The function of D1 is to provide a forward current channel for C1 charging when Q1 is off, and prevent the current from flowing into the control voltage Vcc when Q1 is on. In each PWM cycle, the circuit charges C1 to keep its voltage unchanged.
Since the voltage of C1 is constant and VB floats with the increase of VS, C1 is called bootstrap capacitor. When the input signal Hin is turned on, the drive of the upper bridge is powered by C1. At this time, C1 charges through VCC and D1. When Q1 is turned off, the potential of point a returns to zero due to the freewheeling current of Q2. In Figure 2, C1 is bootstrap capacitor and D1 is fast recovery diode. The key of half bridge driving circuit is how to realize the driving of upper bridge. Working principle of 3 half bridge driving circuitįigure 2 shows a typical half bridge drive circuit. Due to the rapid change of IRR, a high di / DT is generated in the IRR loop. When Q1 is turned on again, due to the charge recovery process of Q3 body diode, the body diode cannot be turned off quickly, so there will be reverse recovery current (IRR) flowing through the body diode. When Q1 is turned off and Q5 is turned on, the current in the motor coil remains basically unchanged through the if of Q5 and Q4. When Q1 and Q5 are on, the current (ion) flows from VDD to the ground through Q1, motor coil and Q5, and motor AB is energized. The system adjusts the speed by adjusting the PWM duty cycle of the upper bridge MOSFET. The conduction sequence of MOSFET is q1q5 → q1q6 → q2q6 → q2q4 → q3q4 → q3q5. The working mode of the motor is three-phase and six state. The brushless DC motor realizes electronic commutation through bridge circuit. Half bridge driver chip is widely used in MOSFET Driver circuit because it is easy to design driver circuit, less peripheral components, strong driving ability and high reliability.įigure 1 shows the bridge circuit for driving three-phase brushless DC motor, in which lpcb, LS and LD are the lead inductors of DC bus and phase line, and the motor is a three-phase Y-type brushless DC motor. Because the reference point of the driving voltage of the lower bridge MOSFET is the ground, it is easier to design the driving circuit, while the driving voltage of the upper bridge is floating with the phase line voltage, so how to drive the upper bridge MOSFET well becomes the key to the success of the design. In application, MOSFET generally works in bridge topology mode, as shown in Figure 1. In order to make the MOSFET give full play to its performance in the application, it is necessary to design an optimal driving circuit and parameters suitable for the application. MOSFET is widely used in switching power supply and motor drive because of its high switching speed and low on resistance.
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