SMPS Power Supply Circuits Primer & Tutorial Includes:
Switch mode power supply How does SMPS work Step down buck converter Step up boost converter Buck Boost converter
See also: Power supply electronics overview Linear power supply Capacitor smoothing Over-voltage protection PSU specs Digital Power Power management bus: PMbus Uninterruptible power supply
A simple buck converter can only produce voltages lower than the input voltage, and a boost converter, only voltages higher than the input. To provide voltages over the complete range a circuit known as a buck-boost converter is required.
There are many applications where voltages higher and lower than the input are required. In these situations a buck-boost converter is required.
Buck-Boost Converter basics
The buck-boost DC-DC converter offers a greater level of capability than the buck converter of boost converter individually, it as expected it extra components may be required to provide the level of functionality needed.
There are several formats that can be used for buck-boost converters:
- +Vin, -Vout: This configuration of a buck-boost converter circuit uses the same number of components as the simple buck or boost converters. However this buck-boost regulator or DC-DC converter produces a negative output for a positive input. While this may be required or can be accommodated for a limited number of applications, it is not normally the most convenient format.
When the switch in closed, current builds up through the inductor. When the switch is opened the inductor supplies current through the diode to the load.
Obviously the polarities (including the diode) within the buck-boost converter can be reversed to provide a positive output voltage from a negative input voltage.
- +Vin, +Vout: The second buck-boost converter circuit allows both input and output to be the same polarity. However to achieve this, more components are required. The circuit for this buck boost converter is shown below.
In this circuit, both switches act together, i.e. both are closed or open. When the switches are open, the inductor current builds. At a suitable point, the switches are opened. The inductor then supplies current to the load through a path incorporating both diodes, D1 and D2.
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