Design of Ceramic-Capacitor VRM/VRD's with Estimated Load

Current Feedforward

Angel V. Peterchev

Prof. Seth R. Sanders

Power Electronics GroupDepartment of EECSUniversity of California, Berkeley

Intel 2004 Technology Symposium

2

Microprocessor Supply Trends

[Yao , 2004]

regulation tolerance ± 2 %

challenge to regulation

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Microprocessor Supply Trends (Cont.)

[Yao , 2004]

challenge to regulation

4

VRM Implementations

Low density, high profile High density, low profile

Electrolytic cap 10x10x20 mm3

820 µF, 10 mΩ ESR

Ceramic cap 3.2x2.5x2.5 mm3

100 µF, 2 mΩ ESR100 x

5

New Results

In framework of all-ceramic capacitor VRM’s

Critical Inductance Expression

Dynamic Load Line vs. Static Load Line

Load Current Feedforward vs. Feedback Control

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Microprocessor VRM Load Line

microprocessor

PC “silver box”

12 V

~ 1 V

rC

Vref

ESR

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Critical Inductance

Critical Inductance –largest inductance for which load-line specification can be met

Unloading transient more constraining – low voltage across inductor

rise/fall time const.

control delay

unload. overshootRref ≠ ESR

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Dynamic Load Line with Ceramic Caps

Li ~ 100’s nH for efficient operation at fsw < 1 MHz

Lcrit ∝ C → C > 100’s µF

electrolytic caps rCC = 10 µs, ceramic caps rCC = 0.2 µs

with ceramic caps rC < Rref

Dynamic Load Line

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In conventional feedback designs Rref = rC, and

1/2πrCC < BW < fsw

electrolytic caps (rCC = 10 µs): fsw = 200—500 kHz

ceramic caps (rCC = 0.2 µs): fsw ~ 10 MHz

high switching losses ~ fsw

Feedback Bandwidth Considerations

~

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With ceramic caps design for rC < Rref

1/2πRrefC < BW < fsw

still for small C < 1 mF, fsw > 1 MHz

To use small C: Use load current feedforward to avoid feedback bandwidth constraint, at conventional fsw

Feedback Bandwidth Considerations (cont.)

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Controller Paradigm

ILVo

state variables

converterinputs

Vin

Io

D

exogenous variables

feedback controllerfeedfwd controller

Feedforward handles bulk of regulation action

Feedback compensates for feedforward non-ideality and DC precision

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Io

Load Current Feedforward

Z1

Z2

Vref

feedback power train

Vc Vo

load

Z3

Z4

modulator

Zref

Vx

Vin

feedforward

Io

load line

L

C

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Load Current Feedforward (cont.)

Feedforward bandwidth not limited by stability constraints

load current is (approx.) exogenous variable

Fast response with conventional switching frequencies

Response limited by modulator and switch delay

Modulator must have low turn-off latency

Non-idealities of feedforward attenuated by feedback

Feedback contributes robustness

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Load Current Feedforward (cont.)

Applicable to both Voltage-Mode and Current-Mode Modulation

Voltage-Mode:

Current-Mode:

(high current-loop gain)

Feedforward control law approximately 1st order TF

Load current estimation needed for FB load-line regulation

Little added complexity

sL

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2-Ph

ase

VRM

Dia

gram

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50 A Loading Transient

C = 8 x 100 µF, 4-phase, Li = 390 nH, fsw = 1 MHz,

Vin = 12 V, Vo = 1.3 V

estimated load current

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8 A Unloading Transient

C = 8 x 100 µF, 4-phase, Li = 390 nH, fsw = 1 MHz,

Vin = 12 V, Vo = 1.3 V

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50 A Unloading Transient

duty ratio saturation (Vc = 0)

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Conclusion

In ceramic-capacitor VRM’s, output capacitor ESR < load-line slope

Load current feedforward enables fast VRM response not limited by feedback stability constraints

Feedforward can be used with different modulation schemes, as long as low turn-off latency

The increased effective bandwidth allows for VRM operation with only a few ceramic output capacitors