This article gives the information of SMPS devices. SMPS or switched mode power supply is an electronic power supply unit that incorporates a switching regulator. In linear power supply uses a 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 cut off at a high rate. In SMPS the active device that provides regulation is always operated in a switched mode i.e. it is worked in either in cut off or in saturation. The input given to DC is chopped at a high frequency (10 KHz to 100 KHz) using an active device and the converter transformer.
A sample of the output voltage is used as feedback signals for devices circuit for the switching transistor to achieve the regulation. The use of feedback mechanism to alter the output voltages as per the requirement of the load. Hence SMPS is more efficient than linear power supply.
Working of SMPS:
Block Diagram of SMPS.
Input Rectifier and Filter Stage:
If the SMPS has an AC input then its job is to convert the input to DC. This is called as rectification. The rectifier gives an unproper DC voltage which is then sent to large filter capacitor. If an input range switch is used the rectifier stage is usually configured to operate as a voltage double when operation on low voltage (~120 VAC) range and as straight rectifier when the operation on the high voltage (~240 VAC) range. If an input frequency switch is not used then a full a wave rectifier is used and the downstream inverter stage is simply designed to be flexible enough to accept wide range of dc voltages that will b produced by the rectifier stage.
Inverter “Chopper” Stage:
The inverter chopper stages converts DC whether directly from the input or from the rectifier and stages described above, to AC by going it from a power oscillator whose output transformer is very small with few windings at frequency of tens or hundreds of kilohertz. The frequency may be chosen to be above 20 KHz to make it inaudible to humans.
If the output wants 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.
Output Rectifier and Filter:
If a DC output is required the AC output from the transformer is rectified. The rectified output is then smoothly by a filter consisting of inductors and capacitors. For higher switching frequency components with lower capacitance and inductance are needed.
A feedback circuit monitors the output voltages and compares it with a reference voltage which is set manually or electronically to the completely output. If there is a problem in the output voltage the feedback voltage circuit compensates by adjusting the timing with which the MOSFETs are switched on and off.
Advantages of SMPS :
- Smaller size.
- Better power efficiency
- Lower heat generation
Disadvantages of SMPS :
- SMPS s generally more complex than supplies.
- SMPS generate high frequency electrical sound that may need to be carefully suppressed.