design of ceramic-capacitor vrm/vrd's with estimated load current feedforward
DESCRIPTION
In ceramic-capacitor VRM’s, output capacitor ESR slopeLoad current feedforward enables fast VRM response notlimited by feedback stability constraintsFeedforward can be used with different modulationschemes, as long as low turn-off latencyThe increased effective bandwidth allows for VRMoperation with only a few ceramic output capacitorsTRANSCRIPT
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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
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Microprocessor Supply Trends
[Yao , 2004]
regulation tolerance ± 2 %
challenge to regulation
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Microprocessor Supply Trends (Cont.)
[Yao , 2004]
challenge to regulation
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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
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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