Download An AC High Frequency Quasi Square Wave Bus Voltage for the Next

An AC High Frequency Quasi Square Wave Bus Voltage for the Next
Generation of Distributed Power Systems
Dr. Angel Gentchev
Analog Devices, Inc.
1500 Space Park Drive
Santa Clara, CA 95052
Phone: 408-562-7334, Fax: 408-562-2603
E-mail: [email protected]
Douglas P. Arduini - Consultant
Arduini Design & Development (AD&D)
2415 San Ramon Valley Blvd., #4-415
San Ramon, California 94583-1651
Phone/FAX 925-804-6063
E-Mail: [email protected]
URL: http://www.AD-and-D.com
(Presented and published in Proceedings of High Frequency Power Conversion HFPC ’98 Conference,
November 1998, Revised 10-14-00, Updated October 2009)
Abstract - An advanced Alternating Current Distributed Power System (AC-DPS) is proposed as a new lowprice and high-performance alternative to the typical Direct Current Distributed Power System (DC-DPS). A
high frequency quasi square wave or trapezoidal AC waveform is distributed on the DPS power bus from the
Bulk Power Front-End Rectifier/Inverter Stage, to a variety of AC to DC Secondary Power Converters.
Zero-Voltage (ZV) switching is used in the DC/AC inverter, and various types of AC/DC power conversion
stages are available at the board level or point-of-use loads, including Zero-Current (ZC) Resonant
Switching, for post regulation, if needed. This next generation DPS can provide lower cost, board space,
simplicity, improved reliability, and manufacturability. This technology may also be well suited for
distributed power in the higher power Personal Computer (PC), with solutions for higher di/dt power
demands of the modern microprocessors. A 250W prototype system with 48VAC DPS bus at 250KHz was
tested.
INTRODUCTION
The demand for new features and higher performance digital electronics and CPUs in nearly every application in
modern electronic industry presents a major challenge to the power supply and power distribution design. The trend
to lower the CPU supply voltages with tighter tolerances, higher currents, faster load steps, plus the need for more
output voltages to various analog, digital, and optical support chips, creates difficult problems to maintain power
quality in a cost-effective manner. Centralized power distribution is no longer capable of providing power quality
used in the modern and future digital electronics [1,2], including the new high power personal computer (PC) and
workstation, for the following reasons:
1.
Additional supply voltages, typically including +12V, +5V, +3.3V, -5.2V, and -2.1V.
2.
Very Fast Transient Response (0.1-10A/nS) at low supply voltage (2.0-3.6V), not possible from the bulk
power supply as direct power source of the modern CPU electronics. Therefore most systems use low
voltage DC/DC Converters, which are required as secondary converters for regulation at the point-of-use,
3.
Higher currents for lower CPU supply voltages, up to 50A in some applications. Without a DC or AC
power distribution system at higher voltage level and therefore lower current level, the pure resistive losses
and heavy wiring become an unacceptable solution,
4.
Static and dynamic load regulation with tight tolerances, +/-5% to +/-2% at voltage levels of 3.6V to 2.0V
and lower at various critical loads in the system is not practical from a single bulk supply,
5.
Cross regulation effects between the outputs of a single bulk converter, add to the problems with the tight
regulation requirements of many of the modern digital electronic circuit specifications.
The DC-DPS is a popular solution for high power digital systems, but it requires large quantities of DC/DC
converters on the point-of-use boards and loads, and require distributed input filters for isolation from converter
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interaction and they are sensitive to impedance instability [1,2,3,4 and 7]. Inrush current control switch circuits on
each board for turn-on current limiting are also usually required. The DC-DPS solution is also complex with a large
quantity components, filters, and DC/DC power supplies that add a high cost to the system, take up a large amount of
board space, lower reliability with high parts count, and require second sources that are form, fit, and function
compatible. The DC-DPS is also being considered as the solution for the high power PCs and workstations.
An improved power system would be one that could provide all the advantages of a DC-DPS over the centralized
power supply distribution, but at a lower cost, smaller board space, and fewer components. Low voltage 24VAC
DPS power systems are used today at 60Hz for security and survival systems, outdoor lighting, and control systems.
But, 60Hz is impractical for small size, lightweight, and low cost at high power. Higher frequency AC sine wave
systems above 20KHz have been used, but exhibited power factor problems that would be expensive to correct at
every board or load [1]. The advantages of an AC-DPS using square or rectangular (trapezoidal) waves are
discussed in detail in references [1,4].
DESCRIPTION OF A PRACTICAL AC-DPS
A basic DC-DPS is shown in Figure1 and compared to basic AC-DPS shown in Figure 2, showing the rectifier and
switching stages of each. It is shown that the AC-DPS has one or two less switches (optional inrush control), one
less rectifier stage less at the intermediate voltage level, and one less set of filters at the intermediate voltage level.
The AC-DPS also has smaller capacitance sizes on the low voltage level because the AC distribution voltage is preregulated with little or no need for post regulation. When post regulation is needed for load changes and tighter
regulation, power conversion is optimized with high duty cycles and the need for small PWM regulation bandwidth.
The component savings improves performance at a reduced cost with simple on-board regulators that eliminate the
need for typical DC/DC converter “bricks,” and in some cases need no post regulator. Efficiency savings are
obvious due to fewer switches, rectifiers, and filters, as shown in Figures 1 and 2 as a comparison between the
typical DC-DPS with 73% efficiency example and an AC-DPS with 81% efficiency example.
As in every distributed power system, the main advantages are flexibility, elimination of distribution losses,
improved point-of-load regulation, and fault tolerance. However, the complexity of the system and the multiple
power conversion cycles in series, effect the overall price of these systems, thus the DC-DPS is a poor choice in
price-sensitive commercial electronic products and systems.
Figure 3 shows the 7 power conversion stages from the input Line 50/60Hz to DC/DC converter output of that DCDPS. As a logical improvement, as was known in the power utility industry more than a century ago, AC distribution
is a better distribution system than DC. Two intermediate AC/DC and DC/AC power conversion cycles can be
avoided in the structure of DC-DPS by implementation of an AC-DPS. It is shown in Figure 4 that the “MiddleMan” DC/DC converter is effectively not necessary and can be eliminated, thus cutting the overall power conversion
steps from 7 to 5. The direct result from employing “No Middle-Man” in the AC-DPS approach is a significant price
reduction and performance enhancement. Therefore one full conversion cycle of DC/AC and AC/DC can be avoided
with corresponding improvement of less hardware, less complexity, higher efficiency and price reduction with the
AC-DPS. A good example of such an AC-DSP based system is described in reference [4] with some comparisons
and practical results.
Due to the novelty of the AC-DPS, including for low power electronic systems like the PC, there are some important
questions and considerations regarding an AC-DPS, as follows:
1.
What frequency should be used on the AC Bus? The answer can be any frequency in the range of 20KHz to
>1MHz, which gives proper fit to the system specification goals. If the highest priority is miniaturization, then
higher frequency is better, but if the priority is simplicity at higher efficiency and lower risk, then lower
frequency is better for performance of the AC DPS.
2.
What should be the Bus Voltage Waveform Shape? There are mainly three choices – sine wave and quasi sine
wave, square wave, or quasi square wave (trapezoid shape). The simplest waveform to generate is the square
wave, but it is the most difficult one to be EMC managed, due to the high harmonic content of AC Bus voltages
and currents. The sine wave bus is the best for very low EMI and RFI, but it is the most difficult one to generate
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and regulate efficiently with high power and high frequencies, plus it suffers heavily from power factor or form
factor. The square wave is the simplest waveform to generate, but the switching speeds of voltage and current
provide high EMI and RFI, plus high switching losses. A good compromise is the quasi square wave or
trapezoid wave bus voltage, with controlled rise and fall dead times with transitions times of 5-15% of the AC
pulse period, thus greatly reducing the high frequency power harmonic content of the AC distributed bus
voltage.
Line Regulation Bulk Front-End: The input line voltage is regulated to a DC bus voltage to the DC/AC inverter as
regulated peak-to-peak voltage of the AC-DPS trapezoidal waveform, typically with a PFC AC/DC Converter or a
DC/DC converter. This inverter stage requires no isolation because transformer isolation is automatically provided
at the DPS load AC/DC converter stages, and transformer isolation is optional at the inverter DC/AC inverter frontend stage. Load regulation can be improved with an additional voltage feedback loop with peak detection of the ACDPS trapezoidal voltage waveform.
DC/AC Inverter Bulk Front-End with Zero-Voltage switching is an ideal solution for generating controllable Soft
Switching bus voltage waveforms. The AC trapezoidal waveform voltage can be generated in the front-end converter
by a half (or full) bridge converter where the transitions of the peak voltages from rail-to-rail are set by the Voltage
Mode resonant loop formed by the sum of the node capacitance and the inductance of external ZV mode choke.
Thus, the slew rate of that voltage does not depend on load changes. The simplified schematic and the
corresponding switching node waveforms are shown in Figures 6 and 7. The inverter stage replaces the bulk frontend DC/DC converter and together with the Power Factor Correction in front provides peak-to-peak voltage
regulation of the line to provide a minimum need for post regulation at the loads. The trapezoidal wave is an
improvement over other square wave distribution systems, with approximately 5-15% transition duty cycle requiring
small filter sections, limiting dV/dt for reduced EMI and transient effects. Shielded bus transmissions line
techniques or twisted pair technique are appropriate ways to reduce furthermore still radiated EMI fields in the
backplane where needed. A design goal of less than +/-2% line/load regulation of the AC Bus including ripple and
noise that can be achieved by the PFC line regulator, controlling the DC bus (usually 400V) to maintain the inverter
transformer output voltage to +/-48Vpeak triangle wave on the AC-DPS bus system backplane. Additional sensing
of the backplane peak voltage can be added to the PFC voltage loop control if required.
EMI Considerations: EMI radiation with square or rectangular wave power distribution is an obvious concern.
The use of a trapezoid wave is an improvement over the pure square wave for harmonic content. Noise source
energy is minimized with Zero Voltage and Zero Current topologies that are choices at the Bulk Inverter stage and
Secondary Converter stages of the proposed AC distribution system. In fact, this AC-DP System is a direct
derivative from the innovative designs reported in [5,6,10 and 13]. If is necessary, the transmission line can be
additionally shielded from radiation in coaxial or shielded PC power planes or strip transmission line [1, 4].
Load Regulation: Post regulation and filtering complexities are minimized with the regulated AC trapezoid input
waveform. Small high frequency transformers isolate the power at each board or load area, with secondary rectifier
filters for each voltage requirement. Synchronous rectifiers can also be incorporated to improve efficiency. Load
voltage accuracy and regulation can be held within +/-5% with proper design to eliminate post regulation to some
loads [1][2]. For other load, requirements a variety of different low-cost post regulator designs could be used [1].
Most applications will favor using low dropout regulators (LDOs), Mag-Amps, and a Zero-Current Fully Regulated
(ZC) Resonant Converters, to supplement the Zero Voltage switching of AC/DC Bulk Converter. Another choice
good choice is a secondary controlled synchronous rectifier PWM. Typical board or load regulators for the DC-DPS
are shown in Figure 3, and a variety of choices are shown for the AC-DPS are shown in Figures 4, 5, 10, 11, and 12.
Simple Transformer Rectifier as Low Cost Secondary Converter: The bipolar trapezoidal wave from a
transformer secondary of the AC-DPS bus provides good line regulation of about 3-6% load dependent DC voltage
after a rectifier stage. This is accomplished with low copper and core loss transformer designs and robust sizing of
rectifiers for low dynamic resistance for small forward voltage changes with load current changes. The cost of this
simple transformer/rectifier/filter is 5 to 10 times lower than the price of “bricks” in the DC-DPS counterpart. This
design has no stability concerns from a regulator loop control or negative impedance effects on the input filter or
distributed filters. Also, the filter size is smaller because of a fixed 85-95% duty cycle, with no ride-through time
requirement for PWM step recovery time from load step response.
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However, there may be a need for ride-through or holdup time in fault tolerant system designs with energy storage
for fuse fault clearing that can drop the bus voltage. This energy storage will be at the lower end-point voltage level
versus the at intermediate DC level in a DC-DPS, thereby requiring more capacitance.
The bipolar waveform can use self driven synchronous rectifiers for half-wave (42.5% to 47.5% duty) or full-wave
rectification (85% to 95% duty) as shown in Figures 10,11.and 12. The catch or free-wheeling rectifier in the fullwave rectifier circuit can use a standard Schottky diode in place of a FET synchronous rectifier, with little effect on
efficiency because of the high duty cycle, therefore the low conduction time.
Synchronous Rectifier PWM Secondary Converter: The bipolar trapezoidal wave from a transformer secondary
from the AC-DPS bus can be post regulated with phase modulated synchronous rectifier switches with PWM
secondary control. The bipolar waveform precludes a typical synchronous rectifier phase modulation, but can be
controlled with MOSFETs switched in pairs as bi-directional switches for blocking and conducting.
Mag-Amp PWM based Secondary Converter: The well known Magnetic Amplifier is perfectly suited for ACDPS post regulation function. One consideration is in very high current and low voltage applications, where the
effects of transmission line and transformer leakage reduce the maximal output power capability to the load. But this
solution is one of the simplest and effective to perform the load regulation on a particular DC output, especially
when the output voltage needs to be tightly regulated for the modern fast CPUs.
ZC Phase-Modulated Resonant Converter: This is one or the best post regulation choices because it is the only
way to eliminate the negative effect of noted distributed line inductance plus the leakage inductance of the
transformer(s). Here a controllable series resonant loop is formed by the sum of all and the series resonant capacitor
are used. The resonant loop can be controlled by primary side magnetic amplifier or by phase-modulated electronic
switch at constant frequency and duty ratio. The Zero Voltage and Zero Current switching and the way to achieve
control over the power transfer between input and output of such a resonant system are well described in [5,10,11
and 13]. A magnetic amplifier control can be used as a simple control of the resonant power transfer as shown in [9].
Low Dropout (LDO) Regulator based Secondary Converter: The bipolar trapezoidal waveform from a
transformer secondary of the AC-DPS bus provides regulated 3-6% DC voltage after a rectifier stage. It is easy to
use the line regulated only bus voltage as a regulated headroom voltage for an LDO. This is an inexpensive
regulator, often using a standard 3-terminal regulator. With regulated headroom, efficiency is competitive to DC/DC
converters, with up to 84% efficiency at 3.3V output and 70% overall including the rectifiers [3]. This is an
attractive solution to critical loads requiring low noise and tight regulation tolerances. The high speed dynamic
response, tight regulation accuracy, low ripple and noise of the LDO linear regulator can provide the best power
quality with the least filtering requirements at the lowest cost and smallest board space for critical loads as the new
CPUs and support chips [3].
Pentium II Synchronous Buck Converter modified for AC-DPS Secondary Converter: A version of a
Secondary DC/DC post regulator for an AC-DPS can be build using synchronous switching regulator from the Buck
topology. The AC bus voltage of the AC DPS can be transformed and rectified to DC voltage via small HF
transformer and simple full-wave rectifier/filter. The output DC voltage is post regulated for load changes, where the
AC input line is stabilized at the AC Bus voltage by the PFC/Inverter stage. To achieve a precise stabilized and
programmable DC output as is required by the latest generation of Pentium II CPUs and the next generation, is a
highly efficient synchronous switching regulator as an appropriate solution.
An example of such a controller is the ADP3152 from Analog Devices, specially optimized to supply Pentium II
Processors, having a 5-bit DAC to read a voltage identification code directly from the processor. This controller
meets Intel’s stringent transient specifications VRM8.2 and VRM8.3, and possesses the minimum number of output
capacitors and the smallest footprint. The application schematic is given in Figure 17. The transient response is
shown in Figure 18 for the step 0.8A-19A-0.8A with optimal compensation, reported in the Proceedings of IEEE
APEC’98 [12].
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AC-DPS PROOF-OF-CONCEPT PROTOTYPE
AC-DPS Prototype: An AC-DPS proof-of-concept and practical implementation prototype was built for the
purpose of fine tuning the design procedure and to test and debug the AC solution from both the theoretical and
practical point of view. The input voltage is Worldwide 85 to 265 VAC to an active PFC AC/DC converter. The
output power is only a demonstration level of 250W, the output DC/AC inverter provides an AC Bus at +/-48VAC
peak trapezoidal waveform at 250KHz. Three secondary post regulators were tested, a Mag-Amp PWM, a ZC
Resonant Converter, and an LDO.
Prototype Test Results: Prototype test results from a prototype 250W system with 250kHz AC bus 48V DPS have
shown excellent performance, minimum cross regulation and efficiency of 81% from AC bus to multiple +5V, +12V
and +24V outputs. Figures 13 to 16 show the performance of the prototype and its switching waveforms to
demonstrate the cancellation effect of ZV and ZC switching on transmission line impedances. A power distribution
to three remote loads with up to 5 to 7 foot cable lengths was used to demonstrate the proper operation under the
worst-case bus parasitics.
Cost Comparisons and AC-DPS Saving: A price comparison of a typical DC-DPS system versus an AC-DPS
system at 600W is shown in Table 1, with a 37% overall system cost saving (with a 55% reduction in components
cost for the Secondary Modules) for the AC-DPS. The comparison was made as objective as possible, using the
standard components and off-the-shelf products of well-known power supply companies. The reduction in
complexity and by the conversion to AC distribution reflected the lower component and labor price at both modular
and system level.
CONCLUSION
The advantages of an AC-DPS have been shown to include savings and advantages in cost, board space, component
parts, complexity, and reliability, plus the use of distributed DC/DC converters bricks can be avoided. Flexible and
simple power design changes can be implemented as needed with minimum impact on cost and schedule. The
addition of another voltage or increasing the output power for a board upgrade is as simple as adding a new
transformer winding and changing the rectifier/filter section.
From the brief description of an advanced AC-Distributed Power System, the authors have shown an improved
alternate method to distributed power to different PCB’s, modules, and loads that are used in a variety of modern
electronic equipment. A price comparison between two as parameters typical AC and DC based Distributed Power
Systems has been shown, giving 37% lower cost in favor to the AC-DPS.
A proof-of-concept 250W prototype was built for 250KHz operation, with good operating results. This fully
functional AC-DPS delivered 250W to three remote loads, using three different post regulator secondary converters,
with additional features of the Soft Switching AC Power Distribution. The advantages are shown at the boards or
point-of-use loads, with the elimination of many DC/DC converters to lower costs, weight, complexity, filters
requirements, board space, qualification costs, second source requirements, reliability, design risk, and faster timeto-market schedules.
With the DC-DPS and its variations [1] as the accepted standard replacement to the centralized power distribution
systems, the AC-DPS will find the typical resistance to acceptance and change. Replacement of arrays of DC/DC
converter “bricks” in systems with simpler and lower cost regulated solutions is a drastic concept. The strong
incentive for lower cost, smaller board space, lower parts count, and fast time-to-market with low risk designs should
prove as an incentive to consider the AC-DPS for new designs.
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REFERENCES
[1] D. Arduini, “Distributed Power System using AC to solve DC Distributed Power Problems,” HPFC 1995.
[2] D. Arduini, ”A Distributed Power System with a Low-Cost Universal DC/DC Converters”, Power Conversion
Electronics 1995 Conference.
[3] D. Arduini, “A DC/DC Converter Using a PWM/LDO “Hybrid” for Low Voltage Power Solutions,” Power
Conversion Electronics 1995 Conference.
[4] R.Watson, W.Chen, G.Hua and F.C.Lee, ”Component Development for a High-Frequency AC Distributed
Power System,” Proceedings of IEEE 1996, pp.657-663.
[5] Angel Gentchev, ”A New Low Noise Multiresonant Converter,” Proceedings of PCIM’92 Conference in
Nurnberg, Germany, pp.419-427.
[6] M. Jovanovic, R. Farrington and F. C. Lee, ”Comparison of Half-Bridge, ZCS-QRC and ZVS-MRC,” IEEE
Transactions on Aerospace and Electronic Systems, Vol. 26, No.2. pp.326-336, March 1990.
[7] Per Lindman and Lars Thorsell, ”Applying Distributed Power Modules in Telecom Systems,” Proceedings of
IEEE-PESC’94, pp.777-785.
[8] Barry Arbetter and Dragan Maksimovic, ”DC-DC Converter with Fast Transient Response and High Efficiency
for Low-Voltage Microprocessor Loads,” Proceedings of IEEE-APEC’98.
[9] A.S.Kislovski, ”Linear Variable Inductor in power processing,” Proceedings of APEC’87, pp.87-92.
[10] Kosuoke Harada and Katsuaki Murata, ”Controlled Resonant Converters with Switching Frequency Fixed,”
Proceedings of IEEE-PESC’87, pp.431-438.
[11] A.H.Weinberg and L.Ghislanzoni, ”A new Zero Voltage and Zero Current Power Switching Technique,”
Proceedings of IEEE-PESC’89, pp.909-919.
[12] Richard Redl, Brian Erisman and Zoltan Zansky, “Optimizing the Load Transient Response of the Buck
Converter”, IEEE APEC’98, p.170-p.177.
[13] Angel Gentchev, “A Novel Fully Stabilized Multiresonant Converter”, Proceedings of PCIM’93, Nuerberg,
Germany, p. – p.
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Figure 1. Typical DC-DPS with efficiencies.
Figure 2. Typical AC-DPS with efficiencies.
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Table 1. Price comparisons for typical 600W DC-DPS and AC-DPS.
Front-End
PS
Transfer
Price,
Six 100W
DC/DC or
AC/DC PS
Transfer
Price
Total Price
DC Distributed Power
System 600W
AC Distributed Power System
600W
Absolute
Price
Difference
% Price
Difference
$214.88
$193.98
(-$11.802 component cost)
(-$9.099 labor cost,2.585h x 3.52/h)
$20.90 less
9.73% less
$52.63 x 6 = $315.78
$23.63 x 6 = $141.78
(-$17.21 component cost per unit)
(-$11.79 labor cost, 3.35h x 3.52/h)
$174.00 less
55.10% less
$530.66
$335.76
$194.90 less for 36.73% less for
an AC DSP
an AC DPS
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Figure 6. Bulk DC/AC Inverter section.
Figure 7. DC/AC Inverter loaded with 50 watts ZC to AC/DC Secondary Converter,
CH1 is AC BUS Voltage, CH2 is a Switching Node Voltage of the DC/AC Converter,
CH4 is the AC BUS Current supplying Zero Current AC/DC Secondary Converter.
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Figure 9. Prototype Mag-Amp AC/DC converter.
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Figure 10. Self-driven half-wave synchronous rectifier/filter secondary power supply.
Figure 11. Self-driven full-wave synchronous rectifier/filter secondary power supply, with standard catch diode
rectifier, as a good solution for high duty cycles.
Figure 12. Self-driven full-wave synchronous rectifier/filter secondary power supply, with synchronous rectifier
catch diode.
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Figure13. AC DPS, AC/DC MagAmp Converter, 40 watts output power and
AC/DC Zero Current Converter, 35 watts output power,
CH1 is AC BUS Voltage, CH2 is a Switching Node Voltage of the DC/AC Converter,
CH4 is the AC BUS Current supplying the Zero Current AC/DC Secondary Converter,
CH3 is the AC BUS Current supplying the MagAmp AC/DC Secondary Converter.
Figure 14. AC DPS, AC/DC MagAmp Converter at 40 watts output power and
AC/DC Zero Current Converter at zero watts output power,
CH1 is AC BUS Voltage at the Secondary Converter Input,
CH2 is a Switching Node Voltage of the DC/AC Converter,
CH4 is the AC BUS Current supplying the Zero Current AC/DC Secondary Converter,
CH3 is the AC BUS Current supplying the MagAmp AC/DC Secondary Converter.
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Figure 15. AC DPS, AC/DC MagAmp Converter at zero watts output power and
AC/DC Zero Current Converter at 35 watts output power,
CH1 is AC BUS Voltage, CH2 is a Switching Node Voltage of the DC/AC Converter,
CH3 is the AC BUS Current supplying the Zero Current AC/DC Secondary Converter,
CH4 is the AC BUS Current supplying the MagAmp AC/DC Secondary Converter.
Figure16. AC DPS, both AC/DC MagAmp Converter and
AC/DC Zero Current Converter are at zero watts output power,
CH1 is AC BUS Voltage, CH2 is a Switching Node Voltage of the DC/AC Converter,
CH3 is the AC BUS Current supplying the Zero Current AC/DC Secondary Converter,
CH4 is the AC BUS Current supplying the MagAmp AC/DC Secondary Converter.
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1
2
3
4
D
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4
Figure17. ADP3152 Application Schematic covering VRM8.2 and VRM8.3 Intel Specification
Figure 18. Transient Response of output voltage 2.00V,Current Step 0.8A-19A-0.8A
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