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Hello everyone, today we will learn about the normal waveform of the PWM circuit

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We have learned the working principle of the PWM circuit,

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here we learn some common waveforms in the PWM circuit

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During the maintenance process, the oscilloscope is often used to measure the main power supply of the chip,

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the open signal, the PG signal, the waveforms of the G pole of the upper tube and the G pole of the lower tube,

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the waveforms of the front end of the inductor and the rear end of the inductor,

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and are used to judge the fault of the PWM circuit

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When the common point power supply is 19V, that is the adapter mode

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The peak value of the G pole of the upper tube can reach about 24V

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The G pole of the upper tube refers to the G pole of this Q1

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The waveform we measured is like this

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10 volts per grid, here's two and a half grids already, so it's around 24V

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The G pole of the lower tube only occupies half a grid, about 5V,

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

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Under normal working conditions,

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the peak value of the front end of the inductor is equal to the peak value of the common point voltage,

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19V-20V, which is the normal waveform of the front end of the inductor

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The back end of the inductor is a smooth DC output

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In battery mode, the common point may only be 8V or 10V

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At this time, the G pole voltage of the upper tube is around 14V

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The G pole voltage of the upper tube must be about 4.5V higher than the common point voltage,

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so that the upper tube can be completely turned on

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If the measured G pole waveform does not meet the requirements,

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we will check the BOOT pin

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This is an insufficient bootstrap boost

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When the driving voltage of the G pole of the upper tube is not enough,

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it will cause the upper tube to not be completely turned on,

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and it is easy to burn the upper tube and the components of the rear pole.

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If there is a problem with the filter capacitor of the rear pole,

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it will cause the power supply at the output end to be unstable,

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and it is prone to poor filtering

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Well, this is the normal waveform of the G poles of the upper and lower tubes

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When the tube is not installed, its G pole only has a peak value of 5V

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Because when the tube is not installed,

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there is no voltage at the front end of the inductor, and the bootstrap boost cannot be completed.

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So the highest voltage that can be measured by the G pole is only 5V

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We call it the base voltage waveform

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Set to 2V per grid, which takes up two and a half grids

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Of course, this base voltage waveform will not continuously output

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Because after the chip is working,

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its feedback pin and voltage detection pin cannot detect the output voltage at the rear end of the inductor,

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and it will enter the undervoltage timeout protection state

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Therefore, after the G pole outputs pulses,

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it will not work after a certain period of time,

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and it will enter the protection state

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This is the G-pole waveform without the upper tube, also called the base voltage waveform

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Ok, this is the normal waveform of the PWM circuit