A NEW ZVT PWM CONVERTER FAMILY: ANALYSIS,
SIMULATION AND EXPERIMENTAL RESULTS
Ricardo Nederson do Prado, Member ZEEE
Department of Electronic and Computation
Federal University Santa Maria
97119-900 - Santa Maria - RS
Fax: 55.2262013
BRAZIL
Abstmct -
This paper proposes a new ZVT PWM Converter
Family, where the voltage across MOSFET is clamped to the
input voltage V,. The MOSFET turns on under zero voltage and
the auxiliary switch turns off under zero current. It is possible to
use MOSFET with a smaller Rnh,,, because the voltage acrossit
is clamped to the V,. The ZVS is nearly independent from the
load range.
Principles of operation, analysis and simulation results are
presented.
Experimental results with a ZOOkHz, 310W ZVT-PWM Buck
converter are provided
and current stresses, increasing the conduction losses.
When conventional PWM converters are operated at hgher
frequencies, the circuit parasitics are shown to have
detrimental effects on the converters' performances.
This paper presents a new ZVT PWM Converter Family.
By using a resonant network in parallel with the switches and
two input voltage sources, the proposed converter acheves
zero-voltage switching for the main switch and the
freewheeling dlode, and zero-current switchng for auxiliary
switch. The ZVS is obtained without increasing voltage and
current stress. A soft reverse recovery is acheved to the
freewheeling &ode.
1. INTRODUCTION
High-frequency high-efficiency operation of DC-DC
converters requires a substantial reduction of switching losses
in traditional pulse-width-modulated (PWM) converters.
Several converter's topologies have been studled and proposed
to achieve thls goal [5-111. The switchmg losses in these new
circuits can only be reduced at the expense of much increased
voltage and current stresses of the switches, whch lead to a
significant increase in conduction loss [8].
In a ZVS-QRCs, the active switch is subjected to relatively
low current stress [1,5,6]. However, the power switch in a
single-ended ZVS-QRC suffers from an excessive voltage
stress that is proportional to the load range [1,2,6]. The
operation of ZVS-QRCs is adversely affected by the junction
capacitance of the rectifier. Thls capacitance interacts with the
large resonant inductor causing severe switching noise and
resulting in a possible instability in a closed loop system [4].
The ZVS-h4RC techmque uses major parasitics [2,3]. The
reduction of the switchmg losses and switchng noise is
obtained because all semiconductor devices operate with zerovoltage switchng. The switches are subjected to h g h voltage
0-7803-1456-5194 $4.00 0 1994 IEEE
2. THE BUCK ZVT PWM CONVERTER
a) Circuit Description.
The Buck ZVT PWM Converter will be considered to
analysis in thls paper, and it is shown in Fig. 1. It dlffers
from a conventional Buck P W M by the addltion of a resonant
network consisting of a resonant inductor L,, a resonant
capacitor C,, an auxiliary switch S,, an auxiliary &ode D, and
two input voltage sources V,, V,.
Fig. 1 - The Buck ZVT PWM Converter.
978
~
~~
b) Operating Principle
Simplifying the analysis, the following assumptions are
made:
- The circuit operation is in steady-state.
- All power semiconductors are ideal.
- The output filter inductance is sufficiently large to be
approximated by a current source with a value equal to
the load current 4.
- The two input voltage sources have the same value that
is equal to VJ2.
The six topological stages are shown in Fig. 2 and the main
waveforms are represented in Fig. 3. The operation is
described as follows:
a ) First stage
During thls stage, the load current I, keeps freewheeling
through &ode D,.
b) Second stage
Thls stage begins when S , is turned on. The current on the
inductor L, increases linearly with V,/L,. When it reaches I,,
D, turn off and thls stage ends.
c ) Third stage
Thls is a resonant stage, during whch V,, and I,, change in
a resonant fashon until V,, become equal to V,.
d) Fourth stage
Thls stage begins when Q, is turned on at practically zero
voltage. The inductor current decreases linearly with -VI&.
Thls stage ends when it reaches zero.
e ) Fifth stage
T h ~ sstage begins when Q1 assumes the load current I,
completely.
f) Sixth stage
Thls stage begins when Q, is tumed off. Capacitor voltage
V,, decreases linearly with -I& to zero and D, is directly
polarized and starts conducting.
5
VI
+
F
J
?
p
a ) First stage
b) Second stage
dl
c ) Third stage
d ) Fourth stage
e ) Fifrh stage
f) Sixth stage
Fig. 2 - Topological stages
3. THEORETICAL ANALYSIS
Output Characteristics.
According to the waveforms shown in Fig. 3 and
considering V1=V2,the average output voltage V, is given by:
Fig. 3 - The main waveforms of the Buck ZVT PWM Converter
Where:
D = duty-cycle
f = switchng frequency
f, = resonant frequency
979
v,f
-
4. SIMULATION RESULTS
= resonant stage
4f0
K2.Crf
= linear dlscharging of the voltage capacitor
2 4
b) Commutation Analysis
The zero-voltage switching is acheved to Q, only if V, 5
V,. The ZVS is independent from the load current for thls
circuit.
In order to verify the operating principle of the Buck ZVTPWM Converter, a simulation has been performed. In this
simulation L, = 6.5m, C, = lOnF, 4 = 5A and f = 200kHz
were considered. The waveforms obtained by simulation are
shown in Fig. 4a for V, = V, = Vi/2 = 75 and in Fig. 4b for
V, = 70 and V, = 80. When V, <Vz,a new topological stage
beg" where the anti-parallel &ode of Q, conducts the
difference 4 - iLr,as can be seen in Fig. 4b.
The simulation confirms the techmque proposed and
shows that it is desirable.
c) Current Stress in the Auxiliary Switch
The current stress in SI, is given by:
-57
,
-1ooj
,
6-
This current stress can be minimized by design of L, and
I
,
,
I
,
I
,
I
'
I
'
IQ 1
2oo-
0
I
,
I
I
I
I
I
'
I
'
,
I
VQl
\
7 5
5 0
i
I
I'
i
0 0
1
~
r
i
,
IQ!
2 5
-100;
0 10
,
I
20
-2
980
r
4-
c.
It can be seen that the voltage and current waveforms of Q,
and D, are square-wave-like except during the interval where
the zero-voltage transition takes place. The ZVT time can be
very short concerning the switchmg cycle, so the operation of
the new converter resembles that of the Buck PWM Converter
during most portions of the cycle. Thus, both Q, and D, are
zero-voltage switched and are subjected to low voltage and
current stresses associated with those in the Buck PWM
Converter. The auxiliary switch SI, operates with zero-current
switching. Consequently, the switchmg losses are significantly
reduced at a minimal increase of conduction loss.
In a ZVS-QRC or ZVS-PWM converter, the zero-voltage
switchmg is strongly related to the load current and input
voltage. At light load the ZVS property is dfficult to maintain
since the energy stored in the resonant inductor at light load
is not sufficient to discharge the resonant capacitor before the
active switch is turned on [1,6,7].
In this Buck ZVT-PWM Converter the situation is contrary.
In the Buck ZVT-PWM converter, the zero-voltage switchmg
to Q, is independent from the load current. It depends on the
relation V,/V,. The ZVS is acheved only if V, < V,.
Consequently, the zero-voltage switching property is easier to
maintain at light load.
I
I
0 12
,
JI
I
I
I
0 16
0 14
7
I
L
I
0 18
I
,
0 20
XIO-~
Fig. 4
- The main
waveforms obtained by simulation of the Buck
ZVT-PWMConverter.
~
i
5. A NEW FAMILY OF ZVT-PWM CONVERTERS
The concept of ZVT presented in this paper can be extend
to any PWM topology. Simply by adding a resonant network
(as shown in Fig. l), several ZVT-PWM converter topologies
are derived, as shown in Fig. 5.
The switches in a ZVT-PWM converter are subjected to
low voltage stresses, same as those in their PWM
counterparts.
Fig. 6 shows several isolated topologies of the ZVT-PWM
converters. The limitation of the isolated ZVT-PWM converter
is that they do not utilize the leakage of the power
transformer. The Transformer should be designed with a
minimum leakage.
Fig. 7 shows the full-bridge (FB) and half-bridge (HB)
ZVT-PWM converter. In these circuits the arrangement of the
auxiliary switches is different. The operations of two halfbridges of the FB-ZVT-PWM are completely symmetrical.
The Fl3 ZVT-PWMhas several advantages: (a) much less
circulating energy since no resonant inductor is used in the
main powewr path, (b) no severe secondary parasitic ringing,
(c) soft-switching of the rectifier diode and (d) soft-switching
operation maintained for the entire line range and load range.
-
Fig. 6 Isolated topologies of the ZVT-PWM converters:
(a) Forward, (b) Fiybadr
I
i r
I
T
I
I
-
Fig. 7 Fuii-Bridge and Half-Bridge ZVT-PWM Converter
6. EXPERIMENTAL RESULTS
(C)
A Buck ZVT-PWMConverter has been implemented to
demonstrate the operation. The power stage in Fig. 1 consists
on the following components:
-
Fig. S Basic topologies of the ZVT-PWM converters:
(a) Buck, (b) Buck-boost, (c) Zeta
981
Q, = IRF640
s, = IRF640
D,,D, = MUR1520
L, = 1 . 5 m
c, = lonF
v, = 75v
V, = 90V
f =2001rHz
In practical applications, due to the conduction losses in the
circuit and to ensure ZVS commutation, V,<V, has been
selected.
Fig. 8 shows the oscillograms of the experimental circuit.
It can be seen that the experimental waveforms agree with the
Theoretical analysis. Both MOSFET and rectifier &ode
operate with zero-voltage switchng. The auxiliary switch
operates with zero-current switchmg.
-50
0.000
0 025
0 050
0 075
x10-4
0 125
0.100
0.150
(8)
7. CONCLUSIONS
Thls paper presents a new ZVT-PWM Converter. It
combines the advantages of the conventional PWM and
resonant techques. The advantages of the proposed Buck
ZVT-PWM Converter are summarized as follows:
- The zero-voltage switching is independent from the load
current;
- Both the active and passive switches operate with zerovoltage switchmg;
- The voltage across the main switch is clamped to the
input voltage (V,+V,) whch makes it possible to employ
a MOSFET with a smaller RDson.The situation is
opposite in a ZVS-QRC, where the voltage across the
main switch is strongly related to the load current;
- The auxiliary switch is zero-current switched and the
voltage across it is clamped only to V2;
- The design optimization of the circuit is easily attainable
since the new converter operates with constant frequency;
- A soft reverse recovery is obtained to the rectifier diode
avoiding parasitic oscillations and switching noise;
The operation of the proposed converters was analyzed by
using the buck ZVT-PWM converter as an example. A
200kHz, 310W ZVT-PWM buck dc-dc converter was
breadboarded to show the operation of the proposed
converters.
0 000
0 025
0 050
0 1 7 1 ‘ 0
x 1 r 4
“
d
l
5
0
(b)
2001
Fig. 8 - Oscillograms of the Buck ZVT-PWM Converter for
Po = 310W, f = 2OOkHz and q = 96,3%.
(a) Q , drain-to-source voltage Vql (5OVidiv)
(b) resonant inductor current i,, (1,75Ndiv)
(c) resonant capacitor voltage V, (dOv/div)
982
REFERENCES
K.H. Liu. F.C. Lee. "Zero-Voltage Switching Technique in DC-DC
converters." IEEE PESC'86 Record, pp. 58-70.
W.A. Tabisz, P. Gradzki, F.C. he. "Zero-voltage Switching MultiResonant Technique. A Novel Approach to improve Mormanoe of
High-Frequency Quasi-Resonant Converters." IEEE PESC'88 Record,
pp. 9-17.
M.M.Jovanovic, W.A. Tabisz. F.C. he. "Zero-Volw Switching
Technique in High-Frequency off-Line Converters." IEEE APEC'88
P r d i n g ~ pp.
, 23-32.
M.M.Jovanovic, F.C. Lee. "DCCharacteristics and Stability Analysis
of Pueh-Pull and Bridge-Type Zero-VoltapSwitched Quaei-Resonant
Converters." IEEE Trans. On Power Electronics. Vol. 3 no3, July 1989.
I. Barbi. J.C. Bolacell, D.C. Martins, F.B. Libano. 'Buck QuasiResonant Converter Operating at Constant Frequency: Analysis, Design
and Experimentation." IEEE pEsC'89 Record, pp. 873-880.
R.N. MO,
D.C. Martins, I. Barbi. "Effects of Nonlinear Resonant
Inductor On the Behavior of Zero-VoltageSwitching Quasi-ReMmant
Converters." IEEE pEsC'90 Record, pp. 522-527.
G. Hua, F.C. Les. "A New Class of the Zero-VoltagsSwitched PWM
Converters." HFPC'91 Proceedings, pp. 244-251.
G. Hun, C.S. Leu, F.C. Lee. "Novel ko-VoltageTransition PWM
Converters." VPw3'91 SEMINAR Proceedings, pp. 81-88.
R. Farrington, M.M.Jovanovic, F.C. Leu. "A New Family of Isolated
Zero-VoltegeSwitched Converters.'' IEEE PESC'91 Record, pp. 209215.
[lo] WJ.Gu,K. Hamla. "A Novel, Self-Excited,PWM Forward Converter
with ZVS Resonant Transition Using Two Minor-Loop-optrstsd
Saturable Coma.'' IEEE PESC'92 Record, pp. 85-92.
[ l l ] L.C. Fmitas, VJ. Fnrias, P.S. Capare.lli, J.B. Vieira Jr., H.L. Hey, D.F.
do Cruz. "An Optimum ZVS-PWM DC-DCConverter Family: Andysia,
Simulation and Experimental Results." IEEE PESC'92 Record, pp. 229235.
[12] R.N. MO.
"A New Buck ZVT P W M Converter." HFPC'93
Proceedings, PP.73-79.
-
983