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10/7/2005

Kevin Covi

2005 IBM Power and Cooling Technology Symposium

A Combined Buck and Boost Converter for Single-Phase Power-Factor Correction

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IBM High-End Power Hardware Development

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Introduction

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)

Up to 3 converters per line cord provide as much as 22.5kW of bulk power

Each converter operates line-to-line, without a neutral connection

This results in voltages over 700V at the input to each converter

A typical Boost converter would require a 750V intermediate bus voltage

Buck + Boost topology was chosen to maintain a 400V intermediate bus

Permits use of industry-standard 500-600V devices

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IBM High-End Power Hardware Development

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Buck + Boost Power Train

Buck switch

Freewheeling diode

Switches have independent duty cycles

Boost switch

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IBM High-End Power Hardware Development

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Operating Modes

Boost region

Buck Region Buck Region

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IBM High-End Power Hardware Development

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Boost Mode

Boost is switching

Buck operates at 100% duty cycle

Only boost error signal crosses ramp

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IBM High-End Power Hardware Development

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Buck ModeBuck is switching

Boost is off

Only buck error signal crosses ramp

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Why use Buck+Boost for Single-Phase?Buck switch eliminates boost inrush problem

Buck switch functions as prime-power disconnect

Input current can be controlled

Enhanced PLD immunity

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Controls: Prior Art

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Buck+Boost ControlBuck+Boost converter is difficult to control in continuous-conduction mode

Early applications operated the inductor on the verge of discontinuous conduction to maintain stability impractical for high power applications

In 1993 Dr. Ray Ridley developed a controller that maintains stability even in continuous conduction mode

The addition of an inner current loop provides adaptability to changing power stage operation

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IBM High-End Power Hardware Development

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Ridley Controller

From “Analysis and Design of a Wide Input Range Power Factor Correction Circuit for Three-Phase Applications” by Ridley, et. al.

Additional circuitry

LINE CURRENT CONTROLLER

MULTIPLIER

VOLTAGE ERROR AMPLIFIER

BUCK PWM

INDUCTOR CURRENT CONTROLLER

Line and inductor current are sensed

BOOST PWM

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IBM High-End Power Hardware Development

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Fuld & Kern Controller: eliminated one ramp

From “A Combined Buck and Boost PFC Controller for Three-Phase Applications” by Fuld, et. al.

Only one ramp is used

Two current sensors are used

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IBM High-End Power Hardware Development

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Summary of prior art

Both earlier schemes required two sensors for line and inductor current

Efficiency penalty at lower power levels where resistive shunts are used

Cost penalty at high power levels where Hall-effect sensors are used

IBM controller requires that only inductor current be sensed

Line current is synthesized by controller

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IBM High-End Power Hardware Development

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IBM Controller

BUCK PWM

LINE CURRENT CONTROLLER

INDUCTOR CURRENT

CONTROLLER

LINE CURRENT SYNTHESIZER

MULTIPLIER & DIVIDER

VOLTAGE ERROR AMPLIFIER

PEAK CURRENT LIMITER

BOOST PWM

LOW-PASS FILTER

INDUCTOR CURRENT SENSE

INDUCTOR CURRENT SENSE

RAMP

ZENER CLAMP LIMITS WINDUP AT ZERO

CROSSING

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IBM High-End Power Hardware Development

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Simulated Performance

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IBM High-End Power Hardware Development

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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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IBM High-End Power Hardware Development

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VAC=85V

Line amplifier output

Multiplier out (red) Synthesized line current (green)

Boost Error signal

Line current

Rectified Input voltage

Output voltage

Buck Error signal

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IBM High-End Power Hardware Development

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VAC=300V

Line amplifier output

Multiplier out (red) Synthesized line current (green)

Boost Error signal

Line current

Rectified Input voltage

Output voltage

Buck Error signal

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IBM High-End Power Hardware Development

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Transition from Boost to Buck

Boost region

Buck region

Error signals and ramp

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IBM High-End Power Hardware Development

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Startup

Spikes not really there!

Output voltage walking in

Error signals

Line current

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Lightning Strike

Peak current limited to 30A

Output increases only 5V

Input voltage1200V peak

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Measured Performance of 7.5kW Rectifier

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IBM High-End Power Hardware Development

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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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IBM High-End Power Hardware Development

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Power Factor

Boost region Buck region

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IBM High-End Power Hardware Development

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Efficiency (includes DC/DC isolation stage)

Boost region Buck region

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IBM High-End Power Hardware Development

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Summary

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Advantages of Buck+Boost topologyNo restrictions on output voltage

Enables use of 450V caps and 600V silicon regardless of line voltageEnables operation from 277V while keeping output voltage unchanged

Inherent control of input currentPermits use of fast blow line fuses (semiconductor fuses)Permits N+1 operation from single line cord – fuses clear before upstream CBEnables use of Silicon Carbide rectifiersPermits operation from DC bus - no inrush current

Enhanced PLD immunity: Lightning strike and Ring Wave

Buck switch functions as prime-power disconnect simplifies Hotplug

“Anti-Smoke” compliant

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IBM High-End Power Hardware Development

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“Anti-smoke” Compliance

Inherent protection against a shorted bulk cap or boost FET

Buck switch limits fault current to a safe level and is then turned off to isolate the fault

If a shorted buck switch causes an OV the boost switch functions as crowbar to clear the input fuses

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

Extra floating switch required increased complexity and cost

Discontinuous input current in Buck region bigger input filter

Filter not required if converter operates from low voltage only

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IBM High-End Power Hardware Development

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IGBT bias and gate drive

Additional floating bias and optical isolator required

Cost ~$2.00

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Optional Input Filter

Filter not required if converter runs in boost mode steady state

Damping

3rd order elliptical

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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