DE2138219B - Circuit for controlling a switching transistor for deflection devices - Google Patents

Description

The invention relates to a circuit for controlling the base of a switching transistor for Deflection devices with impulses, which a complete evacuation of the from the previous conduction state the remaining charge.

The switching transistors in horizontal deflection devices must be able to handle large collector currents in the Feed the deflection transformer and the high voltage peaks that occur when the beam returns To endure load inductance. The transistors with high collector voltage developed for this purpose require a strong base current because of the low current gain at high currents. One with this A related property of these transistors is the high turn-off delay or, in the usual therminology expressed, the long clearing time. The latter is subject to strong specimen variations.

In the production of television cameras or television receivers, the spreads are the clearing time very annoying with the switching transistors of the deflection devices. That has to do with the fact that in each case different impulses have to be set so that the beginning and the end of the switching process in a certain ratio to the fixed-time blanking pulses and to the standard regulations certain beginning of the outward run. So far, one has the one to clear out the residual charges required current pulses are generated with a transformer, whose primary circuit is controlled by a driver transistor was controlled. According to German Auslegeschrift 1 248 097, a transformer is used for pulse shaping for the purpose of coupling the driver stage to the base and a double-sided conductive impedance element used. In the German Auslegeschrift 1 072 it is proposed to add a rectifier to the transistor to put opposite forward direction parallel and in the input circuit parallel to the output terminal of the driver transformer to switch a matched choke. Finally, in U.S. Patent 2,962,626 recommended to achieve a line pulse to clear the Base charges to match a leakage inductance of the driver transformer to a high frequency. All of these proposals, however, have the disadvantage that the use of a throttle or a Driver transformer is associated with relatively high costs, since a generated by a resonance process Pulse always has a delay and there is the change in inductance in the case of specimen variations of the transistor is very inconvenient.

The invention is therefore based on the object of a quick and undelayed evacuation of the base charges of the switching transistor without using the driver transformer.

According to the invention, the base of the switching transistor is supplied with control currents from two transistors, namely from a first transistor during the time in which the switching transistor is energized should be, and by a second transistor mainly during the period immediately before the blocking of the switching transistor, the collector of the second transistor with the base of the first The transistor is galvanically connected to the base of the switching transistor via a differentiating element. Preferably, the switching pulse makes the second transistor current-permeable at the beginning of the retrace. The advantage of this circuit is that the clearing time of the switching transistor is short, because you can work with the highest permissible clearing current, and that a special transformer is not needed. The use of an additional transistor is less than this saving does not matter. For cameras with a »Plumbikon mit ACT-gun« type pick-up tube the improvement has a particularly beneficial effect, since return impulses to control the »ACT-gun« (of the jet system) of a defined shape and duration can be obtained.

For a better understanding of the invention and to explain further advantages of the same is in the The following part of the description made reference to the accompanying drawing in which

F i g. 1 reproduces the deflection circuit according to the invention, while

F i g. 2 shows a number of current and voltage oscillograms at different points of the invention Circuit shows. In Fig. 1, 2 is a transistor working as a switch, 8 is a so-called driver transistor, 11 is a Circuit breaker working transistor which feeds the horizontal deflection coil 13 via the transformer 12, and 14 a so-called booster diode to which a capacitor 15 is connected in parallel.

The transistor 8 essentially supplies the base current for the transistor 11 during the period in which it is energized, the transistor 2 essentially the clearing current which is used to discharge the base 11 and which is necessary for the transistor 11 to be switched off as quickly as possible. When explaining the mode of operation of the circuit, reference is made to the oscillograms in FIG. Referring to FIG. 2: the transistor 2 is energized (opened) for the duration T ₁ to T ₄ as a result of the control by the supplied pulses U _B1, and deenergized (blocked) for the remaining time period T ₄ to T _6.

During the opening period of the transistor 2 is

the base of the transistor 8 is kept almost at the final potential. This is the transistor 8 and also the Transistor 11 de-energized.

At time T ₄ , the transistor 2 is de-energized due to the vanishingly small base voltage U _Bl; then the capacitor 5 gradually charges through the resistors 3 and 4 and the base path of the transistor 11 (the current in the relatively large resistor 10 can be neglected for the opening time of the transistor 11, as well as the voltage drop across the relatively small resistor 3) .

As soon as the voltage across the capacitor 5 has exceeded the threshold voltage of the base of the transistor 8, the transistor 8 supplies an additional current to the base of the transistor 11, which current increases gradually. This happens around time T ₅ . Since the increase in the capacitor voltage takes place according to an exponential function, this is also approximately the case for the emitter current of the transistor 8. Equation (F 1) applies to this emitter current.

can. The capacitor 5 loses an amount \ U _{C of} its voltage due to the clearance

h =

β β

R ₉

(Fl)

The capacitor voltage follows the relationship

Ues = (U ₀ - U _B11 ) (l - e ~ ~ public transport). (F 2)

The emitter current of the transistor 8 resulting from (F 1) and (F 2) is in A b b. 2 indicated as / _{3. In addition to the current J 3} , the charging current of the capacitor 5 also flows through the base of the transistor 11 (here, too, the current through the resistor 10 is neglected).

H -

_(F3)

40

The total current in the base of transistor 11 is

¹ BIl

- ! ■>. +

(F 4)

45 AU =

Δ0,

if Δ Q is the charge required to clear it.

The voltage at the base of the transistor 11 jumps to a negative value after it has been cleared, which is necessary to _{maintain the blocking of the transistor 11 for the duration of the return (T 2} - T _3). Secure blocking is guaranteed if

¹ CB rest

RßB

(F 5)

In this

CB rest

U ₁

CB lock

means

Collector-base residual current, base-rail resistance, reverse voltage at the base of transistor 11,
Ußsch ⁼ base threshold voltage. In addition, it must be taken into account that the capacitor 5 by the collector-base residual current by the amount

Δ U =

¹ CB rest

dt (F 6)

is discharged. The capacitor 5 or the minimum time constant can be derived from equations (F 5) and (F 6)

and is in A b b. 2 indicated. It has a value that increases until the end of the outward run, which is desirable is, since, as is well known, the collector current in the deflection transistor 11 gradually increases and an unnecessary high base current would lead to greater heating of the transistors 8 and 11.

At time T ₆ (T ₁ ), termination of the outward run and the start of the return run is desired. This process is initiated by the increase in voltage U _Bl at the base of transistor 2.

The starting collector current in transistor 2 sets the voltage of the base of transistor 8 to 0 and blocks this transistor 8. The most important characteristic of this circuit is to be seen as a discharge current of the capacitor 5 is used, which clears the base of the transistors 11. This stream is through the Resistance 3 limited to the permissible value.

The duration between insertion of the pulse _B1 U and the end of the avoidance T _2, since a delay is missing by the leakage inductance of the conventional driver transformer and advertising easily adjusted by dimensioning of the limiting resistor 3, the maximum permissible depletion current minimum ° 5 ¹ MO

calculate.

The maximum time constant results from the condition that the capacitor should be discharged before the onset of the new base current in transistor 11 (T ₅ ). In a practically implemented device, the following dimensions resulted:

transistor resistance 1Ω capacitor diode 2 BSX 62/16 3 IkO 5 0.1 µ 14 BA 145 8th BSX 62/16 4th 56 Ω 7 10μΡ 11 2N 3584 6th 27 Ω 15 6.8 nF 9 1.5 kü 10

Despite the lack of a driver transformer, the circuit described does not consume a large amount of current of transistor 8, there

a) the energy required to clear out the transistor 11 is not stored in the driver transformer needs to become

b) the base current in transistor 11 during .der Duration in which this current flows, not starting from an unnecessarily high value as usual

decreases the minimum value required at the end of the opening period, but increases from a negligibly small value to this value Iβ minimum . The heating of the transistor 8 can be reduced in that the collector voltage is reduced to a minimum by the resistor R ₆ and the smoothing capacitor C _7.

A thyristor that can be switched off more quickly can also be used to generate the clearing needle will.

1 sheet of drawings

Claims (2)

Patent claims: 1. Circuit for controlling a switching transistor for deflection devices, with the base of the switching transistor supplied switching pulses are preformed so that the base of the switching transistor at the beginning of the blocking time initiated by a switching pulse edge an excessive Receives locking pulse, the duration of which is short compared to the locking time, characterized in that that the base of the switching transistor (11) is supplied with control currents from two transistors by a first transistor (8) during the time in which the switching transistor (11) should be energized, and of a second transistor (2) mainly during the period immediately before the switching transistor (11) is blocked, the collector of the second transistor (2) galvanically with the base of the first transistor (8) and with the base of the switching transistor (11) is connected via a differentiating element (3, 5). 2. A circuit according to claim 1, characterized in that the switching pulse (U _Bl ) makes the second transistor (2) current-permeable at the beginning of the return.