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Hello everyone, in this lesson we will talk about the bootstrap boost in the PWM circuit

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We said in the last lesson that to generate power, the upper and lower tubes need to be turned on in turn

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Then the upper tube and the lower tube are both N tubes, and the input voltage is 5V

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For the upper tube to be turned on, its G pole must have a voltage above 9.5V

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The lower tube needs a power supply above 4.5V to be turned on

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The 4.5V power supply of the down tube is easy to meet, and the 5V directly supplies power to the chip

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When the chip has power supply, its G pole also has 4.5V voltage

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But how does this 9.5V voltage come from?

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It comes from the bootstrap boost

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There is a dedicated pin in the PWM chip, which is used for self-boosting boost,

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that is, the BOOT pin

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A resistor and a capacitor are connected to the BOOT pin

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This capacitor is the core of the bootstrap boost, also called the bootstrap boost capacitor

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First, the 5V power supply is supplied to the chip, and the power supply pin of the chip has 5V

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It will be given to the BOOT pin along a diode inside the chip

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At this time, the BOOT pin also has a power supply similar to 5V.

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The left end of the bootstrap capacitor also has a voltage of 5V

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Next, the chip starts to work, and the upper and lower tubes will be turned on in turn.

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The high level of the upper tube comes from the BOOT pin

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At the beginning, the BOOT pin is only 5V, so the upper tube drive pin is only 5V at the beginning

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Because the circuit has not started working yet, its S pole is 0V

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At this time, the upper tube will be partially turned on, let's assume that it is turned on by 2V

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This 2V is not only given to the inductor, but also to the other end of the bootstrap boost capacitor

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In the previous electronic circuit, we said that the voltage across the capacitor cannot change suddenly

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The right end of the capacitor suddenly becomes 2V, which is a sudden change

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In order to continue to maintain a voltage difference of 5V,

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the capacitor will superimpose 2V on top of 5V, which becomes 7V

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The voltage of the BOOT pin becomes 7V

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Then the voltage of the upper tube drive pin becomes 7V

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7V can conduct more voltage down, such as 4V

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Then this side becomes 4V

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It stacks further and here it becomes 8V

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In the end, turn on the 5V of the input power supply completely

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After it is fully turned on, the right end of the bootstrap pin becomes 5V

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The two 5Vs are superimposed together to become a voltage of about 10V,

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which is greater than the 9.5V required for the upper tube to fully turn on

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At this time, the upper tube can be fully turned on, and the bootstrap boost is completed.

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This is the bootstrap boost in PWM circuit

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We call this capacitor connected to the boost pin as the bootstrap capacitor

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The initial power supply of the bootstrap pin is called the base voltage