Working principle of power circuit BUCK
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Working principle of power circuit BUCK

Posted Date: 2024-01-13

1. Definition of BUCK circuit

The BUCK circuit is a DC-DC converter based on the principle of inductive energy storage, which involves the basic principles of electromagnetic induction and electric energy conversion in physics. In the BUCK circuit, the DC voltage provided by the input power supply is converted into an adjustable low-voltage output by controlling the PWM wave with variable input duty cycle to switch the on and off states of the switch, thereby meeting the power supply needs of different circuits. .

2. Basic topology

In the BUCK circuit, the switching tube switches periodically, allowing the inductor to store energy and transfer it to the load. BUCK circuitThe output voltage is related to the input voltage and the duty cycle of the switch tube. Specifically, when the switch is turned off, the current in the inductor will continue to flow and generate a reverse electromotive force, causing the output voltage to increase. When the switch is turned on, the current in the inductor will continue to flow and generate a positive electromotive force, causing the output voltage to decrease.By adjusting the duty cycle of the switching tube, the output voltage can be controlled.

The output voltage of the BUCK circuit is related to the input voltage and the duty cycle of the switching tube. The output voltage can be calculated using the following formula:

Vout=Vin×D

Where Vout is the output voltage, Vin is the input voltage, and D is the duty cycle of the switching tube. The duty cycle of the switching tube can be achieved by controlling the on-time and off-time of the switching tube.

1) Three working modes of inductor current

(1) There are three working modes of inductors, namely continuous conduction mode (CCM), critical conduction mode (BCM) and discontinuous conduction mode (DCM).

(2) In CCM mode, the inductor current will not reach 0, which means that the inductor magnetic flux never returns to 0 during the switching cycle. When the power tube is closed, there is still current flowing in the coil. In DCM mode, the inductor current will always reach 0, which means that the inductor is properly "reset", that is, when the power switch is closed, the inductor current is zero. In BCM mode, the inductor's conduction time and magnetic field changes are between CCM and DCM. 342.

(3) The difference between the above three modes lies in the conduction time of the inductor and the change of the magnetic field. In CCM mode, the inductor current never reaches 0, which means that the inductor flux never returns to 0 during the switching cycle. When the power tube is closed, there is still current flowing in the coil. In DCM mode, the inductor current will always reach 0, which means that the inductor is properly "reset", that is, when the power switch is closed, the inductor current is zero.In BCM mode, the inductor's conduction time and magnetic field changes are between CCM and DCM 342. (4) How to choose the appropriate working mode

① Selecting the appropriate inductor operating mode requires consideration of multiple factors, such as the load of the circuit, input voltage, output voltage, switching frequency, etc. In practical applications, selection usually needs to be based on specific circuit parameters.

② If you need to operate under high load conditions, continuous conduction mode (CCM) is a good choice. In CCM mode, the inductor current never reaches 0, which means that the inductor flux never returns to 0 during the switching cycle. When the power tube is closed, there is still current flowing in the coil. If operation at low loads is required, Discontinuous Conduction Mode (DCM) is a better choice. In DCM mode, the inductor current will always reach 0, which means that the inductor is properly "reset", that is, when the power switch is closed, the inductor current is zero. If you need to operate at moderate loads, you can consider using critical conduction mode (BCM), where the on-time and magnetic field variation are between CCM and DCM.

3. Working principle

The switch S chops the input voltage Vin into a PWM wave, and then outputs the DC voltage Vout through the LC filter. By adjusting the duty cycle of the PWM wave, different output voltages can be obtained. Vout =Vin *D

Loop: A. Loop 1, the current path when the switch is closed; Loop 2, the current path when the switch is open (the loop through the freewheeling diode). In a closed loop, the changing current generates a magnetic field. In order to reduce EMC, when designing the PCB, the loop design should be as small as possible. At the same time, it should not interfere with analog circuits, such as feedback loops, gain compensation, soft start, enable circuits, etc.

B. In order to reduce power consumption and improve step-down efficiency, the power inductor should be selected with low DCR, and the saturation current should be 4/3 of the average current (empirical value); the freewheeling diode D should be a Schottky diode, or a synchronous buck IC (Use MOS as a freewheeling diode)

C. In order to reduce the output ripple, the inductor and capacitor need to choose appropriate values. Generally, the datasheet has recommendations. The larger the inductor value, the smaller the relative ripple. However, because the inductor blocks the current change, the response to the load becomes slower; the capacitor Generally, a combination of aluminum electrolytic capacitors and ceramic capacitors (low ESR) is used; when the height is limited or the cost is not sensitive, tantalum capacitors can be selected. Tantalum capacitors have good temperature characteristics, low ESR, and long life, but are high in cost and have poor voltage resistance ( It is best to choose an output voltage greater than 2 times the withstand voltage)

4. Commonly used models

TI: Texas Instruments is a well-known semiconductor company that provides various types of BUCK chips, such as LM2675, LM2677, LM2679, etc.

ADI: ADI (Analog Devices Inc.) is a well-known semiconductor company that provides various types of BUCK chips, such as ADP2384, ADP2385, ADP2386, etc.

ST: STMicroelectronics is a well-known semiconductor company that provides various types of BUCK chips, such as L7987, L7986, L7985, etc.

ON: ON Semiconductor is a well-known semiconductor company that provides various types of BUCK chips, such as NCP3065, NCP3066, NCP3067, etc.

5. Basis for selection

There are many factors to consider when selecting a BUCK circuit chip, such asInput voltage, output voltage, output current, switching frequency, efficiency, size, etc.. Here are some factors to consider when selecting:

Input voltage and output voltage: The appropriate input voltage and output voltage need to be selected according to the needs of the circuit. Generally, the input voltage range of the chip should be wider than the actual input voltage range.

Output current: The appropriate output current needs to be selected according to the load requirements. Usually, the maximum output current of the chip should be larger than the actual output current.

Switching frequency: The appropriate switching frequency needs to be selected according to the application scenario. Generally, the higher the switching frequency, the higher the efficiency of the chip, but the cost of the chip will also increase accordingly.

Efficiency: High-efficiency chips need to be selected to reduce power consumption and heat. Generally, the more efficient a chip is, the less heat it generates.

Size: A chip of appropriate size needs to be selected according to the application scenario. Generally, the smaller the size, the higher the cost of the chip.

Some other conditions such as power consumption, etc.

6. Application scenarios

Power supply: BUCK circuit can be used for various types of power supplies, such as laptop power supplies, mobile phone chargers, LED drivers, etc.

Automotive electronics: BUCK circuits can be used in automotive electronics, such as car audio, car chargers, etc.

Industrial control: BUCK circuit can be used in industrial control, such as PLC, servo driver, etc.

Communication equipment: BUCK circuits can be used in communication equipment, such as routers, switches, etc.

LED lighting: BUCK circuit can be used in LED lighting, such as LED light strips, LED bulbs, etc.

7. Application circuit

1), MP1584

2) TPS54331

Review Editor: Huang Fei


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