The_next_level_of_MCUs_for_power_conversion_Article

The_next_level_of_MCUs_for_power_conversion_Article
The Next Level of Power
Conversion with
Cortex-M4 based MCUs
Abstract
In the power conversion world, the tendency of migrating into digital control loops has been gaining
popularity over the last few years. Not only does a digital implementation provide better immunity to
process and operating conditions, but it can also increase the resource usage density. With the
powerful ARM® processors often found in today’s microcontrollers, together with a diversified
peripheral arrangement, this type of solution is attractive to control power conversion stages, with a
good design reuse factor and reduced overall cost.
Using a microcontroller for power conversion applications may not always be straightforward because
of the big effort of mapping previously and newly developed topologies from the pure analog (or power
controller) form into a more complex microcontroller system. This paper however will demonstrate how
the ARM® Cortex™ M4 CPU of the Infineon XMC4000 product family and the associated peripherals,
are tailored to power conversion applications, and how you can take advantage of this to build a
design with a better resource density and reduced bill of material.
Introduction
Over the last fifty years, mankind has doubled the world population and tripled average energy
consumption. With increases of this magnitude, the paradigm of resources and energy versus the
number of consumers is no longer local to developed countries, but global. The world is now in a
constant search for resource and energy optimization, with the aim of creating a better and sustainable
life for all of us.
Power conversion is a big part in energy usage and distribution optimization. Power conversion
engineers are driven into a constant search for improvement of power density and efficiency. A high
power density will enable a smaller footprint, while improved efficiency will provide “greener” designs,
with a better cost versus energy consumption ratio.
With the aim of optimizing power conversion designs, industry has started the migration of the
common analog control to a digital control. Digital control provides better immunity to process
variations, and can compensate power stages dependent on several parameters such as different
temperatures and operating conditions like load, input or output voltage. Digital control also offers a
high level of portability and reliability.
State of the art Infineon microcontrollers, such as the XMC4000 family, have a complete set of
peripherals that together with the powerful ARM® Cortex™ M4F CPU, can easily perform all the tasks
needed to optimize a power conversion stage. The comprehensive set of peripherals can also help
lead to an increase of resource density and decrease the manufacturing costs (by for example
controlling several power stages, handling communication, monitoring operating conditions, etc.).
Vout Monitor
Vin Monitor
Load cond.
Dead-time
Temp.
Switch
Drive
X
System
-
CPU
- high resolution
- dither pwm
- current mode control
- voltage mode control
Compensator Block
Current Sense
Amplifier
A/D
PWM Drive
Current Sense
Amplifier
- compensator
- PID
- cycle-by-cyle update
- system handling
- error conditions info
- operating cond. adj.
Communication
X
Voltage
reference
+
- status indication
- system information
D/A
- reference generation
- pattern generation
Comparators
CODE
RAM
a) Analog Implementation
- voltage conversion
- cycle-by-cycle conversion
- cycle-by-cycle monitor
- error conditions
- PWM direct control
b) MCU Implementation
Figure 1 – Example of a Power Conversion application control with a microcontroller: a) Typical Analog
Implementation; b) MCU implementation
With most generic MCU solutions, the migration jump from a complete external component solution or
an implementation with a power controller is often met with hurdles and problems that are difficult to
predict. The Infineon XMC family approach enables a cleaner identification of resources for each and
single power conversion application. This includes single power conversion stages such as Buck or
Boost Converters, Flyback, Half-Bridge, Resonant converters, but also extends itself to the most
complex applications in the market today, such as Interleaved PFC, Phase-Shift Full Bridges, N-Phase
Buck Converters, Multi-Level converters, and so on.
With the Infineon XMC family of products, you can adjust your MCU resources to your power
conversion needs, instead of the targets to be met being constrained by the available resources.
XMC4000 Microcontroller System
The XMC4000 devices are built on the powerful ARM® Cortex™ M4 processor with a Floating Point
Unit (FPU), a Memory Protection Unit (MPU) and a Nested Vector Interrupt Controller (NVIC). The
communication between the peripherals and the ARM® Cortex™ M4 is based on a multi-layer, multimaster bus system, optimized for real-time applications. The Direct Memory Access (DMA) can be
used to perform data transfers from peripheral-to-peripheral, memory-to-peripheral, or peripheral-tomemory. This offloads the processor for the demanding real-time computation tasks.
XMC4400
ARM®
Cortex™-M4
FPU
Memory
Safety, Power &
Clock
BROM
WDT
Debug
Flash
RTC
ARM®-JTAG
MPU
PSRAM
FCE
Serial Wire
Debug
NVIC
DSRAM
ECC & Parity
Embedded
Trace
DMA (12 channels)
Bus Matrix
Peripheral Bus 1
Peripheral Bus 2
Enhanced Connectivity (ERU)
Analog
HMI
Communication
Industrial Control
ADC
Capacitive
Touch
Ethernet
CCU4 (x4)
DAC
LED Matrix
USB
CCU8 (x2)
Delta-Sigma
Demodulator
Ports
CAN
POSIF (x2)
USIC (x2)
(serial comm.)
HRPWM
Figure 2 – XMC4400 Device Diagram
The peripherals included in the XMC4000 MCU family contain dedicated features for HMI or
Communication applications. The Industrial Control subset contains an extensive list of peripherals
especially developed for power conversion and motor control:
 CCU4 and CCU8 for PWM generation and signal conditioning
 POSIF for Motor Control and Multi-Channel PWM control
 HRPWM for High Resolution PWM generation with built-in high speed Comparators and slope
generators (for current control applications)
The analog group contains several ADC channels that are able to perform sequential and chained
conversions, and offer a fast compare mode for demanding power conversion monitoring. The DAC
channels enable fast digital to analog conversion with a dedicated pattern generator.
Adjust Your MCU to Your Power Conversion Application
In the power conversion world there are several converter topologies and normally two major control
loop implementations: current control or voltage control. The converter topology chosen directly
dictates the minimum amount of resources that are needed. If for example a synchronous Buck
Converter is present in the application, it is a safe assumption that at least two complementary PWM
signals and a current and/or voltage sensing loop are needed. The choice between a current or a
voltage control loop is normally dictated by how fast the response of the system should be, versus
updates on the line or output load.
Already we can see that there are several variables to be considered when choosing a microcontroller
for a given power conversion application. If we also add the different modulation schemes that
currently exist in the power conversion world (Figure 3), then we have even more resource variables,
and these can be interconnected or shared:
 Resources to drive the chosen converter
 Resources for the current or voltage loop
 Resource for controlling the chosen modulation
VC
Clear
iL*R
iL*R
Set
VC
Set
Timer
Timer
PWM
PWM
Clear
a)
b)
Vca
VC+
Clear
iL*R
Clear
Set
Set
VC-
Timer
PWM
PWM
c)
d)
Verror
Clear
Set
PWM
e)
Figure 3 – Different modulation schemes: a) peak current control; b) valley current control; c) hysteretic
control mode; d) average current control mode; e) voltage control mode
The decision of which microcontroller to take becomes even more complex when several products or
derivatives need to be developed (each one of them with different requirements for power conversion).
The resource arrangement of the new XMC4000 microcontrollers gives you the possibility of adjusting
the MCU to the specific power conversion needs, reducing the initial complexity of analyzing each
single requirement.
Adjust the XMC4000 resources for driving the converter
The direct drive resources (PWM) of the power conversion can be taken from the CCU4 (Capture &
Compare Unit 4) of the XMC4000 in a straightforward manner if non-synchronous converters are
being used (Figure 4). Each CCU4 contains four, sixteen bit timers, with dedicated functions for
controlling non-synchronous topologies: select the active level of the switch, over current protection
with trap function, dither for increasing the PWM resolution, etc.
2
3
CCU4
0
1
PWM3



16 bit timer
Edge aligned
Mosfet level
dither
Center aligned
trap
PWM2
Non-Synchronous
Converter topologies
PWM1
PWM0
Buck
Converter
Boost
Converter
Flyback
Converter
Forward
Converter
Up to 4 non synchronous converters
Safe shutdown control
Resolution boost via dither
Figure 4 – CCU4 (Capture & Compare Unit 4) resources for generating the PWM
If synchronous converters or more complex topologies are used (like phase shift, interleaved or multilevel conversion), then the CCU8 (Capture & Compare 8) is used for the direct PWM drive. CCU8
contains four identical sixteen bit timers with two compare channels, that are able to generate up to
four PWM signals (per Timer Slice) adding up to a maximum of sixteen output PWM signals. Deadtime control for synchronous converters is also built-in (Figure 5).
Synchronous and complex
converter topologies
3
CCU8
2
PWM3y(y= 0..3)
1
PWM2y(y= 0..3)
0
Phase shift
16 bit timer
Edge aligned
Mosfet level
dither
Center aligned
2x Compare
trap
Dead-time
PWM2y(y= 0..3)
Full Bridge
Converter
Phase-Shift
Full Bridge
Converter
Interleaved
Boost
Converter
Multi-Level
Converter
Half-Bridge
Converter
Push-Pull
Converter
PWM0y(y= 0..3)
N-Phase
Converter




Up to 16 PWM outputs
Programmable dead-time
Asymmetric PWM mode
Phase Shift Control
Figure 5 – CCU8 (Capture & Compare Unit 8) resources for generating the PWM
To achieve a higher power density and cost ratio in the development of a power conversion
application, the switching frequency of the converter should be as high as possible. Increasing the
switching frequency will decrease the size of the filtering and coil stages of the power converter,
therefore decreasing the cost of these components. It will also decrease the heat dissipation concerns.
To address the market of high switching frequency converters, the XMC4000 as one HRPWM (High
Resolution PWM) unit can achieve a 10 bit resolution for converters with a switching frequency of up
to 6 MHz (Figure 6).
Synchronous/non-synchronous
and complex converter topologies
with High Switching frequency
3
HRPWM drive
2
PWM3y(y= 0,1)
Interleaved
Boost
Converter
Half-Bridge
Converter
Multi-Level
Converter
N-Phase
Converter
Push-Pull
Converter
0
1
PWM2y(y= 0,1)
150 ps resolution
PWM2y(y= 0,1)
PWM0y(y= 0,1)
trap
Dead-time
Turn-ON adjustment
Turn-OFF adjustment






Full Bridge
Converter
Phase-Shift
Full Bridge
Converter
up to 10 bits with fs ≤ 6.5 MHz
Up to 8 PWM outputs
Adjustable dead-time
Phase Shift Control
150 ps resolution for duty-cycle
150 ps resolution for phase-shift
Figure 6 – HRPWM resources for generating the PWM
Adjust the XMC4000 resources for the converter loop
There are two major control loops for a typical power converter: voltage mode control and current
mode control. While the voltage control follows a simpler implementation because it only contains one
loop, it has the disadvantage of having a slower response to line or load updates. This will impose a
non-favorable transient response. The current control “updates” the voltage control with an additional
loop. This loop will sense the circuit current. This control scheme is therefore more complex to
implement, but it can compensate for a change of the input voltage or the load before this affects the
output voltage of the converter (Figure 7).
iL
VOUT
VOUT
RL
VIN
RL
VIN
PWM Drive
Current Sense
Amplifier
PWM Drive
Compensator Block
Comparator
Block
Compensator Block
X
X
X
Compensator
Block
+
X
+
VIN
Voltage Feed
Forward Block
a)
Voltage
reference
Voltage
reference
b)
Figure 7 – Control loops: a) voltage control; b) current control
It is common to have power conversion applications that follow different control loop implementations
(voltage or current). The microcontroller therefore needs to provide proper resource arrangements for
both schemes. The XMC4000 can be used with either of these two schemes, and it can even be used
for applications where both schemes need to coexist, so increasing the resource density and
decreasing the bill of materials.
The ARM® Cortex™ M4 powerful processor can be used to implement the required compensator
blocks used for the voltage or current control modes (Figure 8). This can be done by using the ADC to
sense the voltage output and the coil current of the converter.
The ADC results will then be used by the processor to calculate the next cycle PWM values. Because
the compensator blocks are implemented digitally, only a small amount of external resources are
required. The amount of resources can be limited to some signal accommodation components.
ARM® Cortex™ M4
Compensator
Cycle-by-Cycle adjustment
Operating conditions adj.
Duty-Cycle/Frequency adjustment
Temperature adjust.
More demanding loops
in terms of data
processing but less
demanding in switching
freq.
Conversion
Results
Vin Monitor
3
ADC
PWM
Voltage control
0
1
Vout Monitor
Arbiter
Averaging
Sequential triggering
Timed conversion
Current Monitor






Fast comp.
8 chann.
Overvoltage/
overcurrent
2
CCU4
CCU8
PWM
Current
Control
PWM
HRPWM Drive
Averaging conversion
Controllable timed conversion
Several input channels
Direct connection to PWM drive
Fast comparison mode
Sequential conversion
Figure 8 – ADC and ARM® Cortex™ M4 resources for control loop implementation
By implementing a loop with an ADC and the CPU, some latency may be incurred, which may not be
suitable for high switching frequency converters. As the switching frequency of the converter
increases, the acceptable latency for the control loop decreases. This is especially critical for control
loops that implement cycle-by-cycle monitoring, such as peak current control, hysteretic control, valley
current control, etc.
The HRPWM unit contains a monitor that is tailored for fast control loop implementations (Figure 9).
This monitor part contains three dedicated high speed comparators and DACs that enable the cycleby-cycle current to be monitored for different modulation schemes (peak or valley current control,
hysteretic mode up to 5 MHz, etc.), without any CPU interaction. The direct connection to the drive
units (CCU4, CCU8 or HRPWM drive part) is implemented such that there is no need for CPU usage.
Fast Control Loops
(cycle-by-cycle control)
Vout Monitor
Direct Duty-Cycle
and frequency
adjustment
2
HRPWM monitor
1
CMPout2
0
Current Monitor
High Speed CMP
High Speed DAC
Slope Compensation





CMPout1
blanking
filtering
clamp
Hysteretic
Control
Voltage control
Peak current
control
Valley current
control
Variable
frequency fixed
Duty-Cycle
Fixed
frequency
variable ON
time
PWM
CCU4
CCU8
PWM
CMPout0
PWM
HRPWM Drive
Fixed
frequency
variable OFF
time
High Speed CMP for current/voltage monitor
HW slope compensation for peak control
Blanking to avoid commutation spikes
Direct Control of the HRPWM drive/CCU8/CCU4
Two channel CMP for hysteretic mode
Figure 9 – HRPWM monitor resources for control loop implementation
For a resource organization that can cope with any type of power converter topology, control loop or
modulation scheme, the XMC4000 solution can be seen in Figure 10. The HRPWM monitor is used for
fast control loop implementation, while the ADC and ARM® Cortex™ M4 can handle the slow loops
and operating conditions optimization. The CCU4, CCU8 and HRPWM drive units are used to drive
the power converter PWM signals.
Temp.
ARM® Cortex™ M4
Compensator
Operating conditions adj.
Operating conditions adjustment
Temperature adjust.
Vin Monitor
All Control Loops and all
modulation schemes
Conversion
Results
3
ADC
PWM
Average
Current
Control
Hysteretic
Control
Peak current
control
Variable
frequency fixed
Duty-Cycle
Trailing current
control
Fixed
frequency
variable ON
time
Fixed
frequency
variable OFF
time
0
1
Vout Monitor
Arbiter
Averaging
Sequential triggering
Timed conversion
Fast comp.
PWM
PWM
2
Direct Duty-Cycle
and frequency
adjustment
CMPout2
0
1
High Speed CMP
High Speed DAC
Slope Compensation
CCU8
HRPWM Drive
8 chann.
HRPWM monitor
Current Monitor
Overvoltage/
overcurrent
2
CCU4
Voltage control
blanking
filtering
clamp
CMPout1
CMPout0
Figure 10 – XMC4000 resource organization for any type of control loop/power topology
With the powerful arrangement of resources inside the XMC4000 it is now possible to adjust the
microcontroller to the needs of the power conversion stage (topology, control loop and modulation
scheme), and at the same time decrease the number of external components.
Figure 11 shows the implementation of a control loop for a synchronous buck converter using peak
current control modulation. The resources inside the drive units (HRPWM, CCU4, CCU8) can
substitute the following external components: current comparator, slope/ramp generator, the latch
generating the PWM and the dead-time driver. The ADC together with the processor routines can
substitute the sensing and accommodation of the voltage output (this loop is slower than the current
loop handled by the HRPWM).
iL
Vin
Vout
Ri
+
Latch
Dead
Time
Clear
CMP
Set
-
Err.
Amp.
+
-
+
Vref
Ramp
Generator
Clock gen.
HRPWM Drive
CCU4
ADC
HRPWM Monitor
Cortex™ M4
CCU8
Figure 11 – Peak Current control with XMC4000
Adjusting Your XMC4000 to Your Power Conversion
The XMC4000 is a very powerful microcontroller that can be adjusted to your power conversion
topology and control loop. This turns the XMC4000 into a perfect partner for your product line
development by covering the entire range of power conversion topologies (Figure 12), reducing porting
and feasibility expenses, and increasing cross-derivative compatibility.
Figure 12 – Power Converter vs. Resources
By combining the resources of the XMC4000, the complete set of control loops and modulation
schemes can be covered: from the simplest voltage mode control to the most complex current control
loop (Figure 13). Using the XMC4000 resource functions tailored for power conversion (e.g. HRPWM,
ADC, ARM® Cortex™ M4), the resource density of the final application can be increased by reducing
the number of external components, and at the same time reducing the total cost of the product.
Figure 13 – Control Loop vs. External Components
Author Information
Pedro Costa
Concept Engineer for XMC4000 family at Infineon Technologies Munich
Email: [email protected]
Andreas Jansen
Application Engineer for XMC4000 family at Infineon Technologies Munich
Email: [email protected]
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement