A faster current loop
reduces the size ofEV motors
Sang Chon
Automotive marketing manager
C2000™ Real-time MCU
Texas Instruments

Introduction
When it comes to the design and manufacturing of electric vehicles, the goal of
automotive OEMs is to provide an ownership experience that meets or exceeds
ICE-based cars today.
One major goal for OEMs is to reduce size and weight of mechanical components
within the EV vehicle in order to save on cost and due to the limited space available.
One novel way to reduce the physical size of the EV traction motor while maintaining
the same rated power level is by improving the overall performance of the EV traction
inverter. And by improving traction inverter performance, this will also improve overall
efficiency for the motor drive system. Therefore the next major step in advancing EV
motor system design is to significantly increase the speed of the motor in order toachieve smaller motor size with higher efficiency.
Motor power = torque x speed

Only one solution in particular can optimize or eliminate
the design trade-offs involving expensive FPGAs

The key to reducing motor size but maintaining

or increased CPU speeds.

its power level is to reduce the amount of torqueThis solution, which isbased on TI’s C2000™ real-time microcontrollers

needed for the motor system and increase speed.

(MCUs) and the enhanced MotorControl Software

Since the power (size) of the motor is a product ofDevelopment Kit, increases the bandwidth of athe torque times motor speed, OEMs and Tier 1s

EV traction motor drive’s current control loop.

can aim to double motor speed (rpm) in order to cut
the amount of torque needed by ½.

Current loop close-up

However, as motor speed increases, motor

The current loop controls the torque in an EV

frequency increases as well which will require

traction motor by manipulating the pulse-width

higher system current-loop performance (ormodulator (PWM) outputs that drive an inverter. The

torque response) and thus pushes the need for

motor currents are monitored and fed back to thehigher PWM switching frequencies that challenge

current-loop controller and the controller updates

traditional IGBTs.

the PWM outputs if necessary. The current-loopThe current control loop bandwidth becomes thefeedback path quantifies the analog output of thelinchpin upon which the rest of the traction inverter

motor current sensor with a high-precision analog-drive system depends. In order to improve current

to-digital converter (ADC), and then feeds the result

loop bandwidth, more OEMs and Tier 1s are either

to the current-loop controller. Several different

considering adding costly FPGAs to the system or

modules of field-oriented control (FOC) algorithms

demanding more CPU MIPS from MCU supplier toprocess this sample before the controller makes any

improve performance.

adjustments to the PWM’s outputs.

A faster current loop reduces the size of EV motors		

2November 2018

Many conventional proportional integral (PI) current-Clearly, the historical solutions for increasing

loop controllers limit the current-loop bandwidthcurrent-loop bandwidth come with strings attached.

to approximately 10 percent of the PWM carrier

The cleanest, most effective architecture would

frequency, which is typically in the 10kHz – 20kHz

simultaneously address the need for improvedrange. This would yield a current-loop bandwidth ofprocessing capabilities to accommodate shorter

1 to 2 kHz. To increase the bandwidth, some drive

feedback cycles and retain a lower carrier frequency tosystems increase the carrier frequency to 30kHz

avoid the undesirable power and thermal trade-offs.

or higher, which can indeed increase the current
loop’s bandwidth but will wear on the IGBTs in

Fast Current Loop (FCL) technology

the system while increasing the inverter switching

TI has harnessed two decades of real-time industrial

losses. Increasing carrier frequency will also

control system design experience when applying

demand higher processing performance from theC2000 real-time MCUs to the development

digital control system. A higher switching frequency

of a new Fast Current Loop (FCL) technology.

requires increased gate-drive power, which thenFCL, quite simply, breaks many of the long-heldincreases the size and complexity of the system’s

design assumptions in current-loop design. For

power supply. Plus, the higher switching losses will

instance, designers of motor control applications

mean additional heat dissipation, necessitating more

have for years placed limitations on the current-extensive thermal-management strategies such as

loop bandwidth because they assumed that thelarger, heavier and more expensive heat sink,controller could update the PWM only once every

fan or coolant.

cycle of the control loop. Now, with an integrated

Less than 1 microsecond current loop
From start of Conversion to PWM update

Loop control
PI Control
Speed
PI Control
Position

PI Control
Q Current
PI Control
D Current

Computations
InversePark
Transform

Actuation, Protection

SpaceVector
Modulation

PWM
DriverTrip Config

PWMHWIsolatedGate
Drive

ThreePhase
Inverter

PWM TripSensing
Park
Transform

ADC Driver

ClarkeTransform

ADC

Comparator Comparators
ConfigSoftware

Isolated Shunt
Current Buffer
FluxgateSensors

ΔΣ Filter
ΔΣ Filters
DriverΔΣ Comp Comparators
ConfigPM
Motor

Incremental

Position
SensoreQEP

Registers
HardwareFigure 1. Less than 1 uS current loop with FCL (replace “QEP” with “Resolver” in the picture).
A faster current loop reduces the size of EV motors		

3

November 2018

high-performance successive approximation register

Current loop bandwidth(SAR) ADC, ADC post-processing hardware,magtrigonometric math instructions, and other cyclescavenging resources, C2000 MCUs can sample

fmotor currents, convert them to digital data,fprocess the data and update the PWM generator

fin less than 1µs. (Figure 1.) The field-orientedcontrol (FOC) processing and PWM update takes

More than 3 kHz bandwidth
on 10 kHz carrier

less than 500ns. Because of FCL technology andC2000 MCU ePWM’s immediate mode capabilities,
sub-cycle PWM updates occur as soon as possible,

Figure 2. Improving current loop bandwidth while maintaincarrier frequency.

instead of waiting for an entire control loop cycle totranspire.

controller at high speeds. Digital or transport delays

Conventionally, designers assume that increasing

progressively reduce the phase margin of the current

the current-loop bandwidth will require higher carrier

control loop at higher speeds, eventually leading tofrequencies. So, for example, increasing the current-a loss of control. Traditional controllers in the current

loop bandwidth by a factor of 3 from 1kHz to 3kHz
will require increasing the carrier frequency fromloop do not model these digital delays well.

10kHz to 30kHz.

However, FCL software includes an efficient algorithm

But C2000 real-time MCU’s ability to performthat assumes a more appropriate system model,
compensating for the inherent transport delay of thesub-cycle PWM updates in less than a microsecond

system. This is embodied in the complex controller

improves the current-loop bandwidth, making(CC) and results in perfect pole-zero cancellation atit possible to reduce or optimize the switchingfrequency for a given bandwidth requirement. Tests

all times, ensuring stability at high speeds.

have shown that FCL can increase the current-Conclusion

loop bandwidth by a factor of 3 from 1kHz toThe TMS320F2837xD MCUs are highly integrated

approximately 3.3kHz while maintaining a carrier

real-time MCUs for EV traction drive systems. On-

frequency of 10kHz. This avoids the trade-offs

chip features such as sigma delta filters, analogof increased power consumption, greater heatcomparators, and PWM protection circuits savedissipation, and more complex and expensive

system level costs when compared to adding these

thermal-management strategies, all of which

components externally.

become a non-issue with FCL. Therefore, in order
to meet a certain desired bandwidth in closed-loop

The technology breakthrough that contributes tocurrent control, FCL makes it possible to reduce

FCL have emerged from more than 20 years of

the PWM frequency dramatically and eases IGBT

experience in embedded processing and motor-control and thermal design. In other words, FCL

control algorithm development by C2000 MCU motor

enables the motor to run at a higher speed with theengineers. Certainly, the MCUs themselves havesame PWM frequency.

been endowed with powerful, high-performance
processing units, coprocessors; and specialized

Another challenge of high-performance current-loop

instruction sets; but the C2000 MCU’s architecture

design has been keeping stability of the current-loop

A faster current loop reduces the size of EV motors		

4November 2018

is just as essential because it enables a highprocessing as jitter-free as possible, cycles are notdegree of real-time parallel processing through very

wasted recovering from timing issues or runninglow latencies and high processing determinism.

error-correction routines. There are no data-transfer

Ultimately, this makes a C2000 MCU in an EV motor

latencies between the ADC and the C28x cores,

drive application a cycle scavenger, saving processor

or between the cores and the PWM. Transferring acycles over the entire loop and transforming whatsample from the ADC to the C28x cores or updating

may have been longer sequential steps into shorter

the PWM takes only a single cycle. See Figure 3.

simultaneous processes.

Several C2000 real-time MCUs, such as theFor example, 200MHz C28x cores are, ofDelfino™ TMS320F2837xD, include a powerful ADC

course, critical for breaking apart the workload

post-processing block capable of performing certain

to optimize parallel processing, but so is the real-

sample-preparation routines in dedicated hardware

time deterministic architecture that surrounds thethat the C28x cores would normally have executed,CPUs. Because the architecture keeps the system’ssaving cycles on the main CPUs and freeing

Figure 3. C2000 MCU architecture for real-time motor control

A faster current loop reduces the size of EV motors		

5November 2018

them to perform other tasks in parallel. The cores

The Motor Control SDK developers kit features thethemselves have been equipped with coprocessors

latest enhancement, FCL software. Taking advantage

and specialized processing units that prove usefulof a real-time cycle-scavenging architecture,in motor control applications. For instance, the main

high-performance processing resources and theprocessing cores can offload resolver feedback

fast data throughput of C2000 MCUs to significantly

processing to the control law accelerator (CLA)

increase the current control loop bandwidth, FCLcoprocessor, which excels at high-level mathematicalsoftware achieves subcycle updates of the PWM inprocessing. Moreover, both the FOC processing

less than 1µs and without the assistance of external

and the complex controller (CC) use a specialized

processing elements like an FPGA or ADC. And

processing unit, the trigonometric math unit (TMU).

in addition, compared to traditional MCU-based

With the aid of the TMU, you can improve Park

systems, FCL can potentially double an EV traction

transform cycle performance by a factor of 8 overmotor’s maximum speed without increasing thecomparable MCUs in the industry. In addition,

carrier frequency thus reducing the size of an EV

the C28x core’s 32-bit floating-point instructions

traction motor while maintaining its power level.

accelerate the execution of several different types ofThis is how OEMs can save space in an EV vehicle

mathematically intense calculations used by the FCL

with faster current loop.

algorithms, such as minimum/maximum, compare
and square root.

For more information

During the design development of an EV motor

For additional information about Fast Current

traction drive application, designers face a diverse
set of subtle trade-offs, many times placing highLoop software, visit our www.ti.com/c2000ev

performance, cost-effectiveness, power efficiency
and other factors in direct opposition with one
another. Simple yet effective solutions may be hard
to come by, but fortunately, the experience andexpertise in real-time motor drive control systems
that TI has amassed over the last 20 years has led tomany enhancements to the C2000 family of
real-time MCUs.

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