A New Soft-Switched ZCZVT-PWM DC-DC Boost Converter

In this paper, a new soft-switched zero-current transition and zero-voltage transition (ZCZVT) converter is proposed for family pulse width modulation (PWM) DC-DC converters. The proposed new converter assures the main switch turning on under zero-voltage transition and turning oﬀ under zero-current transition. The proposed new converter also decreases electromagnetic interference noises and operates under soft switching in a wide input voltage range. There is no additional voltage or current stress on the main devices. The proposed converter has low cost and simple control and structure. The theoretical analysis of the converter is clariﬁed and the operating intervals are given in detail. The experimental results of the converter are obtained by a prototype with 500 W and 100 kHz. The overall eﬃciency of the proposed converter is 98.5% at nominal output power.


Introduction
Pulse width modulation (PWM) DC-DC converters are often used in industry due to their fast dynamic response, simple structure, low cost, and easy control [1].High switching frequency is desired in DC-DC converters.When the switching frequency is increased, power density increases and the cost decreases since the sizes of the inductance, capacitor, and transformer decrease at high frequencies.However, the high switching frequency causes switching losses and electromagnetic interference (EMI) noises.These issues cause the decrease of the total efficiency and performance of the converter.Therefore, zero-voltage switching (ZVS) and zerocurrent switching (ZCS) techniques are improved in order to reduce switching losses.In the fundamental zero-voltage transition (ZVT) converter, the voltage of the main switch falls to zero with an active snubber cell while the main switch is in the off state [2].Then a control signal is applied to the gate of the main switch while its voltage is zero.Thus, the main switch turns on without loss under ZVT.The main diode turns off under ZCS and the auxiliary switch turns on under ZCS.The main transistor and the auxiliary transistor also turn off under hard switching (HS).Many studies have been done to overcome these problems [3][4][5][6].
In the fundamental zero-current transition (ZCT) converter, first the current of the main switch is dropped to zero with an active snubber cell while the main switch is in the on state [7].The control signal of the main switch is removed while its current is zero, so the main switch turns off under ZCT.The main diode turns on under ZVS and the auxiliary switch turns on under ZCS, but the main switch turns on under HS.The auxiliary switch and the main diode turn off under HS.The main diode has a high reverse recovery loss.Many studies have been made to overcome these problems in ZCT techniques [8][9][10][11].Turning off without switching loss is possible because of ZCT while turning on without switching loss is possible because of ZVT.Many studies have been done to eliminate disadvantages in the ZVT and ZCT techniques.However, these studies have many drawbacks, including HS, high EMI, and current or voltage stress.The best solution to overcome these drawbacks is combining the ZCT and ZVT techniques.Many studies have been made on the subject of combining ZCT and ZVT (ZCZVT) [12][13][14][15][16][17][18].Some disadvantages occur with several soft-switched converters in the literature.In [6], turning off the main switch is not soft switching (SS).In [14], the SS conditionals depend on the duty cycle.The SS is not provided under a 0.5 duty cycle and there is an additional current stress on the main switch.In [12], a magnetic coupled inductor is used in the ZVT-ZCT converter.It needs tight magnetic coupling or else additional losses and parasitic oscillations occur due to the leakage inductance.In [15], there is additional current stress on the main switch.In this case, the values of the semiconductor devices and the losses of the converter increase.In [16], there is additional current stress on the main switch.It leads to both additional conduct losses and cost increase.A new converter is proposed here in order to overcome all of these problems and the proposed new converter is compared to that of [16].The comparative efficiency results are examined with efficiency graphics.

Features of the proposed converter
The proposed new ZCZVT PWM DC-DC boost converter is shown in Figure 1.In this converter, ZVT is provided by turning on and ZCT is provided by turning off for the main switch.The main diode turns on under ZVS and turns off under ZCS.Additional current or voltage stress does not occur on the main switch and the main diode.The auxiliary switch and the auxiliary diode both turn on and turn off under ZCS.The proposed converter decreases EMI noise and operates even under a wide range of line and load voltages.

Operating intervals
Eleven intervals occur in the steady-state operation of the converter in one switching period.The equivalent circuits of these intervals are shown in Figure 2. The key waveforms of the operation intervals are shown in Figure 3.
Interval 1 [ t0< t <t1: Figure 2a ]: Before t = t0, the main switch S1 and the auxiliary switch S2 are in the off state.The main diode DF conducts the input current Ii.At t = t0, iS1 = 0, iDF = Ii, iS2 = i Lb = 0, i La = 0, v Cp = Vo, and vCs = -VC0 are valid.At the beginning of this interval, a control signal is applied to auxiliary switch S2 and the current of the main diode DF decreases while the current of the auxiliary switch S2 and the voltage of the snubber capacitor Cs increase.
The main diode current iDF(t), lower resonant inductor current iLb(t), and resonant capacitor voltage vCs(t) can be expressed as follows: ( ) ( ) ( ) ( ) where is valid.At t = t1, the current of main diode DF falls to zero when the current of the snubber inductance Lb reaches the input current Ii.The voltage of the snubber capacitor reaches a value of (-VC1).
Consequently, the auxiliary switch S2 turns on under ZCS because of series resonance inductances Lb and the main diode DF turns off under ZCS.
Interval 2 [ t1< t <t2: In this interval, a resonance begins between parasitic capacitor Cp and the snubber cell via Cp -Cs -Lb -S2.
The energy of Cp is transferred to the snubber cell.Thus, vCp voltage decreases while iLb current and vCs voltage are increasing.For this interval, the following equations are written: ( ) ( ) ( ) where, In the end of this interval, at t = t2, vCp voltage falls to zero and iLb current is maximum.The voltage of snubber capacitor Cs is equal to zero.
, iLa = 0, vCp = 0, and vCs = 0 are valid.In this interval, a new resonance starts via Lb -La -Cs.At thebeginning of this interval, the D1 diode turns on under ZVS and the energy of the resonance inductor Lb is transferred to La and Cs.It is called the ZVT interval where D1 is in the on state.A control signal is applied to the main switch S1 when its internal diode conducts.In this way, the main switch S1 turns on as lossless with ZVT.At t = t3, the current of snubber inductance Lb falls to the input current level and the D1 diode turns off under ZCS.Then the current of the main switch increases and the current of snubber inductance Lb continues to decrease.For this interval, the following equations are written: where, ( ) At t = t5, the voltage of capacitor Cs reaches the maximum voltage value in the adverse direction and the current of snubber inductor La falls to zero.This interval is then finished.
Interval 5 [ t5< t < t6: Figure 2e ]: During this interval of the conventional boost converter, the main switch S1 conducts the input current Ii.At t = t6, a control signal is applied to the main switch S1 and this interval is finished.
At the beginning of this interval, a control signal is applied to auxiliary switch S2 and it turns on under ZCS because of series snubber inductance Lb.A resonance starts between the snubber capacitor Cs and the snubber inductance Lb via Cs -Lb -S2 -D1.The current of the main switch S1 begins to decrease while the current of snubber inductance Lb is increasing.
At t = t7, the current of S1 falls to zero when the current of Lb reaches the input current value.D1 conducts the excess of input current.This interval is called the ZCT interval where D1 is in the on state.The control signal of the main switch S1 is removed when its internal diode is in the on state.In this way, the main switch S1 turns off as lossless under ZCT.At the same time, the resonance via Cs -Lb -S2 -D1 goes on.For this interval, the following equations are written: ( ) ( ) where, At t = t8, the current of snubber inductance Lb reaches the maximum value (ILbmax) when the voltage of snubber capacitor Cs falls to zero.This interval is then finished.
Interval 7 [ t8< t < t9: 1 cos At t = t10, the current of Lb falls to zero and the control signal of S2 is removed, so S2 turns off under ZCS and this interval is finished.
At t = t12, the current of the snubber inductor La falls to zero and vCs voltage reaches to -VC0 interval is finished.
Interval 11 [ t12< t < t13 = t0: Figure 2k ]: In this interval, in which the main diode DF is in the on state, the input current is transferred to the output through DF.This mode is the conventional PWM boost converter, which is in the off state interval.Thus, it is returned to initial conditions and the intervals expressed are repeated in the next switching cycle.

Design Procedure of Proposed Converter
The characteristic curves of the ZCZVT PWM DC-DC boost converter are shown in Figures 4{7.These curves will be considered when the elements of the converter are determined.In these curves, the value of snubber inductance Lb is kept constant at 3 H and the variations of La and Cs are graphically shown.As shown in Figure 4, the duration of ZVT increases while the value of La is decreasing and the value of CSis increasing.Figure 5 illustrates that the duration of ZCT is constant while the value of La is increasing.
On the other hand, the duration of ZCT increases when the value of Cs increases.In Figure 6, as expressed in the fourth interval, the variations of resonance duration between La and Cs are shown.This duration is more important because it affects the total time of resonance between interval 8 and interval 10.It is possible to see from the curves that tLa Cs durations increase depending on the increasing of the values La and Cs.tLa Cs is the resonance duration between La and Cs.
Finally, Figure 7 illustrates the variations of resonance peak current that the auxiliary switch S2 conducts.This peak value is fairly important in order to provide ZVT and ZCT conditionals.It is shown from the curves that the variation of La does not change the peak current of S2 but the decreasing of Cs is increasing the peak value of this current.As shown in the figure, the resonance current reaches a level of twice the input current. 1.
The parasitic capacitor Cp consists of the parasitic capacitor of the main switch and the other parasitic capacitors.This value is almost 1.2 nF.Therefore, it does not require an extra capacitor.

2.
The snubber inductance Lb must choose to provide ZCS turn on for auxiliary switch S2.The turned on duration tr of S2 is considered while this value is being determined.In this case, the value of snubber inductance Lb is chosen as 3µH and then La is 6µH.
As shown in Figures 4 and 5, a smaller snubber capacitor means shorter ZCT and ZVT durations.Shorter ZVT and ZCT durations are undesirable.If the value of Cs is increased, ZVT and ZCT durations increase.However, in this case, the resonance duration between La-CS and the other resonance durations will be extended.Consequently, the transient resonance intervals increase in the period and an undesirable case occurs because the total transient resonance intervals should be less than 20% of the total time of a period in order to assure PWM operation.For this reason, the value of CS cannot be increased over a determined value.Finally, the peak value of the resonance current should be twice the peak value of the main inductance current because the SS should ensure even in the worst conditions that the input current is the maximum value.Figure 7 can be used for determining this current value.It is seen that the most suitable value for CS is 4.7 nF.

Experimental Results
The photograph of the experimental circuit is given in Figure 8.A prototype of the proposed ZCZVT PWM DC-DC boost converter with 500 W and 100 kHz was realized at the laboratory.The nominal values of elements used in the converter are shown in Table 1 by considering datasheets of manufacturers.In Figure 9b, the control signal and the current of the main switch are shown.As shown in the figure, the control signal is applied and removed while the body diode of the main switch is in the on status, so ZVT and ZCT are perfectly assured.
In Figure 9c, the current and the voltage waveforms of the main switch are shown.As shown in the gure, the voltage of the main switch falls to zero with the snubber cell before the main switch is turned on.
Then the main switch turns on under ZVT while the body diode of the main switch conducts.Likewise, the current of the main switch falls to zero with the snubber cell before the main switch turns off.
Then the main switch turns off under ZCT while the body diode of the main switch conducts.It is seen that there is not any voltage or current stress on the main switch.In Figure 9d Additionally, it is shown from the results that there is additional voltage stress on the auxiliary switch and its value is equal to 1.5 Vo.In Figures 9f and 9g The efficiency graphics of the new converter for SS and HS are shown in Figure 10.While the efficiency value of the converter is 92.8% under HS, it is measured at about 98.5% for SS at the nominal output power.Moreover, the soft-switched new converter is compared with the converter in [16] and it is shown in graphics that the efficiency of the new proposed converter is higher.The theoretical analysis of the new ZCZVT-PWM boost converter is verified with the experimental results.

Conclusions
In this paper, a new ZCZVT-PWM DC-DC boost converter with an active snubber cell is proposed.
The proposed converter has advantages of the previous ZVT and ZCT studies and it also eliminates their disadvantages.The snubber cell of the proposed converter operates in a small part of the period and ensures that the main switch turns on ZVT and turns off ZCT in a lossless manner.Moreover, it ensures that the main diode turns on ZVS and turns off ZCS.Thus, it minimizes the reverse recovery losses of the main diode.It also does not add any extra voltage or current stress to the main devices.The current stress of the auxiliary switch is at an acceptable level.Considering all these features, an improved ZCZVT-PWM DC-DC converter that combines the advantages of PWM and resonance operating in a snubber cell and overcomes the disadvantages is proposed.

Figure 1 .
Figure 1.The basic circuit scheme of the proposed new ZCZVT-PWM DC-DC boost converter.

Figure 2 .
Figure 2. Equivalent circuit schemes of the operating intervals in the proposed ZCZVT-PWM DC-DC boost converter.where Figure 2g]: At t = t8, iS1 = (Ii -ILbmax) , iDF = 0, iS2 = iLb = ILbmax, iLa = 0, vCp = 0, and vCs = 0 are valid.In this interval, a new resonance starts via Lb -La -Cs.The energyof the resonance inductor Lbis transferred to La and Cs.Then the voltage of snubber capacitor Cs begins to become positive and the current of La begins to increase.For this interval, the following equations are written: Figure 2h]: At t = t9, iS1 = 0, iDF = 0, iS2 = iLb = Ii, iLa = IL9, vCp = 0, and vCs = VC9 are valid.This interval begins when D1 turns off.Then a resonance occurs via Cp-La -Lb -Cs under the constant input current.During this interval, the current of La and the voltage of Cs increase while the current of snubber inductance Lb decreases.For this interval, the following equations are written:

Figure 3 .
Figure 3.The key waveforms of the operation intervals of the proposed ZCZVT-PWM DC-DC boost converter.Interval 9 [ t10< t < t11: Figure 2i ]: At t = t10, iS1 = 0, iDF = 0, iS2 = iLb = 0, iLa = IL10, vCp = Vp10, and vCs = VC10 are valid.In this interval, there are two different circuits.The parasitic capacitor Cp is linearly charged under the constant input current and the energy stored in the upper snubber inductance La is transferred to Cs via La -D2 -Cs resonance.For this interval, the following equations are written:

Figure 4 .
Figure 4.The variation of main switch ZVT duration

Figure 5 .
Figure 5.The variation of main switch ZCT duration depending on La-CS.

Figure 6 .
Figure 6.The variation of resonance duration between La-CS.

Figure 7 .
Figure 7.The variation of the auxiliary switch peak current depending on La -CS.
time, the value of snubber inductance La should be at least twice as big as Lb because the auxiliary switch S2 is turned off with ZCS.

Figure 8 .
Figure 8. Photo of the experimental circuit.

Figure 9
Figure9shows the experimental results of the proposed new ZCZVT-PWM DC-DC boost converter taken at full load.These results verify the previously mentioned theoretical analysis.In Figure9a, the control signals of the auxiliary and the main switch are shown.The auxiliary switch operates twice in one switching cycle of the main switch.
, the voltage and current waveforms of Cs and D2 are shown.As shown in the Figure, the voltages of Cs and D2 do not exceed the output voltage.

Figure 10 .
Figure 10.Efficiency graphics of the proposed new converter in the case of SS and HS.

Table 1 .
Nominal values of the semiconductors in the used experimental circuit.