Introduction to power principle

A key component of a DC power management system is a DC-powered power supply for the relevant system. The specific type of DC power management depends on its power input, including:

AC input - accepts AC power input, then rectifies and filters it, and the resulting DC voltage is connected to a voltage regulator circuit that provides a constant DC output voltage. There are a variety of AC DC power supplies with output voltages from less than 1 volt to several thousand volts. This DC power management system typically uses a switching power supply, and some linear power supplies are available.

DC Input - Receives a DC voltage input, typically 5 volts, 12V, 24V, or 48 volts, and produces a DC output voltage. At the low end, this type of power supply can produce less than 1 VDC, while other DC-DC power supplies can generate thousands of volts of DC. Here, power management usually uses a switched mode power supply.

Battery Input (for Portable Devices) - Due to the size and weight limitations of portable devices, this power management feature is often 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. Typically, an AC adapter is used to power the unit and to charge the system battery.

Ultra Low Voltage Input (Energy Harvest) - Energy harvesting provides the energy to charge, replenish or replace the battery. A key component in the energy harvester is a power converter that can operate with ultra-low voltage inputs. In operation, the power converter captures a small amount of energy, accumulates it, stores it, and then maintains the stored energy as a power source. Low voltage inputs can come from solar, thermal, wind or kinetic energy.

Linear and switching power supplies:

There are two basic power configurations with a DC power management subsystem: linear and switch mode. The linear power supply always turns on the current. Switch mode converts DC to a switching signal and then rectifies it to produce a DC output. The differences between these two configurations include size and weight, power handling capabilities, electromagnetic interference and regulation.

The main components of the linear regulator are through the transistor, error amplifier and voltage reference, as shown in Figure 1-1. The linear regulator maintains a constant output voltage by using an error amplifier to compare a portion of the output voltage to a stable voltage reference. If the output voltage tends to increase, the feedback causes the output voltage to be reduced by the transistor and vice versa. OEM linear power supplies can handle several amps of current. They are usually bulky desktop or racking supplies.

In most applications, old, high current linear power supplies have been replaced by switch mode power supplies. Figure 1-2 shows a typical isolated switch mode power supply. Here, the AC input voltage is rectified and filtered to obtain the DC voltage of other power components. A widely used method uses ON and OFF time pulse width modulation (PWM) to control the power switch output voltage. The ratio of the on time to the switching cycle time is the duty cycle. The higher the duty cycle, the higher the power output of the power semiconductor switch.

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

Isolation circuitry is required to maintain isolation between the output ground and the power supply to the power components. Typically, optocouplers provide isolation while allowing feedback voltage to control the output of the power supply.

The inductor-capacitor low-pass output filter converts the switching voltage from the switching transformer to a DC voltage. The filter is not perfect, so there is always some residual output noise called "ripple." The amount of ripple depends on the effectiveness of the low pass filter at the switching frequency. The power switching frequency can be between 100kHz and 1MHz. Higher switching frequencies allow the use of smaller, lower value inductors and capacitors in the output low pass filter. However, higher frequencies can also increase power semiconductor losses, which reduces power supply efficiency.

The power switch is a key component of the power supply in terms of power consumption. The switch is usually a power MOSFET that operates in only two states. In the off state, the power switch draws very little current and consumes very little power. In the on state, the power switch draws the maximum amount of current, but its on-resistance is low, so its power consumption is minimized in most cases. In the transition from the on state to the off state and from the off state to the on state, the power switch passes through its linear region, so an appropriate amount of power can be consumed. Therefore, the total loss of the power switch is the sum of the switching state plus the transition through its linear region. The actual loss depends on the power switch and its operating characteristics. Table 1-1 compares the characteristics of isolated, AC-DC linear, and switch-mode power supplies.