Design and Implementation of DSP-based Voltage Frequency Conversion System

Chih-Yuan Chen Yu-Hsien Lin Jiann-Fuh Chen Ray-Lee Lin Department of Electronic Engineering Department of Electronic Engineering Department of Electronic Engineering Department of Electronic Engineering National Chen Kung University National Chen Kung University National Chen Kung University National Chen Kung University Tainan, Taiwan, R.O.C Tainan, Taiwan, R.O.C Tainan, Taiwan, R.O.C Tainan, Taiwan, R.O.C doctxt228@yahoo.com.tw kengi1123@gmail.com chenjf@mail.ncku.edu.tw raylin@ieee.org

Abstract This paper presents a voltage frequency conversion system with Digital Signal Processor (DSP). The digital control is used to achieve the variable frequency control. The topology of the proposed system is two-stage converter, where the first stage is a power factor correction (PFC) circuit and the second stage is a single-phase full-bridge inverter circuit. The PFC can control input AC voltage, which can achieve high input power factor. Otherwise, it also can supply a regulated DC bus voltage for the full-bridge inverter. The variable output voltage frequency can be controlled in the full-bridge inverter by using the DSP. Finally, a prototype system operates frequency varied from 50 to 400Hz is designed and implemented. The experimental results validate the advantages of the proposed variable frequency control.

I. INTRODUCTIONNowadays, all kinds of electronic products are

widely used in daily life. The different electronic products required the different power. 50Hz and 60Hz are for the utility line voltage frequency, which is applied to the vast electronic products. 400Hz is special specification power to be used in communications, aerospace, marine and military electronic equipment, etc.

The voltage frequency conversion system is often used to test various electronic products in the laboratory or the industry. The converter frequency characteristics also can be simulated as wind turbine. Both of output voltage frequency and amplitude are directly proportional to wind speed which is an unstable energy source.

The proposed voltage frequency conversion system can be divided into single-stage or two-stage circuit structure as shown in Fig. 1 and Fig. 2, respectively. Besides, the single-stage circuit is an AC-AC converter, and the two-stage circuit is consisted of an AC-DC converter and a DC-AC inverter. Compared with the

single-stage circuit, the two-stage circuit has the advantages such as simple circuit and easy to control. As well as the frequency and amplitude of output voltage are wider than single-stage circuit [1-4].

Fig. 1 Single-stage circuit

Fig. 2 Two-stage circuit

II. SYSTEM INTRODUCTION

The topology of the proposed system is a two-stage converter, where the first-stage is a PFC circuit and the second-stage is a single-phase full-bridge inverter. By using the control IC UC3854B, the utilization of the PFC controls the input AC voltage to achieve high input power factor and to supply a regulated DC bus voltage for the full-bridge inverter. The variable frequency can be controlled by full-bridge inverter using TMS320LF2407A. The topology of the proposed system is shown as Fig. 3.

Fig. 3 Proposed system topology

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A. Power Factor Correction Circuit

The boost converter can achieve high power factor and regulate DC bus voltage by using UC3854B. The control principle of the PFC circuit is shown in Fig. 4. UC3854B is an average current control IC which consists of voltage compensator, current compensator and input voltage feed-forward control.

The voltage compensator regulated DC bus voltage to create a reference signal of inductor current. The current compensator made inductor current to rapidly follow the inductor current reference signal which generate by voltage compensator. Input voltage feed-forward control can keep the voltage gain stable when input voltage is unstable [5].

Fig. 6 Sinusoidal-pulse-width-modulation

Fig. 4 Average current control mode

B. DC-AC Inverter

Single-phase full-bridge converter of four power switches is controlled by using sinusoidal-pulse-width-modulation (SPWM) to convert the DC input voltage into AC output voltage. According to VAB which in Fig. 6, the control of SPWM can be divided into the unipolar voltage or the bipolar voltage.

III. THE VARIABLE FREQUENCY CONTROLLER

Fig. 7 is the variation frequency control model of the proposed circuit. Using feed-forward controller to increase response speed for the output voltage and using steady state error compensator to eliminate the steady state error.

It is accomplished variation frequency control model by using TMS320LF2407A of Texas Instruments digital signal processor chip. The output voltage can be regulated by the control IC through an open-loop control reference signal. Output voltage can quickly follow the output voltage reference signal.

The steady state error compensator adjusted the gain of the feed-forward signal to eliminate the output voltage steady-state error by calculating the average output voltage error in one period. The average output voltage error in one period is expressed in Equation (1) and the value of steady-state error compensator is expressed in Equation (2).

p

)p(verr)p(verr

v

p

p21

p21

1

avg,err

∑∑ −+

= (1)

*,vk1

2kec

pkovf ⋅⋅

π= (2)

Fig. 7 Proposed variation frequency control model

In this paper, the frequency range of the output voltage is from 50Hz to 400Hz in the proposed frequency system. Therefore, ts1 is from 25�s to 200�s and ts2 is 100 times of ts1, where the corresponded

Compared with the bipolar voltage switching, the unipolar voltage switching is more complex, and the bipolar voltage frequency is twice of unipolar voltage frequency. That can reduce output filter components size and weight, so the proposed circuit used bipolar voltage switching.

Fig. 5 Single-phase full-bridge inverter

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parameters of different frequency are shown in Table 1. Fig. 8 is the flow chart of program design process for the inverter by using DSP.

Fig. 8 Proposed flow chart of program

TABLE1. ts1 and ts2 for Difference Frequency

Reference frequency (Hz) ts1(�s) ts2 (ms)

50 200 20 60 167 16.7 200 50 5 400 25 2.5

IV. System specification and filter of inverter design

Table 2 is the specifications of the power factor correction, and Table 3 is the specifications of single phase full-bridge inverter in the proposed prototype system.

TABLE2. Specifications of the Power Factor Correction

DC bus voltage 400V Input voltage 110/220Vrms Input voltage frequency 60Hz Switch frequency 100kHz

TABLE3. Specifications of Single Phase Full-Bridge Inverter Rate power 400W DC bus voltage 400V AC output voltage 110Vrms Output voltage frequency 50~400Hz ± 1% Voltage regulation 1

No-load of the inverter should not exceed 20% of

full-load current in order to reduce the loss. The output voltage frequency is directly followed no-load current at

the same output voltage, because output voltage frequency is inversely proportion to capacitive reactance. Output capacitance is selected by the highest frequency. Based on Table 3, the output current of the inverter can be calculated as shown in follows:

A636.3110400

VP

Irms,o

)rate(o)rms(rate,o === (4)

According to the equation (4), output capacitor of

the inverter can be calculated as follows:

F63.21104002

636.32.0Vf2

I2.0C

rms,o(max)o

)rms(rate,oo μ=

××π×

=⋅⋅π

⋅≤

(5)

So the output capacitance is selected 2.6�F.

The output of low-pass filter cutoff frequency should be less than 0.1 times the switching frequency. Besides, the cutoff frequency must be greater than 10 times of the fundamental frequency in order to avoid attenuation of the fundamental wave component, thus the output inductance can be calculated as below:

10f

CL21f10 c

ooo ≤

⋅π≤ (6)

So output inductor is 600�H.

V. EXPERIMENT RESULTThe experimental data are measured by Agilent

54624A digitizing oscilloscope, the experiment waveforms are recorded by Tektronix TCPA300 current probe and the power factor is measured by using PM100.

Figs. 10 11 are the experimental waveforms of the PFC. Figs. 9 to 14 are the experimental waveforms of the inverter. Table 4 is experimental data of the inverter at no load and full load.

VI. CONCLUSIONS

The topology of the proposed system is a two stage converter, where the first stage is a PFC circuit and the second stage is a single-phase full-bridge inverter circuit. Besides, the PFC can control input AC voltage to achieve high input power factor, and supply a regulated DC bus voltage for the full-bridge inverter by using UC3854B. The variable output voltage frequency can be controlled in the full-bridge inverter by using the TMS320LF2407A.

Finally, the 400-watt prototype system is implemented. Output voltage of the prototype system is 110V and frequency is from 50 to 400Hz. The experimental results show that the input power factor can up to 99% at 110 V, 220V 60Hz under full load. The output voltage frequency can be controlled from 50

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to 400Hz, and the voltage regulation was less than 1% under no-load and full load.

TABLE4. Experimental Data of the Inverter

4

No load Full load

fo (Hz)

Vo,rms (V)

Vd (A)

Io,rms (A)

Vo,rms (V)

Io,rms (A)

� (%)

50 110.0 400 1.130 110.0 3.63 88.3

60 110.0 400 1.130 110.0 3.63 88.3

400 110.5 400 1.135 110.5 3.65 88.8

(50V/div, 5A/div, 2ms/div) Fig12. Waveforms of Vo and Io at full load (fo =400Hz)

(50V/div, 5A/div, 2ms/div) Fig13. Measured output voltage frequency from 50Hz to 400Hz

(50V/div, 5A/div, 5ms/div) Fig9. Waveforms of Vin and Iin at full load (input 110Vrms/60Hz)

(50V/div, 5A/div, 2ms/div) Fig14. Measured output voltage frequency from 400Hz to 50Hz REFERENCE

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Fig10. Waveforms of Vin and Iin at full load (input 220Vrms/60Hz) [2] L. Li and Q. Zhong, “Comparisons of Two kinds of

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(50V/div, 5A/div, 5ms/div) Fig11. Waveforms of Vo and Io at full load (fo =60Hz)

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