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DEVICE FOR THE D.C. D.C. CONVERTERS PWM COMMAND

Alexandru MORAR

Petru Maior University of Tg.-Mures, Romania

morar@upm.ro

Abstract: The author presents in this paper an electronic device for the PWM command of the two-quadrant of four-quadrant choppers. A simple construction and enhanced operation safety, very smoothregulation of the duty cycle, 11 values programming for the working frequency (500Hz 5KHz), TTL orCMOS compatibility are some of the advantages presented by this electronic device.

Keywords:PWM device, 2-Q chopper, 4-Q chopper, saw tooth voltage generator.

1. Introduction

A spectacular evolution in the field of the electric energy conversion has been produced

by the emergence and development of new power semiconductor devices which facilitated the

power converter improvement and diversification, interposed between the power supply and the

electrical device motor.

The d.c. d.c. converter, also known as chopper, is frequently used for the separately-

excited d.c. motors speed regulation, being a converter which transforms a d.c. voltage, applied

to the input, into rectangular pulses to the output. The average value of a choppers outputvoltage can be modified between zero and the feeding voltage, using the Pulse Wawe

Modulation principle of constant frequency pulses. In this way it is possible to achieve the

regulation by voltage of the d.c. motors speed [1],,[5].

The block diagram of such an electrical drive system is presented in Fig. 1.

Fig. 1. Block diagram of d.c. electrical drive system.

+

-

PWM1

PWM2

DIGITALPART

PWM

PROT

LEMCOMP

DRIVER

V+ +U

T1 T3

T2 T4

D1

D2

D3

D4

FOUR QUADRANT CHOPPER

M

LOAD

OPTICAL

ENCODER

A B N

+-

Ex

IBM PC

COMPUTER

DBUS ABUS CBUS

INTERFACE

PWM SIGNAL

GENERATOR

K

V+V+

P1(FREQUENCY)

P2(DUTY CYCLE)

fL

u

i

?

I n t er - Ing 20 0 7INTERDISCIPLINARITY IN ENGINEERINGSCIENTIFIC INTERNATIONAL CONFERENCE,TG. MURE ROMNIA, 15 -16 November 2007.

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There are schemes of chopper operating in two or four quadrants, largely used in practice.

The H bridge converters are widely utilized in the adjustable electrical drives with d.c. motors.

An arm of this bridge is obtained by serially connecting two controllable power switching

devices (bipolar transistors, MOSFET, IGBT, GTO, MCT). Each device has an antiparallel

diode, called free-wheeling diode. The two devices of an arm structure work anti-phase like. In

practice, for passing from a state into another, the devices will be simultaneously blocked for ashort length of time, called dead time [4], [5], [9]. The PWM command signal can be

generated either with the IBM-PC computer, in this case being necessary a specialized interface

and the required software tools, or with a special electronic device. Taking into consideration

what has already been revealed the authors present in this paper an electronic device for PWM

commanding the two or four quadrant chopper.

2. Devices description

The command circuit for choppers, realized with traditional electronic means, are widely

used. The block diagram from where it can be deduced the operating principle of the device for

the choppers PWM command is presented in Fig. 2. There has been made the followingnotations: OSC-astable circuit; M-monostable circuit; CD1, CD2-discharging circuits, GTLV1,

GTLV2-variable linear voltage generators; CS-selection circuit; CE-output circuit.

Fig. 2. Block diagram of the PWM device.

The signal from the astable circuit output, of programmable frequency and duty cycle

50%, is applied to a sawtooth voltage generator, composed by the discharging circuit CD1 and

the variable linear voltage generator GTLV1.

In the four-quadrant choppers case, the sawtooth voltage is compared to a reference

voltage fixed with the potentiometer P by the comparators COMP1 and COMP2. At the output of

these comparators there are obtained PWM signals with adjustable duty cycle in the range of

[50-100]% (clockwise drive) respectively [50-0]% (anticlockwise drive).

In the two-quadrant chopper case, the signal from the astable circuit output is applied at

the monostable circuit M input. The very short time pulses from the monostable output command

a second sawtooth voltage generator made in the CD2 discharging circuit and the variable linear

C1

+

-

V+

C2

+

-

V+

C3

+

-

V+

GTLV1CD1

FOUR QUADRANT CHOPPER

T

K2

R2 C2

GTLV2CD2

K3

R4 C4

T

M

R3 C3

TWO QUADRANT CHOPPER

OSC

K1

R1 C1

T

= =

V+

P(DUTY CYCLE)

DUTY CYCLE=50%

[50-0]%

[50-100]%

[0-100]%

I0

I1

I2

I3

K1 K0

A1 A0

BS

CE

T

PWM-TTL

PWM-CMOS

PWM

DP

PROT

T

T

PWM1

PWM2

K

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voltage generator GTLV2. The sawtooth voltage is compared with the same reference voltage by

the COMP3 comparator, at whose output it is obtained PWM signal with the duty cycle

adjustable in the range [0-100]%.By means of the switches K0, K1 and of the selection circuit CS

it is selected: the signal with the duty cycle 50% from the OSC astable circuit output (resting

motor),the PWM signal with the duty cycle adjustable in the range [50-100]% (clockwise

electrical drive motor), the PWM signal with the duty cycle adjustable range [50-0]%(anticlockwise electrical drive motor), or the PWM signal with the duty cycle adjustable in the

range [0-100]% (two-quadrant choppers command).Through the intermediary of the CE output

circuit , the device provides PWM signal TTL or CMOS compatible. Meantime, through the

intermediary of a DP digital command part, there are also generated PWM command signals with

dead time for the power semiconductor devices of the half-bridge and full-bridge converter.

The electrical schematic of the programmable oscillator is shown in Fig. 3.

Fig. 3. Electrical schematic of the programmable oscillator.

Nowadays, the CMOS integrated circuits imposed them selves as a dominant familyamong the other families of digital circuits [6], [7].There are to be emphasized the following

advantages: low consumption power, enhanced immunity at perturbations, absence of input

current. Consequently, it has been utilized the integrated circuit MMC 4047. There has also been

utilized a rotating switch K with 3 sections and 11 positions each. The first section 1K1-1K10 is

utilized for the working frequency programming (11 values in the range 500Hz-5kHz ), the other

2 section (2K1-2K10 and 3K1-3K10) being utilized within the two variable linear voltage

generators. The 11 working frequencies values are established by means of 10 multiturn

adjustable resistances S1-S10.

In Fig. 4 it is presented the electrical block diagram of the PWM generator for the four-

quadrant choppers command.

The variable linear generator is realized with constant current generator and with thecapacitor C1 [8] whereas the discharging circuit is realized with the transistor T2.The 10 multiturn

adjustable resistances S1-S10 are in or off the circuit trough the intermediary of the second section

of the rotating switch K. Thus, for any value of the working frequency, the maximum amplitude f

the sawtooth remains the same, Umax (see Fig.5).Consequently, the smoothness of the duty cycle

regulation remain the same, very good, for every selected value of the working frequency. Led

with the emitter-repeater realized with the operational amplifier I2A of type TLO84, the sawtooth

voltage is applied to the comparators I3A, I3B of type betaM 339, where it is compared with the

prescribed command voltage by means of the multiturn potentiometer P.

The electrical schematic of the PWM generator for the two-quadrant chopper command is

presented in Fig. 6. The constant current generator realized with the transistor T1 together with

the capacitor C1 forms a second variable linear voltage generator.

AST5

AST4

TRIG+8

TRIG-6

EXT RES9

Q11

GND7

Q10

R2

C1

R-CCOM3

+514

RETRIG12

OSCO13

IC1

CD4047BE

C2R2

OSC

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

1K2

1K3

1K1

1K10

1K4

1K5

1K6

1K9

1K8

1K7

R1

C1

+5 +5

OSC

RESET

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devices is shown in Fig. 9. It is has been integrated in a high-performance trial stand dedicated to

the separately-excited d.c. motors driving systems command.

As experimental results, the command PWM signal of a different working frequency, are shown

in Fig. 10.

Fig. 9. General view of the PWM device.

4. Conclusions

The electronic device for the choppers PWM command presented in this paper has the following

advantages:

Very smooth regulation of the duty cycle indifferent of the working frequency value;

The programming possibility, in the range of 500Hz 5kHz, of the working frequency

value;

Offers complementary PWM signals with dead-time;

Offers PWM signal, TTL or CMOS compatible, for commanding two or four quadrantchoppers.

References

1 . Leonard W., Control of Electrical Drives, Springer-Verlage, Berlin, 1985.2 . Mohan N., Undeland T., Robbins W., Power Electronics-Converters, Application and Design, John Wiley

& Sons , New York, 1989.3 . Strinescu I., Variantelor statice de tensiune continu. Editura Technic, Bucureti, 1983.

4. Bogdanov I., Microprocesorul n comanda acionrilor electrice.Editura Facla, Timioara, 1989.

5 . Kelemen A., Imecs M., Electronicde putere. Editura Didactici Pedagogic, Bucureti, 1980.

6. Ardelean I., Giurgiu H., Petrescu L., Circuite integrate CMOS. Manual de utilizare. Editura Technic,Bucureti, 1986.

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Fig. 10. PWM command signals of different working frequencies

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7. Blua Gh., Circuite logice i structuri numerice.Proiectare i aplicaii. Editura Marix Rom, Bucureti, 1999.8 . Mitrofan Gh., Generatoare de impulsuri i de tensiune liniar variabil. Editura Technic, Bucureti, 1980.

9. Blua Gh., Insulated Gate Bipolar Transistors Based Reversible PWM Converter for D.C. Motors. Bul.Inst. Polit., Iai, XLIV (XLVIII), 1-4, s. IV, 49-57 (1998).

10. Morar A., Diaconescu I., Circuite digitale. Circuite logice combinaionale, Editura Universitii PetruMaior Tg. Mure, 2003 .

11. .Morar A., Diaconescu I., Circuite digitale. Circuite logice secveniale, Editura Universitii PetruMaior Tg. Mure, 2005

12. Morar, A.ElectronicdigitalAplicaii i probleme- Editura MEDIAMIRA cluj Napoca, 2007.