1. Schematic diagram
Seeing the circuit design diagram below, I don't know how many friends can see the mystery.
2. Given the known conditions
Given the known conditions first, there is a starting thought:
1). The left-most input PWM of U6B LM258 is a square wave signal output by MCU, the amplitude is 5V, and the duty cycle is assumed to be 75% in the analysis;
2). The input SIN of U3 and U4 LM311 is the same sine signal, the peak-to-peak value is 12.2V, and the frequency is 25kHz;
3). Bridge1, Bridge2, Bridge3, and Bridge4 are output 4 signals;
3. Guess
After seeing the known conditions given above, the following two preliminary conclusions may be drawn:
1). 4-channel Bridge signal output, since it is Bridge or 4-channel, it may be a full-bridge drive PWM signal;
2). 25kHz, could it be used in ultrasonic power supply?
PS: Ultrasonic frequencies are 25kHz, 40kHz, 68kHz, 138kHz, etc.;
4. Principle analysis
Next, a complete analysis of the schematic is performed. Generally speaking, it can be divided into 5 major sections:
1. RC filter, PWM wave to DC signal;
2. Follower;
3. Inverter;
4. Comparator;
5. NAND gate Schmitt trigger circuit.
The details are as shown below:
Analysis Description:
1) At the beginning, the known conditions have been given: the PWM amplitude is 5V and the duty cycle is 75%, so the PWM is converted into a DC voltage of about 5V*75%=3.75V after two-stage RC filtering;
2) After "2" follow and "3" reverse amplification (-1 times), the input voltage of U4 LM311 pin 2 is -3.75V, and the input voltage of U3 LM311 pin 3 is 3.75V;
3) U3 and U4 are voltage comparators, compare the input sine signal with the ±3.75V level respectively, and obtain the square wave signal output;
4). The circuit design of the digital part of the "5" block is also the essence of the circuit; after the analysis, I can't help feeling that its design is ingenious, and the designer is definitely a master of digital circuits!
"1" to "4" block circuit, presumably everyone can understand after the above analysis is given. When analyzing the "5" block circuit, let's first check the CD4093BC data sheet given by Fairchild. In the manual, you can See the following circuit application and analysis, which is crucial to the analysis of the "5" block circuit.
5. Timing diagram, final conclusion
Well, everything is ready, only the final conclusion is left.
Draw the waveform diagram of each point at the same time and analyze it slowly.
The following is the timing diagram analysis of each point
Final conclusion:
1-way PWM and 1-way sine wave signal input, through each circuit block, finally output 4-way phase-shift PWM wave; and by changing the PWM wave duty cycle, the phase-shift angle can be changed.
