Lecture-43. Dr. Qadeer Ahmad Khan. Integrated Circuits and Systems Group Department of Electrical Engineering IIT Madras

Transcription

1 ecture-43 EE5325 Power Management ntegrated Circuits Dr. Qadeer Ahmad Khan ntegrated Circuits and Systems Group Department of Electrical Engineering T Madras 1

2 Current Sensing Current sensing in a dc-dc converter is used mainly for following purposes 1. imit the inductor peak current from getting saturated (Over Current Protection OCP) 2. Use the inductor current information in current mode control 3. imit average power stage current within the maximum rated specification (Over Current Protection OCP) 4. egulate the average output/load current in case of current regulator (battery charger, ED drivers etc.) 5. Transition between PWM and PFM mode based on load current Current sensing can be based on: Peak current sensing: used for 1 and 2 Average current sensing: used for 3-5 nformation obtained from peak current sensing can also be used to calculate the average current but not vice versa 2

3 Current Sensing Methods Sense resistor based current sensing MOSFET DS_ON based current sensing MOSFET DS based current sensing Observer (integrator) based current sensing C filter based current sensing SenseFET based current sensing Transformer based current sensing 3

4 Sense esistor Uses a Sense esistor in series with the inductor f the value of the resistor is known, the current flowing through the inductor is determined by sensing the voltage across it. The technique is simple but not very power efficient due to losses incurred in series sense Assuming sense=100mohm, for an output voltage of 1.8 and load current of 1A, loss due to sense could be ~5% 4

5 MOSFET DS Current Sensing MOSFETs act as resistors when they are ON The equivalent resistance of the device is: DS in this case should be known ; DS is measured. Accuracy is not good due to variation in DS_ON across process, voltage and temperature D C OX W ( GS T ) DS ; for : DS ( GS T ) DS W C OX ( GS T ) 5

6 MOSFET DS Current Sensing Based on sensing DS of main FET and sense FET Assumes main FET and sense FET have same variations across PT DS of sense FET is generated by forcing a fixed bias current ref equires post processing (division) to get the final current value 0 N PWM N Gate Driver gatep gaten Main FETs SW N M P M N + dsp - + dsn - P N O O C O O gatep N M P_SENSE ref + sensep - N ref gaten + Sense FETs MN_SENSE sensen - from main PFET (M P (1) from sense PFET (M EF dsp dsp sensep ds _ sensep P ) P_SENSE (2) ) divide (1) and (2), we get : p ref or p ds _ sensep dsp sensep ref ds _ sensep ds _ P dsp sensep ds _ P ds _ sensep ds _ P (3) if width ratio of main FET to sense FET is N : N substitutingin (3), we get : p N N simlarly : N ref ref dsp sensep dsn sensen 6

7 Observer Based Sensing This method uses the nductor voltage to measure the nductor current nductor current can be calculated by integrating the voltage over time ntegrator time constant should match inductor value Difficult to track the variation in inductors v di dt i 1 v dt 7

8 ssues With Observer Based Sensing Difficult to track the variations in inductor Works only with zero loss across the inductor (i.e. DC =0 or l OAD =0) Can be used to determine the ripple current information by resetting the integrator every cycle nductor: = 1 dt Gm-C: C = G m C dt 0 N PWM N Gate Driver SW N M P M N DC G m O C O C C if = C G m then C = Under no load condition 8

9 C Filter Based Sensing C low pass filter is connected in parallel with inductor (same as used in current mode hysteretic) Average voltage across C F is same as voltage drop across DC provides dc current sensing. DC must be known to find the sensed dc current. f F C F =k. then ripple voltage across C F provides ripple current sensing. K should be chosen such that F C F >>T SW (usually > 5 for good accuracy) X + C - O 0 N PWM N Gate Driver SW N M P F C F DC O C O dc = C dc DC =k C M N 9

10 SenseFETs Based Current Sensing Most commonly used method for current sensing The idea is to build a current sensing FET in parallel with the power MOSFET (Current Mirror). The effective width (W) of the sense MOSFET (SenseFET) is significantly smaller than the power FET, and therefore sensed current is scaled down version of the actual load current: gatep Sense FET N W/ = 1x M SENSE SENSE = /N SENSE SENSE gatep gaten Main FET The width (W) of the Power MOSFET is X times the width of SENSEFET to reduce the power consumption. N M P M N W/ = Nx D O SW C O O W COX ( GS T ) O DS 10

11 Modified SenseFETs Based Current Sensing The Op-amp is used to force DS of mainfet and SenseFET to be equal As width ratio of main MOSFET to SENSEFET increases, the accuracy of the circuit decreases. Bigger SENSEFET improves accuracy but will consume higher current due to increase in sense current (tradeoff between accuracy and quiescent current) equires high gain-bandwidth (GBW) amplifier for sensing Advantages ossless (sensor current is small) Can be easily integrated on-chip elatively good accuracy Sources of Error Op-amp offset and limited bandwidth Mismatch between main FET and sense FET (smaller sense FET has more error) ariation in sense SW gatep Sense FET N W/ = 1x M SENSE SENSE = /N SENSE SENSE gatep gaten Main FET N M P M N W/ = Nx O O SW C O O 11

12 Current Transformers The use of this technique is common in high power systems. The idea is to sense a fraction of the high nductor current by using the mutual inductor properties of a transformer. Main Disadvantages: ncreased cost and size and non-integrablity. The transformer also cannot transfer the DC portion of current, which make this method inappropriate for over current protection. 12

13 Comparison of Current-Sensing Techniques Current Sensing Method Advantages Disadvantages Series Sense esistor Good accuracy ncreases conduction loss MOSFET DS Sensing ossless ow accuracy as DS is not constant MOSFET DS Sensing ossless, relatively better accuracy compared to DS Accuracy depends upon matching between transistors, requires post processing to get the final current value Current Observer ossless Works with known, does not work under load C Filter ossless, occupies large on-chip area Works with known and DC SenseFET ossless, smaller area Poor Accuracy due to DS mismatch Modified SenseFET ossless, smaller area, relatively good accuracy Accuracy depends upon transistor matching and op-amp BW Current Transformer ossless Not cost effective, bulky, can t be integrated on-chip 13

14 eferences 1. H. P. Forghani-zadeh and G. A. incón-mora, Current-sensing techniques for DC-DC converters, in Proc Midwest Symp. Circuits and Systems (MWSCAS), pp E. Dallago, M. Passoni, and G. Sassone, ossless current-sensing in low-voltage high-current DC-DC modular supplies, EEE Trans. nd. Electron., vol. 47, pp , Dec H. P. Forghani-zadeh and G. A. incon-mora, "An Accurate, Continuous, and ossless Self-earning CMOS Current-Sensing Scheme for nductor-based DC-DC Converters," in EEE Journal of Solid-State Circuits, vol. 42, no. 3, pp , March Yang Zhang,. Zane, A. Prodic,. Erickson and D. Maksimovic, "Online calibration of MOSFET on-state resistance for precise current sensing," in EEE Power Electronics etters, vol. 2, no. 3, pp , Sept S. ao et al., "A 1.2-A Buck-Boost ED Driver With On-Chip Error Averaged SenseFET-Based Current Sensing Technique," in EEE Journal of Solid-State Circuits, vol. 46, no. 12, pp , Dec