Different parts of the motor have different design requirements.
1. Input and level shifting section:
The input signal line is introduced by DATA, the 1 pin is the ground line, and the rest is the signal line. Note that 1 foot to ground is connected to a 2K ohm resistor. When the driver board and the microcontroller are powered separately, this resistor can provide a path for the signal current to flow back. When the driver board and the microcontroller share a set of power supplies, this resistor can prevent large currents from flowing along the wires that flow into the ground of the microcontroller board. In other words, it is equivalent to separating the ground line of the driver board from the ground line of the microcontroller to achieve "one-point grounding".
The high-speed op amp KF347 (also available as TL084) acts as a comparator that compares the input logic signal to a 2.7V reference voltage from the indicator and a diode and converts it into a square wave signal that is close to the power supply voltage amplitude. The input voltage range of the KF347 cannot be close to the negative supply voltage, otherwise an error will occur. Therefore, a diode that prevents the voltage range from overflowing is added to the input of the op amp. One of the two resistors at the input is used to limit current, and one is used to pull the input low when the input is left floating.
The LM339 or any other open-circuit comparator cannot be used in place of the op amp, because the high-level output impedance of the open-circuit output is above 1 kΩ, and the voltage drop is large, and the transistor of the latter stage cannot be turned off.
2. Gate drive part:
The circuit composed of the rear transistor and the resistor and the Zener tube further amplifies the signal, drives the gate of the FET and uses the gate capacitance of the FET itself (about 1000pF) to delay the FET of the upper and lower arms of the H-bridge. Simultaneous conduction ("common state conduction") causes a short circuit in the power supply.
When the output of the op amp is low (about 1V to 2V, it can't reach zero completely), the lower transistor is turned off and the FET is turned on. The upper transistor is turned on, the FET is turned off, and the output is high. When the output of the op amp is high (approximately VCC-(1V to 2V) and cannot fully reach VCC), the lower transistor is turned on and the FET is turned off. The upper transistor is turned off, the FET is turned on, and the output is low.
The above analysis is static. The following is a discussion of the dynamic process of switching: the on-resistance of the triode is much less than 2 kΩ, so the charge on the gate capacitance of the FET can be quickly released when the transistor is switched from off to on. Closed quickly. However, it takes a certain time for the transistor to be charged by a 2 kΩ resistor when the transistor is switched from on to off. Correspondingly, the FET switches from on to off at a faster rate than from off to on. If the switching action of the two triodes occurs at the same time, this circuit can make the FETs of the upper and lower arms break and then pass, eliminating the common-state conduction phenomenon.
In fact, the output voltage of the op amp needs to change for a certain period of time. During this time, the output voltage of the op amp is in the middle between the positive and negative supply voltages. At this time, the two transistors are turned on at the same time, and the FET is turned off at the same time. So the actual circuit is safer than this ideal situation.
A 12V Zener diode for the FET gate is used to prevent FET gate overvoltage breakdown. The voltage resistance of the general FET gate is 18V or 20V, and the voltage directly applied to 24V will break down. Therefore, this Zener diode cannot be replaced by a normal diode, but it can be replaced by a resistor of 2 kΩ. 12V partial pressure.
3. Field effect tube output part:
In the high-power FET, there is a diode connected in reverse parallel between the source and the drain. When connected to the H-bridge, it is equivalent to four diodes used to eliminate the voltage spike in the output terminal. Therefore, there is no external diode. Parallel connection of a small capacitor (between out1 and out2) at the output has certain advantages in reducing the peak voltage generated by the motor. However, there is a side effect of peak current when using PWM, so the capacity should not be too large. This capacitor can be omitted when using a low power motor. If you add this capacitor, you must use a high withstand voltage, ordinary ceramic capacitors may break through the short circuit.
A circuit consisting of a resistor and a light-emitting diode and a capacitor connected in parallel at the output end indicates the direction of rotation of the motor.
4. Performance indicators:
The power supply voltage is 15~30V, and the maximum continuous output current is 5A/per motor. It can reach 10A in short time (10 seconds) and 30KHz in PWM frequency (usually 1 to 10KHz). The circuit board contains four logic independent units, and the output terminals are connected to form an H-bridge power amplification unit, which can be directly controlled by a single-chip microcomputer. Realize the bidirectional rotation and speed regulation of the motor.
5. Wiring:
The high-current line should be as short and thick as possible, and try to avoid passing through the via hole. If it is necessary to pass the via hole, make the via hole larger (>1mm) and make a small via hole on the pad. Solder fills up, otherwise it may blow. In addition, if a Zener diode is used, the source of the FET should be as short and thick as possible for the power supply and the ground. Otherwise, at high current, the voltage drop across the conductor may pass through the positively biased regulator and The turned-on transistor burns it. In the initial design, the source of the NMOS transistor was once connected to a 0.15 ohm resistor to detect the current. This resistor became the chief culprit in the continuous burning of the board. Of course, if you replace the voltage regulator with a resistor, there is no such problem. In the 2004 Robocon competition, we mainly used this circuit for motor drive.
