The PWM generating circuit consists of IC 1TL494 and peripheral devices. TL494 is a PWM switching power supply integrated circuit. The pin function and internal block diagram are shown in Figure 2.
C 10 and R 19, which are connected to pins 5 and 6 of IC 1, are timing elements, which determine the oscillation frequency of the sawtooth oscillator, F= 1. 1/RC, and the numerical value in the figure is 50KHz. Pin 14 is a +5V reference voltage output terminal, which can be used not only for internal use of the chip, but also for pins 2, 4, 13 and IC2 directly or after voltage division. The 13 pin is the control terminal of the output mode, and it is a single-ended output mode when it is connected to a low level. In the figure, the 14 +5V high level is connected to a double-ended output mode. The fourth pin is the dead-time voltage control pin, whose voltage determines the dead-time time. With the increase of potential, the dead time is prolonged and the output pulse width is narrowed. When the voltage is greater than the sawtooth voltage, the output pulse width will become very narrow and even stop oscillating. When full-bridge or half-bridge switching circuit is used at the output end, the dead time should be set correctly to avoid the danger of short circuit caused by the simultaneous conduction of two switching tubes. In the figure, the foot potential is obtained by dividing the reference voltage by R24 and R20, and the measured voltage is 0.46V V. Pin 1 2 and 16 and 15 are the positive and negative input terminals of two voltage comparators in IC 1, which are used for charging voltage sampling and charging current sampling respectively. The charging voltage of +44V is divided by R28, R27 and R26 and fed back to the 1 pin. C 15 is the soft start capacitor. The second pin potential is obtained by dividing the reference voltage by R23 and R3, and the measured value is 3.2V The higher the pin voltage is/kloc-0, the narrower the output pulse width and the lower the charging voltage. On the contrary, the pulse width becomes wider and the charging voltage increases. So as to achieve the purpose of +44V charging voltage. Ra is the charging voltage debugging resistor, and the smaller the parallel value of Ra and R26, the higher the charging voltage. R29 is the current sampling resistor for pin charging, and the voltage change obtained from this resistor is sent to the 15 pin of IC 1 through R 13. The larger the charging current, the lower the potential of the 15 pin. When the potential of 15 pin is lower than the potential of 16 pin (ground), the IC output is turned off to realize overcurrent protection. Rb is the over-current protection debugging resistor, and the preset value of this machine is1.8a.