A switch-mode power supply is an electronic power supply unit that incorporates a switching regulator.
In linear power supply uses a linear regulator. Linear regulator uses a transistor biased in its active region to specify an output voltage. But in SMPS actively switches a transistor between full saturation and full cutoff at a high rate.
In SMPS, the active device that provides regulation is always operated in a swithched mode i.e. it is operated either in cut off or in saturation.
The input DC is chopped at a high frequency (10KHz to 100KHz) using an active device and the converter transformer. So the transferred chopped waveform is rectified and filtered.
A sample of the output voltage is used as feedback signal for drive circuit for the switching transistor to achieve the regulation.
The use of feedback mechanism to alter the output voltage as per the requirement of the load. Hence SMPS is more efficient than linear power supply.
SMPS Block Diagram Explanation:
The diagram above shows the SMPS block diagram explanation in which 5-blocks are present i.e. input rectifier and filter, inverter chopper, output transformer, output rectifier and filter, and chopper controller.
Input rectifier and filter stage:
If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification.The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor.
If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 VAC) range.
If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of DC voltages that will be produced by the rectifier stage.
Inverter Chopper stage:
The inverter Chopper stage converts DC, whether directly from the input or from the rectifier and filter stage described above. To AC by running it through a power oscillator, whose output transformer is very small with few winding at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20kHz, to make it inaudible to humans.
The output voltage is optically coupled to the input and thus very tightly controlled . The switching is implimented as a multistage MOSFET amplifier. MOSFETS’s are a type of transistor with a low on-resistance and a high current-handling capacity.
Output transformer or PWM:
If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer.
Similarly this converts the voltage up or down to the required output level on its secondary winding. So the output transformer in the block diagram serves this purpose.
Pulse-width modulation (PWM) sets the on and off duty cycle of the devices. The output voltage of the power supply is determined by this timing. Other features designed into the PWM controller help regulate, stabilize and provide the requirements for higher load current and instantaneous current.
Output rectifier and filter:
If a DC output is required the AC output from the transformer is rectified.
For output voltages above ten volts or so. ordinary silicon diodes are commonly used. For lower voltages, Schottky diodesaew commonly used as the rectifier elements, they have the advantages of fasteer recovery times than silicon diodes and a lower voltage drop when conducting.
The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.
A feedback circuit monitors the output voltage and compares it with a reference voltage, which is set manually or electrically to the desired output.
If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the mosfets are switched on and off.
This parts of the power supply is called the switching regulator. The Chopper Controller shown in the block diagram serves this purpose.
Advantages of SMPS over a linear power supply:
Better power efficiency.
Lower heat generation.
Disadvantages of SMPS over a linear power supply:
SMPS’s are generally more complex than linear supplies.
Generate high-frequency electrical noise that may need to be carefully suppressed, and have a characteristic ripple voltage at the switching frequency.