One. Power Conversion Module
The power supply voltage refers to the input voltage that is converted through a voltage conversion circuit to achieve boosting or bucking for other modules. In smart car development, the input battery voltage is 7.2V, and the required voltages for other modules are as follows: the minimum system board, OLED, and eagle eye camera require 3.3V, the operational amplifier needs ±5V, the servo motor requires 6V, the drive circuit needs 12V, and the CCD and encoder operate at 5V. Below are some detailed explanations of the voltage conversion circuits.
Voltage regulator circuits are mainly divided into several types:
LM2940 to 5V:
The LM2940 is a linear voltage regulator (only for buck). It has a simple structure, high stability, but also higher power consumption, larger size, and lower filtering efficiency. It requires large input and output filter capacitors.
MC34063 to 12V:
The MC34063 is a switching DC regulator chip that can be used for both boost and buck. This type of circuit has low power consumption, high efficiency, small size, and a wide voltage regulation range. However, its circuit structure is more complex, and it may have a higher failure rate. The working principle involves the internal switch tube being turned on continuously (the frequency depends on the timing capacitor), with energy stored in an inductor and capacitor being charged and discharged rapidly to provide power to the load. The output voltage is calculated using the formula Uo = 1.25*(1 + R4/R3).
MIC29302 to 6V:
The MIC29302 is a linear adjustable regulator, similar to the 34063 circuit, and can handle high current loads. Its output voltage formula is Uo = 1.242*(1 + R1/R2). In this case, the 6V output powers the servo motor.
LM2663 to -5V:
The LM2663 is a simple negative voltage converter that only needs two external capacitors.
Note: Avoid reverse connections due to manual errors, and consider adding anti-reverse protection to the circuit.
Using diodes for unidirectional conduction:
MOSFET-based anti-reverse protection circuit:
An NMOS transistor is connected to the negative terminal of the power supply, and the gate is turned on at a high level. A PMOS transistor is connected to the positive terminal, and the gate is turned on at a low level. (NMOS has a smaller on-resistance, so it's preferable to use NMOS.)
Various anti-reverse interfaces.
Two. Motor Drive Circuit
First, the MCU can output a DC signal, but its driving capability is limited. Therefore, the MCU usually uses the driving signal to control a large power transistor, such as the LR7843, to generate a large current and drive the motor. By adjusting the duty cycle, the average voltage applied to the motor can be controlled, thus achieving speed regulation.
The motor drive primarily uses N-channel MOSFETs to build an H-bridge circuit. An H-bridge is a typical DC motor control circuit, named for its shape resembling the letter "H." The four switches form the vertical legs of the H, while the motor acts as the horizontal bar. To make the motor run, a pair of diagonal switches must be turned on, and the direction of current determines whether the motor rotates forward or backward. The circuit is shown in the figure below.
H-bridge drive principle:
In actual drive circuits, switches are typically controlled by a hardware circuit. The motor drive board mainly uses two types of driver chips: the full-bridge driver HIP4082 and the half-bridge driver IR2104. The half-bridge circuit consists of two MOSFETs, while the full-bridge circuit uses four. The IR2104 is a half-bridge driver chip that can drive high-side and low-side N-channel MOSFETs, providing a large gate drive current and including features like hardware dead time and anti-same-arm conduction. Two IR2104 chips can be used to form a complete H-bridge driver circuit for DC motors. Compared to HIP4082, IR2104 is more cost-effective and offers better functionality, though its output power is slightly lower.
Additionally, since the driver circuit may generate a large back-EMF current, it’s advisable to isolate the chip to prevent damage to the microcontroller. Common isolation methods include using chips like 74LVC245, 74HC244, or PS2801. These chips are often used as control buses to improve driving capability. After meeting certain conditions, the output mirrors the input, allowing one-way signal transmission from the microcontroller to the driver and vice versa.
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