Soft Switched DC-DC PWM Converters

Soft Switched DC-DC PWM Converters

Soft switched DC-DC PWM Converters

Mr.M. Prathap Raju(1), Dr. A. Jaya Lakshmi(2)

Index Terms PWM, DC to DC, buck, boost, fly back, Zero Current Transition (ZCT)

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1. PULSE WIDTH Modulated (PWM) dc/dc converters are vastly used in industry because of their high-power capability and fast transient response. To reduce the volume and the weight of these converters, higher switching frequency operation is preferred. In high power frequency requirements, power semiconductor switches are subjected to high switching stresses and switching losses, which limits the operating switching frequency. Generally, a snubber circuit reduces the switching losses and stresses, but increases the total power loss in the converter. In resonant and quasi-resonant converters, the switching losses are reduced; however, the converter control system is usually a frequency control instead of a PWM control. Furthermore, in these converters, high voltage or current stresses are applied to semiconductor devices due to the nature of the resonance. Zero Current Transition (ZCT) and Zero-Voltage Transition (ZVT) techniques incorporate a soft switching function into standard PWM converters [1][9]; thus, the switching losses can be reduced. In these converters, an auxiliary circuit is added to the main PWM converter, which functions only at switching instances and recovers the switching losses. For high-power applications, an Insulated Gate Bipolar Transistor (IGBT) is the preferred device. However, the IGBT exhibits tailing current at turnoff, which increases turnoff switching losses. Hence, ZCT techniques provide better results than ZVT techniques. Several ZCT converters have been previously proposed in [1][9], but they suffer from one or more of the following drawbacks.

   1. In some topologies, the main switch turn-on is not soft and, thus, limits the gain in efficiency [1].

   2. The main switch peak current is increased considerably [2] [4].

   3. The switches turn off are not soft [5].

   4. There are additional semiconductor devices in the main power path that increase conduction losses [6], [7].

   5. The proposed technique cannot be applied to fly back converters [1], [2], [5].

   A new family of ZCT converters lacking the aforementioned disadvantages was introduced [9]. In these converters, the main switch and the auxiliary switch are turned on and off under Zero-Current (ZC) condition, so that the switching losses and stresses are significantly reduced. The energy of the proposed auxiliary circuit is absorbed from the input voltage source and is transferred to the output, which boosts the effective output duty cycle. However, in other converters, usually, the auxiliary circuit energy is just a circulating energy [1],[2],[5][9]. One of the advantages of the proposed auxiliary circuit is that it can be applied to fly back converters. A ZCT fly-back converter was introduced in [4], which has disadvantages in comparison to the proposed ZCT fly back as discussed in Section V. The idea of the proposed auxiliary circuit is discussed in section II. The ZCT buck converter was analyzed in [1]; however, since it is the base of this converter family, its operation is briefly discussed in Section III. In Section IV, fly back ZCT converter is explained. As the requirement of boost converter is high in industries, detailed performance characteristics are detailed in section IV. The performance parameters derived for different ZCT PWM converters are presented in Section V. Simulation results and comparative analysis for the same is presented in the proceeding sections.

2. Generally, in non isoated fundamental converters, one way to create ZC condition for switch turn-on is to have a snubber inductor in series with the switch or diode. However, at turnoff, this inductor will cause a voltage spike on the switch. Therefore, a pulse current source is required to provide the output current, and, thus, the switch can be turned off under ZC condition while preventing the voltage spikes. To reduce the number of circuit elements, the pulse current source path and the required snubber inductor to decrease turn-on losses can be combined as shown in Fig. 1 for a buck converter. A pulse voltage source can be applied to the snubber inductor and create the required pulse current source at switch turnoff. Design of resonant inductance and capacitance should be done such that the underdamped nature of the LC circuit at a certain resonant frequency is ensured. Switching frequency of main

   and auxiliary switches is to be considered with the reference of the resonant frequency for which the LC components are designed.

   Fig.1 Schematic ZCT buck converter (a)Snubber Inductor in series with a diode and (b)Snubber inductor in series with a switch.

3. The proposed ZCT buck converter is shown in Fig.2. The circuit is composed of the main switch S, the main diode D, the auxiliary switch Sa, the auxiliary diode Da, the auxiliary inductors La1 and La2, and the auxiliary capacitor Ca. La1 is the snubber inductor, and the pulse voltage source is basically represented by Ca and Sa. La2 and Da are used to recharge the capacitor at every cycle. The converter has seven distinct operating intervals during one switching cycle. Before the first interval, it is assumed that the auxiliary capacitor is charged to 2Vin, and the main switch is conducting. Key theoretical waveforms of the converter are shown in Fig. 3.

   Fig.2 Proposed ZCT buck converter.

   Interval 1 [t0t1]: This interval begins with turning the auxiliary switch on. This starts a resonance between the auxiliary capacitor Ca and the auxiliary inductor La1. During this resonance, the La1 current increases until it reaches the output current I0 and reduces the switch current to zero.

   Interval 2 [t1t2]: In this interval, the main switch body diode starts to conduct, and the resonance between the auxiliary inductor La1 and the auxiliary capacitor Ca will continue.

   Thus, the main switch can be turned off under the ZC condition before the La1 current is reduced back to the output current I0.

   Interval 3 [t2t3]: In this interval, the La1 current is equal to I0, and the auxiliary capacitor linearly discharges until its voltage becomes zero at the end of this interval.

   Interval 4 [t3t4]: This interval starts when the main diode begins to conduct under the zero-voltage condition, and the auxiliary switch can be turned off under the ZC condition. In this interval, the main diode current and the La1 current are equal to Io. This interval ends when the main switch is turned on.

   Interval 5 [t4t5]: When the main switch is turned on, the La1 current begins to linearly decrease, and the La2 current starts to increase in a resonance fashion with Ca. Therefore, the main switch turns on under the ZC condition.

   Interval 6 [t5t6]: This interval starts when the current of La1, which flows through the main diode, reaches zero, and the main diode turns off under the ZC condition. At the end of this interval, the resonance between La2 and Ca ends when Ca is charged to 2Vin, and Da prevents the current from going negative.

   Interval 7 [t6t0 + T]: The output current runs through the main switch, and the circuit behaves like a regular PWM buck converter.

   Fig.3 Key theoretical waveforms of the ZCT buck converter.

   1. (b)

reaches Ip, The main switch is turned off under the ZC condition.

1

1

L a 1.C a

(c) (d)

(e) (f)

nV0, and the rectifying diode D is forward bised. During this interval, the energy stored in Ca is transferred to the output, which is equivalent to boosting the effective duty cycle.

2

(L a1

1

• L l ).C a

  (g)

  Fig.4 Equivalent circuit for each operating interval of the

  proposed ZCT buck converter. (a) (t0 t1). (b) (t1 t2). (c) (t2 t3). (d) (t3 t4). (e) (t4 t5). (f) (t5 t6). (g) (t6

  3

  (L a 2

  1

• L l ).C a

t0+T)

An equivalent circuit for each operating interval of the improved ZCT Fly-back converter (Fig. 5) is shown in Fig. 7, and the main theoretical waveforms are illustrated in Fig. 6, Ip is the transformer magnetizing current in the primary side, and LL is the transformer leakage inductance. The transformer magnetizing inductance is assumed to be large enough, so that

4

the transformer total ampere-turns are considered constant in a

Where

1

switching cycle. Before the first interval, it is assumed that the Ca voltage is V1, and the main switch is conducting. The converter operating modes are as follows.

Fig.5 proposed Improved ZCT flyback converter

L a 2 .C a

Fig.6 Main theoretical waveforms of the ZCT flyback converter

Assumed that C a 1 F

Resonant Frequency Fr = 10khz

Leakage inductance L L

2 H

1

1

L a 1 .C a

La 1 25.33 mH

4

1

L a 2 .C a

Fig.7 Equivalent circuit for each operating interval of the improved ZCT flyback converter. (a) (t0 t1). (b) (t1 t2). (c) (t2 t3). (d) (t3 t4) (e) (t4 t5). (f) (t5 t6). (g) (t6 t7). (h) (t7 t8). (i) (t8 t1 + T).

1. Vi	K	Vo
   250	0.25	83.33
   250	0.5	250
   250	0.75	750

   Input voltage Vi = 250 v

   Duty cycle K=0.25, 0.5,

   0.75, 1.

   Transformer turns

   N 2

   Ratio: N2/N1 = 1

   V

   .V S .K

   N 1

   Assumed that

   C a 1 F

   Output voltage =

   0 1 K

   Resonant Frequency fr = 10khz

   1 ;

   1

   La 25.33 mH

   The performance of the ZCT boost converter is also similar to

   L a 1 .C a

   the remaining converters. Design and performance perameters are detailed here below.

   1

   2

   w 1

   La 25.33 mH

   L a 2 . C a 2

   Vi	K	Vo
   250	0.25	62.5
   250	0.5	125
   250	0.75	187.5
   250	1	250

   Input voltage Vi = 250 v

   Duty cycle

   K= 0.25,0.5,0.75,1

   Output voltage

   Fig.8 ZCT Boost converter

   Assume that Cx 1F

   Frequency fr = 10khz

   V T on

   .V

   0 T i

   Lx 2.533 mH

   Vi	K	Vo
   250	0.25	333.33
   250	0.5	500
   250	0.75	1000

   Input voltage Vi = 250 v

   Duty cycle

   K=0.25, 0.5, 0.75, 1

   1

   V 0 1 K .V i

2. With reference to the design and performance parameters, the proposed ZCT converters and conventional converters for a frequency are simulated. Simulated models along with results are detailed here below.

   1. Fig.9 Simulink model for hard switched buck converter

      Fig.10 waveforms for hard switched buck converter

      Fig.11 Simulink model for proposed ZCT Buck converter

      Fig.12 Main switch gate pulse and current waveforms (soft switched buck converter)

      Fig.13 Auxiliary switch gate pulse and current waveforms

      Fig.14 Waveforms of ZCT buck converter

      1. Fig.15 simulink model for hard switched flyback converter

         Fig.16 waveforms of hard switched flyback converter

      2. Fig.17 Simulink model for ZCT flyback converter

         Fig.18 Waveforms of ZCT buck converter

      3. Fig.19 Simulink model for hard switched boost converter

The proposed topologies with Zero current Transition and conventional hard switched are analyzed based on the simulated results obtained. Calculation of the very important factors switching (power) loss and stress are done and are detailed here below .

P loss

1

2 V i .I

m f s

t f

Fig.20 Waveforms of hard switched boost converter

di/dt rating across the switch when the switch is turned off:

di Transition in current during fall time

dt Fall time

As per the performance parameters figured in the earlier section. Detailed analysis is given below.

FOR HARD SWITCHED BUCK CONVERTER

Vi = 250 v,

Im = 2.1 A (as per the figure 10)

Fs = 10 kHz

and fall time of switching transition(assumed) Tf = 3 s

Switching powerloss : P loss 7 . 87 watts

Switching stress:

di

dt

0.7A/ sec

FOR SOFT SWITCHED BUCK CONVERTER

Fig.21 Simulink model for ZCT boost converter

The current through the switch is modulated such that

Im = 0.9A (as shown in figure 12), other parameters remain same.

Switching Power loss: Ploss 3 .37 watts

Switching stress:

di

dt

0.3 A / sec

FOR HARD SWITCHED FLYBACK CONVERTER

Im = 4.5A (as shown in figure 16 ), other parameters remain same.

Switching Power loss: P loss 16 . 87 watts

Switching stress:

di

dt

0.3 A / sec

FOR SOFT SWITCHED FLYBACK CONVERTER

Fig.22 Waveforms of ZCT boost converter

Im = 1A (as shown in figure 18), other parameters remain same.

Switching power loss:

Ploss 3 .75 watts

Switching stress:

di

dt

0.3 A / sec

FOR HARD SWITCHED BOOST CONVERTER

Im = 7A (as shown in figure 20), other parameters remain same.

Switching power loss:

1. This paper emphasizes different PWM DC/DC buck, boost, Flyback converters with Zero Current Transition. The concept of resonance is implemented by using capacitance and

   Switching stress:

   Ploss 26.25watts

   inductance (Auxiliary circuit) in all the converters. An suitable

   di

   dt

   2 . 33

   A / sec

   converter PWM strategy is designed for the entire control range of frequencies, such that the power switching device IGBT current and voltage waveforms do not overlap, which

   FOR SOFT SWITCHED BOOST CONVERTER

   Im = 2A (as shown in figure 22 ), other parameters remain same.

   Switching power loss: Ploss 7.50watts

   di 0 .66 A / sec

   dt

   Switching loss analysis:

   Fig.23 graphical view of power loss analysis

   Stress analysis:

   Fig.2 Graphical view of stress analysis

   result in reduced switching losses results in improved converter efficiency. Design of the same presented for a certain switching frequency and simulated by using MATLAB- Simulink. Switching stress and power loss analysis is also carried out for the proposed topologies. Comparison between hard switched and soft switched converters proves that a dc-dc converter switched with ZCT has reduced switching stress and less switching losses which adversely improve the efficiency and reliability entire converter system.

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M.Prathap Raju , received his B.Tech degree, 2003 from G. Pulla Reddy Engineering College and M.Tech(Power electronics) from JNTUH, Hyderabad, AP, India. He is currently pursuing Ph. D from JNTUH and the paper is carried out

towards the research objective. He is currently working as sr. Assistant professor, Dept of EEI, College of engineering Studies, University of Petroleum and Energy Studies, Dehradun, India.

India.

Dr.A.Jayalaksmi, received her

B.Tech degree from NIT Warangal and

M. Tech from Osmania University. Awarded Ph.D from JNTUH, Hyderabad, AP, India, in the year 2007. She has quite a good experience in the industry and research. She is currently working as an Associate Professor in the Dept of EEE, JNTU College of Engineering, JNTUH, Hyderabad,