How to estimate motor energy feedback and VM power supply pumping

“The problem of motor energy feedback is a common problem that occurs in motor drive systems. Many designers have to choose a motor power supply voltage (VM) level that is equivalent to twice the rated voltage level, which increases system cost. Fortunately, if you can estimate the pump lift first, you can choose the right VM margin. In the DY article of this series (specially for frequently asked questions), Nicholas Oborny provides advice on how to read the motor drive product manual. Today, by introducing a method for estimating the pumping level, I will continue to talk about this topic.

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The problem of motor energy feedback is a common problem that occurs in motor drive systems. Many designers have to choose a motor power supply voltage (VM) level that is equivalent to twice the rated voltage level, which increases system cost. Fortunately, if you can estimate the pump lift first, you can choose the right VM margin. In the DY article of this series (specially for frequently asked questions), Nicholas Oborny provides advice on how to read the motor drive product manual. Today, by introducing a method for estimating the pumping level, I will continue to talk about this topic.

VM pumping waveform

Figure 1 shows a typical VM pumping waveform caused by energy feedback during deceleration. When the input PWM (Pulse Width Modulation) duty cycle changes from 99% to 70%, the VM voltage is pumped from 24V to 32V. (Test conducted on TI motor driver device DRV8840. DRV8840 is a 5A brushed direct current (DC) motor driver.)

Pumping mechanism

Figure 2: Buck and boost circuit

Figure 3: Buck converter in the H-bridge

Figure 4: Boost conversion in the H-bridge

The operating mode of the brushed DC motor can be expressed as equation (1).

Under normal driving conditions, PWM duty cycle = D, the motor will run at a speed driven by the voltage VDRV shown in equation (2).

According to equation (1), we should be able to estimate

The boost effect will make VBST

According to equations (2), (3), (4), we can estimate

Therefore, there is no VM pumping under normal operating conditions.

When the PWM duty cycle is reduced from D1 to D2, just before the time point at which the reduction occurs, we can estimate

Just after the duty cycle is reduced, the speed of the motor cannot suddenly change, so based on the new duty cycle D2, the VBST is calculated

According to equations (6) and (7), we can get

When K * D1/ D2 “1, we can estimate

VBST will be higher than VVM and cause a pumping effect. Assuming K is close to 1, then whenever you reduce the duty cycle and make D2 Pump lift test

In practice, VM pumping cannot be as high as estimated by the above equation (8), because the power supply and VM capacitor will have the ability to absorb electric energy, which helps to reduce the pumping level. In order to verify this estimation method, we can add a diode Ts1 from the power supply to the VM (as shown in Figure 5) in an attempt to obtain a pure pumping effect without the power supply to absorb power.

Table 1 shows the test results. (Note: Some pumping voltage exceeds the VM specification standard of the DRV8840 product manual; this is only applicable to testing. It is never recommended that the device be used under conditions that exceed the specification standard.)

Table 1: Test results and calculation results of pumping voltage

Reduce voltage pumping effect

There are two ways to control the VM pumping:

Use fast decay mode. When the DRV8840 is in fast decay mode, the boost topology shown in Figure 5 no longer exists. Normally, the back-EMF EMF will always be lower than the VM voltage, so VM pumping will not occur at all.But in this case, it will take a long time to decelerate to the target speed

Use a transient voltage suppressor (TVS) to forcibly limit VM pumping. If you choose a TVS with a clamping voltage that is slightly higher than the rated VM level and place it as Ts2 shown in Figure 5, it will be able to forcibly limit the VM pumping (see Figure 8). The author used a 27V TVS, and the VM pumping was effectively limited to 29.6V. The TVS also acts as a dynamic braking device, allowing the motor to decelerate quickly.

Summarize

In the process of motor deceleration, VM pumping is actually an indication of kinetic energy feedback and conversion into electrical energy. Its characteristics are as follows:

The pumping circuit topology included in the PWM drive is the key factor why the back-EMF energy can force the current back to the VM power supply even when it is less than the power supply voltage. In the deceleration phase, if the fast decay mode is used, it will not cause the VM to pump up, but the motor needs a relatively long time to slow down.

The TVS clamping method or other dynamic braking methods can effectively solve the problem of excessively high VM pumping, which can reduce the VM pumping effect while maintaining a faster deceleration speed.