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Hello everyone, in this lesson we will talk about the working principle of the PWM power supply method

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The PWM working mode is that the PWM chip controls the upper tube and the lower tube

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to be turned on and off continuously, so as to step down the output

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Let's first get to know the three very important pins in the PWM power supply mode

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High tube drive pin, down tube drive pin and feedback pin

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At the beginning, both the upper tube drive and the lower tube drive are at low level,

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and both the upper tube and the lower tube are cut off.

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At this time, the circuit does not work, and there is no power output

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When the circuit starts to work, it will first give a high level to the upper tube

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After the upper tube gets a high level, the upper tube is an n-tube,

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it will be turned on and lead the power supply of VIN to the bottom

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At this time, the drive of the lower tube is low, and it is cut-off

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The upper tube supplies the 12-volt power supply from the conduction to the load through the inductor and capacitor

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Because the current flowing through the inductor cannot change abruptly,

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when the current passes through the inductor,

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the inductor will generate a reverse electromotive force to prevent the current from flowing

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At the same time, there is an energy storage capacitor at the rear end of the inductor.

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Therefore, although the front end of the inductor directly jumps to 12 volts,

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because of the energy storage of the inductor and the energy storage of the capacitor,

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the voltage at the back end of the inductor rises slowly

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When the voltage at the back end of the inductor reaches a certain value,

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the drive of the upper tube will become low level, and the upper tube will be cut off.

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At this time, no current flows through the inductor.

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The current of the inductor cannot change suddenly,

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so the inductor will generate a reverse electromotive force,

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which will discharge the electricity stored in the inductor

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At this time, we can treat the inductor as a battery

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Then, the lower tube is driven to a high level, and the lower tube is turned on

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After the lower tube is turned on, the battery is equivalent to a circuit

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At this time, the inductor can discharge to supply power to the load

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When the discharge of the inductance is almost complete,

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the upper tube is turned on again to charge the inductance

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After the inductor is fully charged, the upper tube is turned off,

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the lower tube is turned on, and the inductor is discharged

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After the discharge is almost complete, the lower tube is cut off,

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the upper tube is turned on, and the inductor continues to be charged.

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Reciprocating cycle

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Coupled with the filtering and energy storage of the capacitor, a stable DC current is formed.

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That is, the front end of the inductor is a waveform from 0 volts to the input voltage

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And the inductor rear end is a level

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If there is no filter capacitor, the back end of the inductor will also fluctuate,

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such as high and low fluctuations

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With the filter capacitor, the filter capacitor will clip the ripple and fill the valley,

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turning it into a smooth DC current.

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This is how PWM works