Buck switching regulator circuit diagram explained

Buck switching regulator circuit diagram (1)

This is a circuit diagram based on the LM2678 step-down switching regulator. The LM2678 series of voltage regulators are monolithic integrated circuits that provide all the necessary functions required for a step-down switching regulator and can drive loads up to 5A. The IC has over 90% efficiency and excellent load and line regulation. The LM2678 is available in three fixed output voltages (3.3V, 5V, 12V) and an adjustable output version. The IC also has few features such as thermal shutdown, current limit and ON/OFF control.

The circuit given here is based on the LM2678-5.0 version, which provides a 5V output. The input voltage of the regulator is fed to pin 2 of the IC. Capacitors C1 to C4 are input bypass capacitors. They also provide current to the IC controlled switch when it is first turned on . Capacitor C5 boosts the gate drive of the internal MOSFET , causing it to fully conduct. This minimizes switching losses and helps achieve high efficiency. Pin 7 is the ON/OFF pin, if this pin is connected to ground, the regulator will shut down. The current consumption in shutdown mode will be less than 50uA. Schottky diode D1 is used as a freewheeling diode. When the control switch (internal MOSFET) is closed, the current in the inductor L1 flows through this diode. capacitors C6 and C7 and the output filter capacitor.

Assemble the circuit on a high quality PCB .

The circuit's power supply can be anything between 8 and 40V DC .

The feedback wiring must be as far away from inductor L1 as possible.

Do not use loads that consume more than 5A.

It is highly recommended to use a heat sink for the IC.

Buck switching regulator circuit diagram (2)

As the name suggests, a Buck-type step-down switching regulator means that the input voltage is higher than the output voltage. The conversion principle is shown in the figure below:

①Detect the output voltage and compare it with the reference voltage

②When the output voltage is lower than the set output voltage, the switch is ON and the current flows in the direction of the red arrow.

③Inductor stores magnetic energy

④When the output voltage is higher than the set output voltage, the switch is OFF and the current flows in the direction of the green arrow.

⑤The inductor converts the stored magnetic energy into current for load output, and then returns to the inductor

⑥When the magnetic energy of the inductor disappears and the output voltage begins to drop, the switch will turn ON again.

By controlling the turn-off and turn-on time of the switch tube, a stable output voltage can be obtained.

Synchronous rectification or asynchronous rectification

In the Buck circuit, only one power tube is asynchronous. The asynchronous type relies on the freewheeling of the diode to complete the green area in the picture above. Synchronous rectification has two power tubes, generally called upper tube and lower tube. These two tubes are controlled by a logic device, and only one tube is in the conducting state at the same time.

Characteristics of non-synchronous rectification

Low efficiency, especially when the load is large current

Simple circuit and low cost

Synchronous rectification characteristics

High efficiency, MOS tube conduction internal resistance is very small

Requires additional control circuitry and costs more

Working modeCCMDCMBCM

CCM (Conti nuousConductionMode ) continuous conduction mode: the inductor current is always greater than zero during a switching cycle

DCM (DisontinuousConductionMode) discontinuous conduction mode: the inductor current will return to zero within a switching cycle

BCM (BoundaryConductionMode) critical conduction mode: the minimum value of the inductor current just returns to zero within a switching cycle

It is different for synchronous and asynchronous structures. The use of diode freewheeling in the asynchronous structure can prevent the direction of the inductor current from falling below 0. For the synchronous structure, MOS tubes are used instead of diodes. MOS tubes can allow the inductor current to reverse direction, that is, flow out of the load, causing the inductor current to be below 0.

In actual application scenarios, most DCDCs work in CCM.