2005 IBM Power and Cooling Technology Symposium
A Combined Buck and Boost
Converter for Single-Phase
Power-Factor Correction
ƒ Kevin Covi
10/7/2005
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IBM High-End Power Hardware Development
Introduction
¾ The AC/DC converters in IBM’s high-end servers connect to any 3-phase utility
world-wide (up to 480V nominal, 576V for 2 seconds)
¾ Up to 3 converters per line cord provide as much as 22.5kW of bulk power
¾ Each converter operates line-to-line, without a neutral connection
¾ This results in voltages over 700V at the input to each converter
¾ A typical Boost converter would require a 750V intermediate bus voltage
¾ Buck + Boost topology was chosen to maintain a 400V intermediate bus
¾ Permits use of industry-standard 500-600V devices
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Buck + Boost Power Train
Switches have independent duty cycles
Buck switch
Freewheeling
diode
Boost switch
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Operating Modes
Boost region
Buck Region
Buck Region
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Boost Mode
Buck operates at 100% duty cycle
Boost is switching
Only boost error signal crosses ramp
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Buck Mode
Buck is switching
Boost is off
Only buck error signal crosses ramp
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Why use Buck+Boost for Single-Phase?
¾ Buck switch eliminates boost inrush problem
¾ Buck switch functions as prime-power disconnect
¾ Input current can be controlled
¾ Enhanced PLD immunity
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Controls: Prior Art
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Buck+Boost Control
¾ Buck+Boost converter is difficult to control in continuous-conduction mode
¾ Early applications operated the inductor on the verge of discontinuous
conduction to maintain stability Æ impractical for high power applications
¾ In 1993 Dr. Ray Ridley developed a controller that maintains stability even in
continuous conduction mode
¾ The addition of an inner current loop provides adaptability to changing power
stage operation
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Ridley Controller
Additional circuitry
Line and inductor current are sensed
MULTIPLIER
LINE CURRENT
CONTROLLER
BUCK PWM
INDUCTOR CURRENT
CONTROLLER
BOOST PWM
VOLTAGE ERROR
AMPLIFIER
From “Analysis and Design of a Wide Input Range Power Factor Correction Circuit for Three-Phase Applications” by Ridley, et. al.
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Fuld & Kern Controller: eliminated one ramp
Two current
sensors are used
Only one
ramp is used
From “A Combined Buck and Boost PFC Controller for Three-Phase Applications” by Fuld, et. al.
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Summary of prior art
¾ Both earlier schemes required two sensors for line and inductor current
¾ Efficiency penalty at lower power levels where resistive shunts are used
¾ Cost penalty at high power levels where Hall-effect sensors are used
¾ IBM controller requires that only inductor current be sensed
¾ Line current is synthesized by controller
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IBM Controller
ZENER CLAMP LIMITS
WINDUP AT ZERO
CROSSING
PEAK CURRENT
LIMITER
INDUCTOR CURRENT
SENSE
VOLTAGE ERROR
AMPLIFIER
LINE CURRENT
CONTROLLER
RAMP
INDUCTOR
CURRENT
CONTROLLER
BUCK
PWM
BOOST
PWM
INDUCTOR CURRENT
SENSE
MULTIPLIER
& DIVIDER
LINE CURRENT
SYNTHESIZER
LOW-PASS FILTER
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Simulated Performance
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Line Current Synthesis
Sensed inductor current
After blanking
Output of inverting filter is proportional to line current
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VAC=85V
Output voltage
Rectified Input voltage
Line current
Multiplier out (red)
Synthesized line current (green)
Buck Error signal
Boost Error signal
Line amplifier output
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VAC=300V
Output voltage
Rectified Input voltage
Line current
Multiplier out (red)
Synthesized line current (green)
Buck Error signal
Boost Error signal
Line amplifier output
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Transition from Boost to Buck
Buck region
Boost region
Error signals and ramp
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Startup
Output voltage walking in
Spikes not really there!
Line current
Error signals
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Lightning Strike
1200V peak
Input voltage
Peak current limited to 30A
Output increases only 5V
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Measured Performance of 7.5kW Rectifier
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Line Current Total Harmonic Distortion
Rise in THD largely
caused by filtering
after rectifier
Boost region
Buck region
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Power Factor
Boost region
Buck region
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Efficiency (includes DC/DC isolation stage)
Boost region
Buck region
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Summary
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Advantages of Buck+Boost topology
¾ No restrictions on output voltage
9 Enables use of 450V caps and 600V silicon regardless of line voltage
9 Enables operation from 277V while keeping output voltage unchanged
¾ Inherent control of input current
9 Permits use of fast blow line fuses (semiconductor fuses)
9 Permits N+1 operation from single line cord – fuses clear before upstream CB
9 Enables use of Silicon Carbide rectifiers
9 Permits operation from DC bus - no inrush current
¾ Enhanced PLD immunity: Lightning strike and Ring Wave
¾ Buck switch functions as prime-power disconnect Æ simplifies Hotplug
¾ “Anti-Smoke” compliant
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“Anti-smoke” Compliance
¾ Inherent protection against a shorted bulk cap or boost FET
¾ Buck switch limits fault current to a safe level and is then turned off to
isolate the fault
¾ If a shorted buck switch causes an OV the boost switch functions as
crowbar to clear the input fuses
¾ Input fuses are very fast-acting so this failure does not make a big noise
or smoke!
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Disadvantages of Buck+Boost topology
¾ Extra floating switch required Æ increased complexity and cost
¾ Discontinuous input current in Buck region Æ bigger input filter
¾ Filter not required if converter operates from low voltage only
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IGBT bias and gate drive
¾ Additional floating bias and optical isolator required
¾ Cost ~$2.00
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Optional Input Filter
3rd order elliptical
Damping
Filter not required if converter runs in boost mode steady state
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Filter Response
Very steep fall off beyond the pass band
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References
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References:
1.
E.G Schmidtner, P.W. Busch, “Off-Line Power Supply with Sinusoidal Input
Current and an Active Limit to the Inrush Current”, Power Conversion (PCIM)
Conference Proceedings, Nurnberg, Germany, June 25-27, 1991.
2.
R. B. Ridely, S. Kern, B. Fuld, “Analysis and Design of a Wide Input Range Power
Factor Correction Circuit for Three-Phase Applications”, Applied Power Electronics
Conference and Exposition, 1993. APEC '93. Conference Proceedings 1993.,
Eighth Annual , 7-11 March 1993
3.
B. Fuld, S. Kern, R. B. Ridely, “A Combined Buck and Boost PFC Controller for
Three-Phase Applications”, Power Electronics and Applications, 1993, Fifth
European Conference on Power Electronics, 13-16 September 1993
4.
V. Vlatkovic, D. Borojevic, and F.C. Lee, “Input Filter Design for Power Factor
Correction Circuits”, International Conference on Industrial Electronics, Control
and Instrumentation, November 1993
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