Thiết kế bộ chuyển đổi AC AC cao áp sơ đồ cầu có chuyển mạch không tổn thất và sử dụng điều khiển lệch pha

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 vol

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 1 to

4 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

1 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 3 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 4 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

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 4 secondary coils, which will later be fed to voltage multiplier in the next works

2 IMaterials and Method

Figure 1 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 4 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 4 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

rC automatlc-power-22GVAC ^) rough stablllzer

15V0C ) BTound j I Photomuplef |

12VDC \—^ Microcontroller * "

Figure 1 Mainparts of the AC-AC converter

Figure 2 The schematic of soft-start AC/DC converter (inside the rectangle),

where the V2 represents the power line 220V

v^O

J ^

—rvYYi—.mc

High Frequency P2IC2 Transfonner

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 4 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 6 operation modes [6] The output capacitances of MOSFETs are illustrated in figure 2 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]

To satisfy the ZVS condition, the zero-half 5L between PWM signals of Si and S4 sliould not be at lower than 5L [7],

( C , + Q ) x l ^ „

1 8 0 ' ' 2 180" 2

leading-leg

(fixed)

lagging-leg

-Figure 4 Switch gate PWM signals and the resulting transformer primary voltage (Vf) (dih d/2, dii, anđuore duty cycle of PWM signals correspondingly)

To achieve the ZVS, the value of leakage inductance have to satisfy the following equation [8];

T'>iu-''ấ<(Cj+c,)vi

Also, the dead-time between S2 and S3 must be at least as much as SR [8]

^ « = Y A ' < ( C 3 + Q )

(3)

(4)

With the peak primary current Ip.pK chosen to be 1.85A, and the effective output capacitance of each the IRF22N60K given to be ISOpF, and the leakage inductance of the transformer is measured at 7uH, which effectively satisfies equation (3) (> 6.2uH) From equation (2) and (4), we get 5L = 58 ns and 6R = 470 ns Because of the different

delay time of each MOSFET, we would like to choose the common dead-time to be \\xs

The transformer plays a very important role in the power conversion process The quality of the transformer core greatly influences to the output results In detailed, the

permeability of the transformer's variation leads to the change in the transferred energy,

contributes to the energy loss in the core material Before applying the transformer to

the two terminal of the leading and lagging legs An experimental examination is

implemented to plot the relative permeability-frequency graph to determine the

switching frequency

^

Figure 5 B-H test circuit

Since the voltage multiplier whose input is output of this FB-PS-ZVS requires the

high frequency for ripple voltage reduction, the frequency response of the transformer

should be observed The operating frequency range of the circuit mainly depends on the

permeability of the magnetic material Therefore, the characteristic in frequency

dependent of the output voltage can be investigated through the permeability

measurement Figure 5 shows the experiment setup for measuring the permeability By

finding the smallest and biggest values of the current, we can calculate the magnetic

field strength H [9]

H = N^,xI^I\, (5) Here, N is the number ofthe turns in the primary coil, /^, is the primary current and 1^,

is the effective magnetic path length By the change in the voltage of secondary coil, the

transformer magnetic flux and the flux density through the cross section area A can be

estimatedby the Faraday's law [9]

AS

V = N^y-Av

with Ns is the number of the turns in the secondary coil The maximum change in magnetic

field B occurred in half of the switching period, then revert the equation, we obtained

V,

' 2fN,A ^'^^ B

Finally, the relative permeability (dimensionless), or the ratio ofthe permeability ofthe

core to the permeability of free space, [XQ is calculated:

Hy.p^

with /ig = 4*71*10"^ H.m''

3 Results

3.1 Simulation result

The whole system was simulated on ORCAD PSPICE 16.6 and gave out results as shown in Fig 6 There was no distortion found in the input and output wave form because the core was assumed to be ideal By changing the phase delay time, the on-time was changed accordingly and led to change in the power transfer These waveforms of input and output in the simulation results proved the consistency to the theories from the design

WW

Vs Vp

Figure 6 The input (solid) and output (dash) voltage ofthe simulated transformer at 50 kHz

3.2 Experimental results

Figure 7 shows the diagram of the system performed the experimental measurements to investigate the characteristics of the output response waves The transformer core was examined under the frequency range from 600 Hz to 50 kHz without any filtering and the result is shown in Fig 8 figured out the frequency response ofthe permeability

4500

E 2500

^ 2000

~ 1500

1 1000

500

0

• '

•

V •

• •

•

100 1000 10000 100

Frequency-Hz

Figure 8 Relative permeability ofthe core as a function of frequency In log scale

As the figure shown, the permeability isstart reducing from 4 kHz and drop to below 500 at 50 kHz with slope angle of 44.6 , or in other word, the relative

permeability is attenuated by an amount of 2875xlog(/,//) (fi > f \ ) This

experimental result shows that the frequency ranged from 4 kHz to 50 kHz has failed in electrical energy conversion to maintain the stability The appropriate frequencies for this design should be equal or lower than 4 kHz for the permeability to maintain the maximum conversion for the electrical power Since the square wave can be considered

as the synthesized wave from sinusoidal wave, at the range of cut-off frequencies the high-order harmonical wave is reduced resulting to the sine-like waveform for the output wave These properties characterized as the low-pass filters Because of the filtering, the converted energy also reduced, this explains the sine-like waveform and the lower voltage presented in Fig 9c

Fig 10 shows the frequency responses measured in range from 3 kHz to 30 kHz which the diagram of setup has been shown in Fig 7 for the harmonical generation in the output wave in detail In the range of 4 kHz approximately, whole the harmonical waves are started to reduce, but with the different slopes The higher order harmonical

generation is cut-off faster than the lower others This is reasonable because the higher order harmonical wave is generated at higher frequency than the lower ones

Ime-IM^'s)

JAA/ lime(lU-^sj

Figure 9 Output voltage of the transformer and tlie primary current at 3.2 kHz (a andb) and 24 kHz (c andd)

^

JO \ 30'

Freauencv (Hzl

—1—1st - * - 3 r d

—1—7lh —1—9th

—i-5th

—1—11th

Figure 10 Amplitudes in Volt ofthe V, i''', 5''', 7''', ^'\ and if' harmonics are arranged

in orderfi-om series I to 6 (Log scale in both Amplitude and Frequency)

4 Conclusion

In this paper, the design of high voltage AC-AC power source using FB-PS-ZVS converter has been demonstrated and believed to be the reservation for the gas lasers excitation source The converter raises the 220 AC 50 Hz to 1200 VAC approximately with the highest effective frequency operated in the range from 1 kHz to 4 kHz Outside the range, there is a severe reduction in permeability, which leads to losses and drop in power conversion Also, the characteristics o f t h e power source are investigated under

the examination experimentally proving the high feasibility to develop the gas laser

power source in the near future

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Engineering in Japan, Vol 149, No, 4, 2004

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