High-voltage transformer for a television receiver

A transformer to achieve a precise amplitude for the operating voltages produced by the auxiliary windings, and such that the construction and production of the transformer are simplified. An auxiliary winding is split into a plurality of winding elements disposed in different chambers of a chamber-type coil former. One of the chambers is disposed in the region of the air gap of a U/I core or of a U/U core, the two U-core halves having parallel limbs of different lengths.

BACKGROUND 
The invention is based on a high-voltage transformer for a television 
receiver. The auxiliary windings are used to produce additional operating 
voltages, for example for a video amplifier, the heating of a cathode ray 
tube, other bias voltages or pulses for phase comparison circuits. The 
amplitude of the operating voltage produced by an auxiliary winding must 
in many cases be kept particularly accurate, especially in the case of the 
voltage for heating the cathode ray tube. This precise maintenance of the 
amplitude in practice often results in difficulties if the number of turns 
of an auxiliary winding is small. In this case, for example, three turns 
produce an operating voltage which is too small, and even four turns 
produce an operating voltage which is too high. The degrees of freedom for 
achieving an operating voltage with a desired amplitude are thus 
relatively limited in the case of such a transformer. 
SUMMARY OF THE INVENTION 
The invention is based on the object of designing the high-voltage 
transformer such that, without any additional outlay such as stabilization 
circuits for example, the respectively desired amplitude of the operating 
voltage produced by the auxiliary winding can be maintained with 
sufficient accuracy. In addition, the production and construction of the 
entire high-voltage transformer are simplified by the invention. 
The invention comprises the following features: 
a. The auxiliary winding is split into a plurality of winding elements 
which are located in different chambers of a chamber-type coil former. 
b. One of the chambers is located in the region of the air gap of a U/I 
core or of a U/U core, the two U-core halves having parallel limbs of 
considerably different lengths. 
The invention is based on the following knowledge. A winding element in a 
chamber which is located above the air gap has a significantly lower 
degree of coupling than a winding element outside the region of the air 
gap. If a winding element in a chamber outside the air gap, with a high 
degree of coupling, and a winding element in a chamber above the air gap, 
with a lower degree of coupling, are now connected in series, the amount 
of versatility in the amplitude of the operating voltage produced by the 
series circuit formed by the winding elements is increased. An operating 
voltage can then be maintained with an accuracy of about 100 mV. If the 
auxiliary windings are wound continuously, the number of connecting pins 
on the high-voltage transformer, the so-called pins, is thus also reduced. 
In addition, the circulating currents are reduced by the defined position 
of the winding elements in separate chambers in the case of a winding 
having a plurality of parallel-wound wires. 
The winding elements can also be located in more than two separate 
chambers, some of which are then located outside the region of the air gap 
and have a high degree of coupling, while others are located above the air 
gap or in the vicinity of the air gap and have a considerably lower degree 
of coupling. 
The auxiliary winding and/or the primary winding of the transformer are 
preferably wound using stranded wires, it being possible for the winding 
to comprise a plurality of wires which are twisted with a short pitch. 
Winding elements of an auxiliary winding can also be wound in opposite 
winding directions. 
A development of the invention comprises the following: it is known for the 
operating voltage for the video amplifier to be obtained by rectification 
of the pulsed voltage at the high-voltage transformer. The rectifier for 
the production of this operating voltage is switched on briefly by the 
line flyback pulse during its pulse peak, in order to recharge the energy 
storage capacitor of the rectifier circuit. In this case, relatively 
sudden and hard switching-on and switching-off occur at the start and end, 
respectively, of the pulse peak. These hard switching shocks cause the 
high-voltage voltage winding to be excited to oscillate, in an undesirable 
manner, at its resonant frequency or frequencies. This results in an 
undesirable influence on the high voltage, in particular leading to an 
increase in the internal impedance. In addition, undesirably high voltage 
spikes can in consequence occur on the switching transistor in the line 
output stage. Such undesirable current surges can be reduced by a resistor 
or an inductance in series with the rectifier of the rectifier circuit. 
However, such additional components are expensive because of the 
relatively high currents. According to the said development, an auxiliary 
winding for producing the operating voltage for the video amplifier is 
located in a chamber of the high-voltage transformer in the region of the 
air gap, so that the degree of coupling is considerably less than the 
degree of coupling of the chamber containing the primary winding and 
high-voltage winding. The lower degree of coupling means increased stray 
inductance. In the equivalent circuit, this stray inductance is located in 
series with the rectifier for producing the operating voltage for the 
video amplifier, and thus reduces undesirable current surges. The 
increased stray inductance, formed by the reduced degree of coupling, of 
the auxiliary winding in the chamber above the air gap is thus used in an 
advantageous manner to reduce the undesirable influence of the rectifier 
circuit for producing the operating voltage for the video amplifier on the 
high-voltage transformer, without any additional components. The said 
auxiliary winding is preferably located in a chamber at the end of the 
coil former, in the region of the air gap of the core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the high-voltage transformer Tr for producing the high voltage 
in a television receiver. The illustration shows the ferrite core 1 having 
an I-shaped core half 2 and a U-shaped core half 3 as well as a coil 
former 4 having two chambers 5, 6, which are located side by side in the 
axial direction and are separated from one another by a chamber wall 7. 
The primary winding P is located on the base of the chamber 5, and the 
winding element Hi of an auxiliary winding is located above said primary 
winding P. Winding elements S of the high-voltage winding, which are 
connected on the basis of the diode-split principle, are located in a 
large number of narrow chambers 8. 
A second winding element H2 of the auxiliary winding H, which is connected 
in series with the first winding element H1, is located in the chamber 6, 
which is located above the air gap 9 of the core 1 between the core halves 
2, 3. The series circuit formed by the winding elements H1, H2 supplies, 
via the rectifier circuit 10, an operating voltage U1, for example the 
heating voltage for the cathode ray tube, an operating voltage for an 
amplifier or the like. Located in the chamber 6 is a second auxiliary 
winding H3, which supplies, via the rectifier circuit 11, an operating 
voltage U2 for the video-signal amplifier of the television receiver. 
As a result of the fact that the chamber 6 is located with the windings H2 
and H3 above the air gap 9, the degree of coupling is considerably less 
than in the chamber 5, because of the greater scattering of the magnetic 
flux there. The described very large number of amplitudes for the 
operating voltage U1 can be achieved by the different degrees of coupling 
for the winding elements H1 and H2. As described, the interfering 
influence of the rectifier circuit 11 on the high voltage is considerably 
reduced by the likewise lower degree of coupling for the winding H3. 
FIG. 2 shows the construction of the line output stage with the transformer 
Tr according to FIG. 1. The illustration shows the switching transistor 
13, which is controlled by the line-frequency switching voltage 12, the 
flyback diode 14, the flyback capacitor 15, the coupling or tangent 
capacitor 16, the line deflection coils 17, the primary winding P, which 
is connected to an operating voltage +UB, of the transformer Tr, the 
winding elements H1, H2, the rectifier circuit 10, the auxiliary winding 
H3, the rectifier circuit 11, four winding elements S of the high-voltage 
winding between each of which there are located diodes D connected in the 
same sense, and the cathode ray tube 18 which is supplied with the high 
voltage UH produced. 
The operating voltage U2 which is produced by the auxiliary winding H3 and 
the rectifier circuit 11 is supplied as the operating voltage to the video 
amplifier 19 in the path of the video signal. As a result of its geometric 
position according to FIG. 1, the winding element H2 has a considerably 
lower degree of coupling than the winding element H1, as a result of which 
the wide versatility of the achievable amplitude levels for the voltage U1 
is achieved. 
In the case of the described transformer, it is known for the high-voltage 
winding to be tuned to odd-numbered harmonics, in particular the 9th, 
11th, 13th and 15th harmonics of the frequency of the flyback oscillation. 
This tuning allows the form of the flyback pulse, which is used to produce 
the high voltage, on the high-voltage winding to be influenced and the 
internal impedance of the high-voltage source to be reduced. 
Harmonics of the fundamental of the flyback oscillation are produced in the 
flyback pulse at the transformer, in particular on the high-voltage 
winding, as a result of this tuning. The ideal pulse in the form of a 
sinusoidal half-cycle during the flyback in this case has superimposed on 
it interference oscillations, for example in the range from 200-500 kHz, 
both during the flyback and during the forward sweep. These interference 
oscillations increase the power loss of the transformer, can cause 
interference radiation and thus an adverse effect on adjacent circuits, 
increase the internal impedance of the high-voltage source and also cause 
interference patterns in the picture, in the form of so-called "curtains". 
It is thus necessary to damp or attenuate these interference oscillations, 
which are also called "ringing". 
Another development of the invention is based on the object of designing 
the high-voltage transformer without any additional means in structural 
terms, such that these interference oscillations are sufficiently damped 
or attenuated. This object is achieved by the development specified in 
claim 8. Advantageous refinements of this development are specified in 
claims 9-10. 
Thus according to this development, a first auxiliary winding is arranged 
in the first chamber and the second auxiliary winding is arranged in the 
second chamber, and their numbers of turns are designed such that the 
amplitudes of the fundamentals of the flyback pulse produced in the two 
auxiliary windings are of approximately the same magnitude. The 
development is based on the following knowledge and considerations. The 
first auxiliary winding, which is located together with the primary 
winding in the same chamber in the coil former, is closely coupled to the 
primary winding as a result of the fact that it is located close to it. 
The flyback pulse does not have the said interference oscillations 
superimposed on it at the primary winding, because these interference 
oscillations are produced in the high-voltage winding and are suppressed 
in the primary circuit during the forward sweep by the so-called flyback 
diode, which is connected in parallel with the switching transistor and is 
forward-biased during the forward sweep. Only the fundamental of the 
flyback pulse is present, therefore, on the primary winding and on the 
first auxiliary winding, without the interference oscillations, that is to 
say a pure wanted signal. 
However, the second auxiliary winding in the second chamber is considerably 
further away from the primary winding and thus has a considerably lower 
degree of coupling to the primary winding. As a result of its coupling to 
the high-voltage winding, the said interference oscillations are produced, 
in similar form to that on the high-voltage winding itself, on this second 
auxiliary winding. The flyback pulse on this second auxiliary winding thus 
has the said interference oscillations superimposed on it. This now has 
the following advantageous effect: 
By appropriate selection of the number of turns on the first auxiliary 
winding and the second auxiliary winding, the fundamental of the flyback 
pulse, that is to say the half-sine wave, has the same amplitude on the 
first auxiliary winding and on the second auxiliary winding, and also 
occurs at the same time. Thus, for the fundamental, no circulating current 
flows in the parallel circuit formed by the two auxiliary windings, in a 
desirable manner. This is also desirable because this half sine wave, 
which can be regarded as the wanted signal, is not intended to be damped 
or attenuated. 
The said interference oscillations in the meantime are not present in the 
flyback pulse on the first auxiliary winding but are present in the 
flyback pulse in the second auxiliary winding. The parallel circuit formed 
by the two auxiliary windings thus results in circulating currents only 
for these interference oscillations, that is to say in the higher 
frequency range above about 50 kHz. These circulating currents are 
therefore desirable and produce the desired damping or attenuation of the 
interference oscillations on the high-voltage transformer. 
In the case of the development, the desired damping or attenuation of the 
interference oscillations is thus achieved without any additional means, 
namely only as a result of a special arrangement and design of the two 
auxiliary windings. The auxiliary windings thus carry out two tasks in an 
advantageous manner, without any additional outlay, namely the production 
of an operating voltage or pulsed voltage, and the damping or attenuation 
of the interference oscillations. 
The number of turns of the second auxiliary winding in the second chamber 
is preferably somewhat greater than the number of turns of the first 
auxiliary winding in the first chamber. This is generally necessary in 
order to achieve the same amplitude of the fundamental of the flyback 
pulses in the two auxiliary windings, despite the lower degree of coupling 
in the second chamber. 
The number of turns of the two auxiliary windings is roughly in the range 
from 6 - 30. If the number of turns becomes considerably smaller, the 
requirement for the same amplitude of the fundamental cannot be maintained 
sufficiently accurately. If the number of turns is considerably greater, 
the desired circulating currents in the auxiliary windings no longer have 
the required value to achieve sufficient damping or attenuation. 
The second chamber is preferably located in the region of the air gap of 
the core. This is advantageous since it promotes the desired and 
advantageously used lower degree of coupling between the second auxiliary 
winding and the primary winding. 
The auxiliary windings are preferably used for producing an operating 
voltage for an amplifier such as the IF amplifier, video amplifier or 
audio amplifier, for example, as a source for a pulsed voltage or else for 
heating the cathode ray tube. 
The described development is explained in the following text with reference 
to FIGS. 3 - 7. 
FIG. 3 shows the construction of the high-voltage transformer Tr, in 
simplified form. The illustration shows the ferrite core 1 with the 
approximately I-shaped core half 2 and the U-shaped core half 3 as well as 
the air gap 9. The core 1 supports the coil former 4. The coil former 4 
contains a first chamber 6a in which the primary winding P and the 
high-voltage winding S, which is designed as a chamber winding, are 
located. The coil former 4 contains a second chamber 6b, which is also 
called an auxiliary chamber, is axially offset with respect to the first 
chamber 6a and is located in the region of the air gap 9. 
In addition, the coil former 4 contains two auxiliary windings Hi and H2, 
which are connected in parallel and supply, via the rectifier circuit 10, 
an operating voltage U1, for example for the IF amplifier, the video 
amplifier, the audio amplifier, a pulsed voltage for a synchronization 
circuit or else the heating voltage for the cathode ray tube. 
The additional effect of the auxiliary windings H1 and H2 for damping or 
attenuating the interference oscillations is explained with reference to 
FIGS. 4 and 5. By virtue of its position immediately above the primary 
winding P, the auxiliary winding H1 is closely coupled to said primary 
winding P, to be precise considerably more closely than to the 
high-voltage winding S. A flyback pulse R1, in the form of a pure half 
sine wave P without the said interference oscillations, is present on the 
primary winding P, since the interference oscillations are produced only 
in the high-voltage winding and are additionally suppressed on the primary 
winding P during the forward sweep by the flyback diode, which is 
connected in parallel with the switching transistor of the line output 
stage. Thus, as a result of the close coupling between the primary winding 
P and the auxiliary winding H1, the flyback pulse R1 on the auxiliary 
winding H1 according to FIG. 2 has no interference oscillations and the 
amplitude A. 
Because of its axially offset position with respect to the primary winding 
P, the auxiliary winding H2 in the chamber 6b is considerably less closely 
coupled to the primary winding P, but is significantly coupled to the 
high-voltage winding S. Significant interference oscillations N thus occur 
in the flyback pulse R2 on the auxiliary winding H2 according to FIG. 5 
during the start of the flyback and during the forward sweep. The number 
of turns of the windings H1, H2 is selected in such a manner that the 
fundamental is of the same magnitude in the two flyback pulses RI, R2. 
This means that no circulating current flows between the auxiliary 
windings H1, H2 for the fundamental, that is to say the wanted signal. 
However, a circulating current flows for the interference oscillations N, 
because these interference oscillations are present only in the pulse R2 
on the winding H2. This desirable circulating current thus produces the 
desired damping or attenuation of the interference oscillations N. This 
effect, which is advantageous for the damping or attenuation of the 
interference oscillations, does not have any negative influence on the 
operating voltage U1 because such interference oscillations are suppressed 
in any case, in the rectifier circuit 10. 
FIG. 6 shows a line deflection circuit in which the high voltage is 
obtained using a transformer Tr according to FIG. 3. The illustration 
shows the switching transistor 13 which is controlled by the 
line-frequency switching voltage 12, the flyback diode 14, the flyback 
capacitor 15, the coupling or tangent capacitor 16, the line deflection 
coils 17, the primary winding P, which is connected to an operating 
voltage +UB, of the transformer Tr, the auxiliary windings H1, H2, the 
rectifier circuit 10 for producing the operating voltage U1, the 
high-voltage winding S and the high-voltage rectifier D for producing the 
high voltage UH for the cathode ray tube 18. The two parallel-connected 
auxiliary windings H1 and H2 are used for producing the operating voltage 
U1 and for damping or attenuating the described interference oscillations 
N. 
FIG. 7 shows an equivalent circuit for the auxiliary windings H1, H2. The 
stray inductance Ls acts between the windings H1, H2 because of the 
relatively loose coupling between these two windings. The stray inductance 
Ls makes it possible for the interference oscillations N to be present on 
the auxiliary winding H2, but not on the auxiliary winding H1. The 
described effect can be achieved by special selection of the numbers of 
turns for H1 and H2. If the numbers of turns are too small, it is 
difficult to comply with the requirement for the same amplitude A of the 
fundamental of the flyback pulses R1, R2. If the number of turns is too 
great, the value of Ls becomes too high, so that the circulating currents 
ia, which are produced and are desirable, do not have a sufficient 
amplitude to damp or attenuate the interference oscillations N. In 
practice, the said requirements can be achieved with a number of turns 
between 6 and 30 for H1 and H2.