# Principal analysis of BUCK / BOOST circuit, a summary is also in place

2023-03-18Archive

Dollar Converter

Also known as a buck converter, this is a single-tube non-isolated DC-DC converter whose output voltage is lower than input voltage. In figure, Q represents switching tube, and its control voltage is usually a PWM (pulse, pulse width, pulse width modulation) signal. The signal period is Ts, then signal frequency is f=1/Ts, conduction time is Ton, off time is Toff, then period is Ts=Ton+Toff, duty cycle is Dy=, Ton/Ts.

Accelerated Converter

Also known as a boost converter, this is a single-tube, non-isolated DC/DC converter whose output voltage is higher than input voltage.

The switching tube Q is also PWM controlled, but maximum duty cycle Dy must be limited and it cannot work in Dy=1 state. The inductor Lf is at input and is called step-up inductor. The Boost converter also has two modes of operation: CCM and DCM. Up/Down Converter

Also known as a buck-boost converter, this is a single-tube non-isolated DC-DC converter whose output voltage can be lower or higher than input voltage, but output voltage polarity is opposite to input voltage. The Buck/Boost converter can be thought of as a Buck converter and a Boost converter connected in series, and switching tube is combined. The Buck/Boost converter also has two modes of operation, CCM and DCM, and Q switching tube is also PWM controlled.

LDO Characteristics

① Very low input-output voltage difference

② Very little internal loss

③ Slight temperature drift

④ High output voltage stability

⑥ Wide operating temperature range

⑦ Wide input voltage range

⑧ The peripheral circuit is very simple and convenient to use

DC to DC conversion is conversion of a fixed DC voltage to an AC DC voltage, also known as DC interruption. There are two chopper operation modes, one is pulse width modulation mode, Ts is unchanged and ton is changed (common), and other is frequency modulation mode, ton is unchanged and Ts is changed (light). interfere). His specific schema consists of following categories:

(1) A step-down circuit is a step-down chopper whose average output voltage U0 is less than input voltage Ui and has same polarity.

(2) The booster circuit is a step-up chopper, its average output voltage U0 is greater than input voltage Ui, and polarity is same.

(3) Buck-Boost circuit - step-down or step-up chopper whose average output voltage U0 is greater or less than input voltage Ui, polarity is reversed, inductance is transferred.

(4) The Cuk circuit is a step-down or step-up chopper, its average output voltage U0 is greater or less than input voltage Ui, polarity is reversed, and capacitance is transferred.

DC-DC is divided into three types of DC-DC: BUCK, BUOOST and BUCK-BOOST. Among them, DC-DC type BUCK can only step down, and step down formula is: Vo=Vi*D

BOOST type DC-DC can only boost voltage, boost formula: Vo=Vi/(1-D)

BUCK-BOOST DC-DC type, can boost or boost, formula: Vo=(-Vi)* D/(1-D)

D is charge duty cycle, i.e. conduction time of MOSFET, 0 <1.

The efficiency of switching regulated power supply is very high, but output ripple voltage is high, noise is large, and performance of voltage regulation speed is also low, especially when it supplies power to analog circuit, it will have a greater impact.

Due to high efficiency of switching power supply, it can usually reach more than 80%, so when choosing its output current, it is necessary to accurately measure or calculate maximum absorption current of electrical equipment, so that selected switching power supply has a high performance Price ratio, usual formula output power calculation: Is=KIf where: Is - rated output current of switching power supply, If - maximum current consumption of electrical equipment, K - safety factor, usually from 1.5 to 1.8.  Capacitive switching power supply

They can step up or step down input voltage, and can also be used to create a negative voltage. Its internal FET switch matrix controls charging and discharging of flying capacitors in a certain way, so that input voltage is multiplied or decreased by a certain factor (0.5, 2, or 3) to obtain desired output voltage. This special modulation process can guarantee an efficiency of up to 80%, requiring only external ceramic capacitors. As circuit switches, charge pump structure will also generate certain output ripple and electromagnetic interference (EMI). Energy is first stored and then released in a controlled manner to produce desired output voltage.