DC motor learning (3) DC motor input capacitance

The selection of the input capacitor is mainly related to the AC ripple current of the input current. The AC of the input current multiplied by the ESR of the capacitor is equal to the AC ripple of the input voltage.

Using a half-bridge drive motor can be equivalent to the following.

Figure 1: DC motor drive equivalent circuit

When M1 is turned on and M2 is turned off, the current on the motor follows a red path. When M2 is turned on and M1 is turned off, the current follows a blue path. Let’s take a look at the current waveform on VM when driven by PWM, as shown in Figure 2 below.

Figure 2: Motor waveform diagram

C1:M2 source voltage | C2: Input PWM | C3: Input current waveform

In Figure 2, we see that the input current waveform is only present when M1 is turned on. When M1 is turned off, there is no input current. When M1 is turned off, we see that C1 has a reverse voltage. This reverse voltage is about 895mV. This is Because the current of L1 cannot suddenly change, the voltage on L1 connected to the MOS tube M2 is a negative voltage at this time, and the voltage connected to the resistor is a positive voltage, allowing the MOS tube body Diode to conduct in the forward direction. At this time, the upper Body Diode of the MOS tube and the internal diode, etc. The voltage drop across the effective series resistor is equal to the voltage across C1.

At this time, the voltage ripple is equal to the ESR of the △Imotor* capacitor. If the ripple of VM needs to be smaller, the capacitor can be connected in parallel to make the ESR on the capacitor smaller, and ultimately the ripple voltage will become smaller. The PWM speed is usually slow, and the current drawn by the motor can usually be restored to what it was before the next cycle began. At this time, the motor draws the same current every time, which is not good for the motor. It will cause jitter or something. The main reason is that it has a relatively large impact on the power supply. If other devices are connected to the power supply VM, it is necessary to evaluate the ripple of the VM that other devices can withstand.

The above situation is the normal operation of the motor. If the motor is not normal and vibrates, the following situations may occur. As shown in Figure 3 below.

Figure 3: Motor waveform diagram

C1:M2 source voltage | C2: Input PWM | C3: Input current waveform

We can see that the current waveform on the motor is a bit messy, sometimes large and sometimes small. If the motor driver has a current collection function, we can judge that the current working status of the motor is not normal. Check as soon as possible to see what's wrong.

Other unexpected situations may also occur during normal operation. For example, the VM suddenly loses connection during the operation of the motor. At this time, as the motor continues to rotate on the VM, Back-EMF is generated and a voltage is generated on the driver chip VM as shown in Figure 4 below.

Figure 4: Motor VM loses connection

The yellow one is the VM voltage at the board end, the blue one is the VM current, and the red one is the PWM input. The moment the motor VM is disconnected, a relatively high voltage will be generated. Note that this voltage may affect other devices connected to the VM. If it is broken, it is relatively simple to deal with it. Just add a capacitor to the VM. Figure 5 shows the effect of adding a capacitor to the VM. It is relatively smooth when you see it.

Figure 5: Motor VM loses connection and has capacitance

Usually when the motor draws current, it is a current with a relatively high frequency, so the VM pin of the motor driver is usually connected in parallel with an MLCC relatively close to provide an instantaneous current when M1 is turned on. When this instantaneous current is provided After that, the current of the electrolytic capacitor just makes up for the MLCC. Because PCB Trace is equivalent to an inductor. Because the current of the inductor cannot change suddenly, it cannot draw current from the input power supply quickly. The size of the inductor needs to be determined based on your actual situation. The equivalent circuit is shown in Figure 6.

Figure 6: Equivalent circuit

Okay, let’s stop here today,