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Televisionsand Monitors Power Semiconductor Applications Philips Semiconductors CHAPTER 4 Televisions and Monitors (including selection guides) 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 Lc HVT IBon -VBB LB 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 Vce Ic Ic Ic Ic ILc ILc Ib Ib Idiode Idiode ILc=Idiode ILc=Ic Tfb Tscan Tfb Fig. 2. Test Circuit Waveforms 321 [ Pobierz całość w formacie PDF ] |
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