EE152 Green Electronics

Transcription

1 EE152 Green Electronics Embedded Software 9/26/13 Prof. William Dally Computer Systems Laboratory Stanford University 1

2 Logistics Homework 1 due next Tuesday 10/1 Sign up for Lab signoff time Lab 1 must be signed off next week Encourage your friends to join the class 2

3 Review of Tuesday PSSA, Buck, Boost Separate behavior into fast and slow Fast within switching cycle b/a il ip Slow f << f cy Within cycle solve for periodic steady state State variables the same at start and end of cycle. For slow transient response Develop equivalent circuit For same change in each state variable over one cycle 0 Buck/Boost Topologies V 2 (V) i tb t0 tb tcy ta ω =1e+06 ζ =0.2 f = 1.59e+05 Q = time (µs) I L (V) V a b L i L + - V 2 = V 1 D a V 2 V L i L a b V 1 = V 2 = D a + - V 1 V 2 ( 1 D b ) 3

4 Today s Agenda Discontinuous Conduction Mode Embedded Software 4

5 Discontinuous Conduction Mode In buck converter, Suppose switch b only passes positive current V a b L i L + - V 2 5

6 Discontinuous Conduction Mode Switch b may be realized as a diode Or as a FET acting like a diode (synchronous rectification) V a L i L b + - V 2 6

7 Discontinuous Conduction Mode For small duty factor: Current ramps up when switch a is on Current ramps down until it reaches zero Average current is proportional to t a 2 a/b i p i L V 1 -V 2 -V 2 L L 0 t a t on t cy 7

8 a/b Discontinuous Conduction Mode ( ) i p = t a V 1 V 2 L " V t on = t 1 V 2 % a $ ' # & V 2 q cy = 1 2 i p ( t a + t on ) ( ) i L i p 0 V 1 -V 2 -V 2 L L t a t on t cy q cy = 1 " $ 2 # t a ( V 1 V 2 ) L q cy = t 2 " a 2L $ V 1 V 2 # " ( ) V 1 $ # %" " V ' t a + t 1 V 2 $ a $ &# # V 2 V 2 %% '' && I avg = q cy = t 2 " a t cy 2t cy L $ V 1 V 2 # " ( ) V 1 $ # V 2 %% '' && %% '' && 8

9 DCM vs CCM DCM CCM D < V 2 /V 1 D = V 2 /V 1 t cy L 2 I = kt a I = ( D V 2 V 1 ) DCM Current is quadratic with pulse width But control is first order CCM Current is integral of pulse width Control is second order ( ) Hard to build one controller that handles both 9

10 DCM in the Real World High Performance DrMOS Integrated Power Stage DESCRIPTION The SiC778 is an integrated power stage solution optimized for synchronous buck applications offering high current, high efficiency and high power density. Packaged in Vishay s proprietary 6 mm x 6 mm MLP package, SiC778 enables voltage regulator designs to deliver in excess of 40 A per phase current with 91 % peak efficiency. The internal Power MOSFETs utilize Vishay s state-of-the-art TrenchFET Gen III technology that delivers industry bench-mark performance by significantly reducing switching and conduction losses. The SiC778 incorporates an advanced MOSFET gate driver IC that features high current driving capability, adaptive dead-time control, an integrated bootstrap Schottky diode, and a thermal warning (THDN) that alerts the system of excessive junction temperature. The driver is also compatible with a wide range of PWM controllers and supports tri-state PWM, 3.3 V (SiC778ACD) PWM logic, and skip mode (SMOD) to improve light load efficiency. SiC778A Vishay Siliconix FEATURES Thermally enhanced PowerPAK MLP6x6-40L package Industry benchmark MOSFET with integrated Schottky diode Delivers in excess of 40 A continuous current 91 % peak efficiency High frequency operation up to 1 MHz Power MOSFETs optimized for 12 V input stage 3.3 V PWM logic with tri-state and hold-off SMOD logic for light load efficiency boost Low PWM propagation delay (< 20 ns) Thermal monitor flag Enable feature V CIN UVLO Compliant with Intel DrMOS 4.0 specification Material categorization: For definitions of compliance please see Diode Emulation Mode (SMOD) Skip When SMOD pin is low the diode emulation mode is enabled and GL is turned off. This is a non-synchronous conversion mode that improves light load efficiency by reducing switching losses. Conducted losses that occur in synchronous buck regulators when inductor current is negative can also be reduced. Circuitry in the external controller IC detects when inductor current crosses zero and drive SMOD Lo turning the low side MOSFET off. See SMOD operation diagram for additional details. This function can be also be used for a pre-biased output voltage. If SMOD is left unconnected, an internal pull up resistor will pull the pin up to V CIN (logic high) to disable the SMOD function. APPLICATIONS Synchronous buck converters Multi-phase VRDs for CPU, GPU, and memory DC/DC POL modules TYPICAL APPLICATION DIAGRAM Figure 1: SiC778 Typical Application Diagram 10

11 DCM in the Real World SMOD OPERATION DIAGRAM PWM 0V PWM 0V GH GH IL 0A IL 0A GL GL 10nS SMOD# SMOD# Figure 5: CCM Operation with SMOD# = High Figure 6: DCM Operation with SMOD# = Active Toggle 11

12 Discontinuous Boost L i L a + - V 1 V b 12

13 Discontinuous Boost b/a i p = t V a 2 L " V t on = t 2 a $ # V 1 V 2 % ' & i L i p i 0 V 2 V 2 -V 1 L L t b t on t 0 t b t cy q cy = 1 2 i p q cy = 1 " $ 2 # ( t + t a on) ( ) t a V 2 L %" " V ' t a + t 2 a & $ $ # # V 1 V 2 q cy = t 2 " a V 1 V 2 % $ ' 2L # V 1 V 2 & I avg = q cy = t 2 " a V 1 V 2 % $ ' t cy 2t cy L # V 1 V 2 & %% '' && 13

14 DCM Summary In DCM mode current flows during only part of each cycle It stops when it reaches zero 2 Gives I = kt a Non-linear first order control Transition to linear second-order when continuous a/b V a b L i L + - V 2 i L i p V 1 -V 2 -V 2 L L 0 t a t on t cy 14

15 Embedded Software 15

16 Embedded Software Most software is embedded 50 microprocessors in the average car Several in many appliances At the heart of most green electronic systems Embedded software forms the slow but complex part of most control loops The fast, simple parts are in hardware (analog or digital) 16

17 Embedded System Sensors Microcontroller Actuators Fast Hardware 17

18 Example: PV Maximum Power Point Tracker Panel Voltage Panel Current Microcontroller PWM to Boost Converter 18

19 Sometimes Embedded Systems Interact with Humans User Interface Debug Log Sensors Microcontroller Actuators Fast Hardware 19

20 Event Driven Embedded systems execute code in response to events A real-time clock interrupt (e.g., every 1ms) An I/O device completes an operation (e.g., A/D converter) An input transitions (logic low-high, or analog threshold) Code is of the form When trigger, do action When 1ms clock tick, refresh display When A/D completes, process data & start next conversion When current over limit, shut down PWM 20

21 Real-Time Clock Microcontroller has a counter-timer that can be programmed to interrupt the processor periodically (e.g., every 1ms). ATmega164A/PA/324A/PA/644A/PA/1284/P bit Timer/Counter1 and Timer/Counter3 with PWM 16.1 Features 16.2 Overview True 16-bit design (that is, allows 16-bit PWM) Two independent Output Compare units Double Buffered Output Compare Registers One Input Capture unit Input Capture Noise Canceler Clear Timer on Compare Match (Auto Reload) Glitch-free, Phase Correct Pulse Width Modulator (PWM) Variable PWM Period Frequency Generator External Event Counter Four independent interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1) 21

22 AVR Timer/Counter 1 & 3 Figure bit Timer/Counter block diagram (Note:). Count Clear Direction Control Logic clk Tn TOVn (Int.Req.) Clock Select Edge Detector Tn TOP BOTTOM Timer/Counter TCNTn = = 0 ( From Prescaler ) OCnA (Int.Req.) = Waveform Generation OCnA OCRnA DATA BUS = OCRnB Fixed TOP Values ICFn (Int.Req.) OCnB (Int.Req.) Waveform Generation OCnB ( From Analog Comparator Ouput ) ICRn Edge Detector Noise Canceler ICPn TCCRnA TCCRnB 22

23 Library Code Use!timer_start(time, routine, mode)!!to register handler // Start a voltage conversion every millisecond! timer_start(1, start_conversion, PERIODIC);! 23

24 Real-Time Clock Interrupt RTC interrupt Every 1ms rtc rtc rtc rtc rtc Every 3ms rd rd Every 50ms tick50() 24

25 I/O Completion I/O devices interrupt when they complete an operation Handler should Process input data (if an input device) Start next operation 25

26 Example: A/D // Once per millisecond (in RTC handler)! adc_requests = ADC_0 ADC_1;! adc_start();!! // this gets called after! // the second conversion finishes! void adc_1()! {! // process data here! }! 26

27 A/D Timing RTC interrupt 1ms Every 1ms rtc A/D Convert V Convert I A/D Handler ad0() ad1() compute_power() 27

28 A/D Timing RTC interrupt 1ms Every 1ms rtc A/D Convert V Convert I A/D Handler ad0() ad1() compute_power() Ideally we would take simultaneous samples of V and I. Here they are about 100µs apart. What error does this introduce? 28

29 A/D Timing RTC interrupt 1ms Every 1ms rtc tick50() A/D Convert V Convert I A/D Handler ad0() ad1() compute_power() The A/D interrupt handlers may interrupt the RTC interrupt handlers why is this OK? 29

30 A/D Timing RTC interrupt 1ms Every 1ms rtc tick50() A/D Convert V Convert I A/D Handler What happens here? Why is this OK? ad0() ad1() compute_power() 30

31 Integer Arithmetic Most embedded code should use fixed-point arithmetic. Floating-point takes too long (on an uc) Fixed point has plenty of precision for typical control values int (16 bits) [-32,768,32,767] long (32 bits) [-2,147,483,648, 2,147,483,647] But you need to scale your variables and check for overflow Example 16b represents voltage in mv, current in ma Multiply mv and ma to get 32b power in uw Divide by 1,000 and truncate to get 16b power in mw 31

32 Fixed-Point Arithmetic Scale numbers to be in [-1,1) Binary point is just to the right of the sign bit s.bbbbbbb On multiply retain high half of result General fixed-point representation saaaaaaaaa.bbbbbb Represents a value to 1/64 Need to shift after a multiply to put binary point in the correct place Need to think about the range and precision of your variables 32

33 Volatile Variables If you run code with interrupts enabled other code may run at the same time. And long running code (more than 200us) should not disable interrupts Enable interrupts with sei(); That other code may change variables Don t expect variables to be stable while background code runs! 33

34 Control User Input Display Housekeeping Organization of Code 34

35 Layered Control: Example Battery Charger User Interface Battery V, I Input V Mode State Selection Status State Battery V, I Target Selection Current 3A Driver I PWM Control PWM to Buck 35

36 Layered Control: Example Battery Charger Battery V, I Input V User Interface 50ms (or foreground) Mode State Selection 50ms State Status Battery V, I Target Selection 1ms Current 3A Driver I PWM Control 10us FPGA 100ns - Comparator PWM to Buck 36

37 Example Photovoltaic Controller Mode selection Line test, Panel test, MPPT, etc Monitors Line (current, voltage, frequency, ) (anti-islanding) Internal operation (voltages, currents, temps) Panels (current, voltage) Maximum Power-Point Tracking Search algorithm for peak power point Inverter control Convert 400V DC to 240V AC in sync with line User interface Display status, serial output, network connections, 37

38 #include "ui.h"! #include "serial.h"!! uint8_t count = 0;!! // This function is called every 500ms by the software timer system.! // It is called with interrupts disabled, so it shouldn't take very long.! // If you need to do a lot of computation, you can call sei() to enable! // interrupts, but that risks overlapping interrupts and variables! // changing at surprising times, so it should be avoided.! //! // If you do have to enable interrupts inside an interrupt handler,! // be sure to declare as "volatile" all variables that could be changed! // elsewhere and for which you need to see the changes.! void count_callback()! {! ++count;! }! led_dec(count);!! int main()! {! // Initialize the user interface systems (LCD, LEDs, buttons)! ui_init(2);!!! // Initialize the serial port! serial_init();! // Write some text to the LCD! // To store the string in stored in program memory,! // we use the _P version of the function and wrap the string with PSTR.! lcd_printf_p(3, 1, PSTR("Hello, world!"));!!! // Set up a software timer to count on the LEDs.! // count_callback() will be called every 500ms.! timer_start(500, count_callback, PERIODIC);! // Turn on some discrete LEDs.! // This number will be displayed in binary, backwards.! leds[led_discretes] = 0xc9;!! // Write some text to the serial port.! fprintf_p(&serial, PSTR("Hello, serial!\n"));!! // Spin forever.! // If we return from main, interrupts will be disabled and the CPU will! // halt, so the LEDs won't work anymore.! //! // It's best to do all your real work in interrupt handlers.! while (1)! {! }! }! hello.c 38

39 Read the Library Bill-Dallys-MacBook-2:library dally$ wc *! SConscript! adc.c! adc.h! calibration.c! calibration.h! filter.c! filter.h! lcd.c! lcd.h! led.c! led.h! serial.c! serial.h! timer.c! timer.h! ui.c! ui.h! total!! 39

40 Summary Embedded software consists of event handlers When trigger do action Real-time clock is used to perform periodic tasks Schedule A/D converter every 1ms Report status on serial port every 10ms Update mode every 50ms Etc I/O devices trigger handler on completion Process input data Start next I/O operation Layered Control High-level control loop sets parameters of lower level control loops Fastest loops may be in hardware 40

41 In Upcoming Lectures Realizing the switches Power MOSFETs, Diodes, IGBTs and their imperfections Application to motor control Realizing the inductors Using transformers Computing losses and efficiency Batteries and photovoltaic cells 41