Control Strategy for a Buck-boost Type Direct
Interface Converter Using an Indirect Matrix Converterwith an Active Snubber
Koji Kato

Jun-ichi Itoh

Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City Niigata, Japan
katok@stn.nagaokaut.ac.jp

Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper proposes a novel control method for aninterface converter that is using an indirect matrix converter(IMC) with a DC power supply. The proposed converter isconstructed based on an indirect matrix converter and aconverter that combined the function of a boost type converter
and a step-down type converter. The proposed converter
connects an active snubber to the DC link part of IMC alongwith a boost type converter. The voltage transfer ratio from the
AC power sources to the load can be improved by this active
snubber. In addition, the proposed control method able tocontrol the power distribution ratio and the output voltage at asame time. The basic operation of the proposed method has
confirmed by simulation and experimental results.

I.

INTRODUCTION

Recently, renewable energies and hybrid electric vehicle(HEV) systems are receiving significant interest, withconsiderations of global warming and environmental
problems. There are two types of power sources for renewableenergies; AC power sources such as a wind turbine and DC
power sources such as batteries, and fuel cells. In thesesystems, interface power converters have been intensely
studied. A conventional power converter system, which
consists of a pulse width modulation (PWM) rectifier, a
DC/DC converter and an inverter, requires an electrolytic
capacitor. However, this electrolytic capacitor in theconventional system is facing with several subjects, such assize, life time and costs. Particularly this large electrolytic
capacitor has a problem in HEV applications because a power
converter is installed at a high temperature area. Therefore, it
is difficult to use electrolytic capacitors for HEV applicationsin terms of the lifetime that is affecting by the hightemperature.
On the other hand, there is an AC/AC direct converter
without a large energy buffer such as a matrix converter,
which uses nine bidirectional switches, and an IMC, which
consists of a current source rectifier and a voltage source
inverter without a capacitor in the DC link part[1]-[8]. The
utilization of these AC/AC direct converter in renewableenergy systems can achieve the following advantages;
downsizing, long-life cycle and low costs.
There are some studies about how to interface the matrix
converter with a DC power supply as mentioned in [9]-[10]. A
voltage source inverter is connected to the input or output sideof the matrix converter. In this system, the matrix converter
operates in parallel with the inverter. However, the drawback
is size will be extended because of constituting many
components. The inverter is used to interconnect with a DC
power source because the matrix converter has no DC
component. However, this turns out to be a high cost structure
for the matrix converter.
On the other hands, there are two types of interface
converters using an IMC have been proposed, which areclassified as the boost type converter[11] and step-down typeconverter[12]. Since the IMC contains of a DC link part, it iseasy to interconnect with a DC power supply from thatconnection. The boost type converter connects a DC/DC
converter using a chopper to the DC link part of the IMC. The
control strategy of the voltage source converter, which
consists of the inverter and the added DC/DC converter, isimplemented as a four-phase voltage source inverter. With
regard to the voltage relationship between the DC power
supply voltage and the DC link voltage, the proposed circuitprovides a boost-up operation for the DC power supply.
However, the proposed boost type converter does not improvethe voltage transfer ratio of the IMC. That is, the voltage
transfer ratio of IMCs, which defines the ratio between the
output voltage and the input voltage, is well known as being
constrained to 0.866.
The step-down type converter interfaces the DC power
supply using an active snubber circuit in the IMC. The active
snubber circuit is composed by a capacitor connected to aswitching device in series. This snubber circuit pairs with aswitching device to work as a step-down chopper for the DC
power supply. In addition, the voltage transfer ratio can be
improved by using the DC power supply because the DC
power supply should be higher than the DC link voltage toavoid short circuit. However, when the battery is used for the
DC power supply in this system, a lot of batteries have to beconnected in series. In this case, the battery volumes and costs
will increase.
This paper proposes a converter which is combined by aboost type converter and a step-down type converter. The
proposed converter connects an active snubber to the DC linkpart in the boost type converter. The voltage transfer ratio
from the AC power source to the load can be improved by theactive snubber. Besides, the high voltage battery is not
required because the DC voltage source is boosted by the
boost type DC/DC converter. That is, the proposed systemcovers the disadvantage of the boost type converter and stepdown type converter from one to each other. The PWM
generator is implemented based on the space vector
modulation (SVM). The proposed control method controls the
DC/DC converters and the inverter independently. The
fundamental operation of the proposed method has confirmedby experimental results and loss analysis results.
II.

Wind power
Engine generator
etc.

Energy buffer
PWM
RectifierG

Photovoltaics
Fuel cell
Batteryetc.

Inverter

LoadorPower line

DC/DC
converter

Fig. 1. Block diagram of the conventional AC and DC power supply
interface system.

PROPOSED SYSTEM CONFIGURATION

A. Back to Back converter system
Figure 1 shows the AC and DC power supply interface
systems using the back to back converter. A conventional
interface system consists of a PWM rectifier, a DC/DC
converter and an inverter, as shown in Fig. 1. This system
requires a large electrolytic capacitor in the DC link part inorder to smooth the DC link voltage. This system is very
flexible in term of voltage condition among the input andoutput side because of using voltage type converters in bothsides. However, the electrolytic capacitor in DC link part gives
problem such as large volume, short lifetime in hightemperature ambience and high cost. Particularly, the large
electrolytic capacitor has a problem for the HEV application
because the power converter is installed at a high temperaturearea. Therefore, the electrolytic capacitor is not suitable for the
HEV applications.

Figure 2(b) shows the interconnection system, which has
been proposed in Ref. [10]. This system is a kind of deltaconfiguration system which is connected by the rectifier, theinverter and the matrix converter. Furthermore, the power canbe converted directly among the three sources; the generator,
the battery and the motor. Therefore, the efficiency of this
proposed system becomes higher than the BTB system.
Moreover the proposed system does not require theinterconnection transformers or reactors at the connection
point between the matrix converter and the inverter because(b) System configuration of Ref. [10].
Fig. 2. Block diagram of the AC and DC power supply interface systemusing matrix converter.
Indirect matrix converter
Engine generator
Wind power
etc.

G

Energy flow
Input filter

B. Matrix converter system
Figure 2 shows a AC and DC power supply interface
system using the matrix converter[9] [10]. In Ref.[9], the inverter
is connected to the input side of the matrix converter. The
characteristic of this system is that an input filter can be
replaced by the inverter. In addition, the voltage transfer ratioof the matrix converter is improved by using the inverter.
However, the efficiency between the battery and the load is
not high because this system requires twice power conversion
between a battery and load.

(a) System configuration of Ref. [9].

No energy
bufferCurrent
sourcerectifiter

Photovoltaics
Fuel cell
Batteryetc.

VoltagesourceinverterDC/DCconverter

Fig. 3. Block diagram of the proposed system.

LoadorPower lineInput filter

the operation of these two converters share the same time in
one carrier cycle. However, it seems that the Ref. [9]-[10]system requires many components due to the insertion of theinverter and rectifier.
C. Proposed system
Figure 3 shows the block diagram of the proposed directinterface converters for the energy management system. The
proposed interface converter is constructed based on an
indirect matrix converter without the electrolytic capacitor as alarge energy buffer. The DC/DC converter connects to the DC
link of the indirect matrix converter. The proposed system canachieve higher efficiency than the conventional BTB system.
In addition, the cost structure for the indirect control is lower
than the matrix converter system because the number ofcomponents for the proposed system is less than the matrix
converter system. Therefore, in this proposed circuit, since thesize is smaller, the cost is lower as well and yet long life time.

Input filter

III.

Fig. 4. Boost type AC/DC/AC direct converter.

CIRCUIT TOPOLOGY USING IMCA.

Boost type configureation
Figure 4 shows a boost type interface converter proposedin ref. [11]. A chopper is connected to the DC link part in the
IMC. In this system, the DC link voltage becomes higher thanthe DC power supply. Therefore, it is referred to as a "boost
type AC/DC/AC direct converter" in this paper. The chopper
leg is controlled as a fourth leg of the voltage source inverter.
Therefore, the control is implemented in a way that thevoltage command of the chopper compares with the inverter
carrier to obtain the desired PWM pattern.
However, the proposed converter has a problem which isthe voltage transfer ratio between the power grid to the motor
is constrained to 0.866 because the current source rectifier
does not have a boost function. As a consequence, the output
current of the IMC is higher than the BTB converter under thesame output power rating. As a result, the motor loss and theconverter loss are increased. Besides, the low voltage transfer
ratio limits the applications of an IMC.
B. Step-down type AC/DC/AC direct converter.
Figure 5 shows a step-down type interface converter
proposed in ref. [12]. A DC/DC converter is constructed withan active snubber circuit in the IMC. An IGBT is connectedanti-parallel to the snubber circuit diode. This snubber circuitwith the IGBT is used as a step-down chopper of the DC
power supply. In this case, the DC power supply voltage ofthe snubber circuit should be higher than the peak of the AC
input line voltage, because a rush current occurs between the
AC and the DC input power supplies when the peak of the AC
input line voltage is higher than the DC power supply voltage.
Therefore, this converter is referred to as a “step-down type
AC/DC/AC direct converter”. The DC/DC converter iscontrolled as the fourth leg of the rectifier side converter.
Time sharing is applied to the control of the snubber circuit.

Figure 6 shows the relationship between the DC link
voltage and the active snubber duty ratio for the step-down
type converter. For example, when the Db becomes zero, thestep-down type converter operates as a conventional IMC. The

Fig. 5. Step down type AC/DC/AC direct converter.
Vc
Vin
Edc

1
0.866

controllable range

0

0.2

0.4

0.6

0.8

1

Active snubber duty ratio : Db

Fig. 6. Control range for each converter.

output voltage is limited to 0.866vin. On the other hand thestep-down type converter operates as a conventional inverter
when the Db reaches “1”. That is, the proposed circuit operates
as a conventional IMC or a conventional inverter alternately ina sequence to interface the AC and the DC power supply.
Therefore the DC link voltage is decided by the average valueof the output voltage between the rectifier stage and theDC/DC converter. The average value of the DC link voltage
Edc in the proposed circuit can be expressed as,*

Edc = Drec vin + D*vb

(1),

*

where Drec is the rectifier stage converter duty ratio, and Db*isthe DC/DC converter duty ratio.
In the step-down type converter, DC power supply voltage
becomes higher than the DC link voltage. Therefore, a lot ofbatteries need to be connected in series to provide high voltage

In Fig. 7, the proposed converter combines both the boost
type AC/DC/AC direct converter and the step-down type
AC/DC/AC direct converter into one converter. In other words,
the proposed buck-boost type AC/DC/AC direct converter iscomposed by a boost type AC/DC/AC direct converter with anactive snubber circuit. The proposed converter can operate as
the boost type AC/DC/AC direct converter and the step-down
type AC/DC/AC direct converter at a same time. The voltage
transfer ratio in this proposed circuit is improved by using theactive snubber circuit.

Fig. 7. Proposed circuit.

Input filter

C. Buck-boost type AC/DC/AC direct converter.
Figure 7 shows the proposed circuit configuration. As
previously discussed, the voltage transfer ratio of the boost
type AC/DC/AC direct converter is constrained to 0.866, andfor the step-down type AC/DC/AC direct converter, the DC
power supply requires a lot of batteries to obtain high DC
voltage.

Input filter

to DC power supply. In this case, the volume of the battery
will increase.

Figure 8 shows the equivalent circuits of the proposed
converter. The rectifier stage converter is similar to a fourphase current source rectifier including the active snubber.
Therefore, the switches in the three-phase PWM rectifier andactive snubber must be separately turned on in order to avoid ashort circuit between the AC power supplies and the capacitor.
When the DC/DC converter switch Sc is turned off, theproposed converter operates as a boost type AC/DC/AC directconverter. In this case, the DC/DC converter becomes aconventional snubber circuit, as shown in Fig. 8(a). On theother hand, the proposed converter operates as a conventional
inverter with a boost chopper when the active snubber switch
Sc is turned on as shown in Fig. 8(b). Note that all the switches
in the rectifier will be turned off in this mode.
That is, the proposed circuit alternately operates as a boost
type AC/DC/AC direct converter or an inverter with the boost
chopper under a simultaneous control. The proposed converter
can achieve a wide control range by controlling the active
snubber circuit.

(a) Sc is turn off.

(b) Sc is turn on.
Fig. 8. Equivalent circuit of the proposed circuit.

Fig. 9. Control block diagram of proposed converter.

IV.

CONTROL STRATEGY

Figure 9 shows the control block diagram of the proposed
converter. The proposed control method of the inverter stage
is based on a space vector modulation. Therefore it is difficult
to be added a DC/DC converter, because the space vector
modulation is expressed by three dimensions[13].
Figure 10 following items in half control period; switchingmode for each of the converter, and the carrier for inverter and
DC/DC converter. The PWM pulse of the inverter and DC/DC
converter carrier are obtained by comparison carriers andvoltage command, which is calculated by the SVM. In the
proposed method, the DC/DC converter applied the carriercomparison method using the carrier as shown in fig.10. The
zero voltage vector of the inverter and the DC/DC converter
lower arm switch sbn have to be synchronized with the rectifierswitching timing. Therefore, the inverter carrier is inverted
when the active snubber switch is turned on. The ZCS of therectifier stage is achieved at all time. That is, the proposed
control method can control DC/DC converter independently.

Fig. 10. Relation between inverter carrier, rectifier pulse and
DC/DC converter pulse.

Figure 11 shows the relationship between an output
voltage command and a power distribution ratio for the
proposed converter. The proposed converter has two types ofoperation modes. The operation mode is changed by theoutput voltage command vout* as follows.
A)

vout*≥0.866vin : boost type operation modeB)

vout*<0.866vin : buck-boost type operation mode

When the output voltage command is lower than 0.866 of
the input voltage vin, the proposed circuit operates as the boost
type AC/DC/AC direct converter. In this case, the active
snubber circuit is the same as a conventional snubber circuit.
In other words, an active snubber duty command Dc* is set to‘0’. In addition, the DC/DC converter uses an ACR(Automated current regulator) to regulate battery current. The
power distribution ratio is decided by the DC/DC converter
current command.
On the other hand, the proposed converter operates as abuck-boost type AC/DC/AC direct converter when the output
voltage command is higher than 0.866 of the input voltage vin.
In the proposed method, the power distribution ratio is decided
by the active snubber duty ratio and the inverter stage output
voltage vout*. In order to control the output voltage and thepower distribution ratio at the same time, the output voltage
command and the active snubber duty command is describedas follows.
In the buck-boost type operation, the power distribution
ratio is controlled by the operation time division of each
operation mode. The output voltage is obtained by the average
output voltage of inverter stage for the boost type mode andstep-down type mode. Thus, when the active snubber duty isdefined as Dc, the output voltage is obtained by (2).
Ec*
* E
+ λbt (1 − Dc ) rec2
2**= Db vsdt _ out + (1 − Dc )vbt _ out*

vout = λsdt Dc

Fig. 11. Relation between output voltage command and power
distribution ratio.

type operation, vsdt_out is the inverter stage output voltage ofstep-down type operation, λsdt is the modulation index of theboost type operation, λbt is the modulation index of the stepdown type operation, vc is the capacitor voltage of active
snubber, Erec is the output voltage of rectifier stage and thesubscript ‘*’ represents the command.
The power distribution ratio is calculated by multiplying
the output voltage of each operation and the active snubber
duty ratio Dc*. Then, the power distribution ratio is expressedby (3) when the instantaneous power is assumed to constantduring one carrier cycle.
*

*

*

*

where, vout is the inverter stage output voltage of the proposed
circuit, vbt_out is the inverter stage output voltage of the boost(3),

where, Pstd is the output power of the boost type operation and
Pbt is the output power of the boost type operation.
The operation time ratio of the active snubber circuit Dc
can be determined by (4).

*

(2),

*

Psdt : Pbt = Dc v sdt _ out : (1 − Dc )vbt _ out

*

*

Dc =

vbt _ out Psdt
*

*

*
*

v sdt _ out Pbt + vbt _ out Psdt

*

(4).

Thus, the output voltage of the proposed converter isconstrained by the power distribution ratio. In order to control
the output voltage and the power distribution ratio at the sametime, the output voltage command vout* and the active snubber
duty command Dc* of the active snubber circuit is formed in(5) from (2) and (4).
*vout Psdt*

v sdt _ out =

*
*

*

*

Psdt + Pbt (1 −

vout
)
*
vbt _ out

Table 1. Experimental parameters.
Input voltage
200[V]
LC filter
Input frequency
50[Hz]
Cut-off
Carrier frequency
7.5[kHz]frequency
Output frequency
40[Hz]
load
Commutation
DC power supply
100[V]
time

(5).

*

vout Psdt

λsdt =****

Psdt + Pbt (1 −

vout
)
*
vbt _ out

2
vc

1[kHz]
R-L
2.5 [μs]

Input voltage (phase)

200 [V/div]

Input current

10 [A/div]

Output current

10 [A/div]

DC current

5 [A/div]0

The modulation index of the inverter can be determined by(6) from (5).
*2 [mH]
6.6 [μF]

0(6).

In addition, an active snubber capacitor voltage vc is
controlled by AVR (Automated voltage regulator). It is notedthat the modulation index λbt is fixed at ‘1’ to output a
maximum voltage. Therefore, the power distribution ratio andthe output voltage can be controlled at the same time. The
relationship between the output voltage and the active snubber
duty ratio is shown in Fig. 11.

00

10 [ms/div](a) Boost type AC/DC/AC direct converter.

V. EXPERIMENTAL RESULTS
Table 1 provides the parameters and conditions for thesimulation and the experimental. The operation of the
proposed circuit is demonstrated by the simulation andexperimental results.
First, the basic operation waveforms of the previous
proposed converters are shown in Figure 12. Fig. 12(a)demonstrates the waveforms of the boost type AC/DC/AC
direct converter; and Fig. 12(b) shows the waveforms for the
step-down type AC/DC/AC direct converter. The input current
and the output current show good sinusoidal waveforms forboth the operation modes, and the DC current represents thebattery current. As a result, in Fig. 12(a), it is confirmed thatthe total harmonic distortion (THD) of the input current, theoutput current and DC output current is 2.4%, 1.9% and 1.9%
respectively. The input power factor is 99% and the efficiency
is 96.3%. On the other side, the step down type converter
shows the THD of the input current, the output current and theDC output current THD is 2.4%, 1.9% and 1.9%, the input
power factor is 99% and the efficiency is 95.2% as shown in
Fig. 12(b).
Figure 13 shows the waveform of the proposed control
strategy. The dot line in the middle shows the change of theoperation. At first, the proposed converter operates as theboost type AC/DC/AC direct converter. After that, theproposed converter operates as the buck-boost type
AC/DC/AC direct converter. Sinusoidal waveforms without
distortion are obtained for the input current and the output
voltage. In Fig. (13), there is no sag occurs in the voltage andcurrent when the operation mode is attempting a change. The
output voltage of the boost type operation is 161 V. On theother hands the output voltage of the buck-boost type(b) Step down type AC/DC/AC direct converter.
Fig. 12. Experimental waveform

Fig. 13. Experimental waveform of proposed circuit.

Figure 14 shows the THD of the input current, the output
current and the DC input current which is 1.4%, 1.8% and2.3%, respectively. In addition, the dead time error
compensation is already applied in the FPGA control asreported in Ref. [11]. Therefore the proposed converter canobtain a low THD values for these current. It is noted that theDC current THD is defined by (7).
I dc _ THD =I dc _ H
I dc

(7),

Input, output and DC current THD [%]operation is 180 V. The output voltage range is confirmed has
been improved in the proposed circuit.

where, Idc is the RMS value in the DC current and Idc_H is theRMS value in the DC current harmonics.
Figure 15 shows the efficiency and the input power factorof the proposed circuit under two circumstances. In Fig.15(a),all the switches in the chopper are turned off, battery voltage
is neglected in this condition. That is, the converter consists of
the indirect matrix converter and the active snubber only. The
obtained data shows that the input power factor is 99% andefficiency is 95.4%. It is noted that the efficiency of aconventional multi-power supply interface converter with alarge electrolytic capacitor is approximately 93%. Fig.15(b)shows the second circumstances where the inverter side istaken off from the converter by switching off all the IGBTs in
the inverter. Then, the DC/DC converter is operating in boost
type mode. The data shows the input power factor is over 99%
and the maximum efficiency is 93.7%.
In this result, the efficiency drops due to the increment of
the conduction loss in the DC/DC converter. The conduction
loss becomes larger because of the output voltage of the
DC/DC converter is lower than the inverter output voltage,
which is set to 100 V. For that reason, the current in the
DC/DC converter will force to increase. In addition, theefficiency of the indirect matrix converter can be improved by
the applying the reverse blocking IGBTs into the rectifier
stage.
Figure 16 shows the relationship between the efficiency
and the power distribution ratio. In this result, the total output
power is about 1.2kW, the DC/DC converter operates incharge mode and the inverter operates in motoring mode. The
maximum efficiency is 95.4 % when the power distribution
ratio is “0”, that is the generator is 100% supplying the input
power. On the other hand, when the power distribution ratio is“1”, where the battery is providing 100% of the input power,
the efficiency is 93.7%. In this experimental condition, thetotal efficiency of the proposed circuit becomes higher when
the power distribution ratio is lower.
Figure 17 shows the loss analysis comparison between the
proposed circuit and the BTB system that is simulated from acircuit simulator (PSIM, Powersim Technologis Inc) and DLL
file (Dynamic Link Library)[12]. In this result, the power
distribution ratio is “0.5”. The switching losses do not occur inthe rectifier stage of the proposed system because of applying
the zero current switching. The proposed system decreases theswitching loss in the rectifier stage by about 2/3 in comparedwith the rectifier stage of the BTB system. In the view of

Fig.14. THD of input, output and DC current.

(a) DC/DC converter leg is stopped.
P.F.

100
98
96

Efficiency
94
92
90
88
0

500

1000
Output Power[W]1500(b) Inverter leg is stopped.
Fig.15. Efficiency and input power factor.

Fig.16. relation between efficiency and output power ratio.

inverter and DC/DC converter, the losses are almost the sameat both systems since the output power of the inverter and theDC/DC converter is about the same. From the comparison, theproposed converter can obtain higher efficiency in comparedto the BTB system.
VI. CONCLUSIONSThis paper proposes the novel control method and theconverter which is combined by the boost type converter andstep-down type converter. The proposed converter connects anactive snubber to the DC link of the boost type converter. The
voltage transfer ratio of the boost type converter can be
improved by this active snubber. In addition, proposed control
method is based on the space vector modulation (SVM). The
proposed control method can control DC/DC converter andthe inverter independently. The validity of the proposed
strategy was confirmed by both the simulation andexperimental results. As a result, it is confirmed that the AC
input, output current, and the DC output current THD are
1.4%, 1.8%, 2.3%, respectively, and the input power factor isover 99% and the maximum efficiency is 95.4%.The basic
operation of the proposed method has confirmed byexperimental results and loss analysis results.

Fig.17. relation between efficiency and output power ratio.

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