06119397

Replacement of the TCA 785 for a Configurable IC to Drive Single and Three Phase Converters

Eduardo M. Vicente, Paula dos Santos, Caio A. da Costa,

Tales C. Pimenta, Robson L. Moreno, Enio R. Ribeiro Grupo de Microeletrônica – IESTI, Universidade Federal de Itajubá. Itajubá – MG, Brasil

[email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

Abstract – This paper presents the development of an integrated system to replace the TCA 785, with the possibility to drive up to six independent devices, which will be used in the triggering of controllable devices. The developed circuit is capable to drive single and three phase systems, using only one control device, letting the user select what type of converter will be driven. It is also possible to include a digital control system capable to maintain the current or voltage output constant. Another important aspect is the decrease of the external circuits, allowing the reduction of the interference and undesirable effects caused by connections between circuits. The practical results are show through the text, with the firing of different loads, which demonstrates the possibility of implementing this system.

Keywords – TCA 785, PSoC (Programmable System-on-Chip),

pulse generation, current control.

I. INTRODUCTION

Over the years the controlled rectifiers have been used in different types of applications, such as DC motor control, lighting control, battery chargers and DC power sources. With the advance of microcontrolled systems, which enables the execution of different tasks and the insertion of several resources in one chip, it is possible to achieve more flexibility in control systems, enabling these resources to be used in the phase control of the rectifiers [1].

This type of control can be made in several ways. One of the most employed methods is to use a trigger circuit for the rectifiers. The integrated circuit TCA 785 (successor of the TCA 780), developed by Siemens, is a dedicated circuit to the firing of controllable devices, most commonly used today.

Analyzing some applications of the TCA 785, there is [2], where three TCA 780 are used for the firing of six thyristors in the control system of a three phase induction motor. In this work, the control system is accomplished through an analog PI controller and the use of the LM555 integrated circuit to decrease the power dissipated by the control device.

In [3], a power control system, for single phase load, through a thyristor trigger circuit, controlled by a Personal Computer (PC), is shown. The system consists of an R-2R network, connected to the PC's parallel port and a circuit, the TCA 785, to control the firing angle of the TRIAC. Another application of the TCA 785, as the component datasheet suggests [4], is to apply the device as a trigger circuit of two high-power thyristors.

The replacement of components by similar ones, with the aim of reducing the costs of final products, can be easily

observed in several areas. Following this trend, it is proposed in this work, the replacement of the TCA 785 by a System-on-Chip, which are integrated circuits that incorporate different features, configurable and programmable, into a single device, where the features of the TCA 785 will be implemented through the functions of a SoC.

Among many SoCs, the PSoC (Programmable System-on-Chip) was chosen, due to its specific characteristics and the resources available in the device.

To implement the functionality of the TCA 785, the circuits used for the applications mentioned above, [2], [3] and [4] were took into account, adding resources that could be useful in developing other applications.

II. THE TCA 785

The TCA 785 is an integrated circuit, which has 16 pins in DIP package. Among many applications, the TCA 785 is used to control the firing angle, from 0 to 180º, of controllable devices (TRIACS, thyristors and transistors) [4]. Fig. 1 presents the block diagram of the TCA 785 internal circuit.

Fig. 1. TCA 785 block diagram. Its configuration allows the selection of external

components for the modification of parameters of the pulses,

without increasing the size of the circuit. The main features of the TCA 785 are:

• Operation in polyphase circuits using other TCAs connected in parallel;

• There are two main outputs and two complementary outputs;

• Generates additional pulses that can be used by an external control system;

• Possibility of simultaneous inhibition of all outputs; • Length of output pulses determined by external

components.

The level of the internal voltage, which will be used by the control system, is 3.1 V, which should be kept constant regardless of variations in supply voltage, set between 8 and 18 V. The synchronization is obtained through a zero detector, which is available at pin 5, internally connected to a synchronization register [4].

The ramp generator, whose control is in the logical unit, comes from a constant current source that charges the capacitor connected at pin 10, this current is controlled by a resistor or potentiometer on pin 9. Its purpose is to adjust the amplitude of the ramp, which goes to zero when the voltage passes through the reference, due to the saturation of an internal transistor connected in parallel with the capacitor.

The Control Comparator block compares the ramp voltage with the control voltage and when their values match, sends pulses to be used by the logic unit in the firing of controllable devices [4].

From the synchronization data, as well as signals from the Comparator system, is obtained at pin 15, during the positive half-cycle, triggering pulses with a 180 degrees width. The width of these pulses is determined by connecting an external capacitor between pin 12 and the reference signal, and the amplitude is controlled by the supply voltage at pin 16. In a complementary manner, at pin 14, the pulses of the negative half-cycle are obtained. At pins 2 and 4 the inverse or complementary outputs, from pins 14 and 15, respectively, are obtained.

The pulse width can be controlled by connecting a resistor between pins 13 and 16. For applications with TRIACS, the output 7, which performs a NOR logical sum of the pulses at pins 14 and 15, can be used, preventing the simultaneous firing of both outputs and ensuring that will not occur a short circuit between the controllable devices. Pin 6, when connected to ground through a relay or a PNP transistor, inhibits all outputs of the TCA 785, which can be used as a protection for the system [4].

III. PSOC MICROCONTROLER

The System-on-Chip (SoC) circuits are devices that have several resources, whether analog or digital, into a single chip. This type of device is desirable because the components are integrated into a single chip, reducing external interference, from trail connections and electromagnetic effects, making the SoC more reliable and less prone to errors [5].

The PSoC, which is a programmable SoC, consists of a family of mixed-signal microcontrollers developed by Cypress Microsystems. Some of the features found in the PSoC that can be mention are: 8-bit core, arrays of configurable analog and digital blocks, customizable I/O bus, and a dedicated arithmetic unit for multiplication and accumulation [6] and [7]. The PSoC microprocessor is based on Harvard architecture and has a CISC set of instructions.

One of the advantages of using the PSoC is due to the fact that the device development took into account the configuration flexibility. Although in SoCs the pins and blocks are fixed and independent of the designed system, in the PSoC, the blocks and pins are mutually independent, allowing any pin to be used to access any block. In addition, it is possible to configure the device only with the resources that are required in the project, reducing the power consumption of the device, since the disabled features are "turned off", unlike some SoC devices [8].

The clocking speed of the PSoC1 family devices is 24 MHz, while the internal modules may use other frequencies of the system. Some digital modules available for use within the PSoC are: PWMs, timers and counters up to 32 bits, UART, I2C, SPI, IR and USB communication interfaces, LCD drivers, RAM and Flash memory. Among the analog modules is an interesting quote: analog to digital converters up to 14 bit, digital to analog converters up to 9 bits, programmable gain amplifiers, comparators, filters, and analog multiplexers [7].

The programming languages supported in the PSoC are: ANSI C and Assembly. All software and development environments are free and are available in the manufacturer site [9]. Fig. 2 illustrates the block diagram of the PSoC.

Fig. 2. PSoC Block Diagram.

The component used in this project was the CY8C29466-24PXI, which has 28 pins, 16 digital blocks, 12 analog blocks, 2 Kbytes of RAM and 32 Kbytes of ROM. This device can control single and three phase converters. However, if only the single phase converter is triggered, it is possible to use an 8 pin device.

IV. REPLACEMENT OF THE TCA 785

A. Developed Circuits

The following are the circuits developed in the PSoC to replace the TCA 785. Fig. 3 presents the digital blocks used, in the development software, for the firing of a pair of thyristors.

Fig. 3. Digital system block diagram In light green there is the timer 1, responsible for the pulse

generation in the positive half cycle. In light blue is the timer 2, which will generate the pulses in the negative half cycle.

One of the advantages to utilize a System-on-Chip is the ability to handle interrupts, which indicates the events that occur in the system. Any pin or block of the PSoC may be used as an interrupt source. Thus, we used two blocks of digital inverters, highlighted in dark yellow in Fig. 3, in order to detect the zero crossing, which, upon receiving the signal of an optocoupler connected to the input voltage, is able to detect the transition moment of the signal through an interruption, generating the trigger pulse according to this reference.

The output of the timers 1 and 2 is applied to an AND gate, together with the output of the PWM module, in dark red, with a frequency of 10 kHz, which is widely used in power electronics to reduce the power dissipated in the control device [10]. This connection can be seen in Fig. 4.

Fig. 4. AND gate responsible for the connection between the timers and the 10 kHz oscillator.

However, if the devices that are being fired are TRIACS, there is the possibility of modifying the triggering logic, preventing the outputs to be activated simultaneously. This is accomplished through a XOR gate. This configuration is shown in Fig. 5.

Fig. 5. XOR gate to prevent simultaneous triggering of TRIACS.

To control the firing angle of thyristors an incremental analog to digital converter, internal to the PSoC, was used. It is illustrated in Fig. 6.

Fig. 6. Analog system block diagram.

The analog to digital conversion ratio is defined in the project, which can range from 121 Hz to 15.6 kHz, according to the chosen resolution and clock speed. The A/D converter input can be used to set the firing angle of thyristors, by reading the voltage of a potentiometer, or to sample the voltage or current output, to be used in the control system, as will be explained. The block diagram of the developed system is shown in Fig. 7.

Fig. 7. Block diagram of the system.

The zero crossing input receives the signal of an optocoupler, which provides a square wave signal, from 0 to 5 V, in phase with the input voltage. The trigger pulses are applied to the drivers that are responsible to amplify and isolate the control and power systems. There are also some switches that can be used to change system parameters, the control method or the type of converter that will be used.

V. RESULTS

A. Trigger pulse generation The first test aimed in the generation of pulses, which

could be applied in single or three phase converters, in order to prove the effectiveness of the developed system.

Figure 8 presents the pulses for a single phase system. In blue there is the waveform of the input voltage, in purple the pulses of the positive half cycle and in green the pulses of the negative half cycle.

Fig. 8. Relationship between the trigger pulses and the input voltage. It is interesting to observe that the generated pulses have a

frequency of 10 kHz, resulting from the association of the 10 kHz oscillator with the timers. This pulse is shown in detail in Fig. 9. Its period is approximately 20º, which is sufficient to cause the power device to turn on.

Fig. 9. 10 kHz trigger pulses. Analyzing the generation of pulses for a three phase

converter, there are the waveforms of Figs. 10 and 11, which show, respectively, the pulses of the upper keys (which have

the anode connected to the phase signal) and the lower keys (with the cathode connected to the phase).

Fig. 10. Trigger pulses for a three phase converter (switches Q1, Q3 and Q5).

Fig. 11. Trigger pulses for a three phase converter (switches Q2, Q4 and Q6).

In the generation of the three phase pulses, the strategy of

generating the pulses with the 10 kHz oscillator was not used just for convenience, to allow better visualization of the trigger pulses.

B. Firing of a single-phase converter

To demonstrate the operation of the developed system, it was applied in the firing of a single phase rectifier bridge. The controllable device of the rectifier bridge is the thyristor, which will receive the trigger signals from the PSoC through a pulse amplifier circuit. The circuit diagram is shown in Fig. 12.

Fig. 12. Circuit diagram.

The input voltage was set at 127 V (RMS) and the load was a bank of resistors, with six 140 Ω resistors, which linked in parallel results in an equivalent resistance of 23Ω, approximately.

The load waveforms of voltage and current, for a 46Ω load, three resistors connected in parallel, is illustrated in Fig. 13.

Fig. 13. Voltage and current waveforms with a 46Ω load. Modifying the load, including another three resistors in

parallel, with a total load of 23Ω, we get the voltage and current output seen in Fig. 14.

Fig. 14. Voltage and current waveforms with a 23Ω load. Analyzing the response of the control system, included

only to demonstrate the feasibility of implementation, a PI controller was used, which will take the output current signal, provided from a Hall sensor and read through the A/D converter, as a parameter to be controlled. The reference current was set at 1.7 A, the input voltage was kept constant at 127 V and the load was modified, to enable the observation of the control system feedback.

For a load variation from 46 to 23Ω, the response is shown in Fig. 15.

The overshoot signal was about 30% and the settling time approximately 1s. In the opposite situation, i.e. a variation of 23 to 46Ω, the waveform of the output current is presented in Fig 16.

Fig. 15. RMS current for a load modification of 46 to 23Ω.

Fig. 16. RMS current for a load modification of 23 to 46Ω. In the last situation, the undershoot signal was 20% and the

settling time 800 ms. The results presented in Figs. 15 and 16 were considered satisfactory when compared to the results presented in [3]. However, the focus of this work is the generation of pulses for single and three phase converters, to enable the replacement of the TCA 785 with the developed system.

The results show the feasibility of including an internal control system, allowing the addition of resources that are not available in TCA 785.

VI. CONCLUSION

A microcontrolled system, capable of replacing the TCA785, was developed in this work. One of the main aspects to be highlighted is the increasing of the system robustness, since a reduced number of external components were used, thus increasing system reliability.

It was also possible, through the available internal resources, to control the output current of the converter. If it were accomplished with the TCA 785, it would require an integrated circuit capable of implementing the control system.

Another advantage is the possibility of triggering a three phase rectifier with a single IC, whereas to accomplish this task with the TCA 785, would require the use of three control devices.

It is important to mention that PSoC is a fully configurable System-on-Chip and it is possible to change the control strategy of the system and the frequency of the trigger signals by modifying its firmware, which is very useful for triggering high frequency systems.

The PSoC’s development environment allows more than one configuration to be created for the component and, through dynamic reconfiguration, it is possible to use the device in different types of circuits, by selecting through a key or a digital input, the desired trigger system.

Due to the device configurability, it is also possible to add other features, such as: the variation of the pulse width, addition of a logic in the pulse generation (in the firing of TRIACS, for example), and the modification of the control strategy used, both in real time.

ACKNOWLEDGMENT

This work was partially supported by CAPES, CNPq and FAPEMIG. The author Eduardo M. Vicente thanks Cypress Semiconductors for the support.

REFERENCES

[1] Ming-Fa Tsai, Fu-Jing Ke, Ying-De Lin and Jui-Kum Wang, “Design of a Digital Programmable Control IC for Single-Phase Controlled Rectifiers,” Power Electronics and Motion Control Conference, 2006. IPEMC'06. CES/IEEE 5th International, vol. 2, pp. 1-5, Aug. 2006.

[2] Ferreira, R. S., Carrijo, D. R., Rangel, E. R., Dias, D. C., Coelho, E. A. A. (2008). Sistema de Controle de Luminosidade de uma Lâmpada Incandescente via Porta Paralela do Computador, 5ª Semana Acadêmica UFU 2008, pp. 1-8.

[3] Pereira, C. A. G. (2003). Otimização de reguladores para acionamento controlado de motores de indução alimentados por intermédio de inversor de corrente com comutação natural. 2003. 115 f. Dissertação (Mestrado em Ciências em Engenharia Elétrica) - Universidade Federal de Itajubá, Itajubá.

[4] Siemens datasheet, TCA 785 Phase Control IC, Semiconductor Group, 1994.

[5] Júnior, V. C. (2005). Tecnologia SoC e o microcontrolador PSoC (Programmable System on Chip). Revista Integração, São Paulo, Ano XI, n.° 42, p. 251-257.

[6] Nicolosi, D. E. C.; Santos, R. C. (2006). Microcontrolador PSoC: uma nova tecnologia, uma nova tendência. 1. ed. São Paulo: Érica, 414p.

[7] Cypress Semiconductor. CY8C29466 device datasheet, 2010.

[8] Vicente, E. M.; Santos, P.; Gallo, C. A.; Moreno, R. L.; Ribeiro, E. R. (2010). Thyristorized Rectifier Bridge Controlled Through a PSoC, 9th IEEE/IAS International Conference on Industry Applications – Induscon 2010, pp. 1-6.

[9] http://www.cypress.com, accessed in 20/04/2011. [10] Barbi, Ivo. (2000). Eletrônica de Potência. 3. ed.

Florianópolis: Edição do Autor.