Power High

Soft Switched Ac Link Buck Boost Converter

Anand Balakrishnan Hamid A. Toliyat William C. Alexander

Student Member, IEEE Fellow, IEEE Member, IEEE

Advanced Electrical Machines and Power Electronics Laboratory Ideal Power Converters, Inc.

Department of Electrical & Computer Engineering Austin, TX

Texas A&M University Phone: 512-***-****

College Station, TX 77843-3128 Email: abpm3u@r.postjobfree.com

Phone: 979-***-****

Email: abpm3u@r.postjobfree.com

Abstract- A novel soft switching high frequency link converter

for medium and high power ac-ac and ac-dc applications is

proposed. The proposed topology overcomes the shortcomings of

conventional dc and ac link schemes and uses 12 bidirectional

switches for a three phase to three phase configuration. An

inductor-capacitor pair with low reactive rating forms the link.

Switches turn on at zero voltage and their turn offs are

capacitance buffered, resulting in low switching losses. Phase

currents are synthesized using precisely controlled current pulses,

which also allows for any desired input or output voltage or power

Fig. 1: Schematic of proposed topology

factor. The converter can perform buck and boost operations in

forward and reverse directions. The proposed topology promises

switches specifically designed for high frequency applications

size, weight and cost reduction while offering improved

are becoming available. The use of resonant circuits in high

performance compared to existing converters.

frequency dc-dc converters has since been reported [7].

I. INTRODUCTION Ac-ac and dc-ac converters employing high frequency ac

links have also been reported [8]-[12]. Most of these converters

Variable frequency drives typically have employed dc

are designed for specific type of source/loads. Reference [1]

voltage or current links for power distribution between the

reported a topology that provided one-step bidirectional power

input and output converters and as a means to temporarily store

conversion for different kinds of loads/sources. This

energy. The dc link based power conversion systems have

configuration used twelve bidirectional switches and employed

several inherent limitations. One of the important limitations is

Pulse Density Modulation (PDM) as a means to control the

the high switching loss and high device stress which occur

currents. The use of PDM reduces the system response because

during switching intervals. This severely reduces the practical

of usage of integral pulses of currents. Topologies that make

switching frequencies. Additionally, while the cost, size, and

use of twelve unidirectional switches, by providing a dc offset

weight of the basic voltage sourced PWM drive is attractive,

to the dc link, have also been suggested. Reference [13]

difficulties with input harmonics, output dV/dt and

proposes a topology with twelve unidirectional switches,

overvoltage, EMI/RFI, tripping with voltage sags, and other

without the dc offset. However, it is limited in operation

problems significantly diminish the economic competiveness

response due to its inability to supply output current at low

of these drives. Add-ons are available to mitigate these

voltages or power factors, at link frequencies sufficiently high

problems, but may result in doubling or tripling the total costs

to avoid input/output filter resonances. Also, there is a large

and losses, with accompanying large increases in volume and

dead time due to the resonant 'fly back' which reduces the

weight.

power capability by about 30%. This largely negates its

High frequency ac link converters have been suggested as an

advantage of using a lower numbers of switches compared to

improved alternative. A high-frequency link allows the

the proposed topology.

flexibility of adjusting the link voltage to meet the individual

In this paper, a new soft switching ac link converter that

needs of the source and load sides and at the same time

overcomes the aforementioned drawbacks while offering

provides isolation between the two [1]. High frequency link

superior control and significant economic advantage is

converters improve the speed of response, and if the frequency

proposed. It consists of 12 bidirectional switches and an ac link

is outside the audible range, reduce acoustic noise [2]. High

composed of a low reactive rating inductor-capacitor pair. The

frequency link power conversion has been employed very

link is charged via high frequency current pulses from the

successfully in dc-dc converters [3]-[6]. This demonstrated the

inputs. The link so charged discharges into the outputs in a

advantages and also the difficulties in working with high

similar fashion. The current pulses are precisely modulated

frequency links. Problems were associated with circuit

such that when filtered, they achieve unity power factor at the

topologies and also device capabilities. With increase in

input while also meeting the reference output currents.

demand and with advancements in semiconductor technology,

Fig. 2: Block diagram to show how the input reference is derived

Filtering is done by low-cost, low-loss and light weight

capacitors. Inputs never directly connect to the outputs and

hence there is inherent isolation between the two, which avoids

any common mode voltage and enables grounding of both

input and output neutral points. Full I/O galvanic isolation may

be provided by a split winding version of the link inductor.

Symmetrical arrangement of the power circuit also provides

fully regenerative operation together with buck-boost

capability. Simulation results are presented to back up the

proposed topology. A 15 kW prototype is presently under

construction.

II. PRINCIPLE OF OPERATION

The proposed soft switching bidirectional ac link converter is

shown in Fig. 1. It consists of two full bridge circuits

composed of reverse blocking bidirectional switches, a link

composed of low reactive rating inductance and capacitance,

and the filter capacitors.

The converter operates by charging the link from the inputs

and then discharging the stored energy to the output. The Fig. 3: Typical waveforms illustrating the operating principles of the

converter is fed with the output current references. The link is proposed converter

charged to an amount which makes the discharging current

exactly meet these references. Since charging and discharging may also be achieved if desired.

take place separately, an estimate of how much the link needs

to be charged to supply the output correctly is required. The Modes of Operation:

controller handles this by translating the output references to Each link cycle is divided into 16 modes, with 8 power transfer

input references. The input reference is derived by the simple modes and 8 partial resonant modes taking place alternatively.

equation that Fig. 3 shows the important current and voltage waveforms over

one link cycle. For a 15 kW, 460 V converters the link

Input Power = Output Power + Losses. (1) oscillates at about 10 kHz. Power is transferred twice during

each link cycle. This is roughly at 20 kHz, thereby resulting in

Fig. 2 shows a block diagram of how the system works. superior control and lesser filtering requirements. Zero voltage

RMS value of the output reference current is used to determine turn-on and capacitance buffered turn-off enables operation at

the RMS of the input current for an ideal converter. A loss this frequency. Medium voltage converters employing this

component is added to this from the loss estimator to get the topology are expected to have a link frequency of about 2.5

exact input command. The instantaneous value of the output kHz.

reference commands could be phase shifted with respect to the There are three basic operations taking place through the 16

output voltages as the load demands. Normally, the modes: energizing, partial resonance, & de-energizing. Modes

instantaneous values of the input current commands are in 2, 4, 6, 8, 10, 12, 14 and 16 are the partial resonant modes and

phase or are phase adjusted with respect to the input voltages as evident from Fig. 3, they make up only a very small fraction

so as to achieve unity power factor, but non-unity power factor of the link cycle time. The link is energized from the inputs

during modes 1, 3, 9 and 11 and is de-energized to the outputs

during modes 5, 7, 13 and 15. The various operating modes are

explained below and their respective circuits are given in Fig. 4

and Fig. 5.

Mode 1 (Energizing): The link is connected to the input

voltage pair having the highest voltage via switches which

charge it in the positive direction. For the waveforms shown in

Fig. 3, the link is connected to input phase pair BC through

switches S3i and S2i. The link charges till Ibi averaged over

cycle time, meets its reference value calculated from (1). The

switches are then turned off.

Mode 2 (Partial resonance): The link capacitance acts as a

buffer across the switches during turn off. This results in low

turn off losses. All switches remain turned off and the link

resonates till its voltage becomes equal to that of the input

phase pair having the second highest voltage. This is the phase

pair the link charges next from. In the example shown in Fig. 3,

the link resonates till the link voltage becomes equal to Vaci.

Mode 3 (Energizing): Switches are turned on to allow the

link to continue charging in the positive direction from the

input phase pair having the second highest voltage. At the end

of mode 2, the link voltage equals the voltage of this phase

pair. Hence at the instant of turn on, the voltage across the

corresponding switches is zero. This implies that the turn on

occurs at zero voltage as the switches transition from reverse to

forward bias. In the example in Fig. 3, the link charges till Ici

averaged over cycle time, meets its reference value calculated

from (1). The switches are then turned off.

Mode 4 (Partial resonance): During this mode the link is

allowed to swing to one of the output line voltages. The sum of

the output reference currents at any instant is zero. One of them

is the highest in magnitude and of one polarity while the two

lower ones are of the other polarity. The converter uses this

simple property to avoid any resonant swing back in the link.

The charged link transfers power to the output by discharging

into two output phase pairs. The two phase pairs are the one

formed by the phase having the highest reference current and

the second highest reference current, and the one formed by the

phase having the highest reference current and the lowest

reference current, where the references are sorted as highest,

second highest and lowest in terms of magnitude alone. For

example, if Iao=10 A, Ibo=-7 A and Ico=-3 are the three output

reference currents then phase pairs AB and AC are chosen to

transfer power to the output. If Vab_o and Vac_o are the

instantaneous voltages across these phases and Vlink is the link

voltage, the phase pair whose voltage has minimum difference

with respect to Vlink is chosen as the first one to discharge to.

For example if Vlink=500 V, Vab_o=400 V and Vac_o=300 V, AB

is chosen as the first phase pair to discharge to.

Mode 5 (De-energizing): The output switches are turned on

at zero voltage to allow the link to discharge to the chosen

phase pair till the output current averaged over the cycle equals Fig. 4: Operating modes 1 to 8

the reference value of the lower phase.

Mode 6 (Partial resonance): All switches are turned off and

the link is allowed to swing to the voltage of the other output

phase pair chosen during Mode 4. For the example discussed

before, the link voltage swings from Vaco to Vbco. This is also

illustrated in Fig. 3.

Mode 7 (De-energizing): During mode 7, the link discharges

to the selected output phase pair till there is just sufficient

energy left in the link for it to swing to the input phase pair

having the highest voltage. When the losses are determined

accurately, this would mean that the output references are

accurately met. Any deviation from this is detected and the

losses re-estimated to eliminate this error.

Mode 8 (Partial resonance): The link swings to the input

phase pair having the highest voltage to be ready to charge the

link in the reverse direction.

Modes 9 through 16 are similar to modes 1 through 8, except

that the link charges and discharges in the reverse direction.

For this, the complimentary switch in each leg is switched

when compared to the ones switched during modes 1 through

8. This is seen comparing Fig. 4 and Fig. 5.

It is observed that the input is never directly connected to the

output resulting in proper isolation between the two. Fully

galvanic isolation can be achieved by using an isolation

transformer in place of the link inductor. It can also be

observed that the converter can operate without that link

capacitor. However, since the topology is tolerant to such

capacitance, a low cost, light weight, and efficient link inductor

with high parasitic capacitance can be used. The inductor being

used in a 15 kW three phase prototype weighs less than 5 Kg,

with less than 3 Kg for the input line reactance, as compared to

over 70 Kg for the input and output filters alone for a 15 kW

VS-PWM drive required to produce comparable low harmonics

on the input and output. Additional capacitance may be

advantageously added to buffer turn-off losses, with the

optimal link capacitance determined by balancing reduced

turn-off losses against the resulting slight decrease in power

throughput.

It must be noted as important that the current pulses are

precisely modulated so that they are sinusoidal when filtered

via the LC filters. Even slightly improper modulation triggers

off ringing in the LC filters which should be avoided. One

situation where there is ringing in the LC filters in spite of

proper modulation of the current pulses is when the voltage of

two phases cross each other. This situation is illustrated in Fig.

6. At the crossovers, the order of charging from the input

phases reverses. The result is that the current pulse from the

increasing phase is advanced whereas that from the decreasing

phase is delayed. This causes the respective phase voltages to

deviate from being sinusoidal and this triggers off small

ringing in the filters. To avoid this problem, the controller uses

a predictive algorithm to detect crossovers and draws a small

Fig. 5: Operating modes 9 to 16

Fig. 7: Input Currents

Fig. 8: Link current and voltage

Fig. 6: Required correction at voltage crossovers

Fig. 9: Input and output voltages phase shifted to demonstrate isolation

between input and output

percentage of extra charge from the decreasing phase. The

correction and its effect are illustrated in Fig. 6.

III. SIMULATION RESULTS

Simulations were carried out for a 15 kW converter with

unity power factor load. Both input and output were at three

phase at 460 V. The link capacitance was 0.2 F and the link

inductance was 140 H. Input current ripple with a 1500 Hz

filter as shown in Fig. 7 is so small as to be almost

imperceptible. The link voltage and current waveforms are

shown in Fig. 8. Fig. 9 demonstrates the ability of the converter

to operate with differing input and output common mode

levels. Input and output voltages are phase shifted by about 50o

Fig. 10: Circuit board and link inductor of prototype under construction

in Fig. 9. Fig. 10 shows the circuit board and the link inductor

of the 15 kW prototype under construction. Fig. 11 shows the

schematic used for simulation.

Fig. 11: Schematic used for simulation

Specialists, pp. 20-26.

IV. CONCLUSION

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