Brief discussion on Buck buck circuit

I talked about it some time ago BOOST circuit , Now it's time to talk BUCK Circuit .

BUCK Circuits and BOOST The devices used in the circuit are almost the same , If you understand BOOST Boost principle of the circuit , Actually BUCK The principle of reducing blood pressure is also easy to understand .

Let's briefly talk about BUCK circuit .

The diagram shows a simple BUCK Circuit diagram .

This circuit has a power supply , Switch tube , inductance , diode capacitance , Load resistance device .

among , The switch tube is MOS tube , You can also choose triode . Its source is connected PWM wave .PWM The ups and downs of MOS Conduction and disconnection of tubes .

inductance ： It can convert electric energy into magnetic energy and store it , Magnetic energy can also be converted into electrical energy for release . In the whole circuit , Note the following two points ：
1 When the inductor stores and releases energy , Its polarity will be reversed .
2 The inductive current cannot change suddenly , It can only be linearly amplified and reduced . From the formula of inductance Ldi/dt=U You can see that .*

capacitance ： It has the characteristics of storing and releasing energy , Opposite to inductance , Its polarity will not change . When the external voltage is greater than the capacitor voltage , It is charged , When the external voltage is less than the capacitor voltage , It starts to discharge .

diode ： It has one-way conduction , Characteristics of reverse cut-off . In some circuits , This diode is also changed to MOS Tubular , We call it synchronization BUCK circuit . However, today we are going to talk about asynchrony BUCK circuit .

In order to better explain the circuit , It is still divided into two stages to explain .

Now in the ideal state , Analyze the circuit .

MOS The pipe is open
When MOS When the tube is on , The current flow direction is shown in the figure .
here V1 Supply power to the whole circuit , inductance L1 Convert electric energy into magnetic energy for energy storage . Its polarity is represented by the positive pole at the left end , Right end negative pole . The capacitor starts charging at this stage . Circuit output voltage UO1. The pressure difference between the two ends of the inductor is equal to V1- UO1.

MOS The pipe is disconnected

When MOS When the pipe is disconnected , The current flow direction is shown in the figure .
At this time, the inductance is equivalent to the battery , Discharge externally . Because the inductive current cannot change suddenly , Therefore, the current flowing through the inductor does not change . Polarity changes , The performance is left negative and right positive . At this time, the diode plays the role of freewheeling . At this time, the output voltage is UO2.
UO1= UO2.

When the switch is closed and disconnected , The polarity of the inductance will deflect , At this time, the load is powered by the capacitor .

We mentioned earlier , Second volt theorem of inductance . According to the second volt theorem of inductance ：
UO2（1-D）T= （V1-UO1）DT
That is to simplify ：UO1(1-D)=（V1- UO1）D
UO1=DV1
because D Always less than 1, therefore UO1 Always less than the input voltage V1.

In order to better understand BUCK circuit , We can think of it this way . Suppose we want to specify the voltage Uo Output .

stay MOS When the tube is closed , Because the inductive current cannot change suddenly , The right end potential can only store energy with the power supply , The right end potential is from 0V Gradually increase , When it rises to Uo When above ,MOS The tube breaks instantly , Stop the continuous rise of output voltage , At this time, the output voltage is slightly higher than the set value .
stay MOS After the pipe is disconnected , It begins to release energy , When the output voltage at the right end is lower than Uo when ,MOS The tube closes again instantly , Prevent the continuous drop of output voltage .
Cycle back and forth , Keep the output voltage at Uo Hover slightly up and down , This fluctuating differential pressure can be understood as the ripple value we often say . It can reduce ripple by adjusting the parameters of output capacitance , Maintain stable output voltage .

Due to diode and MOS All tubes are in a non ideal state , Therefore, there is a deviation between the output voltage and the above formula . therefore . For efficiency , Diodes and MOS tube The choice of inductance is also very important , I will explain the following articles .