Lecture-36 EE5325 Power Management Integrated Circuits

Transcription

1 Lecture-36 EE5325 Power Management Integrated Circuits Dr. Qadeer Ahmad Khan Integrated Circuits and Systems Group Department of Electrical Engineering IIT Madras

2 Buck-Boost Converter

3 Types of DC-DC converter BUCK : out out in D, in 1 I L I o BOOST : out 1 1D out in, in 1 I L 1 1D I o BUCK -BOOST : out in D, I out 1D in L I D o Also works as buck only or boost only 3

4 Why Buck-Boost? Considering the Li-ion battery discharge profile, either buck or boost fails to operate for the output voltage of Converter needs to be operated in buck-boost mode for most of the time Phone Power () 4

5 Drawback of Conventional BB converter out D 1D in (1) I L 1 1D I o (2) Single Duty cycle, D, controls all the switches. Switching losses are higher due to simultaneous operation of 4 switches Conduction losses are higher due to larger Inductor current (nearly 2x when in out. 5

6 Tri-Mode Operation of BB Converter Increasing IN Buck Buck-Boost Boost OUT IN > OUT : Buck Mode IN < OUT : Boost Mode IN ~ OUT : Buck-Boost Mode 6

7 Conventional vs. Tri-Mode Efficiency in = 2.7 to 5.5, out = 3.3, I load = 500mA Efficiency [%] Proposed Tri-Mode Buck-Boost Boost (Measured) Conventional Buck-Boost Boost (Simulated) Input 4.5 oltage Input oltage in [] 7

8 Tri-Mode: Buck Increasing IN Buck Buck-Boost Boost OUT P BO always ON I LOAD D BUCK controls buck switches OUT D BUCK IN IL I LOAD 8

9 Tri-Mode: Boost OUT Increasing IN Buck Buck-Boost Boost OUT P BU always ON I LOAD D BOOST controls boost switches OUT (1 D IN BOOST ) I L I (1 D LOAD BOOST ) 9

10 Tri-Mode: Buck-Boost OUT Increasing IN Buck Buck-Boost Boost OUT I LOAD D BUCK controls buck switches D BOOST controls boost switches OUT (1 DD BUCK BOOST ) IN I L I 1 D LOAD BOOST 10

11 Issue with Tri-Mode Buck-Boost Battery oltage () OUT D (1 D BUCK BOOST ) Buck Region Buck-Boost Region IN (1 0.1) 3.3 Buck Boost OUT Mode :D Mode :D BOOST BUCK 0 1 Transient due to mode-transition Boost Region Buck Region Buck-Boost Region Mode transition causes large voltage transient Boundary condition must be satisfied aries with load current and losses OUT D D Boundary Condition (buck) BUCK_max BUCK D OUT D 1-D BUCK_max (buck - boost) BUCK BOOST_min (1-D BOOST_min ) 11

12 Solutions for Mode Transitions Appropriate Feed-forward voltage vf1-4 is subtracted to instantaneously change the duty cycles during mode transition. Analog Implementation makes is susceptible to PT and requires external compensation capacitor S. Bang, D. Swank, A. Rao, W. McIntyre, Q. Khan and P. K. Hanumolu, 1.2A 2MHz tri-mode Buck-Boost LED driver with feed-forward duty cycle correction, CICC, Sept EE5325 Power Management Integrated Circuits Integrated Circuits and 12

13 Digital Constant ON/OFF Time Buck-Boost Converter Uses constant ON/OFF technique Enables High Switching Frequency Operation All digital implementation eliminates the need of external compensation capacitor SW1 SW2 L P I L IN BU P BO OUT I LOAD N BU N BO C Non-overlapping clock generator D BUCK D BOOST All-digital fractional-n controller 10MHz Clock BGR REF Q. Khan, et al, A mA Digital Buck-Boost Converter with 92% Peak Efficiency Using Constant ON/OFF Time Delta-Sigma Fractional-N Control, Proc. ESSCIRC 11, Sept

14 Constant ON/OFF Time Operation Inductor ripple current, I T ON D T T OFF L (1 D) T IN L OUT T ON (1) Max ripple occurs at D=0.5 (Ton = Toff) The converter can be operated at high switching frequency when D=0.5 From eq. 1, D increases with in Fixing OFF time and making ON time function of in does not affect the inductor ripple Causes variable switching frequency out [] ΔIL [ma] Toff=constant=50nS Ton=137nS in=4.5 Ton=97nS in=5.0 Ton=75nS in=5.5 14

15 Fractional-N Control Buck Mode: N cycles of 50% Buck : 1 cycle of 100% Buck % Buck 100% Buck Buck-Boost Mode: 1 cycle of 50% Buck : 1 cycle of 50% Boost Boost Mode: N cycle of 50% Boost : 1 cycle of 0% Boost % Boost 0% Boost 15

16 Fractional-N Control Logic Predefined states are stored in the lookup table providing the coarse voltages Uses 18-bit acc for integrating the error (4 MSBs, 7 LSBs, 7 dropped bits. Any intermediate states are resolved by ΔΣ Modulator Operating States Control Code Fraction N:1 Buck Mode Buck- Boost Mode Boost Mode ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 ST9 ST10 ST %Buck:100%Buck 5:1 4:1 3:1 2:1 1:1 50%Buck:50%Boost 1:1 50%Boost:0%Boost 1:1 2:1 3:1 4:1 5:1 5.5 Decreasing IN

17 Start-up Control Startup time is the function of no. of bits dropped in the accumulator and converter resolution No. of ACC bits dropped = 7 The startup time may be more than 10ms Speeded up by dropping only 3 bits in accumulator and switch to 7 bits once the output settles comparator o/p Desired out 17

18 Inductor Current Profile ST1: ΔIL=340mA ST2: ΔIL=314mA D BUCK [] D BUCK [] [] ST4: ΔIL=240mA D BUCK [] D BUCK [] ST5: ΔIL=165mA 18

19 Controller Response Settled LSB Code Dithered Output (Control Code) 19

20 Output oltage Ripple Buck Mode (in=5) Buck-Boost Mode (in=3.6) 20

21 Mode Transition IL [A] D BUCK [] OUT [] D BOOST [] buck buck-boost buck buck-boost 21

22 Hybrid PWM Fractional-N Control in DBUCK DBOOST Gate Driver MP1 SW1 MN1 SW2 MN2 MP2 out FB R1 R2 D buck D boost Dc<6:0> 1x/16x multiplier Fraction-N Control PWM Da<17:7> m 0 c DAC c_min = 0.1m c_max = 0.9m Da<17:7> ACC Dm<17:0> RESET CLK C mult Error Bit (0 or 1) Mult Logic ref Buck Mode Buck-Boost Mode Boost Mode (Fractional-N) DBOOST = 0 90%Buck:100%Buck DBOOST = PWM DBUCK = PWM 90%Buck:10%Boost DBUCK = 1 10%Boost:0%Boost 22