A high efficienct DC DC converter

8/7/2019 A high efficienct DC DC converter

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Novel Design Of Soft Switching For Multi-Level InvertersV.Suresh, M.Ramasamy

PG Scholar, Assistant Professor

Department of Electrical and Electronics Engineering,

K.S.R College Of Engineering,

Tiruchengode, India

Abstract A modular soft-switching scheme is proposed

for three-level inverters. The mirror symmetrical pair of

resonant link modules is key for soft switching of three-

level inverters. The dc-link voltages are pinched to a

neutral point when PWM pattern changes. The circuit is

also designed as an optional module that can be attached

to a standard inverter bridge and converted into a soft-

switched inverter.

Index TermsMirror symmetrical pair of dc-link resonant

circuits, negative-bus auxiliary resonant circuit (NBARC),

positive-bus auxiliary resonant circuit (PBARC), pinchedlink zero-voltage switching (ZVS).

I. INTRODUCTION

Soft switching techniques have been proposed in

recent years for power converters to achieve high switching

frequency, low switching losses and improved electromagnetic

compatibility [1-7]. An early proposed resonant circuit [1],[2]

has only passive power components of inductors and

capacitors on dc-link, but it provides high voltage stresses on

inverters switching power devices. It also lacks

synchronization with discrete pulse-width modulation (PWM).

Quasi-resonant circuits [2]-[5], [10], such as the activeclamped resonant (ACR) dc-link circuit, use additional active

clamping switches and capacitors, thus being able to

synchronize dc-link resonance with the inverters PWM

operation and meeting the requirement of a systems

modulation. However, their peak dc-link voltage is still high,

i.e., 1.5 ~ 1.8 times of nominal bus voltage. An auxiliary

resonant- commutated pole (ARCP) circuit [6] can minimize

the device voltage stresses through placing two auxiliary

switches in series with a resonant inductor at each inverter

phase pole. The downside of this circuit is it requires six

auxiliary power switches for a three-phase inverter bridge and

complicated control logic. In this paper, we propose a mirror -

symmetrical resonant circuit modules for zero-voltageswitching (ZVS) of three level inverters [9]. Moreover, new

resonant circuits can be designed as optional modules for a

standard inverter product unit. By attaching such a resonant

module on the dc-link, the conventional hard-switched

inverter can be converted to a ZVS inverter

II. SOFT-SWITCHING TOPOLOGY

The proposed pinched-link soft-switching circuit is

shown in Fig. 1. The three-level inverter is fed from a diode

bridge or a controlled rectifier bridge. The original dc-link of

the hard-switched inverter consists of large electrolytic

capacitor banks. Two quasi resonant circuit banks form a

mirror-symmetric module in the dc-link, where one circui

bank is at the positive bus rail and another is at the negative

bus rail, as shown in the shaded blocks. Further in Fig. 1, SCand SC2 are the clamping switches. When they are in the on-

state, both positive and negative bus are clamped at the ful

dc-link voltage levels, that is the normal steady stateoperation. In a commutation period, SCl and SC2 are in their

off-state and release the inverter buses from the dc-link

capacitor banks. This frees the voltages on the inverter input-

terminals, P and M, to descend to zero for a soft-switched

transient. While is the second auxiliary switch Sa1 and Sa2 tha

activates or deactivates the resonance circuit by controlling the

starting and ending time point of the resonance. Cr1 and Cr2 are

resonant capacitors, and their values are very small. These

capacitors, along with the inductors and auxiliary switches

form the dc-link soft switching module as shown in Fig. 1

The values of the resonant capacitors and inductors in each

bank are the same, i.e., Crl=Cr2 and L1=L2.

Since the upper and lower resonant banks arecomplementary, they work with the same operating principle

and share a commonly designed circuit with minor connection

changes. When a commutation is required, the upper resonan

bank brings the positive bus voltage down to the neutral and

the lower resonant bank brings the negative bus voltage up to

the neutral, thus pinching the entire dc bus voltage at the input

terminals of the main inverter bridge to zero. This creates a

transient at zero voltage or very low voltage during which a

soft switching can be achieved for the main switching devices

T1-T12.

An equivalent circuit of the lower soft-commutation

bank is given in Fig. 2 (a) for an analysis of the resonant

transient. Figure 2 (b) shows the waveforms of the negativebus voltage and resonant current. In the figure, the

commutation current, Io2, is assumed zero. The peak resonant

current, Ap, and one half resonant period, T0, are defined as

(1)

(2)


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Fig. 1. Circuit diagram of a pinched-link soft-switched

three-level inverter

a) Equivalent circuit for analysis of resonant transient

b). Negative bus voltage and resonant current.

Device A B C1 C2 D E

Sc On Off Off Off Off On

Sa Off On On Off Off Off

Dc DOC Off Off Off Off DOC

Da Off Off Off On On Off

DOC: Depend on the current direction in motoring or

regeneration.

Fig. 2. Equivalent circuit and resonant waveforms of the

lower soft switched bank.

A. Operating Modes and Equivalent Circuit Analysis

The soft-commutation stages are shown in Fig. 3

The resonant current flowing in -the different paths has been

indicated in each stage.

Stage A: The resonant capacitor, Cr2, has been charged up to

one-half of the normal bus voltage. The clamping switch, S c2

is in the on-state, and the negative bus voltage is clamped to

the capacitor's level, which is equal to Vdc/2.

Stage B: The clamping switch, Sc2, turns off so that the

negative bus terminal is released from the capacitor bank. By

turning on the auxiliary switch, Sa2, a resonant path is formed

with Lr2 and Cr2. The energy stored in Cr2 is transferred to the

large capacitor bank through the inductor, and the voltage

across Cr2, is decreasing. The peak resonant current and

resonant period are defined in (1) and (2).

Stage C: At t=To, the end of one-half resonant cycle, the

voltage across Cr2 has been discharged to zero. Any exces

current in the inductor will flow through the anti parallel

diodes of the inverter switches, as the voltage remains at zero

During this time, the negative bus, Vm, is in the same potentia

as the neutral line. If the positive bus has also swung to the

neutral line at that time, all the inverter switches wil

experience zero crossing-voltage, and they are ready to safely

turn on and off according to the new PWM gating patterns.

Stage D: As the inductor current reverses, the anti parallel

diode of the auxiliary switch, Da2, will conduct and provide a

path to charge Cr2. As a result, another resonance occursbetween Lr2 and Cr2, with an opposite direction current. The

resonant energy is being transferred back to Cr2.

Stage E: When the voltage across Cr2 reaches its peak value

the clamping switch, Sc2, is turned on at zero voltage, and the

negative bus is clamped to the capacitor bank again. Thus, the

entire soft switching is completed. The circuit returns back to

the steady state as shown in Stage A, and is ready for the nex

switching period


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Fig. 3. Resonance stages for soft-switching

A. Key Features of modular soft-switching Circuit

Key features of the modular soft-switching circuit topology

are summarized as follows.

No main power flows through the auxiliary resonant devices

in contrary with the ACR topology, thus requiring less curren

ratings and thermal management.

Use four auxiliary switches and two set of componentsinstead of six auxiliary switches and three sets of components

compared with the ARCP topology.

Voltage rating of the clamping switch, Sc1 and Sc2, is the

same as the dc-link voltage.

Voltage stress of the auxiliary switch, Sa1 and Sa2, is rated

one half of the dc-link voltage.

Zero voltage turn-on for the inverter switches and clamping

switch, Sc1 and Sc2.

Zero current turn-off for the auxiliary switch, Sa1 and Sa2.

No requirement for increasing the voltage and current ratings

of inverters main power devices.

Circuit can be applied to an existing hard-switched inverter

as an optional add-on module.

III. CONTROL AND SYNCHRONIZATION FOR PWM

PATTERN

A block diagram of electronic control circuit for the

proposed modular soft-switch circuit is shown in Fig. 4(a)

The control circuit receives incoming PWM patterns generated

by a microprocessor or DSP based regular PWM controller

using space-voltage modulation approach.

The incoming PWM pattern, for a three-phase bridge

is also given to control block as shown in Fig. 4(a) and 4(b

shows the three phase three level inverter. Fig. 5 presents the

PWM pattern changes and the control signals which activate

the resonant circuit in the dc-link to achieve ZVScommutation. The detection of the new PWM pattern is

accomplished by performing the exclusive operation. A

positive result of such an exclusive or operation produces

control signal which activate the resonant circuit

(a)


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(b)

Fig 4. (a) Block diagram of ZVS control. (b) Three-phase

three level inverter

.

Fig. 5. logic waveforms, and control signals derived from

PWM pattern

IV. SIMULATION RESULTS

Simulation work has been done in MATLAB to

verify the operation principle of the proposed soft-switching

scheme. In our initial simulation, the dc supply voltage is

given at 220 V, 50 Hz. The resonant parameters are Crl= Cr2=

0.094F. and Lrl= Lr2= 10H. The design parameters are

suitable for higher voltage ratings.Fig. 6 shows the transient waveforms of the positive and

negative bus voltages in a three-level inverter. The inverter

bus voltages are pinched to zero or nearly zero by mirror-

symmetrical resonant modules. At the moment Vpm=0, the

inverter main switches can triggered at reduced voltage

stresses. In the simulation, the synchronization controls are

employed by triggering PBARC and NBARC in a proper

sequence and delay time to force the positive and negative bus

voltages to reach the neutral point at the same time.

Fig. 6. Equivalent circuit and waveform of pinched dc-link

using mirror resonant circuit pair of PBARC and NBARC

(a) Equivalent circuit of pinched-link stage for three-leve

inverter. (b) Voltage waveforms of positive and negative

bus during pitched mirror resonant transient.

Fig.7. Gate pulse of Sc1, Sc2 and Sa1, Sa2.


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Fig. 8. Bus voltages and resonant currents at soft switching

transient under light load current.

Fig. 9. Inverter output voltage

Fig. 10.Inverter output voltage and Bus voltages at soft

commutation transient under light load current.

V. CONCLUSION

This paper presents a modular soft-switching topology for

three-level inverters. The positive and negative bus voltages

can be pinched to the neutral potential point so that the main

switching devices can turn on and off under zero voltage

condition. The main advantages of the proposed topology are

fewer components in the auxiliary resonant circuit which islocated at dc link, modularity design, and the main devices

have the same voltage and current ratings of as a conventiona

inverter. The control logic and operating principle have been

verified by simulation in MATLAB. The resonant-link

modules can be designed and implemented as an optional

module to a standard inverter bridge to achieve ZVS.

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A high efficienct DC DC converter

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