SIITME2009 – 15th International Symposium for Design and Technology of Electronics Packages

978-1-4244-50330309/$26.00 ©2009 IEEE 17-20 Sep 2009, Gyula, Hungary 245

Behavior of Bipolar and MOSFET Transistors Used in Power Stages of

a Controlled Current Amplifier

Radu Arsinte 1), Dorin Petreuş2)

1) Technical University of Cluj-Napoca /Communication Department 2) Technical University of Cluj-Napoca /Applied Electronics Department

Str. Gh. Baritiu 26-28, 400027, Cluj-Napoca, Romania Email : radu.arsinte@com.utcluj.ro

Abstract: The paper presents a comparative analysis of power efficiency and speed for different solutions of driving inductive loads. The main application is a power stages in special CRT display systems, requiring high efficiency, low rise and fall times for the current (less than 1µs for full scale current variation - 2App). The paper analyzes a controlled (hybrid) main bridge topology, implemented both in Bipolar and MOS technologies for the final stages. The results for simulation of the theoretical and optimized circuits are presented.

1. INTRODUCTION

The behavior of semiconductor power devices charged with inductive loads is important in various fields of electronic production. Stepping motors, inductive heating, CRT deflection, are few examples of systems, continuously improved to match the requirements of efficiency, speed and integration. In this paper the case of the nonstandard CRT deflection is analyzed with applications for medical imaging. Traditional CRT deflection systems [1], as used in TV or computer monitors is optimized taking into account the main parameters of the conventional TV (or monitor) system: horizontal frequencies of 15-80 KHz, vertical frequencies of 50-120Hz. For this type of applications (especially in vertical deflection) linear circuits are the perfect choice. Normally closed loop power op amp circuits offer advantages in current control over open loop systems. In addition, sweep systems with substantially improved linearity can be designed using power op amps.

Non-conventional CRT deflection systems [2], are different compared with traditional CRT displays: the necessity to control the current with a large precision (less than 1% compared with 5-8% in conventional systems), low raise and fall times (1-2 ms compared with 10-12 ms for vertical deflection), the necessity to control the electronic beam (and of course the deflection current) over the whole surface of the

screen. In this case use of (pure) linear circuits is not the best choice. Our analysis showed that low raise and fall times over an inductive load is possible only with special devices and very complicated compensations to avoid self-oscillating behavior. Of course, linear solutions lead to a low efficiency, and large thermal management. Our research implies a CRT driver able to fulfill severe requirements regarding beam deflection in special application like medical applications. In fact medical applications are still based in many fields on CRT displays, instead of LCD for multiple reasons [1]. But generation of high amplitude controlled current pulses is a common issue in many applications with inductive loads [3], so the results could be useful in various fields.

2. DESCRIPTION OF THE CONCEPT

2.1. Current amplifier concept

The purpose of this applied research was to design a current amplifier, able to receive an input signal (current pulse) of 0 to 2 mA, and generate into the coil a variable pulse current up to 2A (proportional with the input current).

An additional major requirement of the design specifications is to have only one signal to drive the circuit: if the amplitude of the input pulse is less than Imax/2 the output pulse will be positive (flowing from

SIITME2009 – 15th International Symposium for Design and Technology of Electronics Packages

978-1-4244-50330309/$26.00 ©2009 IEEE 17-20 Sep 2009, Gyula, Hungary 246

coil in one direction), otherwise (I > Imax/2) the output current will be negative (flows in opposite direction).

The main requirements of the deflection system to be implemented are basically the following:

• Current swing: -2 A to +2 A

• Rise time / Fall time: 2 A in less than 1 µs

• Ripple: ~ 0.5% (worst case < 1%)

• Load inductance: max. 15 µH

The voltage necessary to guarantee the current swing is described in equation (1).

dt

dILV = (1)

where L – is the coil inductance, dI – variation of the current, dt – time interval.

Analyzing quickly the basic requirements, it is obvious that the minimal admissible value of the necessary power supply is V = 30V. Of course, in real circuits the voltage could be at least 20% over this minimal value. We started the with a generic full bridge design as presented in Fig. 1. The design requirements lead us to combination of linear and switching circuits.

In this schematic the classical full-bridge design is inserted in series with the controlled current source, able to control the magnitude of the current in the coil.

Fig. 1. The basic concept of the full bridge amplifier.

To avoid a bipolar power supply the best solution is a half (or full) bridge circuit. The idea is to put the coil (load) inside of a transistor bridge (power bipolar or MOS) working in a switching mode in order to reduce the total power dissipation.

As presented, a requirement of the design specifications is to have only one signal to drive the circuit: if the amplitude of the input pulse is less than Imax/2 the output pulse will be positive (flowing from coil in one direction), otherwise (I > Imax/2) the output current will be negative (flows in opposite direction).

This fact lead us to modify the basic full bridge concept (presented in Fig. 1) into a new circuit (Fig. 2) making possible to combine two transistors from the bridge with the transistors necessary in controlled current sources. This configuration brings some important advantages over the classical circuit: is reducing the number of power devices and is reducing the voltage for the main power supply. Starting from this configuration it was possible to implement few practical circuits described in Part III (Simulation and Implementation).

Fig. 2. The implemented concept of the power amplifier.

2.2. Controlled Current Sources

Controlled current sources are used to generate the current step imposed through inductor.

The sources are realized with a high precision, high speed, operational amplifier. To control the current digitally, a classical 8 bit DAC is employed. The output of the DAC is feeding simultaneously the inputs of the two current sources.

SIITME2009 – 15th International Symposium for Design and Technology of Electronics Packages

978-1-4244-50330309/$26.00 ©2009 IEEE 17-20 Sep 2009, Gyula, Hungary 247

2.3. Driving (Command) Circuits

There were experimented different topologies for the command logic: a first version with positive feedback, trying to accelerate the switching processes, and a second one with separate driving circuits for the bridge transistors (close to the generic solution from Fig. 1). The modified bridge circuit with positive feedback is presented in Fig. 3. This configuration, looking feasible in some simulation environments (Orcad Pspice 10) was found inadequate, since it has an oscillating behavior. This behavior is visible even from the simulation stage in different modeling environments (like MicroCap used in our experiments). But this apparently failure in this part of the experiments, emphasized the importance of the reliability of simulation environments used in initial simulation and modeling.

Fig 3. The configuration with positive feedback.

The second method (using a separate driving circuit) is essentially a differential amplifier acting as a comparator, driving the appropriate transistor from the bridge.

3. SIMULATION AND IMPLEMENTATION

The main purpose of the research was to find quick a cost-effective technical solution, but also to investigate the possible versions of the circuit for future improvements.

This is the reason that the current amplifier was designed in two versions for the final stage: with bipolar and MOS transistors [2] and in different versions for the command circuit (positive feedback and differential).

Two environments were employed for simulation: Microcap and Orcad. The circuits presented in this part are simplified and optimized for simulation, this fact facilitating the understanding the particularities of the topology.

3.1. Positive feedback with bipolar transistors

The complete schematic of the bipolar version with positive feedback is presented in Fig. 4. In this version there was an attempt to generate the driving signal of the bridge from the other side via a connection forming clearly a positive feedback.

Fig. 4. Topology of the positive feedback version.

This version, as mentioned before, has an oscillating behavior (clearly seen in Fig. 5). In this diagram, there are represented the output current (via L1- bottom waveform) and the input signal (top waveform). this circuit was verified on the breadboard, and the real circuit has confirmed the simulation.

Fig. 5. Waveforms for the positive feedback version.

SIITME2009 – 15th International Symposium for Design and Technology of Electronics Packages

978-1-4244-50330309/$26.00 ©2009 IEEE 17-20 Sep 2009, Gyula, Hungary 248

An interesting fact is that this effect is not observable if a different simulation environment like PSpice form Orcad 10 package is used.

It is clearly visible that this version does not match the requirements of the specifications and cannot be taken into account.

3.2. Differential with bipolar transistors

The second version, as mentioned, is introducing a differential circuit to drive the opposite branch of the bridge. To perform this function it uses the output voltage of the controlled current source of the opposite branch. This version is keeping the power stages of the previous circuit. The simulated circuit is presented in Fig. 6.

Fig 6. Topology of the differential version with bipolar transistors.

The behavior of this circuit is far superior compared with the previous one as one can see in Fig. 7. In fact this was the best solution obtained in this research, and the topology was retained to be improved further by simulation (and of course implemented) using specialized components (high speed opamps, best switching transistors). The results are the same regardless of the simulation environment used (Microcap or Orcad).

Time

0s 5us 10us 15us 20us

I(L1)

0A

0.4A

0.8A

1.2A

Fig. 7. Current waveforms for bipolar differential topology.

Fig. 7 presents the results using Orcad. This ensures a reliable solution and is guaranteeing, in some degree, the stability of performance if the type of components or any other parameters are changed. The circuit, implemented on the breadboard, revealed a behavior close to the simulated circuit.

3.3. Differential with MOS transistors

Starting from the previous topology, a MOS version of the circuit was developed. This circuit version is presented in Fig. 8. It is possible also to choose from a large number of circuit topologies using MOS transistors presented in [4]. The waveforms are similar with the bipolar version, with the parameters required in the initial specifications.

Fig. 8. Topology of the MOS version.

3.4. Sensitivity of the topology to the component

quality

A final test was to investigate the influence of components in the same topology. For example, a careless choice of the switching components (transistors, diodes and opamps) generates a result far from expectations (Fig. 9).

Fig. 9. Waveforms for bipolar configuration with inappropriate components.

SIITME2009 – 15th International Symposium for Design and Technology of Electronics Packages

978-1-4244-50330309/$26.00 ©2009 IEEE 17-20 Sep 2009, Gyula, Hungary 249

Both versions (bipolar and MOS) were satisfactory regarding rise and fall times, with a slight advantage for bipolar version. The bipolar version was implemented on the breadboard, and the measured values are close to the simulated values.

4. CONCLUSIONS

This brief description of the development process make necessary to synthesize the main achievements of this work.

It was necessary to examine and determine the best topology suitable for this application.

The result, according to our bibliographic research, seems to be original, at least for CRT deflection systems, since the initial specification (requirement) was unique.

Some papers [6], from the golden age of CRT displays, are treating theoretically similar ideas, without offering a solution comparable with this one based on modern components.

This make difficult to compare our results with results of different works in the same field.

An important conclusion is that bipolar and MOS are equally useful in this current amplifier design. Different aspects of the design do the preference for a solution: cost, component availability, consumption.

The resulting topology, a combination of linear and switching technologies is highly dependent of the components quality.

5. FUTURE WORK

Since the first phase of the research finished successfully, fulfilling the specifications, the future work needs to explore only aspects left open in the first phase. A first idea is to improve the MOS version, presented in the previous chapter. Using BiCMOS structures could be an idea [5].

Another path left open is to explore different applications of this circuit. The initial application, developed for CRT displays, could be extended in unexpectedly different fields like DC servo motor control or, for example, automotive applications.

ACKNOWLEDGMENT

The application was developed on the frame of D_1019 PN II Romanian Program, project no.79/1.10.2007.

REFERENCES

[1] Geert Carrein, "Characteristics of CRT and LCD displays", White Papers, Barco Systems , pp. 5-7, 2003

[2] Jed Margolin, "The Secret Life of XY Monitors", http://www.jmargolin.com/xy/xymon.htm

[3] S. Scharnholz, R. Schneider, E. Spahn, A. Welleman, S. Gekenidis, “Investigation of IGBT-devices for Pulsed Power Applications”, 14th IEEE International Pulsed Power Conference, 2003. Digest of Technical Papers. PPC-2003. , pp.349-352

[4] R.W. Ericson, D.Maksimovic, Fundamentals of Power Electronics, Second Edition, Kluwer Academic Publishers, 2001

[5] Andrew Skinner, "Pick the right power supply topology", EE Times-India, April 2009, pp.1-4, http://www.powerdesignindia.co.in

[6] Ranendra N. Biswas, K. Gopalan, "A Programmable CRT Display Scheme for Memory Output", IEEE Transactions on Instrumentation and Measurement, Vol. 29, pp.77-79, 1980