Multiple Alternatives of Regenerative Snubber Applied to SEPIC and CUK Converters Valdeir A. Bonfa Paulo J. M. M e n e g h JosC L. F. Vieira Domingos S. L. Simonetti Laboratory of Power Electronics and Electric Drives - Department of Electrical Engineering Federal University of Espirito Santo P. 0.Box 01-9011 - Vit6ria - ES - 29060-970 - Brazil
[email protected] [email protected] [email protected]. br d.simonetti@ele. ufes. br A6stract - This paper presents several topological alternatives for using a magnetically coupled regenerative turn-on and turnoff snubber configuration applied to the Sepic and 6uk converters operating in continuous conduction mode (CCM). The proposed snubber uses the main core of the DC-DC converter to build the resonant inductor. In addition to reducing the stresses in the switch, providing soft transitions in its turn-off voltage and turn-on current, it transfers the energy stored in the snubber capacitor to the output (load) or input. An analytical study of the converters is presented. Experimental results are shown for a proposed structure.
NOMECLATURE
fs ics
Switching frequency. Cs capacitor current.
Rated input current. LR inductor current. ~ L R Rated output current. 10 Switch current. iMO Coupling turns ratio. n Rated output power. PO PPER additional power transferred to the load through the snubber circuit (% rated output power). Rated output resistance. RO Cs capacitor turn-on discharge time. tR Maximum turn-off Cs capacitor voltage. VC Cscapacitor voltage. vcs Rated input voltage. vIN Drain-source voltage of the switch. vMO hN
Rated output voltage. ZROFF Turn-off characteristic impedance. VO
ZRON Turn-on characteristic impedance. Resonant OROFF
turn-off frequency.
ORON Resonant turn-on frequency.
I. INTRODUCTION The use of high switching frequencies is mandatory when one needs to obtain DC-DC converters with low weight and size. The penalty is the reduction in the reliability and efficiency of the converter due to the high stresses and the increase in switching losses of the semiconductor elements. Some circuits (snubbers) can be employed to minimize such losses. The usual turn-on snubber employs an inductor in series with the switch in
0-7803-7474-6/02/$17.00 02002 IEEE
order to control its current rising rate during the turn-on process. In the turn-off snubber, the current is diverted from the switch into a parallel capacitor, delaying the switch voltage rise and reducing turn-off losses. Many solutions can be employed to reset the capacitor voltage at each switching period. The simplest one is to dissipate the energy stored in the capacitor into a resistor (RC snubber)[l]. Another solution is to transfer this energy to the load or to the input source by an auxiliary circuit (regenerative snubbers)[2-41. This work presents some topological alternatives of a regenerative snubber using magnetic coupling [ 5 ] when applied to the Sepic and Cuk converters operating in Continuous Conduction Mode (CCM). The energy stored in the snubber capacitor is transferred to the load or to the input source by an inductor coupled to the main inductor of the converter, reducing the number of the snubber components. The regenerative snubber applied to the buck, boost, buck-boost and Zeta converters was presented in [6].
11. THE OPERATION PRINCIPLE The description of the operation stages is done for one of the possible solutions obtained for the Sepic converter (Sepic #I). The other possible alternatives are shown in Fig. 1 for the Sepic converter, and in Fig. 2 for the kuk converter. Fig. 3 presents the expected voltage and current switch waveforms. During the turn-off, the current is diverted from the switch to the snubber capacitor (Cs), allowing a soft-transition of the switch voltage (ZVS - Zero Voltage Switching). When the switch turns on, the small inductor in series with the switch (Ls) provides soft-transition of its current (ZCS - Zero Current Switching), reducing the reverse recovery losses of the output diode (Do). At this moment, the energy stored in the snubber capacitor (Cs) is transferred to the load by a coupled inductor (LM)in a resonant way. Ideally the leakage inductance of the coupled inductor should be the LR inductance, in order to obtain a compact structure. The output diode current iDo decreases at a low di/dt rate due to the turn-on snubber inductor influence (LA. At the end of the process, the recovery charge is low. After the diode turns off, the connection point of Ls. LO and Do becomes floating. Without the DG diode, a resonance between the equivalent diode capacitance and Ls occurs, until the connection point voltage settles at V,N+VO. This resonance appears in the DO voltage as an overvoltage. This voltage stress usually is not high, but must be considered in the diode selection (e.g., considering the diode reverse voltage equal to 2*( ViN+Vo) as a start point). The output diode does not present any current stress. To overcome the Do overvoltage problem the diode DG is used, clamping the output diode voltage to VfN+VO.
123
-
(b) Sepic #2 n
L:
m
N
%
1
a (d) - Sepic #4
-
(e) - Sepic #5
(f) Sepic #6
Fig. 1- Topological alternatives for the Sepic converter.. 111. TURN-ON AND TURN-OFF ANALYSIS In the following analysis, the switch output capacitance (Coss) was neglected. This assumption can be made since its value is small if compared with the auxiliary capacitance Cs. One should pay special attention on the cases that Coss value is close to Cs one. In these cases, its value must be included in the analysis and design equations of the circuit devices.
A . Turn-Offswitching Initially, the Cscapacitor voltage is zero. During the turn-off, the capacitor charges with constant current (equal to the sum of Ll and L2 average currents, i.e. 11+12)until its voltage reaches the sum of the input and output voltages (KN+VO).At this moment, the output diode Do starts conducting and a resonance between Cs and inductor Ls occurs. The capacitor
c (a) - Cuk #I
(b) - Cuk #2
, A-
(d) - Cuk #4
(c) - Cuk #3
Fig. 2 - Topological alternatives for the Cuk Converter.
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Analyzing the circuit in Fig. 5, one can obtain the voltage across and current through the capacitor during the resonance as: vcs ( t ) = r + (v, - r) cos(^^,, t )
(7)
r=(vo-nvIN)
(9)
where:
Fig. 3. Expected switch voltage and current. resonant circuit of the capacitor charging process is shown in Fig. 4, in the s domain. The initial capacitor voltage is equal to (r’lN+
YO).
From the analysis of Fig. 4, one can obtain the voltage across and the current through the capacitor during the resonance as:
At the end of the resonance the capacitor Cs will be discharged, allowing the switch to turn-off with zero voltage in the next switching period.
IV. DESIGN EQUATIONS FOR SEPIC #1 The desired capacitor voltage and current at the end of the turn-on resonance are zero. Applying this condition in (7), one can obtain the turns ratio n of the coupled inductor as: n = (2 vo - vc)/(2 VI&.)
Depending on the capacitor voltage value, the auxiliary diode DR can start conducting. If it happens, two additional resonant stages can occur. Their analysis is complex and the effects caused by them establish some operation limits (such as switch overvoltage and limitations on the converter duty cycle) [7]. To ensure that the auxiliary diode DR will not conduct, the maximum capacitor voltage (VJ must be limited to:
Vc 5 Vo ( n + 1) where:
(It is important to mention that for Sepic #3, #4, #5, #6 and for Cuk converters the desired capacitor voltage at the end of the turn-on is -VIN). The auxiliary resonant inductance value is chosen from the desired resonance time tR, which is obtained doing WRONtR = n.Therefore: (13) It is important to observe that, in practice, LR will be the leakage inductance of the coupled inductor itself. The turn-on snubber inductance value Ls is chosen in order to reduce the reverse recovery losses of the output diode Do.It is obtained by limiting the switch (di/dt)on, so: L.S
B. Turn-On Switching During the turn-on, the discharge of the Cs capacitor through the LR inductor and the DR diode into the load will occur. The capacitor discharge resonant circuit is shown in Fig. 5, in the s domain. The initial capacitor voltage is equal to Vc (the final value of the capacitor voltage at the last resonant stage during turn-off).
(12)
= (v/N + vO )/(diMo l d t )
(14)
The capacitance Cs must be chosen so that ( 5 ) is satisfied, avoiding the auxiliary diode DRconduction. Defining: converter voltage gain
Fig. 4 - s-domain equivalent turn-qf circuit.
Figura 5 - s-domain equivalent turn-on circuit.
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a
M,,
is the switch overvoltage respect its original value, without snubber (is the ratio V M 0 , s n u b h / V M O , ~ ~For ~ , ,the , ~ Sepic ~ ) . #1 converter, MsW = MVC. normalized turn-off impedance: a
z P = zRoff
(17)
RO
Using (15) e (16), the limit established by (5) can be defined as : M,
(2m2
+ 2m)/(m’ + 3m + 2 )
a
(18)
From (6), MvCcan be given by:
+ zp)
(19)
M, = 1 (m
Fig. 6 was obtained from (18) and (19). The upper curve represents the maximum value of (18) that assures the nonconduction of DR during turn-on. The lower curve is obtained doing Z, = 0 in (19). The shadowed area of Fig. 6 shows the operational region of the proposed snubber applied to the Sepic #1 converter. The converter design in the mentioned area avoids undesirable conduction of DR. Furthermore, the acceptable switch overvoltage, Mvc, is established. Analyzing Fig. 6 it can be noted that for proper operation of Sepic #1 converter the converter voltage gain must be greater than 2 (m>2). This kind of limitation also applies to the following converters: Sepic #2 (rn>2), Sepic#5 (m
