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.
