Practical Implementation Of A High-frequency Current Sense Technique For VRM
presented by :
Sarath T S
AE&I
College Of Engineering, Trivandrum3

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Overview

Introduction
Traditional methods
DCR method
Theoretical basis analysis
Effect of temperature
Current sense methods comparison
Problems and solutions
Experimental results
Example of VRM using DCR method
Future works
Conclusion

Introduction

VRM is voltage regulator module

Tracking inductor current in high frequency dc/dc converters is difficult task when high output current and low output voltage are demanded by load.

The main idea is to obtain an equivalent voltage using the dc resistance of the inductor and which used to produce the corresponding current

Traditional methods
Shunt resistor method
Good accuracy
Losses due to power dissipation in the sense resistor
Current observer method
No problem of losses, frequency
Problems is choice of the converter and the observer models
Equivalent serial resistance (ESR)
Choice of components is limited
May cause significant decreased accuracy
MOS method
Complicated due to non linearity
Less accuracy due to the Rds temperature dependence
Rds depends on the applied VGS and the VDS.

DCR Method

This sense method makes use of the inductor dc resistance (DCR) to obtain a proportional voltage of the inductor current

Advantage of this technique

Better accuracy
Lossless
Any resistor can be added.
Can be used to supply laptop computers.


Sense circuit using DCR

Theoretical Basis Analysis

Capacitor voltage Vc is proportional to the inductor current

For a correct working of the sensor, we should match both time constants of (1) to have a linear relation between Vc(s) and IL(s) as in equation

Effect Of Temperature

Drawbacks of this sense technique
loss of accuracy produced by temperature changes.
temperature changes, DCR also varies,
if temperature rises, DCR also increases Thus, higher gain for high frequencies

Solving this temperature problem
add a compensation circuit using a positive temperature coefficient thermistor

Current Sense Methods Comparison

Tracked the inductor current of a 4-phased buck converter using following techniques

DCR technique, the shunt resistor (Rs), and the Rds method
Shunt resistor and the DCR technique methods gives good accuracy

Rds method failed to give the real measurement of the current.

Comparison Between Rds and DCR method

Decrease in efficiency due to Significant losses introduced by the shunt resistor.

Even if both time constant are correctly matched in DCR method addition of the second-order current-sense system causes a reduction of the transient response

Quantitative Comparison between different methods
Problems And Solutions Associated To Sense Method

The respective values of L and DCR must be known
Rcs and Ccs are accordingly chosen
Main difficulty is to recover the voltage image of the inductor current at the boundaries of Ccs.
must be very careful with high frequency noises

Overcome this noise problem
Add a previous stage to the sense circuit in order to filter this high frequency noise.
Selection of instrumentation amplifier
(INA103)
(AMP03)

Experimental Results

This dc–dc converter utilizes a current-mode-controlled architecture for an easy compensation and a better line rejection.

Experimented in different cases

low current application

medium current application

high current application

DCM operation


INA103



AMP03
Future Enhancements


Integration of passives to enhance power density since integration allows a drastic reduction of capacitors and inductors size


Explore the integration of passive elements of
this current-sense technique into a full-integrated DCR current sensor and in its new operation in VRM applications

Conclusion

Analysed the practical use of the lossless DCR inductor current-sense technique for a high-current high frequency application.
Also repeated the analysis for low current high-frequency dc/dc converters operating in CCM and DCM.
Implemented and explained in detail each stage of the sense process in order to recover correctly an accurate equivalent voltage image of the inductor current.
Experimental tests employing VRMs have been completed by demonstrating higher performances of the DCR current sense technique compared with Rshunt sense one.

References

[1] J. S. Glaser and A. F. Witulski, “Application of a constant- output-power converter in multiple-module converter systems,” in Proc. 23rd Annu.IEEE PESC, Jul. 1992, vol. 2, pp. 909–916.

[2] R. Giral, L. Martinez-Salamero, and S. Singer, “Interleaved converters operation based on CMC,” IEEE Trans. Power Electron., vol. 14, no. 4,pp. 643–652, Jul. 1999.

[3] H. Mao, Y. Liangbin, C. Wang, and I. Batarseh, “Analysis of inductor current sharing in non isolated and isolated multiphase dc–dc converters,” IEEE Trans. Ind. Electron., vol. 54, no. 6, pp. 3379–3388, Dec. 2007.