Using Coupled Inductors to Enhance Transient Performance of Multi-Phase Buck Converters Jieli Li Anthony Stratakos,, Aaron Schultz Volterra Semiconductor Corp. Charles Sullivan Dartmouth College 1
Processor Power Supply Trends Increase Capacitance 200 Supply Voltage [V] 3 2 1 Voltage Capacitance Current 180 160 140 120 100 80 60 40 Current [A] Normalized Capacitance 20 0 1996 1998 2000 2002 2004 0 With successive processor generations: V cc, I cc C (I cc / V cc ) 2 > 200x increase in C out from 99 to 04 Processor decoupling is becoming prohibitively large and expensive 2
Composite L Ideal Unloading Transient Review i R + i L ESR ESL + V C _ V CC _ i o up load Parasitic spike ESR, ESL V C = 1 2 L( i O +i R ) 2 V CC C e Current AC Voltag i O Parasitic Spike i O Q i L V CC V C di L dt Time = -V O L V C Time Minimize C through smaller L 3
Small L Hurts Steady-State State R on i L i L R S L I PP R on + V C - C I PP = V in -V out DT L Increased AC current and conduction loss Fundamental trade-off with L: Large L Slow response and large C out requirement Small L High current ripple and loss 4
Volterra s Patented Coupled Buck Topology 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 5
Conventional Multi-Phase Buck Reduced Ripple v x,1 L 1 L 2 v out v x,2 L n C out v x,n Full Ripple Current ripple cancellation in capacitors reduces voltage ripple 6
Volterra s Coupled Buck Reduced Ripple v x,1 L 1 v x,2 L 2 L n C out v out v x,n With coupled inductor, the ripple cancellation is extended to inductors and switches 7
Circuit Model for Two-Winding Structure model of two-winding structure v x,1 ideal transformer v y,1 L l v y,2 v out v x,2 L l C out v x,1 Steady-state, ideal coupling: v x,2 v y? 0 T 8
Circuit Model for Two-Winding Structure model of two-winding structure v x,1 ideal transformer v y,1 L l v y,2 v out v x,2 L l C out v x,1 Steady-state, ideal coupling: Transformer ac currents equal. v x,2 v y? 0 T 9
Circuit Model for Two-Winding Structure model of two-winding structure v x,1 ideal transformer v y,1 L l v y,2 v out v x,2 L l C out v x,1 v x,2 Steady-state, ideal coupling: Transformer ac currents equal. Inductor ac currents equal. v y? 0 T 10
Circuit Model for Two-Winding Structure model of two-winding structure v x,1 ideal transformer v y L l v y v out v x,2 L l C out v x,1 v x,2 v y? Steady-state, ideal coupling: Transformer ac currents equal. Inductor ac currents equal. Equal L l s have equal voltage for equal ac current. 0 T 11
Circuit Model for Two-Winding Structure model of two-winding structure ideal transformer v x,1 v y L l v y v out v x,2 L l C out v x,1 v x,2 v y /2 0 T Steady-state, ideal coupling: Transformer ac currents equal. Inductor ac currents equal. Equal L l s have equal voltage for equal ac current. v y s equal, equal to average of v x s Like doubling switching frequency, halving V in. 12
Ripple Current Reduction uncoupled, Ipp = 11.72A coupled, Ipp = 5.12A Vx1 Vx1 Vx2 il1 il1 2-phase buck converter with V in =12V,V out =1.6V,f s =1MHz Same phase inductance (125nH), same probe scale The ripple is reduced by more than 2x of the uncoupled 13
Ripple Reduction Ratio Ripple Reduction Ratio 0.5 0.4 Vin=12V RR 0.3 0.2 0.1 0.0 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Vout [V] N=2 N=3 N=4 N=5 Example: Vout=1.2V N RR 2 44.4% 3 25.9% 4 16.7% 5 11.1% Compared with same inductance value, without coupling Multi-phase coupling enables greater ripple cancellation 14
Multi-Phase Coupled Inductor The ladder structure has the best coupling 15
DC Flux Path DC flux generated by each winding goes through high reluctance path 16
AC Flux Path AC flux generated by each winding goes to others through low reluctance path 17
Patented Structure Multi-phase coupled inductor structure Scalable to n-phase Surface mount Production worthy 18
Photo of 4-Phase 4 Inductor 4 discrete 100nH inductors 4-phase 50nH coupled inductor 4-phase coupled inductor sample 4 identical core cells Each cell is 9mm x 7mm x 4mm Per phase inductance is 50nH Magnetizing inductances are 279nH, 479nH, 472nH, 273nH 19
System Test Setup Cin Integrated driver/fet ICs (under heatsink) coupled inductor Cout 4 phase coupled buck 50nH per phase Small bank of MLCC output capacitors Volterra s power delivery chipset with integrated FETs & Drivers 20
Steady State Waveforms Measurement Simulation 12V/1.2V/1.2MHz/4-phase Current probed by inserting extra wire in series For phase 2, Ipp = 3.4A. Other phases measured similar Ipp. 21
Transient Waveforms Load step from 80A to 12.5A (85% load step) A small bank of MLCC-only output capacitance V is only 71.6mV 22
Transient Comparison Coupled inductor, 50nH/phase V = 71.6mV Uncoupled inductor, 100nH/phase V = 136.5mV Same converter conditions with same load steps and output cap Coupled inductor reduces the overall voltage window by half 23
Efficiency Comparison 50nH coupled 100nH uncoupled Efficiency Output Current With coupled inductor, transient improves without efficiency pen With coupled inductor, transient improves without efficiency penalty alty If using 50nH uncoupled, efficiency is down by 3~4% 24
Conclusions 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 25