Analysis and Design of A Zero Voltage Transition Dc-Dc Boost Converter

Analysis and Design of A Zero Voltage Transition Dc-Dc Boost Converter

Genesia C Tavares

Electrical and Electronics Engineering Department Goa college of Engineering, Farmagudi,

Ponda, Goa 403401

Noah Dias

Electrical and Electronics Engineering Department Goa college of Engineering, Farmagudi,

Ponda, Goa 403401

Abstract In the last few years, there is an enormous growth in Renewable energy sources. During the extraction of electric power from renewable sources, dc-dc converters play a very significant role. This paper, proposes a boost converter wherein the switching of the main switch is done under Zero Voltage Transition. Here the proposed converter is modeled for a load of 250W. The auxiliary circuit which aids in the soft- switching resonates for a small portion of the switching cycle of the converter and enables the soft transition of the switch from ON state to OFF state and vice versa. This will thus help in improving the converter efficiency by reducing the losses that occur due to hard transition of the switches. The converter is modeled in MATLAB simulink.

KeywordsBoost converter, Auxilliary circuit, Zero voltage switching, Zero current switching.

1. INTRODUCTION

   The conduction losses are very low in conventional hard switching converters, but the efficiency is low due to high switching losses [1] [2]. Thus, in order to make the switching transitions at either zero-voltage condition or zero current condition the technique of soft switching is introduced, so that the main portion of losses can also be reduced and the efficiency of the converters can be highly improved [2].

   In Power Electronic converters, switching in any case when the converter is ON, the power losses are zero. During the OFF state the current will not pass through the switch hence there will not be any power losses. Whenever the switch goes from ON state to OFF state and OFF state to ON state power losses will occur if we do not have any way to reduce either the current or voltage to zero [1]. This happens in hard-switching converters and power losses will occur if there will be a voltage applied across and a current flowing through the switch. When the switching device turns ON or OFF, the voltage and current are huge and it will result in high losses. [2]

   The power losses that occur when switching are the product of the voltage appearing across the terminals of the switch and the current which flows through the switch. The entire loss is the product of energy lost and the switching frequency. These losses are referred to as switching losses.

2. SOFT SWITCHING TECHNIQUES

   1. Methods of Soft Switching

      There are two methods for soft switching techniques to attain zero losses switching where either the voltage or current through the device is made zero during switching.

      1. Zero current switching (ZCS)

         Zero Current Switching is achieved with a switch having an inductor and a blocking diode in series [2]. The switch turns ON under ZCS as the rate of increase of current after the voltage turns zero is controlled by the inductor. The inductor does not allow sudden change in current, and hence it rises linearly. When a negative voltage appears across the combination of inductor and switch with a resonant circuit, the current flowing through the switch is automatically reduced to zero which turns OFF the switch under zero current switching.

      2. Zero voltage switching (ZVS)

      A switch which operates at ZVS will have an anti-parallel diode and a capacitor across it [2]. During turn OFF since the current goes to zero, the capacitor controls the rate of voltage increase taking place across the switch. Since the capacitor does not allow sudden changes in voltage, it will rise linearly. The turn OFF characteristics of the switch is controlled by a capacitor. This capacitor reduces the voltage rise as current flow reduces to zero.

   2. Proposed Soft-Switching Boost Converter configuration

      The zero-voltage DC-DC boost converter diagram is described in fig 1. It is basically a boost converter with a diode D1, input inductor Lin, main switch S1 and output capacitor Co with a load Rld. This circuit also contains additional circuit called the auxiliary circuit that will resonate for a small portion of time. This circuit consists of an inductor Lr, capacitor Cr, diodes D2 to D5 and a capacitor Cb which will feed the resonant energy to the load. The capacitance Cs is the parasitic capacitance of the switch S1 and not any external capacitance.

      Zero Voltage Transition works on the principle that the auxiliary circuit will carry a current much higher than of the input current [1][4][5], but for a small period of the switching cycle so as to attain the soft switching in the converter. The main switch is perfectly turned on under ZVS and near ZCS by ZVT with a parallel resonance, the main switch is perfectly turned off under ZCS and ZVS by ZCT with a serial

      resonance. The auxiliary switch is turned on with near ZCS [5].

      Zero Voltage Transition converters have higher efficiency unlike the hard-switching converters due to the decreased switching loss.

      This circuit reduces the conduction losses of all the switches; main and the auxiliary ones and also all the diodes. This can be done using a capacitor which will control the turn-off characteristics of the main switch, whose resonant energy will be dissipated into the load [1]. The capacitor Cb is the feed-forward capacitor that controls the turn-off of the main switch. The energy retained by this capacitor will be discharged fully to the load through the diodes D1 or D3. The rms current of the auxiliary circuit is considerably decreased hence the conduction power losses will be reduced too.

      Fig. 1. Schematic of the proposed ZVT DC-DC Boost Converter

      Fig. 2. Waveforms of the different modes of the DC-DC Boost converter

   3. Modes of operation of the above converter.

   We shall look at the circuit operation for one complete switching cycle, which is divided into seven parts. Each of these parts is explained with its equivalent circuit for that particular interval.

   In the beginning the diode D1 supplies the output current and the circuit acts as a simple PWM boost converter. Under this condition circuit is shown in fig 3

   Fig. 3. Schematic of circuit in simple Boost mode

   Mode 1[t0-t1]:

   At t0 switch S2 is turned ON with Zero Current Transition due to the presence of auxiliary resonant inductor which is connected serially to it. The current will start to move slowly from the diode D1. The resonant inductor present will decrease the turn-off current rate through D1 that turns off under ZCS. Now the auxiliary current flowing through the inductor equals to the input current. The circuit for this interval is explained in fig 4.

   Fig. 4. Schematic of circuit for Mode 1

   Mode 2[t1-t2]:

   The auxiliary circuit current will keep on increasing here. The input current supplied shall be assumed to be constant since the input inductance will not allow sudden change in the current. Hence the capacitance Cs of the main switch S1 will start to discharge into the auxiliary circuit to supply for the increased portion of the auxiliary current. In this interval the capacitance discharges completely. The equivalent circuit for this mode is explained in fig 5.

   Fig. 5. Schematic of circuit for Mode 2

   Mode 3[t2-t3]:

   After t2 the diode present in the main switch which is connected in anti-parallel to it starts conducting, which will make the voltage blocked y switch S1 to be zero. This is the Zero Voltage period during which the switch S1 must be supplied with the trigger. End of this interval the current carried by the auxiliary circuit equals the input supply current and the main switch is in a condition to start conduction. The equivalent circuit of the converter for this mode is explained in fig 6.

   Mode 6[t5-t6]:

   During the start of this interval, the auxiliary current will turn zero and the resonant cycle shall end here. The circuit will run as an equivalent PWM boost converter during its charging state. The equivalent circuit of the converter in this interval is explained in fig 9.

   Fig. 9. Schematic of circuit for Mode 6

   Mode 4[t3-t4]:

   Fig. 6. Schematic of circuit for Mode 3

   Mode 7[t6-t7]:

   At t6 during this interval S1 is turned-off. The feed forward capacitor Cb will cause for the slow voltage rise across S1. Voltage across the capacitor Cb will reverse bias the diode D1 and hence it will not conduct. The energy present in the

   The auxiliary current stops the input supply current and the

   remaining of the input supply current starts flowing through the switch S1. During the end of this interval the current in the auxiliary circuit becomes zero. The equivalent circuit of the converter for this mode is explained in fig 7

   capacitor Cb during the cycle discharges through the diode D3 and when this voltage will be zero the diode D1 s tarts conducting and the next switching cycle gets started. The equivalent circuit of the converter for this mode is explained in fig 10.

   Mode 5[t4-t5]:

   Fig. 7. Schematic of circuit for Mode 4

   Fig. 10. Schematic of circuit for Mode 7

3. CONVERTER DESIGN

   The direction of current flow in the auxiliary circuit changes

   and the negative portion of the resonant cycle will start during this interval. Diode D4 in series with the switch makes the branch unidirectional and will not allow the switch S2 to conduct and this current passes through the diode D5 resulting in a Zero Voltage turn-off condition for S2. The current from the diode D2 will pass to the capacitor Cb hence charging it. The equivalent circuit for this interval is explained in fig 8.

   Fig. 8. Schematic of circuit for Mode 5

   Design specifications:

   The specifications for the design of the converter are given in Table I for a load of 250W.

   Sr NO	Parameter	Specification	Value
   1	Output power	Pout	250W
   2	Output voltage	Vout	400V
   3	Input voltage	Vin	90-265V
   4	Switching frequency	Fsw	100kHz
   5	Output voltage ripple	Vrp	1%
   6	Input current peak ripple	Irpp	20%

   Table I

   Find Rload:

   out out

   RLoad=V 2/ P = 640 ; (1)

   For a conventional boost converter:

   Vout /Vin= 1/(1-D) (2)

   Hence D = 33.75%

   Let Dmain = 30% and Daux= 3.75%

   Output Capacitor:

   The peak charging current of the capacitor is:

   = = = (3)

   The voltage ripple across C0 is:

   = (4)

   = = (5)

   = 207

   Input Inductor:

   The maximum peak-peak ripple current is

   = . =x 20% = 0.827A (6)

   The maximum peak input current with ripple is

   = + =4.135 + (7)

   = 4.55A

   The duty ratio of the converter when maximum current occurs is

   Resonant Capacitor:

   Assume Zr=0.21 pu

   = = (11)

   = * = 0.21 x 107.469 = 22.568 (12) = 11nF

   Auxiliary capacitor Cb:

   For proper design we select the curve K=3

   K = =3 (13)

   = = 3.66nF (14)

4. SIMULATION DETAILS

1. Design parameters of the Boost converter

   Sr NO	Parameter	Specification	Value
   1	Resonant inductor	Lr	6µH
   2	Resonant capacitor	Cr	11nF
   3	Feed-forward capacitor	Cb	3.66nF
   4	Boost inductor	Lin	1050µH
   5	Output capacitor	Co	470nF
   6	Input voltage	Vin	265V
   7	Switching frequency	Fsw	100kHz

   The converter circuit is simulated in MATLAB Simulink for a switching frequency of 100kHz, with the design parameters in Table II.

   = 1-= 1 – = 0.682 (8)

   The input inductor value is calculated as follows

   = = (9)

   = 1050µH

   Resonant Inductor:

   Assuming the value of =30ns

   = = (10)

   = 5.8µH 6µH

   Table II

2. PWM Signals of the main and auxiliary switch

   For zero-voltage switching, delay is required in the PWM signal of the main switch, during which the auxiliary switch is turned ON [11]. The PWM signals of both switches is shown in fig 11.

   Fig 11. PWM signals S1 and S2

3. Simulink Circuit

   Below fig 12 is the circuit design in MATLAB Simulink wherein the output voltage of 400V is obtained for 265V

   input.

   Fig 12. Simulation circuit

4. Simulation results

   1. Output Voltage = 400V

   2. Auxiliary Inductor Current

   3. Auxiliary Capacitor Voltage

   4. Feed Forward Capacitor Voltage

   5. Main Switch S1 Voltage

   6. Main Switch S1 Current

   7. ZVS ON of Main Switch S1

   8. ZCS ON of S2

CONCLUSION

In this paper a DC-DC Boost converter with soft switching techniques is designed and analyzed. The proposed converter decreases the switching losses and increases the efficiency of the converter. The converter uses an auxiliary resonant circuit in order to achieve ZVS. The detailed design and analysis are discussed. MATLAB Simulink is used to verify the operation and performance of the proposed converter.

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