Embedded Systems go on a Power Diet

It used to be the case that embedded systems just had to be small enough to fit in the space available, and powerful enough to handle the processing job at hand. Not anymore. Increasingly, embedded electronics are designed with power consumption in mind. The reason, of course, is a combination of new “green” power-efficiency standards and an explosion in battery- powered uses where operating life carries a premium.

One efficiency metric kicking in this year applies to computers and servers. The 80 Plus standard dictates these devices have an energy efficiency of 80% or greater at 20, 50, and 100% of rated load with a true power factor of 0.9 or greater. The only way to reach this level of operation is with a power-factor controller and by using some clever phase manipulation in the ac/dc power converter.

Another area drawing attention is that of soft switching, more commonly known as zero-voltage or zero-current switching. Soft switching has long been viewed as a way to cut down on the generation of electromagnetic interference. In an energy-efficient world, however, it is a way to reduce the amount of power dissipated in semiconductor switches.

Innovation can also be found in the area of microcontrollers. The easiest way to lower power consumption in computer chips is to put the device into a sleep mode when it isn’t busy. But chipmakers are striving to lower power consumption even when chips are sleeping. Another technique is to scale the speed of the clock so chips operate only as fast as they need to for the job at hand.

Interleaving in pulsewidth- modulated power supplies is a key technique employed to hit the Energy Star 80 Plus spec. The idea is to synchronize the PFC and PWM switching to reduce the harmonic content of the resulting current. The MC56F8013 in this diagram is a Freescale processor often used to implement power factor control.

HERE COME EFFICIENCY REGS
If you’ve bought a PC in the last several months, you may already have been affected by the 80 Plus spec. 80 Plus is now part of the Energy Star computer specification. Manufacturers figure 80 Plus supplies are about 33% more efficient than prestandard units. Moreover, they drastically cut down the harmonic distortion induced in the utility lines, thus increasing the life of the distribution transformers in the utility system.

Today, the typical electronic power supply is built with a pulse-width-modulated (PWM) topology. The idea is to rectify ac to dc, then use a PWM circuit to produce pulsed dc at a frequency much higher than that of the ac mains. The high-frequency pulsed-dc is then filtered to produce a constant dc for powering the load.

Power-factor correction (PFC) takes place after rectification and before PWM. The basic behavior that PFC corrects is the creation of current spikes on the ac line that result when current begins to conduct through the ac bridge diodes in the power supply. Current conducts to charge up the capacitor that is across the bridge and to power the load. Conduction takes place at a relatively high point on the ac voltage waveform, so the resulting spikes can have a substantial amount of energy.