Downsized and High Efficient Direct Power
Converters for High-power Servo Application
Jun-ichi Itoh, Toshifumi Hinata*Nagaoka University of Technology, Nagaoka, Niigata, Japan,

e-mail: itoh@vos.nagaokaut.ac.jp
Abstract- This paper proposes to apply an AC/AC directconverter topology for servo systems. In the conventional AC/AC
converter which consists of a PWM rectifier and a PWM inverter,
the p-n junction temperature becomes high at the low output
frequency or the servo-lock operation because the output current
concentrates at the specific switching devices in the inverter. On
the other hands, the AC/AC direct converter such as matrix
converter is able to reduce the junction temperature of theswitching devices. In this paper, the junction temperature of eachtype of the converters is evaluated by using the thermal
simulation of the p-n junction. The output frequency of theconventional matrix converter and the indirect matrix converterare 1/8 and 1/2 smaller compared to the conventional AC/AC
converter when the same temperature ripple is obtained in low
output frequency region. That is, when these converters use thesame current rating device, the matrix converter can operate at afrequency lower than the conventional AC/AC converter.

INTRODUCTION(a) Conventional AC/AC converter.
(Back to Back converter)

Input filter

Recently, conventional AC/AC converters, such as the
Back-to-Back converter [1] which consists of a rectifier, an
inverter and a large electrolytic capacitor have been actively
used for high power servo amplifier applications. The
Back-to-Back system can convert an AC power supply voltage
into an AC voltage of variable amplitudes and frequencies withgood input current waveform.
Servo applications require a large torque at the low speed or
servo-lock operation. The requirement of starting torque may
be reached to 400% of rating torque. In this situation, theoutput current concentrates at the specific switching devices in
the inverter. The p-n junction temperature of the switching
device rises remarkably. Since the current rating of theswitching device is constrained by the junction temperature.
Consequently, the practical inverters limit the output frequency
or use larger current rating switching devices. The use of alarge current rating device increases the cost and the size of theconverter. Other than that, the electrolytic capacitor in the dc
link part of the inverter is also a high cost and bulky device.
Matrix converters that can directly convert an AC power
supply voltage into an AC voltage of variable amplitudes andfrequencies without large energy storage have been actively
investigated [3-11]. These matrix converters have advantagesof high efficiency, long lifetime and downsizing in comparison
to a conventional AC/AC converter. In addition, the matrix
converter can avoid rise of temperature of the switching
devices at the low output frequency operation. Therefore, thematrix converter is suitable for low frequency application suchas the high-power servo application.

(b) Conventional matrix converter.

Input filter

I.

There are two circuit topologies of the matrix converter. The
first one is so called 9-switches matrix converter orconventional matrix converter, which place 9 ac switches on
the grid and there is no dc link part in this topology. The
conventional matrix converter has no problem in terms of thecurrent concentration at the low frequency output because theload current flows in different switching devices according to
the input frequency.
The second one is so called indirect matrix converter whichconnects a current type rectifier to a voltage type inverter
without a dc capacitor. The indirect matrix converter is suitablefor parallel connection of the inverter parts in servo application
because the indirect matrix converter has the dc part. The(c) Indirect matrix converter.
Figure 1. Circuit topologies.

inverter stage able to reduce the switching loss by adopting thezero voltage switching for the inverter stage devices. Therefore,
the indirect matrix converter can also reduce the rise of
temperature in the inverter stage devices.
This paper evaluates the rise temperature at the p-n junctionof switching devices at the low output frequency operation
among the conventional AC/AC converter and the matrix
converters. At first, the conventional back to back converterand behavior of P-N junction temperature are introduced. Next,
the principles of the matrix converters and the control methods
are described. In particular, the control method for the indirect
matrix converter is proposed to reduce the junction temperatureof the inverter part. Finally, the simulation results regarding thejunction temperature are shown to indicate the validity of thedirect converter.
II. CONVENTIONAL AC/AC CONVERTER
Figure 1 shows the conventional AC/AC converter that is socalled the back to back (BTB) converter. This converter
consists of a voltage source rectifier and a voltage source
inverter. The output voltage and input current of each converter
is controlled by auto current regulators and an auto voltage
regulator as referred as in [2].
When the inverter outputs low frequency with large current,
the load current concentrates on a part of the switching device.
The p-n junction temperature of the switching device follows
this current and results a rapidly increment and the temperatureripple occurs according to the output frequency. The
temperature ripple in the semiconductor chip gives stress to thebonding wires, and solder layers in the power module.
Therefore, a large temperature ripple will affect the lifetimeand reliability of the component.
The output frequency or the maximum current at the low
frequency region is limited in order to suppress the temperatureripple. The current rating of the switching device is decided by
the low output frequency condition according to theapplications. Then, the large current rating device is chosen for
low speed with high torque application such as elevator or
servo system. That is, the current rating of the switching device
in the power converter is different depending on the minimum
output frequency even if the rated power at the rated speed isthe same. It is noted that the input frequency is a constantalong with the voltage supply frequency, thus the current does
not concentrate on specific device in the rectifier stage.
III. CONVENTIONAL MATRIX CONVERTER
Figure 1(b) shows the conventional matrix converter using 9
bi-directional switches. The bi-directional switch is constructedby a conventional IGBT with a series diode or a reverse
blocking IGBT [10]. The matrix converter generates output
voltage by chopping the ac input voltage directly. Besides, theinput current is controlled at the same time. That is, the matrix
converter has no input current harmonics components.
The conventional matrix converter has four main advantages
in comparison with the BTB converter;

1) Smaller size due to no large energy storage.
2) Longer life-time because of no electrolytic capacitor.
Then, the matrix converter can abbreviate the period of regular
maintenance.
3) High efficiency conversion because of only one switching
device passing through between the power grid and the load.
4) No load current concentration on the switching device.
When the power converter outputs dc current (servo-lockcondition), this output current flows into three switching
devices which is alternated by the input frequency. However,for the BTB converter, the output current flows into only one
switching device. Therefore the thermal duty of theconventional matrix converter is much lower than the inverterof a BTB converter. For this reason, the conventional matrix
converter does not use the large current rating device incomparison with the BTB converter because the peak of the
p-n junction temperature is small.
The input voltage for the conventional matrix converter
shown in figure 1(b) is [vr, vs, vt]t and the output voltage is [vu,vv, vw]t. The relations between the input voltage and the output
voltage can be expressed as⎡ vu ⎤ ⎡ sru ssu stu ⎤ ⎡vr ⎤⎢ v ⎥ = ⎢ s s s ⎥ ⎢v ⎥(1)
⎢ v ⎥ ⎢ rv sv tv ⎥ ⎢ s ⎥⎢⎣vw ⎥⎦ ⎢⎣ srw ssw stw ⎥⎦ ⎢⎣ vt ⎥⎦where the switching function s is defined as s=1 means theswitch S is turned on, s=0 means the switch S is turned off.
In this paper, the conventional matrix converter is controlled
by the virtual AC/DC/AC conversion [7]. This method
independently controls the input side and output side whileconsidering it is a a virtual rectifier and inverter system.
IV. INDIRECT MATRIX CONVERTER
Figure 1(c) shows a typical configuration of an indirect
matrix converter. This converter consists of a current typerectifier and a voltage type inverter. It should be noted that theindirect matrix converter does not use boost up reactors
because the rectifier stage uses the current type rectifier.
The difference to the BTB converter is that the indirect
matrix converter has no capacitor in the dc link part. Therefore,
the indirect matrix converter can achieve downsizing and highreliability as the same as the conventional matrix converter.
The advantage of this circuit over than the 9-swiches matrix
converter is to have an easy circuit structure. The indirect
matrix converter can use the conventional power IGBT module
for the inverter side including the gate drive units. Furthermore,
the indirect matrix converter is suitable for the applications that
require dc common connection and dc energy storage such asbattery or EDLC [8].
The output voltage controller affects the input current
controller although the indirect matrix converter is divided into
the inverter stage and the rectifier stage. The relations between
the input voltage and the output voltage can be expressed as⎡vr ⎤⎡ vu ⎤ ⎡ sup sun ⎤⎢ v ⎥ = ⎢ s s ⎥ ⎡ srp ssp stp ⎤ ⎢v ⎥(2)
⎢ v ⎥ ⎢ vp vn ⎥ ⎢ s s s ⎥ ⎢ s ⎥⎢⎣vw ⎥⎦ ⎢⎣ swp swn ⎥⎦ ⎣ rn sn tn ⎦ ⎢⎣ vt ⎥⎦where the switching function s is defined as s=1 means theswitch S is turned on, s=0 means the switch S is turned off.
The switching function is important to consider the influence
between the input and output stage. The output voltage ischanged by the switching function from the inverter stage but
also the rectifier stage.
In order to obtain high efficiency, zero current switching is
implemented in the indirect matrix converter. The rectifier partswitches during zero vector period of the inverter part. Since
the dc link current becomes zero during the zero vector, there
is no switching loss occur to the rectifier part. Therefore, theindirect matrix converter achieves higher efficiency than the
BTB converter [8].
However, when the inverter outputs low frequency withlarge current, the load current concentrates on a part of theswitching device because the converter consists of the inverter
stage that is same as the BTB converter. Therefore, we propose
a new control scheme for the indirect matrix converter to solve
the current concentration problem.
In the proposed control scheme, the switching of the inverter
part is performed while the rectifier stage output the zero
vector, where each of the rectifier phase arm (Srp, Srn) or (Ssp,
Ssn) or (Stp, Stn) are turned on. That is, zero voltage switching
is applied in the inverter stage. As a result, the rise of
temperature at the inverter stage can be decreased because noswitching loss occurs in the inverter stage. Additionally, highefficiency is also obtained in the indirect matrix converter. In
this paper, the control strategy for the indirect matrix converter
is referred in [9].
V. EXPERIMENTAL AND LOSS AND THERMAL ANALYSIS

A. Experimental result
Figure 2 shows the input current and the output current
waveforms of the indirect matrix converter using the proposed
method [9]. The input voltage and input current demonstrates
that unity factor is obtained by the proposed method. Further,good sinusoidal waveforms are obtained for the input and theoutput waveform. It should be noted that the output voltage isobserved by using a low-pass filter of 1 kHz cut-off frequency
to monitor the low frequency component distortion.
The proposed method shows the output characteristics
similar to the conventional AC/AC converter and theconventional matrix converter.
B. Loss analysis of the switching devices
The aim of this simulation is to confirm the decrement of theloss in the switching devices. The simulations aredemonstrated by PSIM (Powersim Technologies Inc) with theoriginal Digital Dynamic Library (DLL) as described in [11].
The parameters of the switching devices (Fuji Electric
2MBI50N-060) are referred to datasheets in [12]. The reverse
blocking IGBT is referred in [10].

10 [ms/div]
Input voltage
200[V/div] 0
Input current
10[A/div] 0
Output voltage(line to line) 0200[V/div]
Output current
10[A/div] 0
Figure 2. Steady state operation of IMC.
(Output frequency :40Hz)Ic
DLL file+-

Vce

Switchingloss
Conduction
loss

Ic
Vce
DLL file
Lossapproximately
equation

Turn-off
loss
calculation

Turn-onloss,
Recoveryloss
calculation

Conduction
loss
calculation

Figure 3. Principle of loss calculation method.
Table 1. Simulation parameters.
Simulation condition
Input voltage
Input frequency
Input power factor command
Carrier frequency
Output power
Output frequency
Commutation time

200 [Vrms]
50 [Hz]1
10 [kHz]
1 [kW]
50 [Hz]0 [us]

Figure 3 shows the configuration for the loss calculation
method. The instantaneous values of the current and thevoltage at the switching timing are captured to the DLL.
According to the collector-emitter voltage and corrector
current, the power loss of the switching device is obtained by
the V-I characteristic and the switching loss characteristic in
the datasheet. The conduction loss and the switching loss arecalculated in the DLL.
Figure 4 shows the loss analysis and the simulation
parameters are shown in Table 1. The BTB system obtains
efficiency of 96.3%, the conventional matrix converter obtains
efficiency of 98.5%, and the indirect matrix converter obtains
the efficiency of 96.9%. It should be note that the simulation

Switching loss
Conduction loss
20

15
10
5

0

Rectifier Inverter
Conventional

CMC(a) The inverter side.of BTB and IMC.

Rectifier Inverter
IMC

Thermal model of a module contain 2-IGBT(Sru)
IGBT chip

Figure 4. Power loss simulation results (Output power: 1kW).
II [W]
TjI [°C]

Rth(j-c)[°C/W]

Total loss [W]Ditto.
(Stw)

Ditto.
(Ssu)

Cth(j-c)[°C/J]
Tc[°C]

Tf[°C]
Power module case(b) Conventional matrix converter.
Figure 6. Thermal analysis model.

Total loss [W](a) Conventional AC/AC converter.

Cooling fin
Ta[°C]

Figure 7. Thermal analysis model.
(b) Indirect matrix converter.
Figure 5. Instantaneous loss of the IGBT.

did not include the loss of the boost up inductor, which isnecessary for the conventional AC/AC converter in a practicalstructure. Consequently, the efficiency of the conventional
AC/AC converter is lower than the shown result. In the matrix
converter, since only one switching device is connected
between the input terminal and the output terminal, the power
loss can be greatly reduced. In the indirect matrix converter,
the switching loss of the inverter part becomes zero due to thezero voltage switching. Therefore, a higher efficiency isachieved than the BTB converter.
Figure 5 shows the instantaneous loss of the IGBT in the
inverter side. The conduction loss and the switching loss in the
inverter are generated in the BTB converter as shown in Figure5(a). In contrast, there is only conduction loss at the inverter ofthe indirect matrix converter as shown in figure 5(b).

Table 2. Thermal resistance and capacitance.
12
3
40.00380.15100. 31810.0428Ri [℃/W]
0.26140.06620.314423.3499
Ci [J/℃]
0.08130.26190.85700.1578Rd [℃/W]
0.01230.03810.1167
6.3366
Cd [J/℃]

C. Thermal analysis of the switching devices
Figure 6(a) shows the thermal analysis model for the inverter
stage in both the BTB converter and the indirect matrix
converter. Figure 6(b) shows the thermal analysis model for theconventional matrix converter. In figure 6, Rth is the thermal
resistance and Cth is the thermal capacity. II and ID are theequivalent current source of the instantaneous loss of the IGBT
and the diode, respectively. In addition, the IGBTs and thediodes are packaged in one module for each phase (U, V, W),
respectively. Those modules are put on a cooling fin. The
ambient temperature Ta was set to 25 °C.

Fig. 7 shows the transient thermal resistance and the thermal
capacity from the p-n junction to the case, which are expressedby fourth order model using four resistors and four capacitors.
Thermal simulation of the p-n junction is implemented toevaluate the conventional matrix converter and the indirect
matrix converter. Using this model, the junction temperature of
the diode (TjD) and IGBT (TjI) are obtained.
Each parameter in the thermal model is obtained from of thetransient thermal resistance characteristic using least square
method [10].
Temperature of FWD chip [°C]70Temperature [°C]

70
60
50
40
30
20

Temperature of IGBT chip [°C]
Temperature [°C]

70
60
50
40
30
20

1

70
60

4 ⎧
−t ⎫
)⎬ (8)Rth ( j − c ) = ∑ ⎨ R n ⋅ (1 − expRn ⋅ C n ⎭n =1 ⎩
Table 2 shows the parameters of transient thermal resistanceand capacity from junction to case in experimental system. Ri
and Ci are the parameters of the IGBTs, Rd and Cd are theparameters of the diodes.
Figure 8 and figure 9 show the simulation results of thethermal analysis for each topology at the output power is 7kW.
When the output frequency is high, the peak temperature and

1.2

1.4
1.6
1.8
Time [s]0.2 [s/div](a) Conventional AC/AC converter.

Temperature of FWD chip [°C]

60
50
40
30
20
70
60

Temperature of IGBT chip [°C]

50
40
30
20
0

2

Temperature of IGBT chip [°C]

70
60

1

2
3
Time [s]1 [s/div](a) Conventional AC/AC converter.

4

Temperature of IGBT chip [°C]

50
40

50
40

30
20

30
20
1.2

1.4
1.6
1.8
Time [s]0.2 [s/div](b) Conventional matrix converter.

02

Temperature [°C]

Temperature [°C]

Temperature [°C]

Temperature [°C]

1

70
60

2
3
Time [s]1 [s/div](b) Conventional matrix converter.

4

Temperature of FWD chip [°C]

50
40
30
20
70
60

Temperature of IGBT chip [°C]

50
40
30
20
0

(c) Indirect matrix converter.
Figure 8. Junction temperature of the IGBT and diode.
(Input frequency: 50Hz, Output frequency: 120Hz)

1

1

2
3
4
Time [s]1 [s/div](c) Indirect matrix converter.
Figure 9. Junction temperature of the IGBT and diode.
(Input frequency: 50Hz, Output frequency: 1Hz)

VI. CONCLUSION
This paper discussed and evaluated the conventional matrix
converter and the indirect matrix converter by simulating the
p-n junction thermal of the switching devices under the low
output frequency operation. The simulation results are obtained
as follows;i) The conventional matrix converter can output a low
frequency that is 1/8 times smaller than the BTB
converter, if the same device is applied to the converter.
Similarly, the indirect matrix converter can output a low
frequency that is 1/2 times smaller than the BTB
converter.
ii) The matrix converters can use a smaller size device incomparison with the BTB converter, provided that thesetwo converters output the same frequency since thematrix converters can decrease the peak value of the p-njunction temperature.

40
Junction temperature rippre [℃p-p]

the variation temperature becomes low as shown in figure 8
because the output frequency is much higher than the cut off
frequency of the transient thermal resistance.
On the other hand, when the output frequency is low, thejunction temperature of the diode chip and the IGBT chip are
gradually rise in a pulsating according to the output frequency
because the output current concentrates at the specificswitching device for long time.
In the conventional matrix converter, the temperature
behavior is oscillated because the current flows into theswitching device is dominated not only the output frequency
1Hz but also the input frequency 50Hz.
The temperature ripple in the conventional matrix converter
decreases approximately 55 % of that of the BTB converter. In
the indirect matrix converter, the temperature ripple decreases
about 25 % of that of the BTB converter.
Figure 10 shows the relationship between the inverter output
frequency and the junction temperature ripple of the IGBT chip.
According to the increment of the output frequency, thetemperature ripple of the chip reduces because the thermaltransient response is slower than the output frequency. From
figure 10, the output frequency of the conventional matrix
converter and the indirect matrix converter is 1/8, and 1/2
smaller than the BTB converter when the same temperatureripple is obtained in low output frequency region. That is, theconventional matrix converter can output the low frequency
which is 1/8 times smaller than the BTB converter under thesame condition. The indirect matrix converter can output the
low frequency that is 1/2 times smaller than the BTB converter
under the same condition. In addition, the maximum
temperature of the matrix converters is lower than the BTB
converter. As a result, under the same output frequency for thesame application, the matrix converter can use a smaller IGBT
chip. These advantages become much pronounced in low speed
and high torque applications such as servo system.

Conventional AC/AC

35

Conventional MC

30

Indirect MC

25
20
15
10
5
0

1

10
100
Output frequency of Inverter [Hz]1000

Figure 10. The relationship between the Inverter output
frequency and the Junction temperature ripple.
(Output power: 7kW)

The simulation results are included to prove the validity ofthe matrix converter for the low speed and high torqueapplication such as the high-power servo application.
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