August 11, 2022
  • August 11, 2022

Optimize DC motor drivers with current regulation

By on June 30, 2021 0


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What you will learn:

  • How does a DC motor work and what is the impact of back EMF?
  • Difference between direct current and stall current.
  • How are current regulations enforced in DC motor drivers?

Often the motor driver and power supply are loaded with large currents when starting a DC motor. Using examples of monolithic power systems (MPS), this article examines the current regulation function built into a myriad of motor driver ICs to deal with high current conditions. In many cases, the use of current regulation can allow designers to use a smaller motor-driver integrated circuit.

For simplicity, a brushed DC motor is used in all of the examples, but the processes described here can also be applied to brushless DC motor (BLDC) systems.

Basics of DC motors

Before discussing current limiting and regulating, it is important to consider how a DC motor works.

In its simplest form, a DC motor can be modeled as a voltage – called a reverse electromotive force (EMF) – in series with a resistance. Figure 1 shows this configuration. The back electromotive force is a voltage generated by the motor and it is proportional to the speed of the motor. The series resistance is simply the continuous resistance of the winding.

% {[ data-embed-type=”image” data-embed-id=”60dcd46e2a3ececa158b48b2″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”1. An electrical model of a dc motor includes a power source, series resistance, and back EMF.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_1__DC_Motor_Electrical_Model.60dcd46dd0263.png?auto=format&fit=max&w=1440″ data-embed-caption=”1. An electrical model of a dc motor includes a power source, series resistance, and back EMF.” ]}%

Torque, the rotational force generated by the motor, is created when current flows through the motor.

If there is no mechanical load applied to the motor and a voltage (VSRC) is applied to the motor, then the motor runs and accelerates until the back electromotive force (VBEMF) rises to the same level as VSRC. At this point there is no current flow.

When torque is applied to the shaft, the motor slows down, causing VBEMF decrease by creating a voltage difference between VSRC and VBEMF. This voltage difference generates a current ((VSRC – VBEMF) / RS) that arises from the source.

Note that this is a simplified and ideal approximation. In reality, there are losses and current is still flowing from the source.

Starting the engine

When the engine is stopped, VBEMF is 0 V. Figure 1 shows that when you apply voltage to the motor for the first time, the current is limited only by the series resistance of the motor. This resistance is usually quite low, resulting in a large current flow until the motor begins to spin. The current is generally much larger than the rated direct current of the motor. Figure 2 shows a small DC motor, and the board below shows the engine specifications.

% {[ data-embed-type=”image” data-embed-id=”60dcd483f3a39074108b4894″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”2. For simplification, use a small dc motor to measure the continuous current.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_2__Small_DC_Motor.60dcd48359093.png?auto=format&fit=max&w=1440″ data-embed-caption=”2. For simplification, use a small dc motor to measure the continuous current.” ]}%

the board lists the rated continuous current at 1.25 A, which is the maximum allowable continuous torque load. With this value, one might think that the motor driver only needs to withstand a maximum current of 1.25 A.

% {[ data-embed-type=”image” data-embed-id=”60dcd51a2a3ece7e158b4887″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”Table” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Table.60dcd51a33d91.png?auto=format&fit=max&w=1440″ data-embed-caption=”” ]}%

However, the stall current (the current flowing through the motor at rated voltage when the motor is stopped) is 3.3 A. This means that the motor driver must either be able to drive the stall current to rotate the motor. motor or provide current limitation to start the motor smoothly. Alternatively, the motor driver can activate an overcurrent protection (OCP) function. Devices that do not provide OCP could be damaged.

In addition, the large current required to start the motor must have a power supply that can withstand this high current. In battery powered systems, battery life is reduced by pulling high current pulses, even if they are of limited duration, so it is beneficial to limit the current when starting the engine.

Motor current regulation

Many motor driver integrated circuits include some form of current limiting or current regulation. figure 3 shows an example of a motor driver IC, the MP6522 from MPS, an H-bridge motor driver. The motor current is measured inside the IC by detecting the current in the two low side MOSFETs (LS -FET). This measurement is used by a current regulation circuit.

% {[ data-embed-type=”image” data-embed-id=”60dcd49a54a18e7c138b4827″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”3. The MP6522 is an H-bridge motor driver with a maximum 3.2-A output current.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_3__MP6522_Functional_Block_Diagram.60dcd49a3ffaf.png?auto=format&fit=max&w=1440″ data-embed-caption=”3. The MP6522 is an H-bridge motor driver with a maximum 3.2-A output current.” ]}%

The measured current is scaled to a voltage by a small external resistor on the RISET pin. This voltage is proportional to the motor current. If the current reaches 1.5 V, the MP6522 will shut off the current to the motor for a specified period of time before being reactivated.

A motor has a large amount of inductance. When using current regulation to drive a motor, the current increases when the H-bridge is activated, then decreases when the current trip point is reached, then the driver cuts off the current. The results in a triangular current waveform (green trace in Fig. 4). This waveform shows that the current is regulated by the MP6522 to a peak of about 1.5A.

% {[ data-embed-type=”image” data-embed-id=”60dcd4b254a18e310b8b4902″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”4. The MP6522 can effectively regulate the current at a 1.5 A peak.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_4__Current_Regulation.60dcd4b17723a.png?auto=format&fit=max&w=1440″ data-embed-caption=”4. The MP6522 can effectively regulate the current at a 1.5 A peak.” ]}%

Motor starting (blocking) current

The MP6522 can be used to drive the small DC motor described above. If there are no regulations in force (RISET resistance = 0 Ω) and a 12 V supply, a peak current of approximately 3.6 A is required to start the motor. Figure 5 shows the associated waveform.

% {[ data-embed-type=”image” data-embed-id=”60dcd4e8fac0bd30008b45cb” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”5. The motor startup current has a peak of 3.6 A.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_5__Motor_Start_Up_Current.60dcd4e84188e.png?auto=format&fit=max&w=1440″ data-embed-caption=”5. The motor startup current has a peak of 3.6 A.” ]}%

The motor reaches full speed when the current stabilizes. In this scenario, it takes approximately 50ms for the motor to reach full speed.

Note that switching the motor causes current to ripple in the waveform when the motor is running. As the motor rotates, the switch moves from segment to segment and transfers current to the next winding. During these transitions, the current constantly rises and falls by a small amount. It is not related to the engine driver.

Using current regulation to limit the stall current

By setting the RISET resistance to 10kΩ, the MP6522 can be configured to provide a current limit of 1.5A. Figure 6 shows that the output turns on and off to limit the current through the DC motor.

% {[ data-embed-type=”image” data-embed-id=”60dcd5012a3ece717f8b4985″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”6. With a 10-kΩ resistor, the current in the MP6522 can be limited to 1.5 A.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/06/Fig_6__Current_Regulated_at_1.60dcd500ab012.png?auto=format&fit=max&w=1440″ data-embed-caption=”6. With a 10-kΩ resistor, the current in the MP6522 can be limited to 1.5 A.” ]}%

Compared to how the motor reaches full speed in 50ms with no current limit, it takes the motor 80ms to reach full speed in this scenario.

As the current is proportional to the torque applied to the motor, limiting the motor current on starting also limits the torque. Because the torque accelerates the motor from a standstill to its final speed, limiting the torque also limits this acceleration, which means that the motor takes longer to reach full speed. The inertia of the mechanical system requires torque to accelerate. Therefore, if a large mass is coupled to the motor (for example, a flywheel), then the time is further extended.

Mechanical systems have friction, which is a static force, as well as friction. Friction works the same as friction, but it decreases once the system is in motion. To move, motors must have enough torque to overcome friction and friction. This means that designers cannot set the motor starting current too low. If the current is too limited, the motor may not start at all or it may take too long to reach the desired speed.

Conclusion

This article explains how to use the current regulation function available with motor drivers, such as the MP6522 from MPS, to regulate and control the large currents that flow when starting a DC motor. By understanding how to properly limit a motor’s starting current, designers can not only use smaller motor drivers, but also optimize the current delivery to their system.