using coupled inductors to enhance transient …...1 ibm symposium september 14-15, 2004 using...
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1IBM SymposiumSeptember 14-15, 2004
Using Coupled Inductors to Enhance Using Coupled Inductors to Enhance Transient Performance of MultiTransient Performance of Multi--Phase Phase
Buck ConvertersBuck Converters
JieliJieli LiLiAnthony Anthony StratakosStratakos, Aaron Schultz, Aaron Schultz
Volterra Semiconductor Corp.Volterra Semiconductor Corp.Charles SullivanCharles Sullivan
Dartmouth College Dartmouth College
2IBM SymposiumSeptember 14-15, 2004
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1996 1998 2000 2002 20040
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200
VoltageVoltage
CurrentCurrent
Supp
ly V
olta
ge [V
]Su
pply
Vol
tage
[V]
Cur
rent
[A]
Cur
rent
[A]
Nor
mal
ized
Cap
acita
nce
Nor
mal
ized
Cap
acita
nceCapacitanceCapacitance
Processor Power Supply Trends Processor Power Supply Trends Increase CapacitanceIncrease Capacitance
With successive processor generations: Vcc ↓, Icc ↑
C ∝ (Icc / Vcc)2 ⇒ > 200x increase in Cout from ‘99 to ‘04
Processor decoupling is becoming prohibitively large and expensiProcessor decoupling is becoming prohibitively large and expensiveve
3IBM SymposiumSeptember 14-15, 2004
Ideal Unloading Transient ReviewIdeal Unloading Transient Review
diLdt
-VOL
=
∆Q
iLiO
∆iO
∆VCVC
VCCeC
urre
nt
Time
ESR
ESL+
VC_
iL+
VCC
_
io
uP load
CompositeL
L(∆iO+iR)2
VCC • C12 •
iR
ParasiticSpike• Parasitic spike ∝ ESR, ESL
AC
Vol
tag
∆VC =
Time
Minimize C through smaller LMinimize C through smaller L
4IBM SymposiumSeptember 14-15, 2004
Small L Hurts SteadySmall L Hurts Steady--StateState
+VC-
L
iL
RS
Ron
IPP
iL
Ron C
IPP =Vin - Vout
L DT
Increased AC current and conduction loss
Fundamental trade-off with L:Large L Slow response and large Cout requirementSmall L High current ripple and loss
5IBM SymposiumSeptember 14-15, 2004
Volterra’s Patented Coupled Buck Volterra’s Patented Coupled Buck TopologyTopology
Multi-phase converter magnetically coupled to cancel AC flux and ripple current
Single magnetic structure replaces multiple discrete inductors
Enables use of smaller inductor values to improve transient without increasing current ripple
Allows significant reduction in output capacitance
6IBM SymposiumSeptember 14-15, 2004
Conventional MultiConventional Multi--Phase BuckPhase Buck
L1
L2
Cout
vout
vx,1
vx,2
vin
Reduced Ripple
Full Ripple
…
vx,n
…Ln
Current ripple cancellation in capacitors reduces voltage rippleCurrent ripple cancellation in capacitors reduces voltage ripple
7IBM SymposiumSeptember 14-15, 2004
Volterra’sVolterra’s Coupled BuckCoupled Buck
L1
L2
Cout
vout
vx,1
vx,2
vin
Reduced Ripple
…
vx,n
…Ln
With coupled inductor, the ripple cancellation is extended With coupled inductor, the ripple cancellation is extended to inductors and switchesto inductors and switches
8IBM SymposiumSeptember 14-15, 2004
Circuit Model for TwoCircuit Model for Two--Winding StructureWinding Structurevin model of two-winding structure
Lℓvx,1
vx,2
ideal transformer
Lℓ
vy,1
vy,2vout
Cout
Steady-state, ideal coupling:
T0
vinvx,1
vinvx,2
vy ?
9IBM SymposiumSeptember 14-15, 2004
Circuit Model for TwoCircuit Model for Two--Winding StructureWinding Structurevin model of two-winding structure
Lℓvx,1
vx,2
ideal transformer
Lℓ
vy,1
vy,2vout
Cout
Steady-state, ideal coupling:Transformer ac currents equal.
T0
vinvx,1
vinvx,2
vy ?
10IBM SymposiumSeptember 14-15, 2004
Circuit Model for TwoCircuit Model for Two--Winding StructureWinding Structurevin model of two-winding structure
Lℓvx,1
vx,2
ideal transformer
Lℓ
vy,1
vy,2vout
Cout
Steady-state, ideal coupling:Transformer ac currents equal.Inductor ac currents equal.
T0
vinvx,1
vinvx,2
vy ?
11IBM SymposiumSeptember 14-15, 2004
Circuit Model for TwoCircuit Model for Two--Winding StructureWinding Structurevin model of two-winding structure
Lℓvx,1
vx,2
ideal transformer
Lℓ
vy
vyvout
Cout
Steady-state, ideal coupling:Transformer ac currents equal.Inductor ac currents equal.Equal Lℓ’s have equal voltage for equal ac current.
T0
vinvx,1
vinvx,2
vy ?
12IBM SymposiumSeptember 14-15, 2004
Circuit Model for TwoCircuit Model for Two--Winding StructureWinding Structure
vout
Lℓ
Cout
vx,1
vx,2
ideal transformer
Lℓ
vy
vy
vinmodel of two-winding structure
Steady-state, ideal coupling:Transformer ac currents equal.Inductor ac currents equal.Equal Lℓ’s have equal voltage for equal ac current.vy’s equal, equal to average of vx’sLike doubling switching frequency, halving Vin.0 T
vin
vin
vin/2
vx,2
vy
vx,1
13IBM SymposiumSeptember 14-15, 2004
Ripple Current ReductionRipple Current Reductionuncoupled, ∆Ipp = 11.72A coupled, ∆Ipp = 5.12A
Vx1
Vx2
iL1
Vx1
iL1
2-phase buck converter with Vin=12V,Vout=1.6V,fs=1MHzSame phase inductance (125nH), same probe scale
The ripple is reduced by more than 2x of the uncoupledThe ripple is reduced by more than 2x of the uncoupled
14IBM SymposiumSeptember 14-15, 2004
Ripple Reduction RatioRipple Reduction Ratio
Ripple Reduction Ratio
0.0
0.1
0.2
0.3
0.4
0.5
0.00 2.00 4.00 6.00 8.00 10.00 12.00
Vout [V]
RR
N=2N=3N=4N=5
Vin=12V
Example:Vout=1.2VN RR2 44.4%3 25.9%4 16.7%5 11.1%
Compared with same inductance value, without coupling
MultiMulti--phase coupling enables greater ripple cancellationphase coupling enables greater ripple cancellation
15IBM SymposiumSeptember 14-15, 2004
MultiMulti--Phase Coupled InductorPhase Coupled Inductor
The “ladder” structure has the best coupling
16IBM SymposiumSeptember 14-15, 2004
DC Flux PathDC Flux Path
DC flux generated by each winding goes through high reluctance path
17IBM SymposiumSeptember 14-15, 2004
AC Flux PathAC Flux Path
AC flux generated by each winding goes to others through low reluctance path
18IBM SymposiumSeptember 14-15, 2004
Patented StructurePatented Structure
Multi-phase coupled inductor structureScalable to n-phase Surface mountProduction worthy
19IBM SymposiumSeptember 14-15, 2004
Photo of 4Photo of 4--Phase InductorPhase Inductor
4 discrete 100nH inductors
4-phase 50nH coupled inductor
4-phase coupled inductor sample4 identical core cells Each cell is 9mm x 7mm x 4mmPer phase inductance is 50nHMagnetizing inductances are 279nH, 479nH, 472nH, 273nH
20IBM SymposiumSeptember 14-15, 2004
System Test SetupSystem Test Setup
4 phase coupled buck
50nH per phase
Small bank of MLCC output capacitors
Volterra’s power delivery chipset with integrated FETs & Drivers
Cin
coupled inductor
Cout
Integrated driver/FET ICs (under heatsink)
21IBM SymposiumSeptember 14-15, 2004
Steady State WaveformsSteady State Waveforms
Measurement Simulation
12V/1.2V/1.2MHz/4-phaseCurrent probed by inserting extra wire in seriesFor phase 2, Ipp = 3.4A. Other phases measured similar Ipp.
22IBM SymposiumSeptember 14-15, 2004
Transient WaveformsTransient Waveforms
Load step from 80A to 12.5A (85% load step)A small bank of MLCC-only output capacitance∆V is only 71.6mV
23IBM SymposiumSeptember 14-15, 2004
Transient ComparisonTransient Comparison
Coupled inductor, 50nH/phase∆V = 71.6mV
Uncoupled inductor, 100nH/phase∆V = 136.5mV
Same converter conditions with same load steps and output capCoupled inductor reduces the overall voltage window by half
24IBM SymposiumSeptember 14-15, 2004
Efficiency ComparisonEfficiency Comparison
Output Current
Effic
ienc
y50nH coupled 100nH uncoupled
With coupled inductor, transient improves without efficiency penWith coupled inductor, transient improves without efficiency penaltyaltyIf using 50nH uncoupled, efficiency is down by 3~4%
25IBM SymposiumSeptember 14-15, 2004
ConclusionsConclusions
50% output capacitor reduction is achieved by coupled buck topology without penalty in efficiency
A production-worthy surface-mount scalable 4-phase coupled inductor is demonstrated
The 4-phase coupled inductor reduces ripple current by more than 4x from the uncoupled value
Demanding transient requirement of a modern CPU is met using only a small bank of MLCC capacitors