Loss analysis of a Five-level Active NPC that uses a Phase-shift ControlMethod
Yugo kashihara *Jun-ichi Itoh**Nagaoka University of Technology
Abstract:- This paper establishes a loss analysis method for an active neutral-point-clamped (ANPC) multilevel inverter. The
ANPC consists of a neutral point clump circuit and flying capacitor topology. This circuit can reduce the number of components
in comparison to conventional multilevel converters. Then, the loss analysis using mathematical expressions are obtained. Thisanalysis results are well agreed with the loss simulation results. Operation of the circuit is first confirmed by experiment.
Keywords : Inverter, multilevel, Active Neutral-Point-Clamped, Loss analysis

1.

based on the study of the device condition. The losses of aconverter are such as, conduction loss, switching loss, recovery
loss. The validity of the proposed calculation method of power
loss is confirmed with the simulation and experimental results.

Introduction

Applications of multilevel converters are actively researchedrecently [1-8]. Comparing multilevel converters to theconventional 2-level converters, multilevel converters show better
advantages. A multilevel converter can reduce the voltage stress ofa switching device to 1/(n-1) of the DC input voltage and also
reduce the harmonic component of the output voltage. As a result,multilevel converters can use switching devices, which are highspeed switching and low voltage rating. Therefore multilevel
converters are possible to obtain higher efficiency than theconventional converters.
In general, the multilevel converters are applied to the medium
voltage application such as the power converter for large power
motor drive and also for power transmission line. Recently, lowvoltage applications also has been studied to use multilevel
converter for high efficiency such as uninterrupted power supply(UPS) and power converter for photo voltaic cell (PV).
However, selection criteria of switching devices are necessary
to get higher efficiency than the two-level inverter in low voltage
applications. Loss analysis by using simulator is a simple method
to study the losses among the multilevel converter topologies
under a same device specification.
Figure 1 shows two conventional multilevel topologies; theneutral point clamped (NPC) type and the flying capacitor (FC)type [9-10]. NPC type outputs the voltage level from the neutral
point voltage clamped by using diodes. However the number ofswitching devices increases in proportional to the voltage level.
FC type outputs the voltage level from the DC link voltage byadding flying capacitor voltage. However, FC type needs morecapacitors as the voltage level increased.
The active neutral point clamped (ANPC) which is one of themultilevel topology has been proposed [11]. The ANPC type is a
new topology that combines the NPC and FC type into one
converter. Compared to the conventional NPC and FC typeconverters, the ANPC type can reduce the number of devices.
Therefore, ANPC type is lower cost and higher efficiency than theconventional NPC and FC type.
This paper establishes mathematical expressions to analyze thepower loss of a five-level ANPC inverter. The point of themathematical expression is to observe the losses of a converter© 200● The Institute of Electrical Engineers of Japan.

2.

Five-Level Converter Topology

Figure 2 shows the five-level ANPC inverter, which isconstructed by eight switches and three capacitors. Operation ofthe ANPC inverter can be studied by dividing the ANPC circuit
into two forms.
Figure 3 shows the equivalent circuit of the five-level ANPC
inverter. This equivalent circuit shows the ANPC inverter can bedivided into a power circuit and a three-level FC circuit. The
power circuit outputs a two level voltage, which are +Edc/2 and –Edc /2 with reference to the neutral point. Then, the three-level FC
circuit outputs a three level voltage which, are + Edc /4 and – Edc/4 with reference to the input voltage. As a result, the ANPC
inverter outputs a five level voltage which, combine the output
voltage from the power circuit and the three-level FC type circuit.
Previously discussed the ANPC inverter can obtain high(a)Neutral Point Clamp(b) Flying Capacitor
(NPC)
(FC)
Fig 1. Conventional five-level inverter topology(Single leg)
1

TITLE

efficiency because of the switching loss is small. The reason isthat switching frequency of all switching devices in theconventional topology as show in Fig.1 is same to the carrier
frequency; on the one hand, the switching frequency of the Cell2
switching device in Fig.2 is same with the output frequency. Itshould be noted that the switching devices in Cell2 is required toapply with a voltage that is double of the Cell1 switching device.

Cell2
S5C2

Cell1
S63.

S1

S3Control StrategyaFigure 4 shows the gate signal waveform of the five-level ANPC
inverter. Gate signals of Cell1 switches are generated by phaseshift carrier-based PWM strategy. These gate signals are generated
by comparing to the output voltage command with two carriers
which the phase is reversed. The duty ratio command Dref for
Cell1 is given by;Dref = a sin x(0 < x < π )(1),D ref = a sin x + 1(π < x < 2π )

(2),

Edc

S2S4bC3Vout

S8

where a is the modulation index and x is the reference phaseangle.
When the polarity of output voltage command is positive, thegate signals of S5 and S7 are turned on. When the polarity ofvoltage command is negative, the gate signals of S6 and S8 areturned on. This control method can balance the flying capacitor
voltage automatically, by choosing the discharge and charge
modes according to a cycle of carrier frequency.

4.

C1S7Fig. 2 Circuit topology of a five-level ANPC inverter (single leg).

1
Edc
2

Mathematical Expressions of Power Loss

PLoss = PLoss1 + PLoss 2

1
Edc
4

Edc

This chapter explains the power loss expression of the five-level
ANPC inverters. The ANPC inverter is assumed in an ideal state.
The power loss of ANPC inverter is obtained under two ideal
conditions.
1) The load is current source.
2) Capacitors are DC voltage sources.
Then, the power loss of the ANPC inverter is given by (3).

1Edc
2(3),Fig.3 Equivalent circuit of the ANPC.

where PLoss is total loss and PLoss1 is Cell1 loss and PLoss2 is Cell2
loss. Furthermore, the power loss is consists of the switching loss
and the conduction loss, which are generated at turn on and off, by
the forward voltage drop of a switching device, respectively.
4.1 Power loss of Cell1
A. Conduction loss
The conduction loss can separate a switch side and a FWD side,which are depends on the forward voltage drop of a switching
device and a diode.
When a FET is used for a switching device in the Cell1, theconduction loss of switch side is calculated from the on-resistanceand the square value of effective current. That is, the conduction
loss is obtained from the current effective value of the Cell1. The
current effective value Irms in the Cell1 is given byI rms =()1 π
2
I peak sin (x − θ ) Dref dx
2π ∫0(4),where Ipeak is the peak phase current and θ is the phase factor andDref is the reference value. The Cell1 conduction loss of the switch
side is given by

Fig.4
Modulation scheme for the 5-level ANPC.

2TITLEfrequency. In addition, the Cell2 recovery loss PRec2 is obtained by

PON 1 = I rms Ron
2=I peak
2PRe c 2 =R on

(5),

5.

where Ron is the on-resistance of FET.
The conduction loss of the FWD side is calculated from thediode forward voltage characteristics and average current value.
The average current value Iave in the Cell1 is given by
I ave =

1 π
I peak sin (x − θ )Dref dx
2π ∫0

As a result, the conduction loss of the FWD side in the Cell1 isgiven by(7),where VF is the diode forward voltage and tD is the dead time andfc is the carrier frequency.
It should be noted that the conduction loss of the FWD side areoccurring during the dead time only due to the synchronize
switching between diode and MOSFET. Therefore, when thecurrent flows into the MOSFET instead of the FWD and therefore
the conduction becomes low.
B. Switching loss
The Cell1 switching loss occurs at the half cycle of carrier
frequency. We assume that the variation of the current and voltage
are proportion to the rise time and the fall time of the switching
device, then the Cell1 switching loss Psw1 is given byPSW 1 =(1VCell1 I peak f c t r + t f6π)

6.

(8),

1VCell1 I peak f c t rr
4π(9),

where trr is the reverse recovery time.
4.2 Power Loss of Cell2
A. Conduction LossThe gate signal of S5 is the same as that of S7. However, theconduction loss which occurs at S5 and S7 are different because thecurrent flowing through S5 and S7 is different. The S5 conduction
loss Pon2_A and S7 conduction loss is given as Pon2_B are given byPON 2 _ A =I peak2(cos θ + 1)R23π2

PON 2 _ B =Experimental results

Table 1 Simulation and experimental parameters.
(a) Circuit parameters

(10),

on

I peak  π 22
 − cos θ + 1  R on
2π  2 3(Loss analysis verification

Figure 6 shows the operation waveforms for the five-level
ANPC inverter. The output voltage is 141 V, 50 Hz, the output
power is 1 kW (rated power), and the flying capacitor voltage
command is set to 70V (the experimental parameter is the same as
Table 1). The output current is sinusoidal waveform without
distortion. In addition, the flying capacitor voltage agrees with thevoltage command, where approximately 70 V, is obtained. Further,a five-step voltage waveform is shown at the output voltage of theANPC inverter.
Figure 7 shows the efficiency of the five-level ANPC inverter. Themaximum efficiency is 97.6% at a 0.9 kW load. The Five-level
ANPC inverter can obtain efficiency of over 97% in a wide loadcondition. The efficiency of the experimental results is shown

where VCell1 is the Cell1 switch voltage and tr is the rise time and tf
is the fall time. The Cell1 recovery loss PRec1 is given byPRe c1 =)(b)MOSFET parameters for S1-S4(11).

Furthermore the Cell2 conduction loss of the FWD side is
obtained by
PFWD 2 =1
V F I peak t D f out
8π(12).
(c)MOSFET parameters for S5-S8B. Switching loss
The switching loss in the Cell2 is depending on the output
frequency. The switching loss Psw2 is leaded by
PSW 2 =()

1VCell 2 I peak f out t t sin (0 − θ ) + t f sin (π − θ )6π(14).

Figure 5 shows the loss analysis of the five-level ANPC inverter
based on the theoretical calculation and loss simulated results withsimulation parameters from Table 1. The power loss of simulated
results is composed by linking a circuit simulator (PSIM, Powersim Technologies Inc.) and a DLL (Dynamic Link Library) file.
The DLL file uses a loss table regarding the switching andconduction losses based on the instantaneous values of the current
and the voltage of the power device, as referred in [12]. Thismethod can estimate the power semiconductor loss, regardless ofthe circuit topologies [13]. The efficiency of the five-level ANPC
inverter is 99.1% at 1-kW load as shown in Fig.5. The error of thetheoretical value and simulation value is approximately 0.6% of
the total losses. In the five-level ANPC inverter, the conduction
loss almost dominant among other losses. The feature of thefive-level ANPC inverter is that only conduction loss and a small
amount of switching loss are occurred in the Cell2 switching
device and consequently high efficiency will be achieved.

(6).

PFWD1 = V F I ave t D f c1VCell 2 I peak f out t rr4π(13),where VCell2 is the Cell2 switch voltage and fout is the output
3TITLE

lower than the calculated results in chapter 5. One of the reasons isthat the conduction loss of the MOSFET and FWD is different
between the datasheet and the actual value. Also, the operation
temperature is not considered in theoretical analysis.

7.

Efficiency [%]
99.1

10

Turn-off loss
Cell1
Turn-on loss
Cell1

9Conclusion

8This paper established mathematical expressions to estimate thepower loss of a five-level ANPC inverter. The five-level ANPC
inverter loss is analyzed by the mathematical expressions and alsoin loss simulation. In analysis, the efficiency of the five-level
ANPC inverter is 99.1% at 1-kW load. The error of the theoreticalvalue and simulation value is 0.6% of total loss. The experimental
results confirmed the efficiency is 97.2% at 1-kW load.
In future study, analysis on the loss of experimental circuit will
be considered. After that, the optimizations of the five-level ANPC
inverter parameters will be discussed.

Power loss[W]7

Conduction loss
Cell26543

Conduction loss
Cell1210References

Theoreticalvalue

Simulation
valueFig.5 Loss analysis at 1kW load.

(2)

(3)

(4)
(5)

(6)
(7)

(8)

(9)

(10)

(11)

(12)

(13)

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Output Voltage[V] 200V/Div

Flying Capacitor Voltage[V] 50V/div

Output Current[A] 20A/div

10ms/divFig 6. Experimental result of the five-level ANPC inverter at 1kW.

Efficiency[%](1)

Fig.7 Efficiency of the ANPC inverter.

4