Getting a handle on brushed DC motor current

Transcription

1 Getting a handle on brushed DC motor current Ross Eisenbeis, Systems Engineer, Texas Instruments - November 11, 2015 Systems that have controlled parameters and closed-loop feedback mechanisms are generally more robust and less susceptible to failure. For example, vehicle engines regularly use temperature sensors and tachometers to ensure that the operating conditions stay within the designed scope. If temperature and engine speed weren t controlled or even monitored, the design would need to be significantly more robust and costly to be able to withstand the worst-imaginable scenarios. Constraints allow designs to be more efficient. With brushed DC motors, a prime example of a constraint that often is not capitalized is the maximum allowed current. In many systems today, maximum current is unbounded, limited only by the small DC resistance of the motor plus the R DS(on) of MOSFETs. Then fault-protection schemes are the only line of defense for preventing component damage. As a result, power-delivery stages are often overdesigned, temperatures can reach high levels, and predicting corner-case behavior can be challenging. Unbounded current When motors are spinning, a back electromotive force (back EMF) develops on the winding. Directly proportional to the RPM, back EMF counteracts the externally applied voltage across the motor terminals. Steady-state current through a brushed DC motor equals the applied voltage minus the back EMF, divided by the resistance of the winding (Equation 1):

2 When a motor is prevented from turning (stalls) while being electrically driven, there is no back EMF, and the current will reach the full applied voltage divided by the resistance. This happens if the load torque is greater than the motor s stall torque, or if there s simply a jam that stops movement. The other situation that involves much higher current than normal operating levels is when a motor begins to spin up. Initially the back EMF is zero, and the current rises as quickly as the motor inductance allows. When the current peaks, the motor will be moving and some back EMF will be present, so the peak will be lower than the stall current. The waveforms in Figures 1 and 2 show measured current during spin-up, runtime and stall using a Maxon Motor RE brushed DC motor. Figure 1: Motor spin-up current

3 Figure 2: Motor stall current With 24 V applied, the motor consumed 29 A during spin-up (700 W) for about 1 ms, while the operational current was 2 A (48 W continuous). When the rotor was held and prevented from moving, stall current was 34 A. This motor requires that continuous current be kept under 3.5 A to prevent overheating and damage. Whenever high amounts of current are involved, two primary consequences come to mind: supplyvoltage drop and heating. Supply-voltage drop Quick demands of current require capacitance to source the energy and maintain a stable voltage. A power supply by itself has limited capacitance on the output, and interconnects to the motor system will have inductance that limits response time. For these reasons, local bypass capacitance is needed.

4 These silos of stored energy must be large enough to handle the biggest demands of a motor system. If the motor consumes more charge than what the capacitor has stored and the power supply can t replenish it fast enough, the motor voltage will drop. Once the voltage rail is unstable, a multitude of bad things can happen: motor rotation will be disturbed, the controller circuitry might stop functioning, and the MOSFETs can even become damaged due to partially on gate voltages that cause high resistance and overheating. Therefore, one must size the bulk capacitance according to the maximum possible current. Heating Heating Most sub-100-v motor systems control speed by pulse-width modulating (PWM) the MOSFETs, with a frequency between 0 to 100 khz, and static power dissipation dominates power loss in the drive stage. I 2 R calculates the dissipated power for each resistive component in the current path. Starting from the power supply, the path typically includes a printed circuit board (PCB) trace, the R DS(on) of the high-side MOSFET, the motor winding, the R DS(on) of the low-side MOSFET, an optional sense resistor and finally a PCB trace back to the power supply. Over time, the dissipated power generates heat. Since motor current exponentially increases this, optimized systems should limit current to what s needed for adequately fast spin-up times and driving the maximum load torque. Closed-loop current regulation Actively measuring and compensating motor current makes it easy to budget for the maximum power draw and heat buildup. This produces more predictable system behavior and the potential for big cost savings in a power design. Perhaps the most commonly used architecture for closed-loop current regulation is shown in Figure 3. This places a low-value resistor in series with the motor s ground path and amplifies the voltage across it. That voltage is compared to a reference, and the comparator output disables the MOSFETs.

5 Figure 3: Common closed-loop feedback circuit The resistor must have a power rating of at least I 2 R, where I is the root-mean-square (RMS) current. Lower resistances will obviously reduce the power dissipation, but also the usable voltage produced. It is common practice to use multiple resistors in parallel to distribute current and heat, since they are sometimes cheaper than a single high-power resistor. Motor driver devices like the DRV8701 have an integrated feedback circuit and rely on an external sense resistor and V REF input. The waveform in Figure 4 shows spin-up current using the same motor that was used for Figures 1 and 2, with current regulation set to 14 A.

6 Figure 4: Motor spin-up using 14-A current regulation Likewise, stall current is also favorably restricted (Figure 5).

7 Figure 5: Motor stall using 14-A current regulation Why isn t current regulation always used? There are plenty of benefits, and the implementation isn t terribly difficult. So why don t more systems use current regulation? There are a variety of reasons: Lack of awareness. Many systems use extra bulk capacitance and higher-current power supplies, along with fault protection (such as a fuse) to catch the high-current scenarios. Brushed DC motors don t require complex control, and crude designs will still work. Small motors can have low stall currents of less than 0.5 A, so regulation isn t always important. Using a high-power sense resistor and other components adds board area, power dissipation and cost.

8 When a motor driver circuit is built with discrete components (rather than a monolithic integrated circuit), incorporating an amplifier, comparator and logic adds extra complexity. New technology This year, Texas Instruments designed an innovative way to measure and regulate motor current without a sense resistor or reference voltage, and integrated it in silicon. The first product to use this advanced technology is the DRV8871, an 8-pin 3.6-A controller, and the current threshold is set by the value of a standard resistor. This efficient solution uses no extra board area; eliminates design effort; and avoids the power loss, heat and cost associated with sense resistors. As this technology makes its way to future devices, it may become an integral part of mainstream DC motor control. References Download these data sheets: DRV8701, DRV8871.