ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

METHODS AND DEVICES FOR MONITORING AND CONTROL OF BIPOLAR OUTPUT (POWER) TRANSISTOR IN SWITCH MODE

Dimitar Ivanov Dimitrov1, Nikola Nikolov2, Tsvetozar Ivanov Hristov3

Technical University – Sofia, 1000 Sofia, 8 Kliment Ohridski Str., e-mail: 1ddim@tu-sofia.bg, 3tsve_hristov@yahoo.com

The method is based on the criteria equations and inequalities, worked out and checked using devices, which parameters are close to the real ones. When the load is permanent and monitoring of the power losses we focus on the solving the problem of reaching the best switch mode parameters and path of the working point when the transistor switches on and off. The research is made using an active load and the possible conditions that can make the load inductive or capacitive. The scheme used for the measurements allows the development of different variations to optimize the base control of the transistor. The relevant time diagrams define the best part of the Safe Operating Area (SOA) of the transistor. The given data can be used in constructing switch mode power supplies based on the forward and flyback converter.

Keywords: MPS – Switch Mode Power Supply, forward, flyback, converter, load, power transistor

1. INTRODUCTION The main characteristic of this method is that it is based on measurements of static

loads in forward and flyback converters. For the static load wire and carbon resistors are used. The tolerances of these

resistors are regulated. The declinations over or below the regulated values of the resistors defines the

capacitive or the inductive character of the load. The dynamic character of the load is typically presented in switch mode power

supplies used in the television sets and displays where the continence of the picture is changed especially from white to black picture. That is how the dynamic characteristics of the picture change.

2. DEVELOPMENT OF A METHOD OF CHECHING AND OPTIMIZATION OF THE SWITCH PARAMETERS OF THE OUTPUT TRANSISTOR

The developing of this method is based on the specialty of the input and output characteristics of the bipolar transistor in switch mode. After defining the segment and saturation regimes we need to find the connection between the on time (ton) and the off time (toff) when passing from the first regime to the other one and vice versa.

The output linearised characteristics of the bipolar transistors used in switch mode power supplies used in the TV sets is characterized using two slopes (Figure 1). In their active area they have smaller slope than the transitional one. This is due to high ohmic and the low ohmic N+ collector areas.

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

Figure 1. Figure 2. Inequalities that define the segment and saturation regimes are given bellow. (1) and 0,0 >< BCBE UU(2) 0,0 >> BCBE UUThe passing from one regime to the other is characterized using the ton and toff

times (Figure 2 a, b and c). (3) rdon ttt +=

(4) fsoff ttt +=

In the classified literature for the transistors these times are given and the development is focused on reaching typical values of these parameters. If only the time constants τs and τon are given the typical times given above can be calculated.

The switching times ton and toff depend especially of the switching characteristics of the transistor, but the load characteristics influence and we can not neglect the unstop base impulse.

When the base impulse is as long as the on and off times the transistor can not be saturated and the losses in this regime are big. When increasing these times is the reason for increasing the current amplification.

Figure 3. Figure 4.

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

Our preferences are the current amplifications 205÷=β and the frequencies (Figure 3). MHzfT 304 ÷=

The ton and toff times remind stable when Tttt offonp .Θ=+> where tp is the time when the collector circuit is switched on.

The transition time when switching the transistor on is closer to the linear one with respect to the condition CsatB II ).105(. ÷>β (Figure 4).

The scheme used for the controlling process (Figure 5) allows to define the collector current Ic, the base on time current IBon and the base off time current when using different types of loads. We use the following method:

Figure 5. The controlling base circuit receives unstop UBon and stop UBoff pulses from the

source of voltage pulses. (5) BoffBoniB UUU +=

The unstopping voltage UBon is given by the equation bellow: (6) BoffiBBEBBBon UUURIU −=+= . The uninterrupted on and off processes are consecutively given by the equations:

(7) BEsatBBBB

iBBB

UURIRRIURRIRI

−−=−+−=+−

11

11

.)()(.

From the equations (7) the result for the base current is:

(8)

iB

iBiB

BEsatBBiBB

iB

B

RRRR

R

UURRRU

I

++

+−+

=

1

1

1

.

)(.

The equation (8) is correct in condition that the time constant BiB

iBiB C

RRRR

R )..

(1

1

++=τ

is significantly bigger from the time when the currents IBon and IBoff flow. When RiB<<Ri the base on current is given with:

(9) iB

BEsatBBiBBon R

UUUI

)( +−=

A full saturation of the output transistor is reached not only where offonpB ttt +> , but when the base current is one fifth part of the collector current or bigger. tpB is the unstop base impulse.

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

(10) CB II .2,0≥

The high sheer of the base current cause to hesitate. They are limited by connecting an external resistor between the base and the emitter. This resistor guarantees low current transmition coefficient that is given by the following equation when UCB=0.

(11) dt

diBdt

di CB .1

0

>

The influence of the input characteristics (IB, UB BE) narrow when the unstopping voltage UBEon is subordinate by the following condition:

(12) , BEonBBon URI >.that is the ideal current control when ∞→RB . The limited changes of the base

current require limiting of the tolerances of RB and UB on=U1>0. The optimal trajectory of the working point in on and

off processes is maximally approximated to the axis (IC and UCE) of the output characteristics (Figure 6).

Figure 6.

The base off current IBoff with respect to Uib is given by the equation:

(13)

iB

BEsatBBBoff

RRRiBRRiB

UUI

++

−−=

1

.1

The collector current is then presented by the equation:

(14) O

CEsatiC Z

UUI

−=

The load Zo can be predominant active, inductive or capacitive type. This defines the time correlations in switch mode and the respective loses. For these three typical cases the trajectory of the working point in on and off process the losses radiated in the transistor are given in table given in Figure 7.

Load Zo predominant

Continuance of ton and toff

Losses in Pon and Poff

General Losses P Trajectory of the working point

L ton < toff Pon < Poff

R ton = toff Pon = Poff

C ton > toff Pon > Poff

Figure 7. The results presented in Figure 7 allow us to develop a basic method for

calculating the losses of switching on and off of the output transistor.

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

3. DEVELOPING OF METHOD FOR DEFINING THE ON AND OFF LOSSES OF THE OUTPUT TRANSISTOR

The processes of switching on depend on the processes of carrying, accumulation and carrying of the current carriers in the base and the collector areas of the transistor.

The current changes in on and off mode is a result of the recharging of the barrier and the parasitic capacitances and inductances.

The linearised changes of the collector voltage and the collector current during the on and off intervals is typical for the output transistor when the load is active (Figure 8).

Figure 8.

Analytically this change is given by the

equations: (15) )1(

onCEMCE t

tUu −=

(16) on

CMC ttIi = in condition ontt ≤≤0

(17) on

CEMCE ttUu =

(18) )1(on

CMC ttIi −= in condition offtt ≤≤0

The average power loss of on and off using 15, 16, 17 and 18 is given by the expression:

(19) ∫ ==offont

offonCMCEMCCE T

tIUtitu

ToffPon

/

0

/

32

4.

)()(1/

The moment losses are:

(20) )(.)( 2/

2

/ offonoffonCMCEM t

tt

tIUtP −=

The maximal power is defined by the equation (21).

(21) 4. CMCEM

CMIU

P = , when 2/ offont

t =

The maximal value of these losses is the upper limit from which the optimization (minimization) begins. This condition is reached by improving and developing the output transistor's base circuits.

4. DEVELOPING OF METHODS FOR OPTIMIZATION OF THE BASE CIRCUITS OF THE OUTPUT TRANSISTOR

In the contemporary TV sets and monitors the load of the switch mode power supplies is dynamical and it is mostly inductive. For the first TV color receivers the load had a typical active load.

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

The optimal regime of the output transistor used with typical output circuit and periodically repeated pulses is reached by using some proper elements used in the base, collector and emitter circuits.

4.1. Optimization of the base controlling of the transistor The analyzes of the possible ways of controlling the off stage of the transistor

shows that the most friendly results are presented when the resistor and the inductance are serially connected (Figure 9 c).

The opposite is the requirement of little switch on losses of the transistor. For getting a positive base current with distinctive jump at the end of the front of the base impulse we need to use a accelerate capacitor that is connected in parallel with the base resistor.

In practice except the main base circuit RB and LB BB the operation of the other components have to be examined carefully. When the elements of the last but one stage and when proper elements are used including the adjoining transformer an excellent result can be achieved. At the same time the base circuit is quite simple. We can conclude that the complication of the base circuit is the last thing to do after exhausting the possibilities of the basic scheme RB and LB BB (Figure 9 c).

Figure 9. Figure 10. The resistor R between the emitter and the base (shown with the dotted line) is

connected for protecting the base circuit from self stimulation. The scheme given on figure 9 g used in forward converter is a reserved mark

protected with copyright No 4345. This decision decreases the start current from 1,44A to 0,64A (Figure 11 a and b).

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ELECTRONICS’ 2006 20 – 22 September, Sozopol, BULGARIA

The switching dissipated power decreases with 25%. The trajectory of the working point from Figure 11 a goes to the one showed on figure 11 b and takes safety place in safe operating area SOA.

Figure 11. 5. CONCLUSIONS 1. The knowledge for bipolar power (output) transistors in switch mode is

methodically mutually related and this leads to new knowledge for their more effective application.

2. The method for control and verification (check up) includes reaching the key parameters through visualization of the switch mode losses, searching and finding optimal trajectory of the working point for reaching the best place in the safe operating area (SOA).

3. The given equations and inequalities show the perfect processes and allow us to synthesize a device for control and verification of the transistor's key parameters.

4. Taken (examined) are the relations between switch mode losses with different loads and trajectory of the working point.

5. The method and the device help solve optimization tasks of the managing base chains and their application in real scheme solutions.

6. REFERENCES [1] Baekhorst A. SSP. A mains isolated synchronous color TV receivers. EDS 7809. Laboratory

report. Central Application Laboratory CAB. Eindhoven, The Netherlands, Philips, 1999. [2] Dimitrov D. Research and development of methods and means of optimizing when

constructing line power supplies for TV receivers, Technical University, Sofia, 1995. [3] Dimitrov D. TV and Video techniques, Russe University, 2000. [4] Valkestijn A. Mains isolated 90 Economy CTV Receiver with Zero East-West Display

System. Report 1 Receiver with GTO driver S2PP

2 system ETV 8101, Laboratory report. Central Application Laboratory CAB, Eindhoven, The Netherlands, Philips, 1999.

[5] Dimitrov D. I. Copyright Certificate No 43845 / 1995. Institute for Inventions and rationalizations. Sofia, Bulgaria.

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