Motor coil discharge processing
Q: Why do we need to pay attention to the discharge of the coil when we turn off the power, and the resistor is not needed?
A: We all know that conservation of energy means that energy does not disappear, but from one form to another. When the current goes through the resistor, the energy becomes heat and is dissipated everywhere. When our electrical energy is connected to the coil, the inductor, the electrical energy is converted into magnetic energy. Magnetic field energy is relatively easy to automatically convert with electrical energy. When we turn off the power, the magnetic energy stored in the coil is converted into electrical energy to compensate for the gradually disappearing electric field. From the external performance, the current of the coil will slowly drop. That is, what we usually say: Inductive load can maintain the continuity of current. At this time, if we not only turn off the power, but open the circuit at one end of the inductive load, we will not only see the continuous drop of current, but also see the rapid rise of the voltage across the inductor. When this voltage is increased to a certain extent, it can penetrate the air to form a spark. This principle explains why the DC brush motor will appear on Mars when it is commutating. It also explains the phenomenon that sparks can be seen when the electric brakes are broken in our lives in the 1970s and 1980s.
Q: Sparks are dangerous, so how to deal with them?
A: Since the basic cause of sparks is that the inductive load will keep the current continuity as tenacious as possible even in the case of an open circuit. Then, we will maintain the accessibility of the circuit as the basic principle for solving the problem. A simple and basic method is to place a diode on both ends of the inductor. When the circuit cuts off the circuit, the voltage across the inductor rises. When the voltage rises to a certain level, the diode will be turned on. Thereby providing a path for the current, and the energy can be safely released
Q: How is the discharge handled in the integrated circuit driven motor coil?
Answer: The discharge of the motor coil is generally called DECAY. The same applies to the applied diodes mentioned above. You can connect the motor as shown below
Of course, integrated circuits have their own characteristics, so we can achieve safe discharge without relying on external diodes by operating the circuit. There are three ways: A-SYNCMODE, SYNCMODEFASTDECAY, and SYNCMODESLOWDECAY.
The figure below shows the connection of the circuit when we normally operate the DC motor. The motor realizes forward or reverse rotation through the four MOS tubes of the H-bridge structure. Now the motor is rotating forward, while the upper left and lower MOS tubes are conducting, and the current is flowing from the left to the right (Figure 4).
At this point we stop the motor and need to process the energy stored in the motor coil.
The first way is A-SYNCFASTDECAY. According to the process of the integrated circuit, a body diode must be integrated in the MOS transistor and connected between the drain and the source. By treating the diodes, we can withstand different current intensities. Then, when we stop, if we turn all four MOSFETs off, the current will drain out along the body diode.
At this time, two phenomena will occur. First, a negative voltage below ground will appear on the left side of the motor coil. The amplitude is the diode breakdown voltage, and the voltage on the right side of the coil has a positive voltage higher than the power supply voltage. Second: the energy on the diode is dissipated as Vdiode × Icoil, and the heat is more powerful.
The second way is SYNCMODEFASTDECAY. In this way, we will open the lower left and upper right MOS tubes when the machine is stopped. The current in the motor is still left to right at the beginning of the shutdown. Energy is circulated into the power system through the connection of two open MOS tubes
There are also two phenomena in this way: First, the voltage applied to the coil is opposite to the direction of the current of the coil itself, so that the current in the coil is attenuated faster. Second: the heat generated on the chip is Rdson×Icoil2, because the on-resistance Rdson of the MOS transistor is generally quite small, so the heat dissipation of the chip is small.
The third way is SYNCMODESLOWDECAY. In this way, when the machine is shut down, we turn on the two lower pipes. The current in the coil is still from left to right. Because of the conduction of the two lower turns, we are equivalent to short-circuiting both ends of the motor coil in the circuit principle. Therefore, the current energy is cyclically consumed in the closed loop system composed of the motor coil and the MOS tube.
This method also has its own characteristics: First, in terms of heat dissipation, it is the same as SYNCMODEFASTDECAY. The heat generated on the chip as a whole is Rdson x Icoil2. Second, the short circuit formed by this mode enables the motor system to implement the self-braking function. Third, this approach is not suitable for large high speed motors. The energy in such a motor system is very large, and when connected by means of SYNCMODESLOWDECAY, an ultra-high current phenomenon occurs. The current value is determined by the motor coil induced electromotive force volts and the internal resistance of the motor. Some extreme conditions can cause overcurrent protection or motor burnout
When we have a basic understanding of the discharge mode of the motor, in practical applications, we can choose according to their different characteristics.
