes ratings for both adapter and open-frame power-supply designs.

A supply built using a TOPSwitch-HX device can deliver 50-W maximum output with 85% efficiency on the main 5-V output, and then under light loads, deliver 2 W of output from a 5-V standby supply with 60% efficiency. Both the main supply and standby supply would be generated using a single TOPSwitch-HX IC, as depicted in Fig. 1.

Although this approach requires an extra winding on the main transformer for the standby supply, TOPSwitch-HX eliminates the need for a separate standby switch and a separate standby transformer. Those components were previously needed to achieve similar efficiency at full and light loads when using previous generations of TOPSwitch or competing devices. In the case of the older TOPSwitch parts, the main power supply was only 40% efficient at light load, so a separate power supply was needed to achieve the required efficiency at light load.

Fig. 2 illustrates the improvement in efficiency TOPSwitch-HX offers for a 1-W input when compared with an existing technology, TOPSwitch-GX. No-load power consumption for HX parts is less than 200 mW.

The improvement in efficiency across the load range is achieved using a new multimode control scheme. The control algorithm seamlessly transitions between four different operating modes to optimize energy efficiency. At high loads, the IC uses a fixed-frequency PWM control technique. As load decreases, the controller transitions to a variable-frequency mode to maintain constant efficiency.

Further reduction in load causes the controller to revert to a lower fixed-frequency PWM mode that avoids audible frequencies. Finally, at very low loads, the controller transitions into a cycle-skipping mode that delivers maximum power to the output for 1- W input and consumes minimal power in standby.

Another feature of TOPSwitch-HX is its tightly controlled power coefficient (I2f) parameter. Control of this parameter greatly reduces overload power, allowing designers to reduce design margins and cut system costs.

Complete documentation, including details of a reference design kit, can be found at www.powerint.com. Pricing in 1000-piece quantities for a representative device, the TOP258PN, a 35-W rated part in a DIP package, is $1.57 each. Production quantities of TOPSwitch-HX ICs are available in DIPs, SMDs, TO-220s and SDIPs.

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