Cutting No-Load Power Consumption in Power Supplies for Industrial Applications

Jan 1, 2011 12:00 PM
Graham Proud CamSemi

Find a downloadable version of this story in pdf format at the end of the story.

There is a universal drive to reduce power consumption that Energy Star and similar initiatives have so successfully promoted. This is starting to take a real hold within industrial systems and embedded controllers, particularly their peripherals. Applications as diverse as fire alarms, process controls, and building automation systems very often operate on low duty cycles that potentially waste significant amounts of power, while they await the next cycle of computation and communications activities. As a result, designers are under pressure to develop systems that intelligently power down whenever it is practical to do so, and run as efficiently as possible under the expected set of active-mode conditions. These considerations demand power supplies that offer very low standby power consumption, together with the ability to quickly return to full output power while maintaining accurate regulation.

Naturally, any power supply solution must also offer a raft of complementary benefits - such as excellent electrical performance, robust fault protection, and minimal EMC issues - at the lowest cost that is consistent with reliable design.

CamSemi's recently-introduced C2163 is a universal-input, primary-side-sensing control IC that meets these criteria while offering a range of output voltages at power levels from about 8 W to 18 W to suit a wide variety of embedded applications. An evolution of the C2161/62 family that suits applications of up to 8 W, the C2163 substitutes an output stage that is tailored for driving the gate of a MOSFET in place of the bipolar-transistor emitter driver that appears in the original devices. This conceptually simple change allows the new six-pin SOT-23 packaged device to control substantially more power at minimal additional component count and cost, while retaining all of the benefits that the original parts deliver (Fig. 1).

OPERATING PRINCIPLES

The C2163's operating principles are easily understood by considering Fig. 1. The general circuit arrangement echoes a conventional flyback converter that includes an auxiliary power winding to supply the controller chip and an additional winding that provides a feedback sense voltage. All power conversion occurs in discontinuous conduction mode, with the C2163 varying its switching frequency and on-time in response to load demands. The device switches the main primary winding via Q1 and Qe, where Q1 is a high-voltage bipolar power transistor (700V blocking for universal-input designs) and Qe is a low-voltage logic-level MOSFET. This cascode-connected pairing is cheaper and typically more efficient than a single high-voltage MOSFET, and it benefits from quasi-resonant switching that turns the devices on when feedback detects that the voltage across primary switch Q1 is at its minimum. Together with a degree of natural frequency jitter that spreads emissions, this “soft” switching technique eases EMI concerns to minimize the need for conducted-emissions filtering.

At power-on, current flows from the high-voltage dc input rail via the high-tension resistors Rht to supply the C2163, which enters its initialization mode. The controller then issues several clock cycles that switch the main primary winding via Q1/Qe. This induces a potential in the auxiliary winding and operating current starts to flow via rectifier Daux. An internal shunt voltage regulator stabilizes the chip's supply and the device enters run mode, with feedback being applied via scaling resistors Rfb1 and Rfb2 and coupling capacitor Cfb. The network Rosc and Cosc sets the maximum oscillation frequency, which lies between 40 kHz and 66 kHz at full load.

The value of Cosc also determines the C2163's integral cable compensation level that automatically allows for a variety of PSU to load cable, track, or output filter inductor resistances, with a 1% minimum being set by design. Resistor Rcs sets the range of pulse-by-pulse current control and the nominal overcurrent point. Inspection reveals that the voltage developed at the chip's CS current-sense pin is negative with respect to chip ground, which is intentional if unusual.

The output range that this basic circuit offers spans voltage levels from 3 -24 VDC and 0.5-5 A at power levels of up to about 18 W. It betters the operating efficiency requirements of Energy Star EPS 2.0 and its recent embodiment within IEMP Level V - with at least 2% margin and has a no-load power level below 100 mW, which is a factor of three times lower than this benchmark specification requires. Importantly, a few minor circuit modifications can improve upon output power levels and drive down no-load power consumption to make the topology's application potential significantly more flexible, enabling cost reductions by amortizing design effort and rationalizing inventory holdings.

Continue on next page

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.