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A MULTI POWER BAND HIGH VOLTAGE DC-DC

CONVERTER

Shahid Iqbal

School of Electrical and Electronics Engineering, Engineering Campus,University Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia

Email:shahidsidu@hotmail.com,shahid.iqbal@eng.usm.my

Keywords: Multi-band, voltage-multiplier, high-voltage.

Abstract

The conventional pulse-frequency-modulated (PFM) zero

current switching (ZCS) series resonant (SR) inverter-fedhigh voltage (HV) dc power supplies have nearly zeroswitching loss. However, they have several limitations like;poor controllability at light load and large output voltage

ripple at low switching frequencies. These limitations makethem relatively less suitable for the applications whichdemand wide range of output voltage variation. To addressthese problems this paper proposes a multi-power-band high

voltage dc-dc converter. The proposed converter has sixteenpower bands (B0 to B15). It has essentially zero output powerin the band B0, lowest output power in band B1 and highest

output power in band B15. Thus the output power of proposedconverter increases in steps from its lowest value to highest

value as power band is changed from B1 to B15. Switchingfrequency of converter is tuned within its narrow limits toadjust the output power within a power band. Proposed highvoltage converter have several features such as wide range of

control, less voltage ripple and low current stress on powerswitches. Simulation and experimental results confirm theexcellent performance of proposed converter.

1 Introduction

There are numerous applications of DC high voltage powersupplies such as lasers, accelerators, ultra-high voltage

electron microscopes and X-ray power generators. However,

the design and development of dc high voltage X-ray powersupply especially for medical imaging application is mostcomplicated [1]-[6]. This is because in medical imaging an X-ray power supply is required to control its output voltage and

current over wide range and its voltage ripple should be assmall as possible. The output voltage across the tube isusually set to a particular value between 20 kV and 150 kV,

depending upon the type of application, and the current isvaried from 0.5 mA to 1.25 A. Thus an X-ray Power supplymust be capable to control its voltage and current over widerange.

In literature several types of dc-dc converters such as seriesresonant (SRC), parallel resonant (PRC), and series-parallelresonant converters (PRC-LCC) have been proposed for an

X-ray generator for medical applications [2], [8]-[10].

However the large turn ratio (300 to 500) of the high-voltagetransformer used in these DC-DC converters exacerbates the

transformer non-idealities (i.e. the leakage inductance andwinding capacitance). Both of these non-idealities affect theperformance of the high voltage DC-DC converter for X-ray

generator. These non-idealities cause voltage and currentspikes and increase loss and noise [3]-[6], [11]. The large turn

ratio and hence the aforementioned non-idealities can greatlybe reduced by using voltage multiplier on the secondary sideof high voltage transformer [3]-[6], [11].

A ZCS-SR inverter is preferred to drive voltage multiplierbased high voltage dc-dc converter because of its features

such as zero switching loss, no saturation of transformer andinherent over load protection etc [3]-[6], [12]-[15]. There aretwo methods to control the output voltage of SR inverter fedhigh voltage dc-dc converter. These are known as PulseFrequency Modulation (PFM) and Pulse AmplitudeModulation (PAM) [14]-[15]. First method is simple;

however, it has limitations like poor control at light load andlarge output voltage ripple for low desired output voltage dueto low switching frequency. The second method is

complicated because it requires an additional power converterto control the voltage across SR-inverter [12]-[15]. Authors in[14] proposed an input voltage modulation control scheme to

overcome the problem associated with PFM control scheme;however this approach need an extra circuitry connected atthe lower arm of the high frequency inverter to control the

output voltage of power supply. In this paper a multi-power-band high voltage dc-dc converter is proposed to achievewide range of controllability and minimum output voltage

ripple.

The organization of this paper is as follows. In section 2 thecircuit topology and principle of operation of proposed multi-power-band HV dc-dc converter is described. Section 3describes the principle of output voltage control. Thesimulation and experimental results of proposed converter arepresented in section 4. Finally, Section 5 presents the

conclusions from simulation and experimental results.

2 Circuit description and principle of operation

Figure 1 shows the circuit diagram of the proposed multi-power-band high voltage dc-dc converter. It consists of an

input DC source, a multi-output full-bridge resonant inverter,four resonant capacitors ( , a high voltage

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Figure 1: Proposed multi-power-band high-voltage dc-dc converter

-transformer with leakage inductance ( ), a three stagesymmetrical voltage multiplier, and an output load resistance

R

. The series resonant inverter in the proposed convertersystem consists of a common leg and four independent legs(leg 1, leg 2, leg 3 & leg 4). The four independent legs of theinverter produce four outputs (A, B, C & D). These outputs of

inverter are fed to the primary of HV transformer through

resonant capacitors respectively as shown in Figure1. The capacitors resonate with the leakage inductance of high

voltage transformer and enables resonant mode operation ofinverter. The centre-tapped secondary of the high voltagetransformer is connected to a three stage symmetrical voltagemultiplier. The voltage multiplier circuit rectifies and

multiplies the ac output voltage of high voltage transformerand supply the amplified dc voltage to the load. The inverterin the proposed converter operates in discontinuous

conduction, so all the power switches turn ON and OFF underzero current switching conditions as shown in Figures 2 & 3.

The values of the resonant capacitor are selected such that: . Because of different values ofresonant capacitors, the impedance of the tank circuit andconsequently the output power of converter is different foreach of the output. Hence, the four outputs of the resonantinverter can be activated in sixteen different ways to yieldsixteen power bands (B0 to B15) as shown in the Table 1. Thepower switches (SH and SL) of common leg operate with 180

phase difference and their operation is independent of thepower band. In contrast to this, the activation of the powerswitches (S1 - S8) of independent legs depends upon the powerband. These power bands are described as follows.Band B0: All the power switches of independents legs aredisabled. So all the inverter outputs are OFF and consequentlyno power transfer occurs in this power band.Band B1: In this power band the power switches ofindependent leg 1 (S1 & S2) and of common leg (SH & SL)operate, while the power switches of other independent legsare disabled. Thus only output A of inverter is ON and allothers are OFF. The resonant inductor current and gate signalfor this power band is shown in Figure 2. Power Switches S1

& SL turn on during positive half cycle and S2 & SH turns onduring negative half cycle. The resonant tank circuit in thispower band consists of series combination Lr and Cr1. Since

the capacitance of Cr1 is smallest of all the resonant capacitors,the power transfer rate is lowest in this power band.Band B2: In this power band the power switches (S3 & S4) ofindependent legs 2 and (SH & SL) of common leg are enabledwhere as the power switches of other independent legs aredisabled. Thus only output B of inverter is ON and all otherare OFF. Consequently, power switches S3 & SL turn onduring positive half cycle and resonant current flow throughresonant tank circuit (formed by Cr2 and Lr ), primary of thetransformer and back to the source. Similarly power switchesS3 & SL turn on during negative half cycle of operation. Thelevel of power transfer from source to load is higher thanpower band B1.

Band B3 B14: These power bands can be explained fromsimilitude with the aid of Table 1. As we go up from lower tohigher power band, the output power increases step by stepfrom P1 to P15 as can be seen from the Table 1.Band B15: In this power band power switches (S1 S8) of allthe four independent legs and (SH & SL) of common leg areenabled. All the four outputs (A, B, C, &D) are ON. So thepower switches S1, S3, S5, S7 & SL turn on during positive halfcycle and S2, S4, S6, S8 & SL. Because all the four resonantcapacitors are connected in parallel so the impedance of tankcircuit is minimum in this power band. Hence power transferlevel is highest in this power band. The typical waveform ofresonant inductor current and gate signal for this power bandare exhibited in Figure 3.

Figure 2: Key steady state waveform of inductor current in power band B 1.

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Figure 3: Key steady state wave form of indcutor current in power band B 15.

PowerBand

State of inverter outputs

(ON/OFF) Net capacitance oftanks circuit

D C B A

B0 OFF OFF OFF OFF 0

B1 OFF OFF OFF ON

B2 OFF OFF ON OFF

B3 OFF OFF ON ON

B4 OFF ON OFF OFF

B5 OFF ON OFF ON

B6 OFF ON ON OFF

B7 OFF ON ON ON

B8 ON OFF OFF OFF

B9 ON OFF OFF ON

B10 ON OFF ON OFF

B11 ON OFF ON ON

B12 ON ON OFF OFF

B13 ON ON OFF ON B14 ON ON ON OFF

B15 ON ON ON ON

TABLE 1:SIXTEEN POSSIBLE POWER BANDS OF THE CONVERTER ANDCORRESPONDING TANK CIRCUIT CAPACITANCE

3 Output voltage control

The output voltage of converter can be varied by changing its

power band from B1 B15. When the power band of converteris changed to higher one the net capacitance of the tankcircuit is increased (shown in table 1) and consequently the

impedance of the tank circuit is reduced. The decrease in tank

impedance increases the rate of power flow from source toload. As a result the output voltage of converter is increased.

However since changing power band the impedance of tankcircuit changes in steps therefore the output voltage ofconverter rises/fall in steps as shown in Figure 4. Theswitching frequency of the inverter can be varied within its

narrow limits (fs(min) to fs(max)) to adjust the output voltagewithin a power band as shown in Figure 4. Thus in proposedconverter output voltage is regulated both by changing powerband and switching frequency. The power band of converteris changed when a larger variation of output voltage isrequired and frequency is slightly varied only to adjust the

output voltage with in a power band. Thus proposed control

approach significantly alleviates the problem of voltage rippleof conventional PFM control approach.

Figure 4: Graphical representation of output voltage variation by chaningpower band and switchinig frequency.

4 Simulation and experimental results

4.1 Simulation results

The proposed multi-power-band high-voltage dc-dc converterwas simulated using PSPICE for verification of its principleoperation and feasibility. The specifications of simulation

circuit are as follows: ,

, Ro = 200 k: and the capacitance

of multiplier capacitors is . The proposed circuit wassimulated for wide range of output voltage and load settings;

however selected results are presented here. When converteris operated in highest power band B15 the net capacitance of

tank circuit is and corresponding resonant frequencyis 91 kHz. So the converter would operate in discontinuous

conduction mode up to switching frequency of 45 kHz.

Figure 5 shows the output voltage and resonant inductorcurrent of proposed converter when operating in highest

power band B15 at switching frequency of 40 kHz. As in thiscase all the four inverter outputs are fed in parallel so tankcircuit capacitance is maximum (300 nF). It can be seen thatthe output voltage of the converter is approximately 38 kV

and peak resonant current is 80 A. So the output power of

high voltage dcdc converter is approximately 7.2 kW. Figure6, shows the output voltage and resonant indcutor current ofinverter when operating in intermediate power band B7 atswitching frequency of 40 kHz. The output voltage of

converter is approximately 20 kV and peak inverter current is55 A. The output power of converter in this case is 2 kW.

Next to evaluate the range of control of proposed coverter, theswitching frequency is set to its minimum value (30 kHz) andit is operated in lowest power band. The obtained waveformsof output voltage and inverter current are shown in Figure 7.The output voltage of converter for this case is approximately2 kV, peak inductor current is 18 A and output power is only35 W. This proves that proposed converter is able to control

its output voltage over very wide range. Furthermore, with thereduction in output power, the peak resonant current of

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inverter is also reduced, consequently the condcution loss inthe power switches is reduced. Therefore proposed converterhave high light load efficiency and its output voltage ripple isalso very small.

Figure 5: Simulated waveforms of output voltage and resonant inductorcurrent in power band B15 at fS =40 kHz and Ro = 200 k:.

Figure 6: Simulated waveforms of output voltage and resonant inductor

current in power band B7 at fS =40 kHz and Ro = 200 k:.

Figure 7: Simulated waveforms of output voltage and resonant inductor

current in power band B1 at fS =30 kHz and Ro = 200 k:.

4.2 Experimental results

A low power prototype of the proposed multi-power-bandhigh voltage dc-dc converter was implemented to evaluate its

performance experimentally. The specifications of the

experimental circuit are as follows: ,

, and the

capacitance of multiplier capacitor is . A conventionalconverter with PFM control was also implemented with

similar specification to compare the performance. Theobtained results are presented in Figure 8. Figure 8a showsthe maximum output voltage and the corresponding resonant

inductor current of both the proposed and conventionalconverter. Because the specifications and switching frequencyof both the converters are identical at maximum power, the

maximum output voltage and resonant current are exactlyidentical. The maximum output voltage for this case is about1050 kV and the peak resonant current is 1.7 A. The percent

ripple in the output voltage is roughly 6%.

Next, to evaluate the range of controllability, both converterswere set to produce their minimum output voltage. Figure 8b

shows the experimental waveforms of the proposed converterfor the case of minimum output voltage. In order to produceminimum output voltage the proposed converter was operated

in lowest power band B1 and its switching frequency was setto 9 kHz. The minimum output voltage is 90 V and the peakresonant current is around 0.3 A. The percent ripple in theoutput voltage is less than 5%. The results reveal that the

range of output voltage achieved by the proposed system isfrom 90 to 1050 V for an output load of 10 N 7KHminimum output voltage is approximately 12 times lower

than the maximum value. In addition, the peak value of theresonant current for the case of maximum output voltage is1.7 A, whereas it is only 0.3 A for the case of minimum

output voltage. Hence, the conduction loss of the powerswitches decreases with reduction in output power in theproposed converter.

Figure 8c shows the experimental waveforms of theconventional PFM converter for the case of minimum outputvoltage. To produce the minimum output voltage the

switching frequency of converter was set to 9 kHz (i.e. theminimum frequency). The minimum value of the outputvoltage that conventional converter produced at this switchingfrequency is approximately 539 V, which is more than three

times higher than the minimum voltage produced by theproposed converter. This shows that the proposed converterhas a wider range of output voltage controllability.

Furthermore, the percent ripple in the output voltage of theconventional PFM converter (Figure 4c) is approximately25% and the peak current stress is 1.8 A. This indicates thatthe peak current stress on power switches at the minimumvoltage is higher than that at the maximum output voltage forthe conventional PFM converter.

To compare the output voltage ripple and peak current stresson power switches at equal output voltage/power, theproposed converter was set to produce an output voltage of539 V. The proposed converter achieved this value of output

voltage in power band B3 at a switching frequency 23 kHz.The resulting experimental waveforms of the output voltage

and resonant current are shown in Figure 8d. The percentageof voltage ripple is less than 7% and the peak resonant current

Time

900us 920us 940us 960us 980us 1000us

I(Lr)

-100A

0A

100A

SEL>>

V(Ro:1)

0V

10KV

20KV

30KV

40KV

Time

900us 920us 940us 960us 980us 1000us

I(Lr)

-100A

0A

100A

SEL>>

V(Ro:1)

0V

10KV

20KV

30KV

40KV

Time

900us 920us 940us 960us 980us 1000us

I(Lr)

-20A

0A

20 A

SEL>>

V(Ro:1)

0V

10KV

20KV

30KV

40KV

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Figure 8: Experimental waveforms of output voltage, resonant inductor current, and gating signals IRUDQRXWSXWORDGRIN. (a) Maximum output voltage

of both the proposed and conventional converter at fS = 37 kHz. (b) Minimum output voltage of the proposed converter (power-band = B1 and fS = 9 kHz). (c)

Minimum output voltage of the conventional converter with PFM control at fS = 9 kHz. (d) Output voltage of 539 V of the proposed converter (power-band =

B3 and fS = 23 kHz).

-is only 0.7 A. This shows that the proposed converter hasthree times smaller percent ripple and two times smaller peakcurrent stress on the power switches than the conventionalPFM converter. The lower current stress results in lower

conduction loss and high efficiency at light loads.Additionally, smaller peak current produces less EMI, which

is another advantage of the proposed control scheme.

5 Conclusion

A multi-power-band high voltage dc-dc converter has beenproposed in this paper. The circuit topology and principle ofoperation of proposed converter has been described in detailin this paper. It has been shown that proposed converter has

sixteen power bands. Its output power can be changed overwide range by changing the power band from lowest powerband B1 to highest power band B15. Switching frequency of

converter can be changed within its narrow limits to vary the

output power within a power band. As change in switchingfrequency is small so output voltage ripple is quite small in

the proposed converter. Furthermore, all the power switchesof proposed converter operate with ZCS so switching loss isnearly zero. The performance of proposed converter has beenevaluated both by simulation and experimental results. It has

been shown that proposed system has several advantages suchas wide range of output voltage control, low output voltage

ripple over the entire control range and low current stress atlight load.

Acknowledgements

The author would like to thank Universiti Sains Malaysia forproviding all necessary facilities and equipment to make thisresearch possible. The research is supported by incentive and

short term grant (grant no: 304/PELECT/60311002) fromUniversiti Sains Malaysia.

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