APPCHP4.pdf

Televisionsand Monitors

Power Semiconductor Applications

Philips Semiconductors

CHAPTER 4

Televisions and Monitors

4.1 Power Devices in TV Applications

(including selection guides)

4.2 Deflection Circuit Examples

4.3 SMPS Circuit Examples

4.4 Monitor Deflection and SMPS Example

317

Televisionsand Monitors

Power Semiconductor Applications

Philips Semiconductors

Power Devices in TV & Monitor Applications

(including selection guides)

319

Televisionsand Monitors

Power Semiconductor Applications

Philips Semiconductors

4.1.1 An Introduction to Horizontal Deflection

Introduction

Cycle of Operation

Briefly going through one cycle of operation, the sequence

of events is as follows. (This can be followed through on

the waveforms shown in detail in Fig. 3, by starting on the

left and following the stages numbered 1 to 8).

1. Turn on the deflection transistor by applying a positive

current drive to the base. The voltage on the collector is

now approximately 0.5V because the device is fully on. This

means that the voltage across the coil, Lc, is the full line

voltage; in this case 150V.

2. According to the law, V = L • dI/dt, the current in the coil

Lc will now start to rise with a gradient given by 150V/Lc.

This portion of the coil current (ILc), is the sawtooth portion

of the collector current in the transistor (Ic).

3. Now turn the transistor off by applying a negative current

drive to the base. Following the storage time of the

transistor, the collector current (Ic) will drop to zero.

4. The current in Lc (ILc) is still flowing! This current,

typically 4.5A for testing the BU2508A, cannot flow through

the transistor any more, nor can it flow through the reverse

biased diode, BY228. It, therefore, flows into the flyback

capacitor, Cfb, and so the capacitor voltage rises as ILc

falls. Because Cfb is connected across the transistor, the

rise in capacitor voltage is seen as a rise in Vce across the

transistor.

Lc will transfer all its energy to Cfb. The capacitor voltage

reaches its peak value, typically 1200V, at the point where

ILc crosses zero.

5. Now we have a situation where there is zero energy in

Lc but there is a very large voltage across it. So ILc will

rise, and since this current is supplied by Cfb, the voltage

across Cfb falls. This is, of course, a resonant LC circuit

and essentially it is energy which is flowing, first from the

inductor, Lc, to the capacitor, Cfb, and then from the

capacitor, Cfb, to the inductor, Lc. Note that the current in

Lc is now flowing in the opposite direction to what it was

previously. It is, therefore, a negative current.

6. This resonance would continue, with the coil current and

the capacitor voltage following sinusoidal paths, were it not

for the diode, BY228. When the capacitor voltage starts to

go negative the diode becomes forward biased and

effectively clamps the capacitor voltage to approximately

-1.5V, the diode VF drop. This also clamps the voltage

across Lc to approximately the same value as it was when

the transistor was conducting, ie the line voltage (150V).

Note that the coil current is now being conducted by the

diode, and hence ILc = Idiode.

This section starts with the operation of the power

semiconductors in a simple deflection test circuit leading to

a functional explanation of a typical TV horizontal deflection

circuit. The operation of the common correction circuits are

discussed and the secondary function of the horizontal

deflection circuit described.

Deflection Test Circuit

The horizontal deflection test circuit used to assess Philips

deflection transistors is shown in Fig. 1 below. Lc

represents the horizontal deflection coils.

+150V nominal

adjust for Icm

HVT

IBon

-VBB

Cfb

BY228

Fig. 1. Test Circuit for Deflection Transistors

This circuit is a simplification of a practical horizontal

deflection circuit. It can be used to produce the voltage and

current waveforms seen by both the transistor and the diode

in a real horizontal deflection circuit. It is, therefore, very

useful as a test circuit for switching times and power

dissipation. The waveforms produced by the test circuit are

shown in Fig. 2.

Vce

ILc

Idiode

ILc=Idiode

ILc=Ic

Tfb

Tscan

Tfb

Fig. 2. Test Circuit Waveforms

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