Zero Voltage Switching Multi Resonant Converter using ... - IEEE Xplore

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Lubbock, USA [email protected]. Texas Tech University. Lubbock, USA [email protected]. Ahstract- Technology used for high performance DC-DC ...
Zero Voltage Switching Multi Resonant Converter using O.6J.lffi Technology Ashish Mishra Electrical Engineering Dept. Texas Tech University Lubbock, USA [email protected]

Dr. Stephen B. Bayne Professor, Electrical Eng. Dept. Texas Tech University Lubbock, USA [email protected]

Ahstract- Technology used for high performance DC-DC voltage regulator based Integrated Circuits is evolving. Zero voltage switching buck converter is one such DC-DC regulation schemes integrated in the power conversion circuitry and is designed such that power density is maximized and the switching losses are reduced.

This paper discusses the design of AMI06 technology

based zero voltage switching buck converter operating at I MHz

switching frequency. A design example accommodating input voltage range from SV-SV DC at a constant DC output voltage of 3V was considered. The simulations were performed on Cadence tool to verify the feasibility of the proposed converter.

Finally

this zero voltage switching buck converter is compared to the conventional buck converter in terms of switching losses and efficiency. Keywords-Resonant Switching; Zero Voltage Switching; Multi resonant;Half Wave; DC/DC Converters

I. INTRODUCTION With the advancement of technology from millimeter scale to nanometer scale, more passive devices and components are being integrated to increase the power density and reduce the manufacturing costs. Size of those passive components is inversely proportional to the operating frequency. Therefore in order to reduce the size of the components the switching frequency needs to be increased. Conventional DC/DC converters can operate in lower frequency range i.e. in KHz range for achieving high efficiency, but further increase in the switching frequency results in reduction of efficiency. At low switching frequency, the size of the components are large enough, which leads to high cost, large size and weight of the converters. The switching of MOSFETs in DC-DC voltage converter can be performed in megahertz range and thus reducing the size and weight of the components. But high switching frequency leads to higher switching losses and stress on the MOSFET [I]. In order to reduce those switching losses and increase the efficiency of the converter, the resonant switching methods are used. Resonant switching or soft switching takes place either at zero voltage or at zero current [2]. So power loss due to switching is drastically reduced. The soft switching technique using zero voltage switching are developed to increase the efficiency [7- 10]. Zero Voltage Switching Multi Resonant Buck converter (ZVS-MRC Buck Converter) is one of the advanced ZVS schemes widely used. To overcome the

97S-1-4673-6S40-6/1S/$31.00 ©20 15 IEEE

Dr. Changzhi Li Associate Professor, Electrical Eng. Dept Texas Tech University Lubbock, USA [email protected]

drawbacks of the conventional Buck converter, additional elements such as resonant inductor, diode and capacitor are connected to the switch. Different topologies to reduce the conduction losses and thereby improve the efficiency of the converter have been proposed [ 12- 17]. Although the conduction losses are reduced, the voltage stress on the MOSFET increases deteriorating the performance of converters, and thus high efficiency for DC-DC converter is not obtainable. Series LLC resonate converter (SRC) is also a solution. But these converters cause high temperature stress and output voltage increases at light loads, so external dummy loads are connected to it at no load condition [II]. ZVS-QRC approach discussed in previous literature helps in achieving low switching losses but suffers from high voltage stress on MOSFET. Here in this approach, for half wave operation, switching losses are reduced during turn-on time only [2, 6]. This paper discusses about zero voltage multi resonate half wave converter design using Cadence simulation tool. Technology used for the design and simulation is AMT06. AMT06 technology is also called as 0.61lm technology in which the minimum length and width of the MOSFET is 600nm. The 0.61lm technology is a non-silicided CMOS process, it has three metal, two poly and a high resistance layer. This process is generally used for low volts application [ 19].The half wave multi-resonant converters are able to reduce the switching losses for the both turn-on and turn-off time of the switch. So the converter which is designed is in the scale of micrometer and can be utilized in various applications which require small size and high efficiency such as energy harvesting in solar cells. The paper contains following sections; Section IT gives a brief description on the working principle of zero voltage multi resonant converters, Section TIT discusses about the design steps and simulation results, Section IV provides a comparison between the resonant converter and conventional buck converter and the conclusion is provided in Section V. IT. DESCRIPTION In order to reduce the size of filter components, etc., converters are designed for switching frequency in MHz range. But switching at this high frequency results in stress on MOSFET and switching losses [3]. With the concept of

resonant switching the major loss in high frequency switching SMPS (Switch Mode Power Supply), the switching losses are reduced. The resonant circuit consists of the switch and resonant elements Lr, Cs and Cd. The switch can be unidirectional or bidirectional [4]. Resonant switching can be classified into two types; Zero Current Switching (ZCS) and Zero Voltage Switching (ZVS). The stress due to switching can be reduced by connecting the dissipative snubber circuit as shown in Figure 1. The snubber circuit consist of diodes and passive components in series and parallel to the switch. However the snubber circuits can only shift the switching power loss from switches to the snubber, but does not reduce the switching power loss [3].

Zero Voltage Switching Multi- Resonant Converter In steady state a conventional buck converter is shown in figure 4(a) can be assumed as a constant current source Ii which supplies power to a constant voltage load V0 by adjusting the duty cycle of the switch S0 as in figure 4 (b). When the switch S0 is replaced by S0-Lr-Cs and Cd in parallel to diode D, we get a voltage mode multi resonant buck converter as shown in Figure 5. The operation of this circuit is dependent on the values of L, Cs and Cd. The capacitor Cd increases the number of resonance in the circuit to three.

Figure 4. Buck Converter. (a) Basic circuit structure. (b)Steady state equivalent circuit. [4] Figure 1.Dissipative Snubber Circuit [3] In ZCS inductor L is connected in series with the switch to achieve switching at zero current as shown in Figure 2. For the unidirectional switch, the current in the switch can be resonant only for the positive half cycle [4, 18]. Diode connected anti parallel to the switch makes it bidirectional and hence the resonant switch operates for the full wave.

1.

Characteristic impedance

2.

Resonant angular frequency

3.

Resonant frequency

4.

Normalized load resistance

[1] The circuit in Figure 5 can be divided into four stages of operation which depend on the switch and the on and offstages of the diode.

Figure 2. Zero current resonant switch [4] For ZVS, resonant capacitor Cs is connected in parallel to the switch to achieve switching at zero voltage .For the unidirectional switch, the voltage in capacitor Cs oscillates in both positive and negative half cycles, a diode is connected to the source of the MOSFET. So, resonant switch operates in full wave mode. If the switch is not unidirectional it operates in half wave mode [4, 18].

(i)

(ii) Figure 5. (i) Multi resonant zero voltage switch, (ii) ZVSMRC Buck Converter [6] Figure 3. Zero voltage resonant switch [4]

A. Stage 1: [t0,t1] At this stage the switch conducts and the current through resonant inductor is less than output current . The difference of this current flows through the diode [5, 6].

(7) (8) Where

B. Stage 2:[t1,t2] Resonant Inductor current now reaches till output current , diode is reversed biased and resonance of and begins. The capacitor voltage increases and which results in zero voltage turn-off of the diode [5, 6]. C. Stage 3:[t2,t3] Stage 3 ends with the discharge of the capacitor and diode being forward biased. At this stage the resonant capacitor and inductor form the series resonant circuit [6]. Current through the inductor crosses zero and capacitor voltage reaches peak value and again starts to decrease and reaches zero. D. Stage 4:[t3,t4] The stage starts when the diode is forward biased vD is equal to zero. The resonant inductor current increases linearly and flows through the switch. So the switch is on at zero voltage and the zero current increases till [5, 6]. The switching cycle ends with voltage across Cs is equal to zero and switch being turned on III.

DESIGN AND SIMULATION RESULTS

Buck Converter Design Resonant Buck Converter is designed for following requirements: Input Voltage Vin: 5V-8V

Assumption: is assumed to be from 30% of the load current Buck converter is designed at worst case scenario i.e. at lowest duty cycle Calculation of Inductor and Capacitance values: (1) (2) (3) Calculation of output filter components of Buck converter (4) (5) Resonant Circuit (6)

(9) (10) (11)

Calculation of resonant inductor and capacitor (12) (13)

Comparator Design for PWM generation Comparator is designed to achieve slew rate of (14) So the comparator used for the simulation has the slew rate of (15) Here ICM is the common mode current Table1. Components for ZVS-QRC Buck Converter Component Value Industry Specification Inductance Lf 20.83µH 22 µH Inductance L 3.01µH 3 µH Capacitance Cf 562.5nF 560nF Capacitance Cs 3.36nF 3.6nF Capacitor Cd 10.8nF 11nF Slew Rate of > 17V/µs Camparator Table 2. Width and Length of MOSFET used in ZVS-MRC Buck Component Width Length Number of parallel components N0 50µ 1µ 100 N1 50µ 1µ 100 N2 50µ 1µ 300 Table 3. Width and Length of MOSFET used in Comparator Component Width Length Number of parallel components P0 9µ 1µ 1 P2 9µ 1µ 1 P1 76 µ 1µ 1 N5 9µ 1µ 1 N3 6µ 1µ 1 N7 70 µ 1µ 1 N6 9µ 1µ 10 N4 6µ 1µ 1

Schematic and Simulation results

Figure 8. Output Voltage at 5V,6V and 8V input

Figure 6. Schematic Diagram of ZVS-MRC half wave with feed back ZVS-QRC buck converter the components from Table 1-3 was arranged as shown in Figure 6 and outputs are shown in Figures 7-9. Switching behaviour in Figure 7(ii) was obtained with LTSpice Simulation tool.

Figure 9. Output Voltages for different inputs Output voltage of 3V is obtained for input voltages of 5V, 6V and 8V respectively as seen in Figure 8. On comparing the nature of the output voltages obtained for the three inputs it can be seen that the input voltage of 8V attains the stable output quickly, but 6V and 8V input voltage takes 1.5ms to attain stable output as seen in Figure 9. IV.

(i)

COMPARISION BETWEEN ZVS-MRC AND CONVENTIONAL BUCK CONVERTER

In this Section the outputs of an ideal Buck converter is compared to ideal ZVS-MRC Buck Converter operating at input voltages of 5V, 6V and 8V respectively. Both the comparators are without feedback. The outputs of the conventional Buck Converter and ZVSMRC Buck converter is compared and is shown in the Table 4.

(ii) Figure 7.(i)Gate Voltage on Switch,Voltage across Switch (ii) Capacitor Voltage, Capacitor Current and Switch Current From Figure 7, when the switch is ON the voltage across the switch desceases and crosses zero, but the current across the switch is at 0A, so the switch turns on at zero voltage and zero current.After the switch is ON the current across the switch increases and reach till Io. Before the switch turn-off time the voltage across the switch is below zero voltage and the switch current is above zero. When the switch is turned off the switch current starts decreasing to reach zero and the switch voltage at the turn on crosses the zero voltage. So the switch turns off at zero volts.

Where And is current rise time is voltage fall time is voltage rise time is current fall time

Table 4. Comparison of Conventional Buck and ZVS-MRC Buck Converter

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From Table 4 it can be seen that as the switching losses are decreased due to zero voltage switching, so this causes less stress on the MOSFET and hence efficiency is increased. V.

CONCLUSION

This paper discusses use of CMOS AMT06 technology for the implementation of the resonant converters. The ZVS MRC is able to reduce the switching losses on the converter and achieve high efficiency. The performance of the resonant converter compared to the conventional converter is better in terms of reduced switching losses and increased efficiency. Due to the limitation in 'width to length' ratio and threshold voltage of AMT06 based MOSFET technology, higher efficiency cannot be obtained. The future work includes the improvement in the efficiency through implementation of a modified resonant converter on 45nm technology. The modified topology constitutes self-biased OPAMP integrated with triangular wave generator.

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