A Reconfigurable Three- and Single-Phase AC/DC Non-IsolatedBi-directional Converter for Multiple Worldwide Voltages

Daniel F. Opila, Eun Oh, Keith Kintzley, Jedediah Lomax

Abstract— AC/DC converters for worldwide deployment ormultiple applications are often required to operate with single-or three- phase AC connections at a variety of voltages.Electric vehicle charging is an excellent example: a typicalAC/DC converter optimized for a 400 V three-phase Europeanconnection will only produce about 35% of rated power whenoperating on a 240 V single-phase connection in North America.This paper proposes a reconfigurable AC-DC converter withintegral DC-DC stage that allows the single-phase AC power toroughly double compared to a standard configuration, up to 2/3of the DC/DC stage rating. The system is constructed with twoconventional back-to-back three-phase two-level bridges. Whenoperating in single-phase mode, rather than leaving the DCside under-utilized, our proposed design repurposes one of theDC phase legs and the unused AC phase to create two parallelsingle-phase connections. This roughly doubles the AC outputcurrent and hence doubles the single phase AC power.

I. INTRODUCTION

Power converters in mobile or worldwide applications

must connect to single- and three-phase electrical connec-

tions at various voltages when the device moves, or when

deployed to a different country. Manufacturers would much

prefer one converter for worldwide deployment. A common

example of this design requirement is laptop power supplies,

which are used worldwide with only a plug change. The

added expense of this versatility is that converters must

be designed with extra capability to handle the varying

operating conditions.

For higher-power converters it is less practical to design

for multiple operating points due to increased costs. One

clear example is electric vehicle (EV) chargers [1]–[8], which

use both single- and three-phase connections depending on

country and location. Vehicle charging rates vary from one to

tens of kW in typical applications. There is also burgeoning

interest in bi-directional power flow to provide grid support

services [9]–[13].

AC/DC converters designed to handle three-phase inputs

can be easily used with single-phase connections by simply

neglecting one phase, but this method is generally a poor

solution and reflects a typical design problem of optimizing

for multiple operating points.

Often the AC stage is paired with a DC/DC stage. A com-

mon application for this type of converter is a bi-directional

inverter/rectifier for a DC source or load with a minimum

voltage below the rectification limit of the maximum AC

voltage. Typical examples include an electric vehicle (EV)

All authors are with the faculty of Electrical and Computer En-gineering United States Naval Academy, Annapolis, MD 21402, USAopila@usna.edu

battery charging/discharging unit [1]–[8] with bi-directional

power flow, a microgrid battery, or a solar inverter. For a

nominal case, the DC voltage ranges from 280 - 400 V and

the charger may connect to a variety of worldwide voltages

including 480, 400, or 208 V three-phase, and 120, 220, or

240 V single-phase [14].

While this range of application voltages would lend itself

to specially designed converters for each voltage or at least

each country, manufacturers want to minimize part variations

as much as possible, especially in relatively low-volume

products.

As an example, a challenging design is a converter for the

Level 2 EV charging common in both the US and Europe

[1], as shown in Table I.

TABLE I: Global Voltages for Level 2 EV Charging

Connector Volts Phases Amps Power (kW)SAE J1772 (US) 240 1 80 19.2IEC 62196 (E.U.) 400 3 32 22.1

The similar output power ratings might imply that one

converter could serve both purposes. If the converter is

designed for the European three-phase 32 A version and

the AC bridge is current limited regardless of voltage, when

operating in the typical single-phase configuration at 240

V and the same 32 A it only produces 35% of the rated

power. Similarly, if designed for the 80 A input, the three-

phase variant is oversized. The same issues appear in many

applications.

This paper presents a bi-directional AC/DC converter that

can be easily reconfigured from three-phase to single-phase

operation with substantially less reduction in power than the

typical approach of neglecting one phase. The key idea is

that voltage and power in single-phase operation are typically

lower, and therefore the DC/DC converter is under-utilized.

A simple reconfiguration allows some of the DC side devices

to improve the AC stage, potentially doubling the AC current

throughput. Other reconfigurable converters have also been

developed [15]–[18] in various applications.

This paper is organized as follows: Section II describes

the converter topology, Section III provides basic analysis of

power ratings, and Section IV describes testing of a nominal

22 kW prototype, followed finally by the conclusions.

II. CONVERTER TOPOLOGY

A relatively standard non-isolated three-phase converter

with DC-DC stage is shown in Fig. 1a. This drawing shows

978-1-5090-2998-3/17/$31.00 ©2017 IEEE 1708

+DC

ConnectionAC

(a) Three-phase operation

AC

+DC

Connection

(b) Standard single-phase operation.

AC+DC

Connection

(c) Reconfigured single-phase operation.

Fig. 1: Three phase converter used with a standard single-phase connection, and reconfigured for higher power.

all switches and diodes populated on the DC side to allow

bi-directional power flow. This design can be manufactured

from two identical three-phase two-level bridges to maximize

part commonality. For single-phase operation, the same con-

verter can operate using two of the three phases, as shown

in Fig. 1b.

In this single-phase configuration, the DC-side bridges are

under-utilized in most applications. If we assume the DC-side

devices and inductors are identical or similar to the ones on

the AC side, the converter can be reconfigured so that one of

the DC phase legs is used on the AC side, for a total of four

available AC side phase legs. As shown in Fig. 1c, two can

be used in parallel on each phase to create a reconfigured

single-phase AC connection with approximately double the

current rating of the standard single-phase configuration.

These paralleled bridges can either be gated synchronously

or interleaved depending on the application. If only uni-

directional power flow is required, switches and diodes can

be depopulated from the two DC phase legs.

The reconfiguration only involves changing some AC

and DC output connections and can be easily implemented

using mechanical switches or plugs. One example is with

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AC

+ DC Connection

AB

C

1

(a) Single pole double throw (SPDT) manual or electricswitch for reconfiguration.

AC

+DC Connection

ABC

3 adapter

+DC Connection

1 adapter

(b) Single- and three-phase plugs for reconfiguration. The dotted lines represent a physicaladapter, which the user would plug in when connecting the device to an appropriate powersource. The converter side would have 5 pins, and the grid power connection would have2 or 3 pins in an appropriate plug type for the power source.

Fig. 2: Two methods to reconfigure a three-phase converter for single-phase use.

an automated double-pole double-throw (DPDT) no-load

switch as shown in Fig. 2a. An adapter plug with built-in

reconfiguration can also be used, as shown in Fig. 2b. Given

that various power supplies have different plug types and

an adapter is required anyway, the proposed reconfiguration

capability would not require any additional end-user compo-

nents. For products that will mostly stay in one country, a

jumper could also be installed in the factory based on the

desired application.

III. RATING ANALYSIS

Rather than study one particular design, consider the

general case assuming that for a given converter, one can

calculate the allowable AC current per phase IAC based on

DC link voltage, AC line-to-line RMS voltage VAC , and

power factor angle φ. For clarity, we neglect efficiency and

current sharing considerations as they do not change the

general conclusion.

A. Symmetric AC/DC and DC/DC converters

First, we show that the DC side currents per converter

leg or channel are roughly equivalent to the AC side phase

currents, so it is reasonable to design a relatively symmetrical

system as proposed.

The power balance in the three-phase case with three DC

side phase legs holds that

3VDCIDC =√3V 3φ

ACI3φAC cos(φ), (1)

where VDC is the DC output voltage to the load, and IDC

is the average current per phase leg. For a nominal example

case of 250 Vdc output voltage, 400 Vac input voltage, and

unity power factor, the ratio of the DC output current per leg

to the AC phase current RMS value is 0.92.

B. Power ratings with reconfiguration

Next, we quantify the potential benefits of this reconfig-

uration. Assume the converter has three- and single-phase

operating voltages, V 3φAC and V 1φ

AC respectively. The RMS

current limits in each case are I3φAC and I1φAC . Typically the

single-phase voltages are less than those for three-phase, and

the allowable AC converter current is similar.

For single-phase operation, assume IDC is the maximum

current given the DC link and DC output voltages. For

the standard configuration, the maximum single-phase power

throughput is limited by the lesser of the AC/DC or DC/DC

bridges,

P 1φ standardmax = min{3VDCIDC , V

1φACI

1φAC cos(φ)}. (2)

After reconfiguration, one of the DC legs is reassigned to

the AC side, so the DC power drops by 1/3 and the AC

power doubles, so that the maximum single-phase power

after reconfiguration is

P 1φ reconfigmax = min{2VDCIDC , 2V

1φACI

1φAC cos(φ)}. (3)

The reconfiguration thus allows a doubling of the single-

phase converter power up to 2/3 of the DC/DC converter

power rating. The DC/DC converter power rating is the same

for single- and three-phase modes if the DC link voltage is

the same.

In many applications the single phase voltage is typically

lower than the three-phase case, so AC input power in the

single-phase case leaves the DC/DC converter under-utilized.

To illustrate these concepts, an example is provided in Table

II for a Level 2 electric vehicle charger at 400 V three-

phase (as in Table I), that is also designed to handle 240

V single phase. A second example is provided in Table

III for a 208/120 V reconfiguration, where a 3.6 kW 208

V connection can still provide 2.4 kW on a 120 V, 20 A

connection. In both cases the converters are assumed to be

optimized for the three-phase application, and the DC/DC

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stage rating does not change for single phase operation. As

seen in these two examples the reconfiguration is particularly

well matched when voltages are√3 different, as in a line to

neutral connection of the original 3-phase voltage.

TABLE II: Ratings for a Reconfigured 400/240 V Converter

Configuration Standard Standard ReconfiguredAC Volts 400 240 240Phases 3 1 1AC Amps per leg 32 32 32AC Amps per phase 32 32 64PDC per leg (kW) 7.4 7.4 7.4PDC total (kW) 22.1 22.1 14.8Apparent Power (kVA) 22.1 7.1 14.2Maximum Power (kW) 22.1 7.1 14.2

TABLE III: Ratings for a Reconfigured 208/120 V Converter

Configuration Standard Standard ReconfiguredAC Volts 208 120 120Phases 3 1 1AC Amps per leg 10 10 10AC Amps per phase 10 10 20PDC per leg (kW) 1.2 1.2 1.2PDC total (kW) 3.6 3.6 2.4Apparent Power (kVA) 3.6 1.2 2.4Maximum Power (kW) 3.6 1.2 2.4

Depending on voltage levels, reconfiguration may only

provide marginal benefit. Consider a three-phase converter

with matched AC/DC and DC/DC power ratings. If the

single-phase AC voltage is the same as for three-phase, this

would normally decrease the AC rated power by√3 to

58%. Reconfiguring to remove one DC-DC leg would yield

66% of original rated power on the DC/DC stage. Although

reconfiguration yields much higher apparent power capability

on the AC side, the maximum active power throughput

improves only marginally.

As a general rule, if the single-phase and three-phase rated

currents are of similar magnitude, reconfiguration provides

some benefit to active power throughput when the single-

phase voltage is less than the three-phase line-to-line voltage.

IV. TESTING

A. Setup

A mock-up prototype has been constructed from commer-

cially available three-phase two-level inverters from the Ap-

plied Power Systems (part number IAP150T120) as shown

in Fig. 3. The technical characteristics of the unit are shown

in Table IV.

These are connected back-to-back and paired with 32 A

450 μH output inductors per phase on AC and DC sides to

create a three-phase AC-DC bi-directional charger nominally

rated for 32 A per phase, and up to 400 V line-to-line (22

kW). The prototype converter is shown in Fig. 4. The two

sets of DC capacitors are connected in parallel on the DC

link, so the total converter DC link capacitance is 6600 μF.

TABLE IV: Three-phase Two-level IGBT Inverter Parame-

ters

Nominal DC bus voltage 650 VSemiconductor Module CM150DC-24NFMModule max voltage VCES 1200 VCollector Current at 25 C 150 APeak Collector Current at 25 C 300 ADC capacitance 3300 μF

Fig. 3: Single three-phase two-level inverter.

To test this converter, an additional inverter and trans-

former are used to provide a “grid” power supply that can

circulate power in a “back-to-back” or “pumpback” configu-

ration. The full test setup is shown in Fig. 5 and a schematic

of the system in Fig. 6. The 15 kVA single-phase transformer

is tapped for 240V/240V and rated at 3.27% impedance.

The inverter providing grid emulation has a 30 μF capacitor

connected across its output terminals between the inductors

and the transformer, as shown in the schematic. A separate

DC source is connected to the DC “load” interconnect to feed

the losses from power circulation. This interconnect has an

additional 840 μF capacitor between the converter under test

and the grid-sourcing converter, which increased to 5800 μF

during the reconfigured test to provide output filtering for a

larger external DC power supply.

Fig. 4: Bi-directional AC-DC converter formed from two

separate three-phase two-level inverters.

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DC

IAIA DC

IC DC

IB DC

VAB (cap)+

VAB+

AC GridEmulator

IA GS

Fig. 6: Complete back-to-back test schematic with an additional inverter and transformer to provide full rated power flow.

Fig. 5: Complete back-to-back test setup with an additional

inverter and transformer to provide full rated power flow.

The additional inverter, its inductors, and AC capacitor are

to the left behind the main converter. The 15 kVA 240 V/240

V transformer is visible on the floor to the right.

B. Results

To demonstrate the usefulness of this method, the con-

verter was tested in single-phase operation for the standard

and reconfigured topologies, under conditions shown in Table

V.

Voltage and current traces for test case 1 in the standard

configuration are shown in Fig. 7a. The AC voltage is

measured at the output converter terminals on the right side

of Fig. 1b, between the inductors and transformer on the

TABLE V: Test Conditions

Test Case No. 1 2Configuration 1-phase standard 1-phase reconfiguredVAC (V RMS) 240 240IAC per phase (A RMS) 32 32IAC Total (A RMS) 32 64VDClink (V) 435 435VDCoutput (V) 370 370Power (kVA) 7.7 15.4

far right side of Fig. 6. There is only one AC current in

this case, but three separate DC currents. Positive current is

“into” the phase leg, from low to high voltage. This case

represents charging or rectification near unity power factor,

so the AC voltage and current are in phase. The DC currents

are negative on average to represent charging on the DC side

to a nominal battery. This case is the maximum available

power given the 32 A current rating, yielding 7.68 kVA.

In contrast, the reconfigured single-phase variant is tested

in case 2 as shown in Fig. 7b. In this configuration, there

are two AC currents shown, one from the original AC A

phase leg, and one from the newly-added “A” phase on the

DC side. Switching for this DC phase leg is synchronized

with the original AC phase A, and they operate in parallel.

The B and C phases are similarly synchronized and operate

in parallel with each other. The two phase A currents are

nearly identical and overlap on the plot. The total converter

output current is the sum of the two and is also shown. This

case shows a nominal 32 A RMS per phase leg on the AC

side, so 64 A RMS total, yielding 15 kVA. Sharing is not

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0 5 10 15 20 25 30 35 40 45 50

Time(ms)

-500

0

500V

olta

ge (

V)

-150

-100

-50

0

50

100

150

Cur

rent

(A

)

Inverter AC voltages and currents

VAB

IA

0 5 10 15 20 25 30 35 40 45 50

Time(ms)

-60

-40

-20

0

20

40

60

Cur

rent

(A

)

DC-DC converter currents

IA-DC

IB-DC

IC-DC

(a) Standard single-phase operation at 240 Vac and 32 A RMS AC current,which provides 7.7 kVA. All three DC legs provide DC current, switchingat 10 kHz. Phase A and B AC current is provided by one phase leg each,switching at 5 kHz.

0 5 10 15 20 25 30 35 40 45 50

Time(ms)

-500

0

500

Vol

tage

(V

)

-150

-100

-50

0

50

100

150

Cur

rent

(A

)

Inverter AC voltages and currents

VAB

IA

(Inv)

IA-DC

IA

(Inv)+IA-DC

0 5 10 15 20 25 30 35 40 45 50

Time(ms)

-60

-40

-20

0

20

40

60

Cur

rent

(A

)

DC-DC converter currents

IB-DC

IC-DC

(b) Reconfigured single-phase operation at nominal 240 Vac and 64 A RMSfor 15 kVA. Each AC phase leg provides approximately 32 A RMS, andtwo operate in parallel to provide 64 A RMS total. In this case IA is 31.9A and IA−DC is 30.9 A for a total of 63 A. The DC-DC converter hasonly two channels as shown in the bottom plot. All bridges are switchingat 10 kHz.

Fig. 7: Standard and reconfigured single-phase use of a three-phase converter. The top plots show AC voltages and currents

during operation at 240 V in active rectification to provide DC load charging. The trace labels match Fig. 6.

perfect so the output current is 63 A RMS. Only two DC

currents are shown, as the third DC leg is providing AC

current.

Perhaps the most striking feature of Fig. 7 is the clear

transition of one DC phase leg to the AC side, where it

significantly augments the AC current. The multiple phase

legs share current effectively, particularly on the AC side.

The DC side current sharing in case 1 (Fig. 7a) shows legs

A and B matching well, but the only minor anomaly appears

to be the leg C DC current. The exact reason is unclear, but

after further inspection, the wiring on that output is slightly

different than the other two and may contribute to the issue.

For completeness, the voltage and current on the “grid”

side of the transformer, at the output of the inverter used for

power circulation are shown in Fig. 8 during case 2. A small

AC filter capacitor there ensures a smooth voltage waveform,

while the increased voltage ripple visible in the top of Fig.

7b is due to transformer impedance.

V. CONCLUSIONS

This reconfigurable inverter topology uses standard

bridges, switching technology, and two-stage AC/DC DC/DC

design, but allows a much wider range of application voltages

through a simple reconfiguration. It uses common parts for

both the inverter and DC-DC stages. Many inverters are

0 5 10 15 20 25 30 35 40 45 50

Time(ms)

-500

0

500

Vol

tage

(V

)

-100

-50

0

50

100

Cur

rent

(A

)

Grid source AC voltages and currents

VAB

(cap)

IA-GS

Fig. 8: Traces from the grid source converter

already constructed this way, so the practical impact could be

significant in that many existing designs could extend their

operating range with relatively minor adjustments. An appli-

cation example was provided for electric vehicle charging,

but many applications could benefit from this single- and

three-phase reconfiguration.

Testing demonstrated the effectiveness of the reconfigu-

ration in doubling the AC power output compared to the

standard single-phase case, and showed balanced current

sharing between parallel bridges.

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