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Part 1: The Power Supply
Chapter 1: Power Supply Fundamentals (Preview)

The key component of the dc power management system is the power supply that provides dc power for the associated system. The specific type of dc power management depends on its power input, which includes:

AC input—A power supply that accepts an ac utility power input, rectifies and filters it, then applies the resulting dc voltage to a regulator circuit that provides a constant dc output voltage. There is a wide variety of ac-dc supplies that can have an output voltage from less than 1V to thousands of volts. This dc power management system usually employs a switch-mode power supply, although some linear supplies are available.

DC input—A power supply that accepts a dc voltage input, typically 5 V, 12V, 24V, or 48 V and produces a dc output voltage. At the low end, a supply of this type can produce less than 1Vdc, whereas other dc-dc supplies can produce thousands of volts dc. Here, power management usually employs a switch-mode power supply.

Battery input (for portable equipment)—Because of size and weight restrictions of portable equipment, this power management function is usually integrated with the rest of the electronic system. Some of these systems also include an ac adapter, which is a small power unit that plugs into the ac wall outlet and provides a dc output voltage. Usually, the ac adapter is used to power the unit and can also recharge the system battery.

Ultralow voltage input (energy harvesting)—Energy harvesting can provide the power to charge, supplement, or replace batteries. A key component in energy harvesting is a power converter that can operate with ultralow voltage inputs. In operation, this power converter captures a minute amount of energy, accumulates it, stores it, and then maintains the stored energy as a power source. Low-voltage inputs can come from solar power, thermal energy, wind energy, or kineticenergy.

Linear vs. Switch-Mode Power Supplies
There are two basic power supply configurations used with dc power management subsystems: linear and switch-mode. Linear power supplies always conduct current. Switch-mode supplies convert dc to a switched signal that is then rectified to produce a dc output. Differences between these two configurations include size and weight, power-handling capability, EMI, and regulation.

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The linear regulator’s main components are a pass transistor, error amplifier, and voltage reference, as seen in Fig. 1-1. The linear regulator maintains a constant output voltage by using the error amplifier to compare a portion of the output voltage with a stable voltage reference. If the output voltage tends to increase, feedback causes the pass transistor to lower the output voltage and vice versa. OEM linear supplies can handle several amperes of current. They are usually bulky benchtop or rack-mounted supplies.

In most applications, older, high-current linear supplies have been superseded by switch-mode supplies. Shown in Fig. 1-2 is a typical isolated switch-mode supply. Here, the ac input voltage is rectified and filtered to obtain a dc voltage for the other power-supply components. One widely used approach uses the on and off times pulse-width modulation (PWM) to control the power-switch output voltage. The ratio of on time to the switching period time is the duty cycle. The higher the duty cycle, the higher the power output from the power semiconductor switch.

The error amp compares a portion of the output voltage feedback with a stable voltage reference to produce the drive for PWM circuit. The resulting drive for the PWM controls the duty cycle of the pulsed signal applied to the power switch, which in turn controls the power-supply dc output voltage. If the output voltage tends to rise or fall, the PWM changes the duty cycle so that the dc output voltage remains constant.

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