PCB Technical - Motor driver circuit design PCB recommendations

Design objectives of dc motor drive circuit

In the design of dc motor drive circuit, mainly consider the following points:

1. Function: is the motor unidirectional or bidirectional? Do you need speed adjustment? For unidirectional motor drive, a high-power triode or mosFEtt or relay can drive the motor directly. When the motor needs bidirectional rotation, h-bridge circuit composed of four power PCB components can be used or a double-pole double-throw relay can be used. If you do not need to speed, as long as the use of relay can be; But if you need speed, you can use triode, fET and other switching components to achieve PWM(pulse width modulation) speed.

2. performance: for PWM motor drive circuit, mainly have the following performance indicators.

1) Output current and voltage range, which determines how high power motor the circuit can drive.

2) Efficiency, high efficiency not only means saving power supply, but also reducing the heating of the drive circuit. To improve the efficiency of the circuit, we can guarantee the switching state of power devices and prevent common-state conduction (H bridge or push pull circuit may appear a problem, that is, two power devices at the same time to make the power supply short circuit).

3) Influence on control input. The power circuit should have good signal isolation on its input to prevent high voltage and high current from entering the main control circuit, which can be isolated by high input impedance or photoelectric coupler.

4) Influence on power supply. Common-state conduction can cause the instantaneous drop of power supply voltage and cause high frequency power supply pollution. A large current may cause the ground wire to fluctuate.

5) Reliability. The motor drive circuit should be as safe as possible no matter what control signal or passive load is added.

a. Input and level conversion:

The input signal cable is introduced by DATA. Pin 1 is the ground cable and the rest are signal cables. Note that pin 1 is connected to the ground with a resistor of 2K ohms. When the drive board and the MCU are powered separately, this resistor can provide a path for signal current backflow. When the drive board shares a power supply with the MCU, this resistance prevents interference caused by high current flowing into the ground wire of the MCU mainboard along the wire. In other words, it is equivalent to separating the ground wire of the drive board from the ground wire of the MCU to achieve "a point of grounding".

The high-speed op amp KF347(also available as TL084) functions as a comparator, comparing the input logic signal with the 2.7V reference voltage from the indicator light and a diode to a square wave signal that approximates the voltage amplitude of the power supply. The input voltage range of KF347 should not be close to the negative supply voltage, otherwise it will make mistakes. Therefore, a diode to prevent voltage range overflow is added to the op-amp input terminal. There are two resistors at the input - one to limit current and one to pull the input to low level when it is

suspended.

LM339 or any other comparator with open circuit output can not be used to replace the operational amplifier, because the high level output impedance of open circuit output is above 1000 ohms, and the voltage drop is large, and the following triode will not be able to cut off.

b. Gate driving part:

The circuit composed of the triode, resistor and voltage regulator further amplifies the signal, drives the gate of the FETT and uses the gate capacitance of the FETT itself (about 1000pF) for delay, preventing the simultaneous conduction of the FETT on the upper and lower arms of H bridge (" common-state conduction ") from causing short circuit of the power supply.

When the output end of the op amp is low (about 1V to 2V, cannot reach zero completely), the following triode cuts off and the FETS turn on. The top triode is turned on, the FETS are cut off, and the output is high level. When the output of the op amp is high (about VCC-(1V to 2V), it cannot reach VCC completely), the following triode turns on and the FETS cut off. The top triode cuts off, the FETS turn on, and the output is low.

The analysis above is static, while the dynamic process of switching is discussed below: The conduction resistance of the triode is far less than 2 KHM, so the charge on the grid capacitor of the FETT can be released quickly when the triode switches from cutoff to on-off, and the FETT quickly cuts off. However, it takes a certain amount of time for the triode to be converted from on-on to the cut-off fET gate to be charged through the 2kHM resistor. Accordingly, the speed of the MOSFEts from on-on to cut-off is faster than that of the mosFEts from cut-off to on-on. If the switching action of two triodes occurs simultaneously, this circuit can make the upper and lower mosfets break first and then turn on, thus eliminating the common state turning on phenomenon.

In fact, it takes a certain amount of time for the operating amplifier output voltage to change, and during this period of time, the operating amplifier output voltage is in the middle value between positive and negative supply voltage. At this time, both triodes turn on at the same time, and the MOSFEts cut off at the same time. So the actual circuit is a little bit safer than this ideal.

The 12V voltage stabilized diode of the MOSFEtt grid is used to prevent over-voltage breakdown of the MOSFEtt grid. General MOSFEts gate voltage is 18V or 20V, directly add 24V voltage will breakdown, so the voltage stabilizer diode can not be replaced by ordinary diode, but can be replaced by a resistor of 2KOW, also can get 12V partial voltage.

c. Output part of field effect tube:

There are diodes in reverse parallel connection between the source and drain inside the high-power MOSFETS. When connected to the H-bridge, it is equivalent to the output end has been connected with four diodes for eliminating voltage spikes, so there is no external diode. A small output parallel capacitor (between OUT1 and OUT2) has certain benefits to reduce the peak voltage generated by the motor, but in the use of PWM has the side effect of peak current, so the capacity should not be too large. This capacitance can be omitted when using a low power motor. If you add this capacitor, be sure to use high voltage, ordinary ceramic capacitor may appear breakdown short circuit failure.

A circuit consisting of resistors, leds and capacitors in parallel at the output indicates the direction of rotation of the motor.

d. Performance indicators:

Power supply voltage 15~30 V, continuous output current 5A/ each motor, short time (10 seconds) can reach 10A,PWM frequency can use 30KHz(generally 1 to 10KHz). The circuit board contains four logically independent power amplifier units, which can be directly controlled by single chip microcomputer. Realize the bidirectional rotation and speed regulation of the motor.

e.PCB layout and wiring:

The high current line should be short and thick as far as possible, and try to avoid passing through the hole. If you must pass through the hole, you should make the hole bigger (gt; 1mm) and make a small hole in the pad and fill it with solder during welding, otherwise it may burn off. In addition, if the regulator tube is used, the fET source to the power supply and ground wire should be as short and thick as possible, otherwise in high current, the voltage drop on this wire may be burned through the positive bias regulator tube and the transistor on. In the original design, a 0.15-ohm resistor was inserted between the source and ground of the NMOS tube to detect the current, and this resistance became the culprit of constant burning of the board. Of course, this problem does not exist if the regulator tube is replaced by a resistor.

The PCB that drive the circuits require special cooling techniques to address power consumption. Printed circuit board (PCB) substrates, such as FR-4 epoxy glass, have poor thermal conductivity. Copper, on the other hand, conducts heat very well. Therefore, from a thermal management perspective, increasing the copper area in the PCB is an ideal solution. Thick copper foil (e.g. 2 oz. (68 microns thick)) conducts heat better than thinner copper foil. However, using thick copper foil is costly and difficult to achieve fine geometry. As a result, the use of 1 ounce (34 micron) copper foil became common. The outer layer is usually used? Ounce to 1 ounce of copper foil. The solid copper surface used in the inner layer of multilayer circuit board has good heat dissipation. However, because these copper surfaces are usually placed in the middle of the circuit board stack, heat accumulates inside the circuit board. Increasing the copper area of the PCB's outer layer and connecting or "stitching" it to the inner layer through a number of through-holes helps transfer heat to the outside of the inner layer.