Introduction to power principle (II)

In most applications, old, high current linear power supplies have been replaced by switch mode power supplies. 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.

Voltage regulation IC:

The output voltage of all power supplies is almost always dependent on the voltage regulator IC. These ICs draw a DC input from a rectified AC or battery. In operation, the voltage regulator feeds back the percentage of its output voltage compared to a stable reference voltage. If the output voltage tends to rise or fall compared to the reference voltage, the feedback keeps the output constant.