A Novel DC Link Soft-Switched Inverter with a Coupled Inductor
Jianwen Shao, Ray L. Lin, and Fred C. Lee
Center for Power Electronics Systems
The Bradley Department of Electrical and Computer Engineering
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061 USA
Abstract— In this paper, a novel DC link soft-switched
inverter with a coupled inductor is presented. The new inverter
provides soft-switching feature while keeping pulse width
modulation (PWM) capability without high voltage stress. First,
this paper gives a brief literature review of the research in DC
link soft-switched inverters. Then, a novel DC link soft-switched
inverter is described. The paper presents a detailed analysis of
the circuit and the operation principles. Finally, the simulation
and experimental results are shown to verify the theory.
I. INTRODUCTION
The emergence of high-frequency power semiconductor
devices, such as IGBTs and MOSFETs, has revolutionized
the power conversion industry. However, the high- frequency
devices cause new problems: high switching frequency
induces high switching loss, which might cause thermal
problems; and high switching frequency generates conducted
and radiated electromagnetic interference problems. To solve
these problems, soft-switching techniques are proposed. The
basic idea is that if the devices can switch under softswitching conditions (zero-voltage turn-on or zero-current
turn-off), these two problems can be avoided.
Because of the simple structure, resonant DC link softswitched inverters have received considerable research
interest in past ten years. The major drawbacks of this kind of
DC resonant inverter are that it suffers from high voltage
stress and has no PWM capability. Seeking a solution has led
to lots of new DC link soft-switched topologies. In this paper,
a novel DC link soft-switched inverter is presented, which
can have PWM capability without high voltage stress. This
paper is organized as follows. Section 2 provides a brief
literature review of the research in DC link soft-switched
inverters. Section 3 describes a novel DC link soft-switched
inverter, providing the detailed analysis of its circuit and the
operation principles. Finally, the simulation and experimental
results are shown to verify the principle behind the circuit.
II. EXISTING TOPOLOGIES OF DC LINK SOFT -SWITCHED
INVERTERS
This work was supported by Matsushita Electric Industrial Co., Ltd and
the ERC Program of the National Science Foundation under Award Number
EEC-9731677.
In the past ten years, researchers and engineers have
proposed many interesting topologies for resonant DC link
inverters. This section gives a brief literature review of
reported topologies.
A. Resonant DC Link Inverter (RDLI)
This is the first type of resonant DC link inverter [1],
which sets the DC link voltage into oscillation so that zerovoltage conditions are created periodically for soft-switching
purposes. It is very nice that RDLI only adds an inductor and
a capacitor to a hard-switched inverter. However, it suffers
substantial voltage stress (about 2.5 per unit) across the
devices. Moreover, pulse density modulation (PDM) has to
be adopted, which will increase the line harmonics
significantly. To produce the same spectrum quality, the
switching frequency must be four to five times higher than
occurs in a hard- switched PWM strategy.
B. Active Clamped Resonant DC Link Inverter (ACRDLI)
In order to reduce voltage stress in an RDLI, the active
clamped resonant DC link inverter is proposed [2]. The
proposed circuit uses another auxiliary switch to clamp the
DC link voltage. The peak voltage across the devices can be
clamped at 1.3~1.5 per unit. However, the clamp switch is
switched at resonant frequency and turned off at high current.
As a consequence, this switch suffers from a large energy
loss. Also, the charge balance of the clamp capacitor is
required to maintain link resonance. Thus, a complicated
control strategy is necessary. Additionally, PDM still has to
be adopted in the ACRDLI.
C. Passive Clamped Resonant DC Link Inverter (PCRDLI)
Since the auxiliary switch induces excessive losses in
active clamped resonant DC link inverters, passive clamped
resonant DC link inverters have been introduced [3]. In the
passive clamped resonant DC link inverters, three coupled
windings and one diode are used to clamp the peak voltage.
With the help of the coupled windings, the peak voltage can
be clamped at 1.3 times the dc voltage and PWM can be
applied. The disadvantages are that the clamped diode suffers
from twice the voltage stress, and the leakage between
primary winding and clamping winding causes a high voltage
©2000 IEEE. Reprinted, with permission, from IPEMC 2000.
456
spike. In order to minimize the leakage inductance, special
transformer structure needs to be considered.
D. Parallel Resonant DC Link Inverters (PRDLI)
different operation modes, and Fig.3 shows the operation
waveforms of the circuit in the time sequence. Initially, the
auxiliary switch Sa1 is on and the auxiliary switch Sa2 is off.
Due to the effect of active clamping, the voltage of the
capacitor C0 is KVs, 1<K<2.
Unlike the previous resonant DC link soft-switched
inverters, a PRDLI inverter has its LC resonant tank in
parallel with the DC link [5,6,7]. Someone names some
similar topologies as quasi-resonant DC link inverters. In
parallel resonant DC link inverters, the resonant circuit is
activated only during the resonant transient. Since the
resonant circuit is paralleled with the main power path, the
commutation loss in auxiliary circuit is reduced. Another
advantage of PRDLI is that the PWM strategy is available.
The major disadvantage is that because there is an active
switch in series with the dc bus, this switch consumes a large
amount of energy.
L
Sa1
Lx
Da
Vs
C1
M
C0
Sa2
Fig. 1 A novel DC link soft -switched inverter with a coupled inductor.
E. DC Bus Notched Commutated Inverters (DBNCI)
The operation principles of DC bus notched commutated
inverters are quite different from those of the RDLI [4]. In the
DC bus notched commutated inverters circuit, a simple active
snubber subcircuit is used to realize soft-switching condition.
While maintaining the conventional PWM, the circuit can
restrict the voltage stress to within 1.3 times the dc voltage.
However, since the auxiliary switch is hard turn-on and turnoff, there is a large amount of switching loss in the auxiliary
switch. Furthermore, high frequency ringing exists in the dc
link. The high frequency ringing causes severe
electromagnetic interference problems.
All these topologies have their advantages and
disadvantages. In general, the preferred topology can provide
the soft-switching feature and PWM capability without big
penalties such as high voltage stress and more loss.
Based on the foundation of previous research, a novel DC
link soft-switched inverter is proposed. In this circuit, the
high voltage stress of devices can be avoided and the PWM
operation is available.
III. DC LINK SOFT -SWITCHED INVERTER WITH A
COUPLED INDUCTOR
The proposed circuit is shown in Fig. 1. In the circuit,
there are two auxiliary switches Sa1 and Sa2. Inductors Lx
and L are coupled with each other. C0 is the clamp capacitor.
A. Principles of operation
The operation of the circuit is explained by referring to
Fig.1 and 2 and 3. Fig.2 shows the equivalent circuit in
Mode 0) steady mode (Fig.2a): The auxiliary switch Sa1 is
on and the link voltage is clamped by the capacitor C0.
Meanwhile, the other auxiliary switch Sa2 is off.
Mode 1) resonant mode (with Sa1 off and Sa2 on, Fig.2b):
To initialize a resonant transient, assume at time t0 that Sa1 is
turned off and that Sa2 is turned on just before a switching
action of the main switches of the inverter. When the
auxiliary switch Sa2 is turned on, the capacitor C1 resonates
with L and Lx. The resonance between L, Lx and C1 causes
the capacitor to discharge and pulls down the link voltage to
zero. In the meantime, inductor L shares the resonant current
with Lx. At time t2, the capacitor voltage reaches zero, the
resonant process stops, and the current of inductor Lx
freewheels through diodes paralleled with main switches. The
zero-voltage condition is created for main switches and can
be maintained for a short period of time that is determined by
the link parameters. During this short period, the main
switches of the inverter can be triggered under zero-voltage
condition.
Mode 2) feedback mode (with Sa1 off and Sa2 on,
Fig.2c): When the Lx current freewheels, the feedback mode
starts. Because of the coupled inductor feedback mechanism,
the freewheeling current decreases linearly until it is zero at
time t3. Thus, the switch Sa2 can be turned off under zerocurrent condition.
Mode 3) charge mode (with both Sa1 and Sa2 off, Fig.2d):
After the resonant stage finishes, C1 is charged by the voltage
source through L. During this charge period, Da blocks the
inductor Lx so that the inductors L and Lx are decoupled. The
voltage source will charge the inductor L linearly until the
inductor current reaches load current. When the inductor
current is equal to the load current, L and C1 begin to
resonate. The resonance between L and C1 keeps driving the
link voltage to the clamp voltage KVs. At time t4, the dc link
voltage equals to clamp voltage KVs, the parallel diode of the
Sa1 starts to conduct, and excess energy is fed into the clamp
capacitor C0. The conducting of Sa1’s parallel diode creates
457
the zero-voltage condition for Sa1. After time t4, Sa1 can be
turned on at any time with zero-voltage condition. The circuit
operation then goes back to normal steady mode.
Sa1
Sa2
B. Analysis of the soft-switched inverter
An analysis of the inverter operation can be performed as
follows, based on the equivalent circuit shown in Fig.2. To
simplify the analysis, the dc link current can be replaced by a
constant current source.
In charge mode (Fig.2d), the inductor is first charged
linearly to the dc link current Io,
I L1 = Io .
(1)
After the inductor current reaches the load current, the
inductor L begin to resonate with C1. The equations can be
expressed as:
Vs
sin θ
Zo
V c = Vs − Vs cos θ
I L = I L1 +
Smain
Ll
Ix
(2)
Vc1
Where Zo = L
.
C1
By solving equation (2), the current of inductor at the time
t4 (Vc = KVs) can be found to be
I L 2 = I L1 + k ( 2 − k ) Vs 2 Zo 2 .
(3)
t0
Il
L
Vs
C1
Io
Vs
t3
t4
dt
= ( k − 1)Vs
L
.
(4)
I
L
C0
t2
In the steady mode, the link voltage is clamped by the C0
and the inductor is discharged by the C0:
dI L
Io
t1
Fig.3. Key waveforms in soft -switching commutation.
C1
Lx
Io
Using the energy balance of capacitor C0, the period of the
operation, which is the same as period of PWM of the main
switches, can be calculated as:
Ix
T=
(b)
(a)
Vs
Il
Il
L
L
Lx
C1
Io


k (2 − k )
LC1cos −1 (1 − k ) + 2
.
k − 1


(5)
Fig. 4 shows a plot of T/(LC1)1/2 as a function of K. From
this curve, we know the relationship between T and L, C1
under certain voltage stress K. This relationship will offer the
guideline for the circuit design.
Vs
C1
Io
Ix
IV. SIMULATION AND EXPERIMENTAL RESULTS
(c)
(d)
Fig.2. The mode diagrams of DC link soft -switching operation, (a) steady
mode, (b) resonant mode, (c) feedback mode, and (d) charge mode.
In this novel DC link soft-switched inverter with a
coupled inductor, parameters L, Lx, C1 and K need to be
chosen to satisfy some specifications, such as di/dt, dv/dt and
voltage stress. Given a desired clamping voltage factor K and
switching period T, (LC1)1/2 can be chosen. A simple
approach to estimating the EMI performance according to the
di/dt value is illustrated in [9].
458
100
100
80
T
LC1
Ix
60
40
20
I
1
3.142 0
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
Clamp voltagekindex K
Fig.4. The relationship between circuit parameters and clamping voltage
Here is a design example. Using K=1.05 and the
switching frequency of the hard-switching inverter of 20kHz.
From Fig.4, we can get the value of (LC1)1/2 . Based on
computation and simulation work, a set of parameters is
chosen as L=40uH, Lx=16uH, Lleak=4uH and C1=47nF.
From a control point of view, and in comparison with
other DC link resonant inverters, this new topology has very
simple control requirements to realize the soft-switched
PWM strategy. There is no need to add another control loop
to adjust the voltage of the clamp capacitor because the
voltage balance of the clamp capacitor is maintained by
nature. It only requires a trivial revision to an original hardswitching PWM control scheme.
A simulation is performed to verify the analysis. Fig. 5
shows the simulation results. As can be observed from Fig.5,
the dc link voltage is pulled down to zero when auxiliary
circuit is activated and the coupled inductors share the
resonant current.
1kw prototype inverter is also built and Fig. 6 shows the
experimental results. The experimental results verify the
analysis and the simulation.
Vc
1
Fig.5. Simulation results of the novel inverter: Ix is the current of inductor
Lx; I1 is the current of inductor L; and Vc1is the voltage of the dc link.
I1
Il
10A/div
Ix
10A/div
Ix
V. CONCLUSION
In this paper, a novel DC link soft-switched inverter is
presented. In comparison with other resonant link inverters,
the new topology can provide soft-switched PWM strategy
without suffering high voltage stress. With the help of a
coupled inductor, the current stress of the auxiliary switch is
greatly reduced and the switch has zero-current turn-off
condition. The control of the soft-switched inverter is simple,
and the voltage balance of the clamp capacitor is fulfilled
naturally. The operation principles of the novel inverter are
analyzed and verified by the simulations and experiments.
Vds(Sa2)
250V/div
Vds(Sa2)
Vc1
250V/div
Vc1
T/div=2us
Fig.6. Experimental results of the novel inverter: I1 is the current of
inductor L; Ix is the current of inductor Lx; Vc1is the voltage of the dc link;
and Vds(Sa2): drain to source voltage of auxiliary switch Sa2.
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5.
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2.
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4.
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