Switching Mode Power Supply (SMPS)

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Summary: Switching Mode power supplies are a smaller and more efficient than the simpler linear power supply.

SMPS

  • Switch Mode Power Supply
    • This term is not used due to Motorola claiming a trademark on SWITCHMODE.
  • Switching Mode Power Supply
  • Switcher
  • Switching power supply
  • Switching regulator
  • SMPS

History

One of the main users of this type of power supply is the computer industry. Switching power supplies (SMPS) have been available commercially since about 1970. Hewlett Packard used one in the HP-35 calculator (1972) to provide power for the LED display and timing signals.

The concept dates back to Kettering's inductive discharge ignition system (1910), based on the discovery in 1836 of the ability of induction coils to boost voltages. In 1936 the "vibrator" power supply appeared, used in car radios to boost the 6V battery voltage to a higher voltage needed for the vacuum tubes.

Later advances in semiconductor technology resulted in solid state switches which could be used to build a switching power supply. In 1977 the Apple ][ used a switching power supply to build a small, lightweight home computer.

Advantages

Switching power supplies are preferred when small size and high efficiency is desired. The typical linear power supply is very inefficient, losing a lot of energy to heat and noise in the transformer and filters. To provide adequate regulation of current or voltage, even more energy will be dissipated in the form of heat. This requires a larger footprint for the heatsinks and adequate airflow around the unit for convection cooling. Forced air cooling will generate additional noise and consume some energy in the process. A switching mode power supply avoids many of those losses, resulting in a smaller, more efficient unit.

A switching mode power supply rectifies the incoming Alternating Current directly, which is fed to a chopper. The chopper is a high frequency switch. The resulting waveform is fed to a transformer to be boosted or reduced, as needed. The output is then rectified and filtered to provide a very smooth and stable Direct Current output. By controlling the chopper, the SMPS is capable of very tight regulation (control) of the output voltage. Since the SMPS works with high frequencies, it is easier to filter and smooth the second rectifier's output using small chokes and filter capacitors. A linear power supply would use large chokes and capacitors to filter the 120 Hz (or 100Hz if the power supply is 50Hz) output from the rectifier. The SMPS uses frequencies which can be 50 kHz or more. This allows for a small package with little loss to heat, and almost silent operation.

SMPS Block Diagram

Compared to a Linear Power Supply

While a linear power supply is simple to construct, a SMPS is smaller and more efficient. Their design also allows it to operate on a wide range of voltages and line frequencies, with quiet operation and good regulation. The linear power supply, while simpler, wastes a lot of energy in the transformer, the filters, and the regulation circuits. It can also be much larger compared to a similar SMPS.

  • Size
    • Linear power supplies are much larger
  • Efficiency
    • Linear: 30 - 40%
    • SMPS: Can be as high as 95% in an optimized design. Typically 60 - 70%
  • Output Voltage
    • Linear: Can very very good when regulated. With no regulation output will vary with the input voltage
    • SMPS: Very stable output regardless of input

Features of the SMPS

Double Insulation Symbol (Class 2)

Both designs incorporate transformers to isolate the line (mains) side from the low voltage (LV) outputs.

Many power supplies are double insulated, indicated by the box within a box symbol. For safety, never connect the GND terminal to your earth ground.
  • Regulation
  • Small Size
  • Can be almost silent in operation
  • Efficient

Notes

  • Regulation is the ability of the power supply to maintain the output under changing load and input voltage conditions
  • A choke is a coil of wire wound around an iron core, also known as an inductor. It stores energy (current) in a magnetic field, resisting any change in the current.
  • A filter capacitor stores energy (voltage), and resists any change in the voltage
ZVS SMPS
Rectification