“First, let’s take a look at several factors that can affect EMI/EMC: the Circuit structure of the driving power supply; switching frequency, grounding, PCB design, and reset circuit design of the smart LED power supply. Because the original LED power supply is a linear power supply, but the linear power supply will lose a lot of energy in the form of heat during operation. The working...
“The H-bridge power drive Circuit can be used to drive stepper motors, AC motors, and DC motors. The excitation windings of permanent magnet stepping motors or hybrid stepping motors must be powered by a bipolar power supply, which means that the windings sometimes require forward current and sometimes reverse current, so the winding power supply needs to be driven by an H-bridge. This text takes the two-phase hybrid stepping motor driver as an example to design the H-bridge drive circuit.
The H-bridge power drive circuit can be used to drive stepper motors, AC motors, and DC motors. The excitation windings of permanent magnet stepping motors or hybrid stepping motors must be powered by a bipolar power supply, which means that the windings sometimes require forward current and sometimes reverse current, so the winding power supply needs to be driven by an H-bridge. This text takes the two-phase hybrid stepping motor driver as an example to design the H-bridge drive circuit.
Figure 1 shows the block diagram of the circuit connecting the H-bridge drive circuit and the AB-phase winding of the stepping motor.
The 4 switches K1 and K4, K2 and K3 are respectively controlled by the control signals a and b. When the control signals make the switches K1, K4 close and K2, K3 open, the current flow in the coil is shown in Figure 1(a) , When the control signal makes the switches K2, K3 close, and K1, K4 open, the current flow in the coil is shown in Figure 1(b). The four diodes VD1, VD2, VD3, and VD4 are freewheeling diodes, and their role is: Take Figure 1(a) as an example, when K1 and K4 switches are controlled from closed to open, because the coil windings The current on AB cannot change suddenly, and still needs to flow in the original current direction (ie A→B). At this time, VD3 and VD2 provide the loop. Therefore, at the moment K1 and K4 are turned off, the current forms a freewheeling loop from ground→VD3→coil winding AB→VD2→power +Vs. Similarly, in Figure 1(b), when the switches K2 and K3 are turned off, diodes VD4 and VD1 provide freewheeling of the coil windings. The current loop is ground→VD4→coil winding BA→VD1→power +Vs. In stepper motor drivers, power MOSFET tubes are often used in actual circuits to implement the above-mentioned switching functions.
It can be known from the principle of the H-bridge drive circuit of the stepping motor that the current flows in the windings in two completely opposite directions. The signal logic of the drive stage should prevent the diagonal transistors from being turned on at the same time, so as not to cause the high and low voltage tubes to pass through.
In addition, the winding of a stepper motor is an inductive load. When it is energized, as the operating frequency of the motor increases, the transition time is often unchanged, so that the winding current is cut off before it reaches the steady-state value, and the average current becomes smaller. , The output torque drops, when the driving frequency is high to a certain level, stalling or out-of-step will occur. Therefore, in addition to the design of the motor to reduce the winding inductance as much as possible, the stepper motor drive must also take measures for the drive power supply, that is, increase the steepness of the leading and trailing edges of the conduction phase current to improve the performance of the motor.
The shortcomings of stepper motors are insufficient high-frequency output and low-frequency oscillation. In addition to the inherent performance of the motor itself, the performance of the stepper motor also directly affects the characteristics of the motor. To improve the frequency characteristics of a stepper motor, it is necessary to increase the power supply voltage.
Figure 2 shows the schematic diagram of the AB phase coil power drive part of the driver.
The selected power MOSFET component is IRFP460, ID=20A, VDss=500 V, RDS(ON)=0.27Ω.
In Figure 2, the power MOSFET tubes VT1, VT2, VT3, VT4 and freewheeling diodes VD11, VD19, VD14, VD22 are equivalent to K1, K2, K3, K4 and VD1, VD2, VD3, VD4 in Figure 1. The control signal of the power MOSFET is provided by TTL logic levels a, a, b, b, where a and a, b and b are logically opposite to each other.
a. Improvement of the front and rear edges of the drive current
From the analysis of the operating characteristics of the stepper motor, it is known that the higher-performance drives require steep front and rear currents to improve the high-frequency response of the motor. In this driver, due to the existence of the gate capacitance of the power MOSFET tube, the driving current of the tube actually shows the charging and discharging of the gate capacitance. The greater the capacitance between electrodes, the greater the drive current required in the switch drive. In order to make the switching waveform have sufficient rise and fall steepness, the drive current must have a larger value. If you directly use an open-collector device such as SN7407 to drive the power MOSFET, when the MOSFET has an inductive load, the rise time will be too long, which will cause the dynamic loss to increase. In order to improve the fast turn-on time of the power MOSFET tube and reduce the power consumption on the front-stage gate circuit at the same time, the left lower arm drive circuit in the dashed frame of Fig. 2 is adopted.
Open-collector device U14 is a buffer/driver that converts TTL level to CMOS level. When U14 outputs low level, the gate capacitance of power MOSFET tube VT2 is short-circuited to ground through 1N4148. At this time, the ability of U14 to absorb current is affected by The current limit allowed by the internal conduction tube of U14. When the U14 output is high, the gate of the VT2 tube obtains the voltage and current through the transistor V3, the charging ability is improved, and the turn-on speed is accelerated.
b. Protection function
In the dashed frame of Figure 2, 1N4744 is an overvoltage protection Zener diode between the gate and source, and its voltage regulation value is 15 V. Because the impedance between the gate and the source of the power MOSFET is very high, the sudden change of the voltage between the drain and the source in the switching state will be coupled to the gate through the capacitance between the electrodes to generate a considerable VCS pulse voltage. This voltage will cause the gate-source breakdown and cause permanent damage to the tube. If it is a positive VCS pulse voltage, although it does not reach the level of damage to the device, it will cause the device to mis-turn on. To this end, it is necessary to appropriately reduce the impedance of the gate drive circuit, and connect a damping resistor or a Zener diode 1N4744 with a voltage regulation value less than 20 V and close to 20 V between the gate and source to prevent the gate source from open circuit operation.
The power MOSFET has an internal fast recovery diode. When VD11, VD12, VD13, VD14 are not connected, it is assumed that the AB phase winding of the motor is driven by the VT1 tube (and VT4 tube) at this time, that is, the VT2 tube (and VB) is cut off, the VT1 tube (and VT4 tube) is turned on, and the current flows through The VT1 tube flows through the winding. When the next control signal turns off the VT1 tube, the freewheeling current of the load winding is obtained from the ground through the internal fast recovery diode of VT2. At this time, the drain-source voltage of the VT2 tube is the on-state voltage drop of the fast recovery diode, which is a small negative value. When VT1 turns on again, the fast recovery diode turns off, and the drain-source voltage of VT2 rises rapidly until it is close to the voltage +VS of the positive power supply, which means that the drain-source voltage of VT2 has to withstand a very high and steep rising voltage. , The rising voltage is reversely applied to both ends of the fast recovery diode in the VT2 tube, which will cause the fast recovery diode to have a recovery effect, that is, a large current flows through the fast recovery diode with reverse voltage. In order to suppress the reverse recovery effect of the fast recovery diode in the VT2 tube, VD11, VD12, VD13, and VD14 are connected to the circuit in Figure 2. Among them, the anti-parallel fast recovery diodes VD11 and VD14 are used to provide freewheeling paths for the AB phase windings of the motor. VD12 and VD13 are to prevent the fast recovery diodes in the power MOSFET tubes VT1 and VT2 from flowing reverse current to ensure VT1 , VT2 can function as a normal switch when working dynamically. The functions of VD19, VD20, VD21, and VD22 are the same.
The analysis of the circuit in Figure 2 shows that the signal a=1, b=1 is not allowed to exist, otherwise the power supply will be directly connected to the ground due to simultaneous conduction, which will cause damage to the power tube; in addition, according to the operation of the stepper motor Pulse distribution requirements, VT1, VT2, VT3, VT4 are often in alternate working conditions, because there is a period of storage time and current drop time during the turn-off process of the transistor, which is collectively called the turn-off time. During this time, the transistor is not completely turned off. Off.
If during this period, another transistor is turned on, it will cause the upper and lower two tubes to pass through and short-circuit the power supply, which will burn out the transistor or other components. In order to avoid this situation, an additional logic delay circuit can be added to make the upper and lower tubes of the H-bridge circuit alternately conduct a “dead time”, which is turned off first and then opened to prevent the upper and lower tubes from being directly connected. Phenomenon.
The power supply of the driver has a simple circuit, and the reasonable design of the front and rear edges of the current reduces the switching loss, improves the high-frequency characteristics of the motor, and has a variety of protection functions. The effect is good in actual use.