How green is my power supply?
Most electronic devices work best with direct current power. Thomas Edison advocated dc power distribution, but we use George Westinghouse’s ac power distribution system because, as Nicola Tesla pointed out, you can move electricity over long distances much more efficiently as high-voltage, low-current ac.
Most electronic devices work best with direct current power. Thomas Edison advocated dc power distribution, but we use George Westinghouse’s ac power distribution system because, as Nicola Tesla pointed out, you can move electricity over long distances much more efficiently as high-voltage, low-current ac. Once you get electric power where you need it, however, the fact remains that you need dc to do anything useful but make light and heat.
DC power supplies consist of a rectifier section that produces unregulated dc, and a regulator section that stabilizes the output level. The final result is 'clean' dc power suited to the final, useful electronic device.
A dc power supply’s job, then, is to convert ac supply current to dc at whatever voltage your end equipment needs. As shown in “Linear regulated power supply,” a dc power supply does the conversion in two stages: rectification and regulation.
Rectification converts ac current, which periodically switches flow direction, to dc current, which doesn’t. The full wave bridge rectifier shown puts out pulsating dc, which includes a heavy dose of ac mixed in with the dc. Generally, a low-pass filter follows to clean out most of this residual ac.
Regulation provides control. Linear regulators control the output voltage or current with a network of variable resistors controlled by feedback from the output.
The problem with linear regulators is that those transistors dissipate electric power as heat. Linear power supplies generally waste as much power as they deliver. Not good!
The essential difference between “Switched-mode power supply” (see graphic; often called “switching supplies” or simply “switchers”) and linear power supplies is how they regulate current. Instead of keeping dc current flowing at all times, then regulating it with partially turned-on transistors, switchers cycle their regulating transistors between fully turned on and fully off. Regulation comes from modifying the duty cycle (fraction of time the switch is on) based on feedback from the supply’s final output.
The regenerative filter following the pulse-width modulator smooths the pulses, averaging the current flow to produce a smooth dc output. I use the term “regenerative” to point out that there must be a dc path to ground bypassing the output under low- or no-load conditions. Otherwise the filter capacitors would charge up and regulation would be lost.
Switched-mode power supplies regulate dc power more efficiently than linear supplies by rapidly switching their regulating transistors between fully on and fully off. Controlling the duty cycle to provide the correct average at the filter's output provides regulation.
When a transistor is fully turned on, its resistance is at minimum—relatively close to zero. The power dissipation is therefore very close to zero because any current (squared) multiplied by zero resistance is zero.
When a transistor is fully turned off, its resistance is very high, but the current through it is practically zero. In that case, the power dissipation is practically zero as well because any high, but finite resistance times zero current (squared) is again zero.
Power loss, therefore, is much smaller than what a linear regulator would dissipate. Because regulator dissipation is so much lower, overall efficiency is higher, and heat generation is lower. Less heat generation means less heat transfer to the environment, which means the supply can be packaged in a smaller block. With less heat wasted, there is less draw from the power lines, so the step-down transformer can be smaller, which reduces its loss as well. Less powerline draw means less environmental pollution at the generating plant as well, making switch-mode power supplies environmentally friendly.
C.G. Masi is senior editor. Reach him at email@example.com .