Looking for a design scheme for a buck-boost converter circuit
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# Looking for a design scheme for a buck-boost converter circuit

Posted Date: 2024-02-02

A buck-boost converter is a DC-DC converter that uses the same principles of buck and boost converters in a simplified combination circuit.

The main feature of a buck-boost converter is that it keeps the output voltage constant even if the input voltage is lower than the output voltage, meaning the circuit can operate in both buck and boost modes depending on the input voltage.

Buck-boost converters are mainly composed of electronic components such as switching tubes, inductors, diodes and capacitors. When the input voltage is applied to the inductor through the switching tube, the inductor stores energy while the current remains unchanged. At the same time, the charge on the output capacitor begins to be released, providing a stable output voltage to the load. When the switch is turned off, the energy in the inductor flows to the output capacitor and load through the diode, causing the output voltage to gradually increase or decrease.

Buck-boost converters are very similar to flyback converters, but buck-boost converters use a single inductor instead of a transformer.

There are two different topologies of buck-boost converters: inverting buck-boost converter and non-inverting buck-boost converter.

Technical Principle of Buck-Boost Converter

Buck-boost converters are equivalent to flyback converters using a single inductor, and they have two main topologies: inverting and non-inverting. The output voltage polarity of the inverting type is opposite to the input voltage, while the output voltage polarity of the non-inverting type is the same as the input voltage. This topology allows the buck-boost converter to be used flexibly in different applications, especially in scenarios where a negative output voltage is required.

Buck-Boost Converter Selection

1 Output voltage range (maximum and minimum output voltage) and converter output type

Buck-boost converters can support fixed or adjustable output voltages, and there are also programmable types.

2 switching frequency

Low-power buck-boost converters typically operate between 500kHz and 3MHz.

3 soft start

It is important to have a "soft-start" function, which allows the output voltage to rise slowly and in a controlled manner to avoid output voltage overshoot at startup.

4 Quiescent current (Iq)

5 Duty ratio (D)

Duty cycle = switch on time/working period. Turning on and off the switch affects the storage and release of inductor energy, thereby affecting the output voltage.

work process

In a buck-boost converter, when the switch is turned on, the input voltage is applied across the inductor, causing the inductor to store energy. At the same time, due to the reverse blocking characteristics of the diode, the output voltage remains unchanged and the load operates normally. When the switch is turned off, the inductor releases energy and current flows through the diode to the output capacitor and load. Due to the energy transfer characteristics of the inductor, the size of the output voltage depends on the energy transfer of the inductor and the discharge of the output capacitor. As the inductor releases energy, the output voltage gradually increases or decreases until the desired output value is reached.

Application scenarios

Buck-boost converters are widely used in a variety of application scenarios. For example, in some scenarios where high voltage needs to be converted into low voltage, such as electric vehicle chargers, LED lighting, etc., buck-boost converters can provide flexible power management solutions. In addition, in some scenarios where low voltage needs to be converted into high voltage, such as wireless charging, USB fast charging, etc., buck-boost converters can also provide optimized power management solutions.