Q: Why do we need to pay attention to the discharge of the coil when we turn off the power, but not the resistor?
A: We all know that the law of conservation of energy means that energy does not disappear, but is transformed from one form to another. When current flows through a resistor, energy is converted into heat and dissipated in all directions. When our electrical energy is connected to a coil, that is, an inductor, the electrical energy is converted into magnetic energy. Magnetic field energy is relatively easy to convert into electrical energy automatically. 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 outside, the current in the coil will slowly decrease. This is what we usually mean when we say that inductive loads can maintain the continuity of current. At this time, if we not only turn off the power, but also completely open the circuit at one end of the inductive load, we will not only see the current continuously decreasing, but also see the voltage across the inductor rising rapidly. When this voltage increases to a certain level, it can break down the air and form a spark. This principle explains why sparks appear when a brushed DC motor commutates, and also explains the phenomenon of sparks appearing when the power is switched off in our lives in the 1970s and 80s (Figure 1).
Figure 1. DC motor spark and switch spark.
Q: Sparks are dangerous, so how should they be handled?
A: Since the primary cause of sparking is that inductive loads stubbornly maintain current continuity even when the circuit is open, we can take maintaining circuit continuity as the fundamental principle for solving this problem. A simple and basic method is to place a diode across the inductor. When the circuit is interrupted, the voltage across the inductor rises. When the voltage rises to a certain level, the diode conducts, thus providing a path for current and allowing energy to be safely released (Figure 2).
Figure 2 shows that energy can be safely released in this circuit.
Q: How is discharge handled in the coils of a motor driven by an integrated circuit?
A: The discharge of the motor coil is generally referred to as DECAY. The method of adding an external diode mentioned above also applies. You can connect the motor as shown in Figure 3.
Of course, integrated circuits have their own inherent characteristics, so we can achieve safe discharge without relying on external diodes by manipulating the circuit. There are three methods: A-SYNCMODE, SYNCMODEFASTDECAY, and SYNCMODESLOWDECAY.
The diagram below shows the typical circuit connection for operating a DC motor. The motor rotates in either direction using four MOSFETs in an H-bridge configuration. Currently, the motor is rotating forward, and the two MOSFETs (top left and bottom right) are conducting, with current flowing from left to right (Figure 4).
Figure 4. Circuit connection method when operating a DC motor.
At this point, we stop the motor and need to process the energy stored in the motor coils.
The first method is A-SYNCFASTDECAY. According to integrated circuit manufacturing processes, a body diode will always be integrated into the MOSFET, connected between the drain and source. By manipulating the diode, we can make it withstand different current intensities. Therefore, when shutting down, if we turn off all four MOSFETs, the current will flow out along the body diode, as shown in Figure 5.
Figure 5, the first method: A-SYNCFASTDECAY
Two phenomena will occur at this time. First, a negative voltage lower than ground will appear on the left side of the motor coil, with an amplitude equal to the diode breakdown voltage, while a positive voltage higher than the power supply voltage will appear on the right side of the coil. Second, the energy dissipated on the diode is Vdiode × Icoil, resulting in significant heat generation.
The second method is SYNCMODEFASTDECAY. In this method, when stopping, we turn on the two MOSFETs at the bottom left and top right. The current in the motor still flows from left to right at the beginning of the shutdown phase. Through the connection of the two turned-on MOSFETs, energy is circulated into the power system (Figure 6).
There are two phenomena in this approach: First, the voltage applied to the coil is opposite to the direction of the coil's current, resulting in a faster current decay in the coil. Second, the total heat generated on the chip is Rdson × Icoil², and because the on-resistance Rdson of a MOSFET is generally quite small, the chip's heat dissipation is relatively low.
The third method is SYNCMODESLOWDECAY. In this method, when the machine stops, we turn on both lower transistors. The current in the coil still flows from left to right. Because the two lower transistors are turned on, in principle, we are equivalent to short-circuiting the two ends of the motor coil. Therefore, the current energy is circulated and consumed in the closed-loop system composed of the motor coil and the MOSFET.
This method also has its own characteristics: First, in terms of heat dissipation, it is the same as SYNCMODEFASTDECAY. The total heat generated on the chip is Rdson × Icoil². Second, the short-circuit circuit formed by this mode enables the motor system to achieve self-braking. Third, this method is not suitable for large, high-speed motors. Such motor systems have a large amount of energy, and when connected using SYNCMODESLOWDECAY, an excessively high current will occur. The current value depends on the induced electromotive force of the motor coil and the internal resistance of the motor. In some extreme cases, this can lead to overcurrent protection or motor burnout (Figure 7).
Figure 7, the third method SYNCMODESLOWDECAY
Once we have a basic understanding of the discharge methods of motors, we can choose the appropriate method based on their different characteristics in practical applications.
