Journal ofScience and Technology.-N 166(2-2015) - Military University ofScience and Technology A DESIGN OF A HIGH VOLTAGE AC-AC FULL-BRIDGE PHASE-SHIFTED ZERO VOLTAGE SWITCHING CONVERTER Dao Duy Tung Trinh Due Trung, Trinh Quang Due This paper introduces a design of a high voltage AC-AC using full-bridge phase-shifted zero voltage switching converter configuration With input source of 220 VAC 50 Hz from the power line, it converts to voltage of 1200 VAC operating in frequency range of to kHz through the FB-PS-ZVS circuit using self-made transformer fabricated from ferrite core Through this work, we demonstrated the feasibility for design of excitation power applied to gas lasers especially the CO? lasers with functions of short optical pulse width and the excited current control which toward to biomedical applications Besides, the method and results of the system properties investigation also are presented to clarify the parameters of the system Introduction In recent years, there are number of high voltage power supply for essential applications have been continued to be developed throughout switched-mode techniques using voltage-fed-type or eurrent-fed-type under generation of high frequency inverters with switches as power MOSFETs or IGBTs [1] Controlling the output characteristics is necessary for any power supply, especially in the case of medical-use [1] - [3] Take into account the biomedical applications, we would like to design an AC-AC converter which has high-voltage output aiming to short pulses excitation gas lasers Since the voltage ripple phenomenon occurred depends on the large number of cascades in Villard configuration, the voltage in few hundreds vohs level is difficult to riiultiply to high voltage of few tens kilovohs level On the other hand, the high fi-equency in range of kHz approximately of the input source for the voltage multiplier to reduce the ripple voltage is required To solve this problem, a primary stage which can generate from the low voltage level to few kilovohs, using transformer should be considered Therefore, in this paper, we present a study on design of the AC-AC converter which towards to the gas laser power supply with capability of short pulses train excitation The basic idea is construction of a full-bridge inverter that enables to raise 300 V achieved from the consumer electricity line to about kV AC Zero switching has been used to reduce the switching loss while increase the power conversion by implementing Hanoi University of Science and Technology 12 Tgp chi Khoa hoc vd Ky thugt-SS 166(2-2015) - Hgc vien KTQS the full-bridge phase-shifted zero voltage switching (FB-PS-ZVS) topology Besides, to minimize the voltage ripple, we divide the output of the transformer into secondary coils, which will later be fed to voltage multiplier in the next works IMaterials and Method Figure shows the block diagram of the converter The 300 VDC is rectified and regulated from the 220 VAC line through the soft-start AC/DC converter circuit (Fig 2) and then fed into the FB-PS-ZVS converter The functions of the soft-start circuh are limitation of the inrush current and protection for the FB-PS-ZVS converter When the voltage converted from the 220 VAC to 12 VDC is charged to the Cl of 50uF, the transistor IRF640 is opened then the AC current of power line is closed to the rectified circuit (D3-D6) and supplies the 300 VDC for the FB-PS-ZVS converter The inrush current charges in to the bank capacitor valued as 3300uF is calculated in the transient time in the condition of 300sin(50 x 27rt) for the input voltage as A»,w,=C;,„„,x—= 3300x10"^ x300x50x2;rlimcos(50x27rO = 311(^) (1) Therefore, a soft-start circuit using combination between resistor and relay is introduced to lower the inrush current to only 28A in several milhseconds High starting current phenomenon also happens in the transformer case when 300 vohs is first applied to, hence, a similar inrush current timiter is placed The controlled signals are generated from a STM32F407VGT6 microcontroller and then through the photo couplers applied to the four MOSFET of the converter to vary the frequencies of the power source In order to control the MOSFET, a small module which operates in voltage of 15 VDC with the same ground from the source of 300 VDC is added To avoid the overload for the power source, an overcurrent protection circuit that detects the overlarge current (>7A RMS) and indicates the signal to the microcontroller to shut down the process is used In figure 3, the proposed high-frequency FB-PS-ZVS converter is illustrated This part involves the controls for the switches to obtain the voltage difference between input and output of the coils available in the transformer Since the gas lasers are typically required few hundreds watt power for excitation source, in tliis design, the MOSFETs coded as IRFP22N60K whose the withstand VDSS and the load current reach up to 600 V and 20 A, respectively, is used for the FB configuration 13 Journal ofScience and Technology.-N 166(2-2015) - Military University ofScience and Technology rC 12VDC 22GVAC ^) rough automatlc-powerstablllzer 15V0C ) BTound j I Photomuplef | \—^ Microcontroller * " Figure Mainparts of the AC-AC converter Figure The schematic of soft-start AC/DC converter (inside the rectangle), where the V2 represents the power line 220V —rvYYi—.mc v^O P2IC2 High Frequency Transfonner J ^ Figure Full-bridge topology with four switches and the transformer having four c Tgp chi Khoa hgc vd Ky thudt-Sd 166(2-2015) - Hgc vien KTQS As the figure show/n, the transformer is driven by two switches couples SI-S2 and S4-S3 alternatively in the pulses wave There are two driven pulses waves fed into the switches Sl, S4 called as "leading leg" and S2, S3 named as "lagging leg" The driven current closed to the transformer including two parts being the positive half and negative half The positive half generated when the switches S1 and S2 opened and turned off when the switch Sl is closed The zero state is maintained unfil the couple S4-S3 is activated for operation Similarly to the positive half, the negative haft is occurred when the switches S4 and S3 are opened and returned zero state when the switch S4 is closed Typically, the zero state of the output waveform through primary conductor in the transformer has shown in Fig can be controlled by either switch Sl and S4 or S3 and S2 of tlie couples In this circuit, we prefer the control through "leading leg" Hence, the phases of the controlled pulses wave fed in to the "lagging leg" should be delayed as calculation from the equation of (O/360")xr5, where Ts is the switching period The actual on-time is the overlap of two on-time of Sl and S4 or S2 and S3, and is Ts represented by dox —, this is also the reason why we call this topology Full-bridge Phased-shift Also, because the zero state of "leading leg" and the "lagging leg" are overlap to the activated state, the leakage current in parasitic capacitors of the switch devices does not close to the converter transformer in their zero state, therefore, the leakage converted energy does not dissipate in the transformer This topology mechanism in energy conversion aspect so called zero voltage switching, represents the energy lossless conversion switch To reserve the power or load current adjustability which is the important function of the gas laser for the applicable availability to the biomedical applications, the duty cycles of the output waveform are controlled by PMW (Pulse Width Modulation) Through the PWM circuit, the duty cycles of the driven input waveform is generated to vary the output power supply with constant voltage, ranging from 100 W to 480 W The switching operating modes of the FB-PS-ZVS inverter include operation modes [6] The output capacitances of MOSFETs are illustrated in figure as C|, C2, C3, and C4 There are external snubber capacitors of 4.7nF are added across S2 and S3 to provide capacitive turn-off snubbing and to ensure that the device can be turned-off with ZVS [4, 5] 15 .Journal ofScience and Technology.-N 166(2-2015) - Military University ofScience and Technology To satisfy the ZVS condition, the zero-half 5L between PWM signals of Si and S4 sliould not be at lower than 5L [7], s,- (C,+Q)xl^„ 180''2 (2) 180" leading-leg (fixed) lagging-leg (phase-shifted) -dTj-T,- Figure Switch gate PWM signals and the resulting transformer primary voltage (Vf) (dih d/2, dii, andduore duty cycle of PWM signals correspondingly) To achieve the ZVS, the value of leakage inductance have to satisfy the following equation [8]; T'>iu-''a^'