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Reduced Electromagnetic Interference (EMI) with TMS320C24x
Martin Staebler Digital Signal Processoring Solutions
This document discusses designer reduce electromagnetic interference (EMI) digital motor control applications using Texas Instruments TMS320C24x digital signal processor (DSP) controller. This document contains printed circuit board (PCB) layout TMS320C24x with reduced electromagnetic interference (EMI) includes techniques implementing code TMS320C24x derive optimum pattern regarding EMI.
Contents
Design Problem.2 Solution.2
Figures
Figure Figure Figure Proposal TMS320F241-FN Single Layer Layout Average) Fixed 20kHz Carrier Frequency Averages) 20kHz with 2kHz Random-Noise-Modulated Carrier Frequency.6
Tables
Table Comparison between Asymmetric, Symmetric Space Vector
Examples
Example Example Example Example Example Code Listing Symmetric/Space Vector Initialization Code Listing Carrier Modulation.7 Code Listing wobble_random().8 Code Listing Updating Space Vector PWM, SV_PWM_Update().9 C24x Register Declaration (Macros), c240.h.10
Digital Signal Processing Solutions
December 1998
Design Problem
reduce electromagnetic interference (EMI) digital motor control applications using TMS320C24x controller?
Solution
This document contains printed circuit board (PCB) layout TMS320C24x with reduced electromagnetic interference (EMI). Since highest currents typically found with PWM-controlled H-bridge optimized switching pattern), likewise space vector and/or wobbling carrier frequency further reduce EMI. This document also describes techniques implement code TMS320C24x derive optimum pattern regarding EMI.
Layout
electromagnetic compatibility (EMC) electronics circuits great extent determined components laid interconnected. Signal lines with their corresponding return line form antenna, which able radiate electromagnetic energy, where magnitude determined current amplitude, frequency geometrical area current loops. There three typical sources EMI:
Power supply lines Signal lines carrying high frequency Oscillator circuit
Power Supply: Whenever CMOS inverter changing output state, both complementary transistors conducting short time. result will considerable increase supply current, which causes current spikes supply lines. These current spikes lead more less direct route power supply lines, which have been found most significant causes EMI. good practice decouple supply voltage close supply pins with 100nF ceramic bypass capacitor. However, shown reference [1], parasitic components circuit, such impedance package leads supply lines, form effective antenna, where bypass capacitor does significantly reduce current peaks hence radiated interference. suppress these current spikes least supply lines) that spikes reach other parts, improvement achieved adding inductive coil (ferrite beat) between blocking capacitor power supply line, shown Figure should close from where interference suppressed. Signal Lines: Signal lines carrying high frequencies, e.g., lower address lines, clock signals, serial ports, etc., usually terminated CMOS input, providing load several 100k parallel. Charging discharging this load results high current peak. possible reduce these currents connect resistance approximately serial with output. Transmission line theory shows that this resistance negative influence speed long output resistance (internal external resistance) smaller equal line impedance typically 70-120W.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
second precaution make antennas, (signal corresponding return line) small possible. most effective method simply keep critical lines short possible, with priority clock lines (1.), lower address lines (2.), other data lines. TMS320C24x clock, provided CLKOUT1 after reset, switched off, which recommended when used application. When external memory used, pull-down/up data lines avoid drawing (internal) current caused floating input. address lines remain last external address hence have terminated. Oscillator: highest (continuous) frequencies digital systems usually found clock generator. When using crystal combination with C24x internal oscillator, reduce high frequency currents, well area enclosed that current paths, reduce EMI. current resonant frequency crystal very small crystal's high resistance several 100k resonant frequency. However, output voltage CMOS inverters square wave signal containing harmonics which crystal longer represents high resistance. This will result significantly higher currents harmonics. serial resistor added reduce these current components. bypass capacitors provide resistance oscillating frequency; hence there significant current flow Cs-X-Cs. minimize radiation, this area should small possible. Figure shows proposal external crystal connected TMS320F241. serial resistor range resistor parallel crystal might added according manufacturers recommendation.
Figure Proposal TMS320F241-FN Single Layer Layout
TMS320F241-FN
XTAL2
XTAL1
VSSD
Reduction Optimized Pattern
When finished, `C24x unit provide optimized switching pattern further minimize EMI. following considerations made 3phase H-bridge with voltage [UDC], which driven `C24x. more information layout power electronics (IGBT), refer [2].
Mode
three typical modes (asymmetric, symmetric space vector PWM) have different influence radiation. modes supported TMS320C24x unit.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
With asymmetric PWM, three (e.g. lower) switches 3-phase H-bridge turned simultaneously switched according duty cycle. With symmetrical turn-on/turn-off time symmetrical with respect half period, hence commutations three phases, mostly occur same time. This reduces related du/dt di/dt approximately compared asymmetric PWM. both modes, when using sine wave modulation, minimum link voltage [UDC] H-bridge, function effective motor voltage [urms], given
urms,motor
space vector symmetrical with respect period, too. However, since only transistors switched during period, switching losses well radiation reduced compared symmetric PWM. second advantage with
urms,motor
minimum link voltage approximately lower than with sinusoidal symmetrical hence also du/dt reduced further.
Table Comparison between Asymmetric, Symmetric Space Vector
Asymmetric Commutations Period Commutations simultaneously Maximum motor voltage U,DC 310V 110Vrms Symmetric 110Vrms Space Vector 127Vrms
TMS320F240 Application Code: following code shows typical setup TMS320F240 unit. relevant registers (compare, period output polarity) shadowed reloaded timer underflow. Either space vector mode chosen (when constant SPACE_VECTOR_PWM defined shown below) symmetric mode (when undefined). on-chip dead-band unit each 3-phases ensures that there overlap between turn-on period upper lower switch, which would cause additional current spikes. F240 unit registers declared C240.h header file, listed appendix.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
Example Code Listing Symmetric/Space Vector Initialization
/*-*/ Initialize Full Compare Unit /*-*/ #define SPACE_VECTOR_PWM T1CON 0x2840; T1PR PWM_PERIOD; #ifdef SPACE_VECTOR_PWM COMCON= 0x1207; up/down count 50ns steps carrier frequency fpwm 50ns PWM_PERIOD SPACE VECTOR CMPRx,T1PR reload Timer1=0 ACTR reload Timer1=0 enable PWM1-6 outputs
#else COMCON 0x0207 #endif ACTR 0x0666; DBTCON 0x14E0; COMCON 0x8000;
SYMMETRICAL 1,3,5 active high*/ 2,4,6 active dead band enable compare unit
Wobbling Carrier Frequency
When related typically constant carrier frequency harmonics high, modulation this frequency used decrease EMI. Modulation strategies e.g., triangle, random noise, etc. Figure shows spectrum pulsed output voltage fixed carrier frequency 20kHz, where peaks carrier amplitude harmonics above ground noise. Figure demonstrates result wobbling carrier frequency 20kHz 2kHz, using random noise generate spread spectrum. Compared fixed carrier, reduced -12dB. further reduction -18dB possible +/-4kHz modulation. random noise generator requires only clock cycles.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
Figure Average) Fixed 20kHz Carrier Frequency
Figure Averages) 20kHz with 2kHz Random-Noise-Modulated Carrier Frequency
Reduced Electromagnetic Interference (EMI) with TMS320C24x
TMS320F240 Application Code: many applications output voltages fractional numbers (Q15) scaled maximum positive/negative output voltage. 3-phase output PWM, these phase voltages u(a), u(b), u(c) case sinusoidal space vector voltage, characterized degree displaced positive fractional vectors u(x), u(x+60), sector (1-6) which this vector belongs, rotation direction vector. sector determined u(x), case anti-clockwise rotation, u(x+60), case clockwise rotation. both modes fractional voltages u(a),.u(c) u(x), u(x60) have multiplied with period, corresponding duty cycle. Hence, wobbling period does overhead when updating compare values! code Example shows include ±10% carrier modulation into service routine current controller interrupt.
Example Code Listing Carrier Modulation
/*-*/ Current Controller Interrupt Service Routine /*-*/ insert here your current controller, which calculates fractional space vector voltage u_x, u_x60, sector, direction /*-*/ Wobble (Timer period ±10% /*-*/ pwm_period PWM_PERIOD wobble_random(PWM_PERIOD/10); /*-*/ Update Space Vector /*-*/
functions wobble_random(), SV_PWM_Update() written assembler minimize runtime provide compatible interface, hence called from allow better readable software structure. Using inline assembler routines takes advantage variable auto-initialization.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
Example Code Listing wobble_random().
wobble_random(int wobble_amplitude) Function: Random noise generator Arguments: wobble_amplitude (Q15) Return value: wobble_amplitude random_number (Q15) /*-*/ static variables page /*-*/ static random_number 21845; static 13849; static MULT 31821; /*-*/ wobble_random(int) /*-*/ asm(" .text asm(" .globl _wobble_random asm("_wobble_random: asm(" ;-"); asm(" ;random noise asm(" ;-"); asm(" setc asm(" #_random_number asm(" _random_number random_number asm(" _MULT MULT*T asm(" ;ACC asm(" _INC ;ACC asm(" sacl _random_number ;random_number=ACC asm(" ;-"); asm(" ;get argument asm(" ;-"); asm(" *;AR1 argument asm(" ;P=random_number*argument asm(" ;ACC number asm(" ;-"); asm(" ;return asm(" ;-"); asm(" sach ;push stack asm(" lacc ;pop into lower asm("
Reduced Electromagnetic Interference (EMI) with TMS320C24x
Example Code Listing Updating Space Vector PWM, SV_PWM_Update()
void SV_PWM_Update(unsigned pwm_period,int u_x,int u_x60, sector, direction); Function: Update T1PR (PWM carrier) space vectors CMPR1,CMPR2, ACTR(DIR,SECTOR) anti-CW (direction=0) CMPR1 pwm_period*u_x/2 CMPR2 pwm_period*(u_x+u_x60)/2 ACTR.bit 14-12 sector (u_x) ACTR.bit else (direction=1) CMPR1 pwm_period/2 u_x60 CMPR2 pwm_period/2 u_x+u_x60 ACTR.bit 14-12 sector (u_x60) ACTR.bit Arguments: pwm_period u_x, u_x60 positive sector, direction Return value: none asm("T1PR .set 7403h asm("CMPR1 .set 7417h asm("CMPR2 .set 7418h asm("ACTR .set 7413h asm(" .global _SV_PWM_Update asm(" .text asm("_SV_PWM_Update: asm(" asm(" ;get arguments asm(" ;-"); asm(" AR1,* asm(" AR2,*,AR2 asm(" *;AR2 pwm_period asm(" asm(" ;CMPR2 (u_x+u_x60)/2 pwm_period (Q15) asm(" ;-"); asm(" #T1PR/128 asm(" lacc asm(" sacl T1PR ;T1PR pwm_period asm(" pwm_period asm(" pwm_period asm(" ;ACC asm(" u_x60*pwm_period asm(" apac ;ACC asm(" sach CMPR2 ;store CMPR2 asm(" asm(" ;update sector/direction (ACTR) asm(" ;-"); asm(" lacc ACTR asm(" #0FFFh
Reduced Electromagnetic Interference (EMI) with TMS320C24x
asm(" *-,12 ;bit 14-12 sector asm(" ;bit direction asm(" sacl ACTR ;update ACTR asm(" asm(" ;CMPR1 u_x/2 pwm_period (direction asm(" ;CMPR1 u_x60/2* pwm_period (direction asm(" ;-asm(" lacc ;ACC direction asm(" bcnd CW,NEQ ;CW, direction=1 asm("ACW: adrk asm(" u_x*pwm_period asm(" CMPR1 ;store CMPR2 asm(" *,AR1 asm(" asm(" asm("CW: adrk asm(" u_x60*pwm_period asm(" CMPR1 ;store CMPR2 asm(" *,AR1 asm("
Example C24x Register Declaration (Macros), c240.h
/*-*/ Register declaration /*-*/ #define T1CNT *(volatile unsigned #define T1CMPR *(volatile unsigned #define T1PR *(volatile unsigned #define T1CON *(volatile unsigned #define #define #define #define #define #define COMCON ACTR DBTCON CMPR1 CMPR2 CMPR3 *(volatile *(volatile *(volatile *(volatile *(volatile *(volatile unsigned unsigned unsigned unsigned unsigned unsigned
int*) int*) int*) int*) int*) int*) int*) int*) int*) int*)
0x7401 0x7402 0x7403 0x7404 0x7411 0x7413 0x7415 0x7417 0x7418 0x7419
References
Haseloff, Printed Circuit Board Layout Improved Electromagnetic Compatibility, Texas Instruments 1996. (#EB215E) Kirchenberger,U., Beierke, Power-Optimized Solution Drives, PCIM, 1998.
Reduced Electromagnetic Interference (EMI) with TMS320C24x
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