Voltage Regulation in SMPS using PWM technique

SMPS

Switched Mode Power Supply (SMPS):

It  is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage                            

Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time.   

They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

SMPS can be operated using a Error Amplifier, but latest development is done using Pulse Width Modulation.1

PULSE WIDTH MODULATION (PWM)

PWM is a powerful technique for controlling analog circuits with a processor’s digital outputs. PWM is employed in a wide variety of applications ranging from measurement and communications to power control and conversion. PWM uses a square wave whose  duty  cycle is modulated resulting in the variation of the average value of the waveform. PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.

High frequency PWM power control systems are easily realizable with semiconductor switch. The discrete on/off states of the modulation are used to control the state of the switch which correspondingly controls the voltage across or current through the load. The major advantage of this system is the switch are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Realistically, semiconductor switches such as

MOSFETs or BJTs are non-ideal switches, but high efficiency controllers can still be built. 

Operation:

            Output of the transistor Q1 is sensed back to the Error Amplifier(EA) through sampling resistors. The other input to the EA is Reference voltage(Vref). The output voltage of EA is fed to the Pulse Width Modulator(PWM), other input to PWM is oscillator signal which can be Sawtooth waveform or Triangular waveform.

            Output of the PWM is a rectangular waveform. The width of the rectangular waveform is dictated by the output voltage of EA. This pulse can be used to drive the transistor Q1 through Driver.

            When the width of the pulse is varied, the ON time of transistor Q1 will also vary and consequently the amount of energy taken from the input voltage. So, by controlling Duty cycle, one can stabilize the output voltage.

            Let consider, the output is less than the required voltage then the output of EA will be more, therefore the ON time will be more and transistor Q1 will feed more power to the load .

·        In real-time scenario this total control is accomplished in an IC

Ø But, the transistor in the IC cannot drive the output. So, a driver is used to drive the pulse needed for the output.