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Hello everyone, today we are going to learn about the self-lift voltage in PWM circuit

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Here we first review the waveform sof the upper and lower tube G poles,

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and then introduce the self-lift voltage

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We have learned that in the last lesson,

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When the common point voltage is 19V,

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the peak value of G pole of the upper tube is about 24V,

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and the peak value of G pole of the lower tube is about 5V

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So where does the upper tube 24V come from?

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That's the self-lift voltage that we're going to talk about

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Like this PWM chip

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When the VIN pin of the chip and the D pole of the upper tube get a common point voltage,

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the integrated LDO power supply inside the chip starts to work

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First, a 5V LDO output is generate

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It will be used by external circuits as well as internal chips

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This 5V linear provides power to this pin through an internal diode

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At the same time, this pin will be connected to the external self-lift voltage capacitor

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This capacitor is actually a normal capacitor, it's not grounded at both ends

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One end of it is connected to the chip's self-lift voltage pin,

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and the other end is connected to the front end of the inductor

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Before T1 time period, the upper tube is not on,

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and there is no electricity at the front end of the inductor

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At this point, the capacitor is being charged

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5V at one end and 0V at the other,

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and it's charged with a 5V voltage

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When entering the T1 period, the upper tube starts to conduct

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At this time, the driving voltage of the upper tube G pole comes from the BOOT pin

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The BOOT pin has 5V, which can provide drive level to the G pole of the upper tube, up to 5V

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But in the case of an S pole without power, 5V is enough to get it fully on in an instant

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The high level that it leads through instantaneously will be superimposed to the other end through the capacitor,

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and the original 5V linear voltage will be superimposed

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Then it gets a boost of 13.6V

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Therefore, in the second cycle, it can output a higher drive level

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Common self-lift voltage pins are BOOT, BOOST, BST, VBST

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The pin is called a starter pin,

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which can also be called a self-lift voltage pin, or a base voltage pin

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The purpose of the self-lift voltage is to drive the upper tube to complete conduction

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If the self-lift voltage is insufficient, the upper tube can not be completely conduction, easy to burn the tube

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Okay, so that's what the bootstrap is for

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This is the introduction to self-lift voltage