Switched Mode Power Supply (SMPS), sometimes called Switcher is gaining popularity in the last decade or so. The SMPS is more efficient and lighter in weight as compared to its conventional counterpart, the linear power supply.
Generally, electronic circuits require a direct current (DC) voltage source in order to operate - typically 3 to 12 volts. Therefore, every electronic device that is connected from the AC mains should include a circuit that will derive a stable DC voltage from the 120 or 240 volts (AC) that the utility supplies. This is made possible by a device called a power supply. In the past, electronic circuits that require DC to operate, utilizes power supplies that are bulky and relatively inefficient. These so-called linear power supplies have a step-down transformer supplying a low voltage to a full-wave (bridge) or half-wave (simple diode) rectifier.
The Switched Mode Power Supply or SMPS has a more complicated design than their linear counterparts. Its design permits either an AC or DC input, and can also provide either an AC or DC voltage output, depending on circuit topology.
Generally, the process involves the following basic flow:
|Basic Diagram of a Switched Mode Power Supply|
1. AC supply voltage is converted to DC using a rectifier.
2. The resulting DC voltage is fed to a “chopper” stage including highly efficient MOSFET switching transistors. It is important to note that the transistors’ gates are fed by an oscillator providing a constant frequency, pulse width modulated (PWM) signal, in order to “chop” the DC into a high frequency AC voltage.
3. Subsequently, the high frequency AC voltage is supplied into the primary winding of a high frequency transformer.
4. Then, the resulting transformer secondary winding voltage is rectified to DC for use in the circuits where it is required.
5. Voltage regulation is performed by a circuit that samples the output voltage, and signals the PWM oscillator to change the ratio of transistor ON time to OFF time to maintain a constant voltage output.
Advantages and Disadvantages
The switched mode power supply design has a couple of remarkable advantages over its conventional counterpart.
· Smaller transformer can be used because of the high frequency of operation, resulting to a lighter power supply unit.
· Higher efficiency of about 80% to 90% means lesser energy is wasted as heat.
Moreover, it is also flexible. For example, if an AC input voltage is not available, such as in an automotive applications, one can simply remove the rectifier stage from the power supply input, and use a high frequency transformer with the appropriate turns ratio to produce the required output voltage.
On the other hand, the main disadvantages of the switched mode power supply are its higher cost and the presence of harmonics in its output voltage. The extra cost is due to the complexity of the SMPS design, although this is a good exchange for the benefits described above. Meanwhile, the pulsed nature of the output contains a high level of harmonics, which can flow back out to the utility’s distribution system, harmfully affecting other equipment and its wiring.