ADVANTAGES OF A SWITCHING REGULATOR
The primary advantages of switching regulators are higher efficiency and smaller size. Conventional linear series and shunt regulators operate in a continuous conduction mode, dissipating relatively large amounts of power. The efficiency of linear regulators is typically around 40% to 50%. When the input-to-output voltage differential is large, the resultant efficiency is much lower than 40%.
Switching regulators have typical efficiencies of 60% to 90%; much higher than either the linear series or shunt regulator. Switching regulators achieve their higher efficiency as a result of three factors:
- The power-transistor switch is always turned completely on or off, except when it is switching between these two states, resulting in either low voltage or low current during most of its operation.
- Good regulation can be achieved over a wide range of input voltage.
- High efficiency can be maintained over wide ranges in load-current.
Switching regulators use the on-off duty cycle of the transistor switch to regulate the output voltage and current. By using a frequency much higher than the line frequency (typically 20 kHz to 500 kHz) the transformers, chokes, capacitors, and other filter elements can be made smaller, lighter, and less costly. The smaller elements used in switching regulators result in smaller power losses than the larger components used in linear regulators. The highest cost elements of a switching power supply are the transistor switches. The remaining costs, in descending order, are due to the magnetic components, capacitors, and rectifiers.
DISADVANTAGES OF A SWITCHING REGULATOR
Switching regulators can generate some electromagnetic and radio frequency interference (EMI/RFI) noise due to high switching currents and short rise and fall times. EMI/RFI noise, which is generated at higher frequencies (100 kHz to 500 kHz), is easily filtered. In those applications where a large series impedance appears between the supply and the regulator, the rapid changes in current also generate a certain amount of noise.
These problems may be overcome or significantly reduced by one or more of the following steps:
- Reducing the series impedance.
- Increasing the switching time.
- Filtering the input and output of the regulator.
Switching regulators with a fixed frequency are easier to filter than regulators with a variable frequency because the noise is at only one frequency. Variable frequency regulators with a fixed “on” time increase or decrease the switching frequency in proportion to load changes, presenting a more difficult filtering problem.