ANew forward type zero voltage switching inverter

Mohammad Ali Kazemi, Ehsan Adib, MEMBER IEEE, Hosein Farzanehfard, MEMBER IEEE

Department of electrical and computer engineering, Isfahan university of technology, Isfahan, Iran

m.a.kazemi@live.com, e.adib@cc.iut.ac.ir , hosein@cc.iut.ac.ir

Abstract -In this paper, a new soft-switching forward type inverter is presented for low power applications. This inverter is composed of a main and a clamp switch on the DC side and low frequency switches on the ac side. The proposed inverter is PWM controlled and soft switching condition is also achieved for the DC side switches. The unique features of the proposed converter are high efficiency, low weight and volume, and good dynamic response. The principles of circuit operation and theoretical analysis are discussed and simulation results are presented.

I. INTRODUCTION

With the rapid development of modem techniques, the main

concerns of converters are reliability, volume, efficiency and

cost of power conversion systems. In order to achieve

relatively small size, light weight and high efficiency,

inverters operating at high switching frequencies are desirable.

Forward converter is one of the most attractive topologies for

low and medium power applications [1]. The transformer in

the forward converter can be used to isolate the electric signal

and step-up the voltage level. Hard switching converters suffer

from low efficiency especially at high switching frequencies.

In the isolated converters, switching losses are higher due to

leakage inductance of the transformer. In order to improve the

performance of the isolated converters, various techniques can

be applied. The conventional passive RCD clamp circuit can

be used to absorb the energy stored in the leakage inductance,

Therefore, the voltage stress of the switch can be reduced;

however the total efficiency of inverter would be reduced due

to power losses of the clamp circuit. Soft switching techniques

are proper solutions to recover switching losses and improve

the efficiency [2]-[5].

In this paper a forward type inverter is introduced and

analyzed. In this inverter active clamp circuit is applied to

recover the leakage inductance energy while achieving soft

switching condition for the switches. In this converter the

achieved soft switching condition is not dependent on the

leakage inductance energy and therefore, soft switching

condition is achieved for a wide load range.

This forward type inverter has several advantages over flyback

and push-pull inverters. The flyback inverter stores energy in

the magnetization inductance of the transformer which results

in higher weight and volume of the transformer [6]-[8]. Also,

due to the air gap applied, the transformer leakage inductance

would be larger which reduces the efficiency.

In the push pull inverter a center-tap transformer on the

primary side is needed and two switches are employed on the

DC side while four switches are applied on the AC side [9].

Due to the inherent differences between switches and

transformer windings on the DC side, the transformer

magnetizing inductance can be saturated especially when the

temperature changes in a wide range.

In this paper, the proposed forward type inverter is introduced

and analyzed in the second section. Also, the converter

operating modes are discussed. The simulation results are

presented in the third section to justify the theoretical analysis.

The proposed converter is a proper choice for renewable

power applications.

IEEE Catalog Number: CFP121IJ-ART 477 ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE

Sac1

Sac2

S1

Fig .1 Proposed forward type inverter

II. CONVERTER ANALYSIS

Coul

Fig .1 shows the circuit configuration of the proposed zero

voltage switching (ZVS) forward type inverter. On the

primary side, the converter consists of the DC input

voltage Vin, isolating step-up transformer and the main

switch Sj. The auxiliary switch S2 and the clamp capacitor Cc

are the active-clamp circuit. The magnetizing inductance is

represented as Lm and transformer leakage inductance is

represented as Lj. On the secondary side, the switches Sacj and

Sac2 operate at the output voltage frequency.

The converter operation can be divided into four operating

modes in each switching cycle. The equivalent circuit for each

operating mode is shown in Fig. 2.

Before to, S2 is conducting and the transformer leakage and

magnetizing current is assumed negative according to the

direction shown in Fig. 1. At to, S2 is turned off and the

magnetizing inductance charges the output capacitance of S2

and discharges the output capacitance of Sj.

During to-tj, Sj is on and magnetizing inductance current

reduces to zero. Then, the difference of input voltage and the

output voltage reflected to the primary side, is placed across

the leakage inductance. Therefore, the leakage inductance

current increases and power is transferred from input source to

the output. Also, the transformer magnetizing current

increases.

V'=Vo/n (1)

Vu= Yin � V' (2)

where v' is the transformer secondary voltage reflected to

primary side.

This mode begins when Sj is turned off. The leakage

inductance current would charge the output capacitance of Sj

while discharging the output capacitance of S2. Then, the body

diode of S2 starts to conduct and thus, S2 can be turned on

under ZVS condition.

The leakage inductance voltage Vu is:

Vu = -V' � Vee (3)

!lic= vu dT

= -v' - VCe

dT Ll Ll (4)

According to volt-second balance of leakage inductance III

mode a and b:

V u x DT = - V u x dT (5)

(Vin � V' ) D = - (-V' � Vee) d (6)

At t2, the leakage inductance current is reduced to the

magnetizing inductance current. During this interval Lm and

leakage inductance are in series and the current through them

reduces to zero and then becomes negative. In this mode the

current through the secondary switch is zero and the

transformer core is reset.

Vu + VLm = Vee

v = -�Lm Lm (Lm+Ll)

VCe vu= - --- Ll (Lm+Ll)

!lie'

(7)

(9)

(9)

(10)

According to the charge balance of capacitor:

!lic xdT = !lic' x D 'T

Therefore

, - � Ll -v - vCe d2 = (Lm+Ll) D,2

Ll Ll

(11)

(12)

By turning S2 off at the end of this mode, Sj output

capacitance is discharged and its body diode would start to

conduct at t3.

In this mode Sj body diode starts to conduct and the leakage

inductance current increases. In this interval, Sj can be turned

on under ZVS condition. Duration of this mode is short.

IEEE Catalog Number: CFP121lJ-ART 478 ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE

According to the volt-second balance of the magnetizing

inductance:

Using MATLAB software, from the above mentioned

equations, the converter gain versus duty cycle (D) is obtained

considering unity turns ratio for the transformer. The gain for

various values of leakage inductance is shown in Fig. 3. v' x(D+d) = (-� Ll) 0' (Lm+Ll)

+

+

(13)

Mode a

11,......----'1'0.

Mode b

l l"'i I --r I

Mode c

Moded

Fig 2. Equivalent circuit for each operating mode.

IEEE Catalog Number: CFP121IJ-ART ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 479

0.9

0.8

0.7

0.6 IJ) > 0.5 (; >

0.4

LM=100u

--------�

� /'

------

0.6

-- LI=1u -- L1=3u - - - - LI=5u -- LI=7u -- LI=9u

_---�-II - - LI=11 -- LI=13

��_���--==] -- LI=15 =-----�:ll - - - - LI= 17

0.7 0.8 0.9

Fig.2. Converter gain ( � ) versus duty cycle for various values of transformer leakage inductance. vs

ISWI ".00

00.00

00

lO.!>:>

0.00

-20.00 Vs''''l l�.OO

125.00 100.00 7�.OO 50.00 25.00 0.00

-25.00 1 & I M.OO

Fig.3. Sl current (top waveform) and drain-source voltage (bottom waveform).

Fig.4. S2 current (top waveform) and drain-source voltage (bottom waveform).

IEEE Catalog Number: CFP121IJ-ART ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 480

Vo 2{)O.OO �----=::--_-----_-----:=--_-----_--=------,

0.00

- 1 00.00

-200.00

1 0.00 20.00 30.00 4(1 ,00 50.00 TimE(rru)

Fig.5. Output voltage

l a c. 1 10.00 8.00 0.00 '.00 2.00 0.00 -2,00 10.00 �-----_-----_. ------�. ------_------, 8.00 0.00 <4.00 -------------

0.00 f--------

· . · . - - - _ ."- - - ---- - ----- - -- - - - � - ---· . · .

.2.00 L-_____ -'-_____ -'-______ -'-_____ -'-_____ -' 0.00 10.00 20.00 30.00

TWM (ms)

Fig.6. Current through Soc! and Sao2

Fig. 7. Control unit

<0.00

LIMITER COMP

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ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE 481

III. SIMULA nON RESULTS

The proposed converter is simulated using PSpice software at

Yin = 65 V, f =100 kHz , VOpeak =300 v , and Po=450 W. The transformer parameter are Lm=100IlH, LL=2IlH, turns ratio = n 1 In2 = 119. Also, R]oad is 100 n, clamp capacitor is 20 IlF, and the output capacitor is 2.2 IlF. The simulation results are presented in Fig. 3 to Fig. 6. Main switch current is negative at switch tum on instant. This means that main switch body diode is conducting and thus zero voltage switching is achieved. Also, the auxiliary switch current is negative at tum on instant which justifies the achieved soft switching condition. The schematic of converter controller is shown in Fig. 7.

IV. CONCLUSIONS

A forward type inverter employing an active clamp for soft­switching is proposed for DCI AC conversion applications. The proposed circuit is capable of operation at high switching frequencies in a wide range of load variations. Steady state performance is discussed and also, operating principles and theoretical analysis are presented. Simulation result justifies the theoretical analysis.

V. REFERENCES

[1] F. D. Tan, "The forward converter: from the classic to the contemporary," in Proc. IEEE APEC, 2002, pp. 857-863. [2] B. R. Lin, K. Huang, and D. Wang, "Analysis design and implementation of an active clamp forward converter with synchronous rectifier," IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 53,no.6,pp.13IO-I3I9,lun.2006 [3] M. Shoyama, G. Li and T. Ninomiya, "Application of common-source active-clamp circuit to various DC-DC converter topologies ",IEEE-PESC Conference, vol. 3, pp. 1321-1326,2003. [4] E. Adib, H. Farzanehfard, "Family of zero voltage transition pulse width modulation converters with low auxiliary switch voltage stress ", lET Power Electronic, Vol. 4, No.4, 2010. [5] E. Adib, H. Farzanehfard, "Zero voltage transition PWM converters with synchronous rectifier ", IEEE Trans actions on Power Electronics, Vol. 25, No.1, 2010. [6] N. P. Papanikolaou, E. C. Tatakis, A. Critsis, and D. Klimis, "Simplified high frequency converter in decentralized grid­connected PV systems: a novel low-cost solution," in Proc. EPE '03, 2003, CD-ROM. [7] T. Shimizu, K. Wada, and N. Nakamura, "A flyback-type single phase utility interactive inverter with low-frequency ripple current reduction on the DC input for an AC photovoltaic module system," in Proc. IEEE PESC '02, vol. 3, 2002, pp. 1483-1488. [8] R. Watson, F. C. Lee, and G. C. Hua, "Utilization of an active-clamp circuit to achieve soft switching in flyback converters ", IEEE Transactionson Power Electronic, vol. 11, no. 1, pp. 162-169, 1996. [9] S. B. Kjaer, "Design and control of an inverter for photovoltaic applications," Ph.D. dissertation, Inst. Energy Technol., Aalborg University, Aalborg East, Denmark, 2004/2005.

[10] A. Lohner, T. Meyer, and A. Nagel, "A new panel-integratable inverter concept for grid-connected photovoltaic systems," in Proc. IEEE ISlE '96, vol. 2, 1996, pp. 827-831. [11] M. Meinhardt, T. O'Donnell, H. Schneider, 1. Flannery, C. O. Mathuna, P. Zacharias, and T. Krieger, "Miniaturised 'low profile' module inte- grated converter for photovoltaic applications with integrated magnetic components," in Proc. IEEE APEC '99, vol. 1, I999,pp.305-3I1.

IEEE Catalog Number: CFP121IJ-ART 482 ISBN: 978-1-4673-0113-8/12/$31.00 ©2012 IEEE