Improved dual-power photoelectric coupling upper tube drive circuit

Improved dual-power photoelectric coupling upper tube drive circuit

In the above dual-power optocoupler upper tube driving circuit solution, the output of the optocoupler is used to directly drive the MOS tube, which will cause serious deformation of the output waveform, especially the slow falling edge of the waveform, which is mainly caused by the capacitance between the G pole and the s pole of the MOS tube. When the output waveform is high, the capacitance between G and s is charged, making the waveform rise slightly slowly; when the output waveform becomes low, the capacitance between G and s is discharged through Rl, making the G pole potential of the MOS tube drop slowly, resulting in severe deformation of the falling edge of the waveform. In view of the fact that the capacitance between the G pole and the s pole of the MOS tube cannot be eliminated, we have improved the above driving scheme based on the pull-up driving circuit of the microcontroller port line, and the improved scheme is shown in Figure 6.

Fig. 6
Advanced H-bridge with Double Power Supplies and Optoisolator

In the improved scheme, the output of the optocoupler drives the differential circuit composed of Q1 and Q2, and then the output of the differential circuit drives the MOS tube. Since the inter-electrode capacitance of the triode is very small, the output waveform of the optocoupler is slightly deformed; and the driving ability of the differential tube is very strong, so the signal deformation can be very small when driving the MOS tube.

This driving circuit still uses an optocoupler as the isolation interface of the two-way power supply system. Since the coupling speed of the optocoupler is limited and cannot be very fast, the waveform of this solution will also be partially deformed when the PWM frequency is high. The higher the frequency, the more deformation there is. Figure 7 shows the waveform distortion when the PWM frequency is 10 k, and Figure 8 shows the waveform distortion when the PWM frequency is 20 k. It can be seen from Figure 8 that even at a frequency of 20 k, the output waveform has only a small distortion compared to the input waveform, so this solution is completely applicable to the EPS upper tube drive.

Figure 8 Signal distortion of the improved driving circuit (frequency 20 k)