2.2 Three-phase full-bridge inverter circuit and drive circuit
The inverter circuit and the drive circuit are the links between the main control chip and the controlled motor, and the transmission performance directly affects the operation quality of the entire system. Its function is to distribute the power of the power supply to the windings of each phase of the stator of the brushless DC motor in a certain logical relationship. The power field effect transistor has the characteristics of high switching speed, good high frequency characteristics, high input impedance, small driving power, excellent thermal stability, no secondary breakdown problem, high safety working area width and high cross-wire property. It is widely used in small and medium power switching circuits.
In this control system, an inverter conversion circuit composed of MOSFETs is used. According to the second section, the control of the half-bridge inverter is relatively complicated, and six sets of control signals are required. The operation of the three-phase winding of the motor is relatively independent, and the three-phase current must be separately controlled. The control of the full-bridge inverter is relatively simple, only three sets of independent control signals are needed, and the two-phase currents that are turned on at any one time are equal. As long as one phase current is controlled, the other phase current is also controlled. This design uses a full-bridge inverter circuit to control the conduction of each phase, as shown in Figure 2.
In this design, the upper and lower arms of the inverter use N-channel MOSFETs. Due to the process, the P-type MOSFET has poor parameter consistency and is expensive, and its internal resistance is larger than that of the N-channel MOSFET, and the loss is also large. Therefore, current brushless controllers generally use two N-channel MOSFETs to form one phase of the inverter. When the power MOSFET is used as a switch and is driven to saturate, that is, when the voltage drop between its two poles is lowest, its gate drive requirements can be summarized as follows:
(1) The gate voltage must be 10~15V higher than the drain voltage. When used as a high-voltage side switch, its gate voltage must be higher than the mains voltage, which may often be the highest voltage in the system.
(2) The gate voltage must be logically controllable, and it is usually referenced to ground.
(3) The power absorbed by the gate drive circuit does not significantly affect the overall efficiency.
The drain voltage of the power MOSFET in this system is 36V, and the highest power supply voltage of this system is also 36V. In order to meet the requirement that the gate is higher than the drain 10V~15V, a boost circuit is required.
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