"PCB (printed circuit board), also known as printed circuit board in Chinese, is an important electronic component, the support of electronic components and the carrier of electrical connection of electronic components. Because it is made by electronic printing, it is called "printed" circuit board.
Design objective of DC motor drive circuit
In the design of DC motor drive circuit, the following points are mainly considered:
1. Function: does the motor rotate in one direction or two directions? Need speed regulation? For unidirectional motor drive, only a high-power triode or FET or relay can directly drive the motor. When the motor needs bidirectional rotation, an H-bridge circuit composed of four power elements or a DPDT relay can be used. If speed regulation is not required, just use relay; However, if speed regulation is required, switching elements such as triode and FET can be used to realize PWM (pulse width modulation) speed regulation.
2. Performance: the motor drive circuit with PWM speed regulation mainly has the following performance indexes.
1) The output current and voltage range determine how much power the circuit can drive.
2) Efficiency, high efficiency not only means saving power, but also reducing the heating of the driving circuit. In order to improve the efficiency of the circuit, we can start with ensuring the switching working state of the power device and preventing the common state conduction (a possible problem in the H-bridge or push-pull circuit, that is, two power devices are connected at the same time to short circuit the power supply).
3) Impact on control input. The power circuit shall have good signal isolation at its input to prevent high voltage and large current from entering the main control circuit, which can be isolated by high input impedance or optocoupler.
4) Impact on power supply. The common state conduction can cause the instantaneous drop of power supply voltage and cause the pollution of high-frequency power supply; High current may cause the ground potential to float.
5) Reliability. The motor drive circuit should be as safe as possible, no matter what control signal or passive load is added.
1. Input and level conversion:
The input signal line is introduced by data. Pin 1 is the ground wire and the rest are signal lines. Note that pin 1 is connected to the ground with a 2K Ω resistance. When the driving board and the single chip microcomputer supply power respectively, this resistance can provide the path of signal current return. When the driving board shares a set of power supply with the single chip microcomputer, this resistance can prevent large current from flowing into the ground wire of the single chip microcomputer main board along the connecting line to cause interference. In other words, it is equivalent to separating the ground wire of the driving board from the ground wire of the single chip microcomputer to realize "one point grounding".
The high-speed operational amplifier kf347 (or tl084) is used as a comparator to compare the input logic signal with the 2.7V reference voltage from the indicator lamp and a diode and convert it into a square wave signal close to the power supply voltage amplitude. The input voltage range of kf347 cannot approach the negative power supply voltage, otherwise an error will occur. Therefore, a diode to prevent voltage range overflow is added at the input of the operational amplifier. One of the two resistors at the input is used to limit the current, and the other is used to pull the input to the low level when the input is suspended.
LM339 or any other open circuit output comparator can not be used to replace the operational amplifier, because the high-level output impedance of the open circuit output is more than 1K Ω, the voltage drop is large, and the triode of the next stage will not be cut off.
2. Grid driving part:
The circuit composed of the rear triode, resistance and voltage stabilizing tube further amplifies the signal, drives the gate of the FET, and delays the time by using the gate capacitance of the FET itself (about 1000pf), so as to prevent the FET of the upper and lower arms of the H bridge from conducting at the same time ("common state conduction") to cause a short circuit of the power supply.
When the output of the operational amplifier is at a low level (about 1V to 2V, which cannot reach zero completely), the lower triode is cut off and the FET is turned on. The upper triode is on, the FET is off, and the output is high level. When the output of the operational amplifier is at a high level (about VCC - (1V to 2V) and cannot fully reach VCC), the lower triode is turned on and the FET is turned off. The upper triode is off, the FET is on, and the output is low.
The above analysis is static. The dynamic process of switching is discussed below: the on resistance of the triode is far less than 2K Ω. Therefore, when the triode is switched from off to on, the charge on the gate capacitor of the FET can be released quickly, and the FET can be cut off quickly. However, when the triode is switched from on to off, it takes a certain time for the FET grid to be charged through a 2K ohm resistance. Accordingly, the switching speed of FET from on to off is faster than that from off to on. If the switching action of two triodes occurs at the same time, this circuit can make the FET of the upper and lower arms cut off first and then turn on, so as to eliminate the phenomenon of common state conduction.
In fact, it takes a certain time for the output voltage of the operational amplifier to change, during which the output voltage of the operational amplifier is in the middle of the positive and negative power supply voltage. At this time, the two triodes are turned on at the same time, and the FET is cut off at the same time. Therefore, the actual circuit is safer than this ideal situation.
The 12V zener diode of the FET grid is used to prevent overvoltage breakdown of the FET grid. The withstand voltage of the general FET grid is 18V or 20V, and the breakdown will be caused by directly adding 24V voltage. Therefore, this zener diode can not be replaced by an ordinary diode, but can be replaced by a 2K ohm resistance, which can also obtain a 12V partial voltage.
3. FET output part:
There are diodes in reverse parallel between the source and drain inside the high-power FET. When connected into an H-bridge, it is equivalent to that four diodes for eliminating voltage spikes have been connected in parallel at the output end. Therefore, there is no external diode here. A small capacitor (between out1 and out2) connected in parallel at the output end is good for reducing the peak voltage generated by the motor, but it has the side effect of peak current when using PWM, so the capacity should not be too large. This capacitance can be omitted when using low-power motors. If you add this capacitor, you must use a high voltage one. Ordinary ceramic capacitors may have breakdown and short circuit faults.
A circuit composed of resistor, led and capacitor connected in parallel at the output indicates the rotation direction of the motor
4. Performance index:
The power supply voltage is 15 ~ 30 V, the maximum continuous output current is 5A / each motor, the short time (10 seconds) can reach 10a, and the PWM frequency can be up to 30kHz (generally 1 to 10kHz). The circuit board contains four logically independent power amplification units with two output terminals connected into H-bridge, which can be directly controlled by single chip microcomputer. Realize the bidirectional rotation and speed regulation of the motor.
5. PCB layout and wiring:
The high current line shall be as short and thick as possible, and the through hole shall be avoided as far as possible. If it must pass through the hole, the through hole shall be made larger (> 1mm) and a circle of small through holes shall be made on the pad, which shall be filled with solder during welding, otherwise it may be burned out. In addition, if a voltage stabilizing tube is used, the conductor from the source of the FET to the power supply and ground should be as short and thick as possible, otherwise, in case of high current, the voltage drop on this section of conductor may burn it through the positive bias voltage stabilizing tube and the conducting triode.
In the initial design, a 0.15 ohm resistor was connected between the source and ground of the NMOS tube to detect the current, which became the culprit of burning the board continuously. Of course, if the regulator is replaced by a resistor, there will be no such problem.
The PCB of motor drive circuit needs special cooling technology to solve the problem of power consumption. Printed circuit board (PCB) substrates (such as FR-4 epoxy glass) have poor thermal conductivity. On the contrary, copper has excellent thermal conductivity. Therefore, from the perspective of thermal management, increasing the copper area in PCB is an ideal scheme. Thick copper foil (e.g. 2 ounces (68 microns thick)) has better thermal conductivity than thinner copper foil. However, the cost of using thick copper foil is high and it is difficult to achieve fine geometry.
Therefore, it becomes common to use 1 ounce (34 microns) copper foil. Is the outer layer usually used? Ounces to 1 ounce of copper foil. The solid copper surface used in the inner layer of multilayer circuit board has good heat dissipation. However, since these copper surfaces are usually placed in the center of the circuit board stack, heat will accumulate inside the circuit board. Increasing the copper area of the outer layer of the PCB and connecting or "sewing" to the inner layer through many through holes helps to transfer heat to the outside of the inner layer.
Due to the existence of wiring and components, the heat dissipation of double-layer PCB may be more difficult. Therefore, it is very necessary to provide as many solid copper surfaces as possible and realize a good thermal connection with the motor driver IC. Copper clad areas are added to both outer layers and connected with many through holes to facilitate heat dissipation between areas divided by wiring and components.
a. Routing width: the wider the better
Due to the large incoming and outgoing current of motor driver IC (more than 10 A in some cases), the PCB routing width of incoming and outgoing devices should be carefully considered. The wider the routing, the lower the resistance. The wiring size must be adjusted so that the wiring resistance will not consume too much power to avoid heating the wiring. Too small wiring can actually be used as an electric fuse and is easy to burn out!
Designers usually use ipc-2221 standard to determine the appropriate routing width. This specification provides corresponding charts showing copper cross-sectional area for various current levels and allowable temperature rise, which can be converted to routing width under given copper layer thickness. For example, the wiring carrying 10 a current in 1 ounce copper layer needs to be slightly wider than 7 mm to achieve a temperature rise of 10 ℃. For 1-A current, the routing width only needs to be 0.3 mm.
In view of this, it seems impossible for 10 a current to pass through the micro IC board.
It should be understood that the routing width recommended in ipc-2221 is applicable to PCB routing with equal width and long distance. If a shorter PCB routing is adopted, it is also possible to pass a much larger current without any adverse effects. This is because the short and narrow PCB routing resistance is small, and any heat generated will be absorbed to a wider copper area, which acts as a heat sink.
The PCB routing is widened so that the IC board can better handle the continuous current.
See the example in the figure. Although the IC boards of the device are only 0.4 mm wide, they must carry a continuous current of up to 3 a. Therefore, we need to widen the routing as much as possible and close to the device. Any heat generated in the narrower part of the route is transmitted to the wider copper area so that the temperature rise of the narrower route is negligible. The wiring embedded in the inner layer of the PCB cannot dissipate heat as fully as the wiring in the outer layer, because the thermal conductivity of the insulating substrate is poor. For this purpose, the inner layer routing shall be designed to be about twice the width of the outer layer routing.
As a general guideline, the following table shows the recommended routing widths for longer routing (more than about 2 cm) in motor drive applications.
If space permits, the use of wider routing or copper-clad area wiring can minimize temperature rise and voltage drop.
b. Hot through holes: use as many as possible
Through holes are small electroplated holes, usually used to thread a wire from one layer to another. Although the thermal vias are made in the same way, they are used to transfer heat from one layer to another. Proper use of hot through holes is very important for PCB heat dissipation, but several technological problems must be considered.
The through holes have thermal resistance, which means that when heat flows through the through holes, there will be some temperature drop between the through holes, measured in ℃ / W. In order to minimize this thermal resistance and improve the efficiency of heat transfer through holes, large holes should be used, and the holes should contain as much copper area as possible.
Large holes (cross section of through holes shown) shall be used and shall contain as much copper area as possible to minimize thermal resistance.
Although large holes can be used in the open area of the PCB, the through holes are often placed in the IC board area to transfer heat directly from the IC package. In this case, large holes cannot be used. This is because large plated through holes may lead to "tin penetration", that is, the solder used to connect IC and PCB flows downward into the through holes, resulting in poor solder joint quality.
Tin penetration can be reduced in several ways. One is to use very small through holes to reduce the amount of solder penetrating into the holes. However, the thermal resistance of small through holes is higher, so more through holes are required to achieve the same thermal performance. Another technique is to "tent" the through holes on the back of the board. This requires removing the notch in the solder resist on the back of the plate so that the solder resist material covers the through hole. If the through hole is small, the solder resist layer will plug the through hole; Therefore, the solder cannot penetrate the PCB.
However, this may cause another problem: flux aggregation. After the through hole is plugged, flux (a component of solder paste) may accumulate in the through hole. Some flux formulations may be corrosive. If not removed, it will lead to reliability problems over time. However, most modern cleaning free flux processes are not corrosive and do not cause problems.
Note that hot air pads must not be used for hot through holes, they must be connected directly to the copper area.
The hot through hole shall be directly connected to the copper area on the PCB.
It is recommended that PCB designers check PCB assemblies together with surface mount technology (SMT) process engineers to select the best through-hole size and structure suitable for the assembly process, especially when the hot through-hole is placed in the IC board area.
c. Capacitor placement
The component layout guidelines for motor driver ICs are similar to other types of power supply ICs. The bypass capacitor should be as close to the device power pin as possible, and the high-capacity capacitor should be placed next to it. Many motor driver ICs use pilot and / or charge pump capacitors, which should also be located near the IC.
Most signals are routed directly at the top level. The power supply is routed from the high-capacity capacitor to the bottom bypass and charge pump capacitor, and multiple through holes are used at the transition of each layer.
The bottom layer of tssop and QFN encapsulated devices has a large exposed IC board. The IC board is connected to the back of the chip to remove heat from the device. The IC board must be fully welded to the PCB to consume power.
The die opening used to deposit the solder paste of the IC board is not necessarily detailed in the IC data sheet. Generally, SMT process engineers have their own rules on how much solder should be deposited on the mold and what pattern should be used in the mold.
If a single opening similar to the size of the IC board is used, a large amount of solder paste will be deposited. This may cause the device to be lifted due to the surface tension when the solder melts. Another problem is solder voids (cavities or notches in the solder area). During reflow soldering, solder voids will appear when the volatile components of flux evaporate or boil. This may cause the solder to be pushed out of the solder joint.
To solve these problems, for IC boards with an area greater than about 2 mm2, solder paste is usually deposited in several small square or circular areas. Dividing the solder paste into smaller areas makes it easier for the volatile components of the flux to escape from the solder paste without shifting the solder.
The solder die of QFN package has four small openings for depositing the central IC board
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