Power supply synchronization

A power supply (1, 2) is synchronized to a repetition frequency of an addressing of lines in a multiples rate line addressing circuit (7). The power supply (1, 2) operates satisfactorily if switched within a prescribed frequency range. The switching frequency of the power supply (1, 2) is equal to the repetition frequency divided by an integer larger than zero. The integer is determined such that the switching frequency of the power supply (1, 2) occurs within the prescribed frequency range for all repetition frequencies of the addressing of the lines.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of, device for, and a control circuit for 
synchronizing a power supply to a line addressing repetition frequency of 
a line addressing circuit, the method comprising the steps of: generating 
trigger moments having a trigger repetition frequency equal to the line 
addressing repetition frequency, determining an integer larger than zero 
in dependence upon the trigger repetition frequency, and generating 
control moments in response to the trigger moments for determining a 
switching frequency of the power supply, the control moments having a 
repetition frequency being the trigger repetition frequency divided by the 
integer. 
Such a method and a control circuit for synchronizing a power supply may be 
applicable in a display system comprising a picture display device and a 
multiple rate scanning circuit suitable for displaying pictures with 
different scan rates, as is the case in computer monitors. The power 
supply may be used to supply stabilized voltages to other circuits of the 
display system, or to generate an anode voltage for the picture display 
device. 
2. Description of the Related Art 
Such a synchronized power supply for a multiple scanning frequency video 
monitor is known from U.S. Pat. No. 4,516,169. The known display system 
includes a horizontal deflection circuit which can be operated on two 
selectable horizontal deflection frequencies. A switching power supply is 
switched on one optimal switching frequency synchronized to the selected 
horizontal deflection frequency, independent on which of the two 
horizontal deflection frequencies occurs. Synchronized operation of the 
switching supply is desirable to prevent switching of components in the 
switching power supply from becoming visible on a picture tube of the 
display system. The known display system shows an embodiment able to cope 
with two different horizontal deflection frequencies: a second horizontal 
frequency of 31.5 kilohertz is a first horizontal frequency of 15.75 
kilohertz multiplied by two. The switching supply is operated on the first 
horizontal frequency of 15.75 kilohertz for both horizontal deflection 
frequencies. A drawback of the known display system is that a fixed 
switching frequency of the switching supply synchronized to the two 
horizontal deflection frequencies is only possible if each of the 
horizontal deflection frequencies is an integer multiple of the fixed 
switching frequency. The known display system does not offer a practical 
solution if the display system should be capable of displaying horizontal 
deflection frequencies which are not an integer multiple of one fixed 
frequency suitable for switching a switching supply. This can be the case 
in computer monitors in which the horizontal deflection frequencies, for 
example, can be 35.2, 37.8, 48, 56, 64, 77.1 or 81.1 kilohertz, 
respectively. 
SUMMARY OF THE INVENTION 
It is an object of the invention to switch a switching supply synchronized 
to a line addressing frequency having a value within a large range of 
various line addressing frequencies without imposing conditions on the 
line addressing frequencies. 
A first aspect in accordance with the invention is characterized in that 
the repetition frequency of the control moments can assume several values 
within a prescribed frequency range, and in that the step of determining 
the integer results in an integer for which it holds that a division of 
the trigger repetition frequency by the integer provides a repetition 
frequency of the control moments within this prescribed frequency range. 
It is a second aspect of the invention to provide a display system 
comprising a picture display device for displaying picture information, a 
multiple rate line addressing circuit for generating the line addressing 
of the picture display device having a line addressing repetition 
frequency, and for generating a trigger signal having a trigger repetition 
frequency equal to the line addressing repetition frequency, a 
synchronized power supply for generating a supply voltage, means for 
determining an integer larger than zero in dependence upon the trigger 
repetition frequency, and means for generating a control signal for 
determining a switching frequency of the power supply in response to the 
trigger signal, the control signal having a repetition frequency being the 
trigger frequency divided by the integer, characterized in that the 
repetition frequency of the control signal can assume several values 
within a prescribed frequency range, and in that the means for determining 
the integer is adapted to generate an integer for which it holds that a 
division of the trigger repetition frequency by the integer provides a 
repetition frequency of the control moments within this prescribed 
frequency range. 
It is a third aspect of the invention to provide a control circuit for 
synchronizing a power supply to a trigger signal having a trigger 
frequency equal to a line addressing repetition frequency of a line 
addressing circuit, the control circuit comprising means for determining 
an integer larger than zero in dependence upon the trigger frequency, and 
means for generating a control signal for determining a switching 
frequency of the power supply in response to the trigger signal, the 
control signal having a repetition frequency being the trigger frequency 
divided by the integer, characterized in that the repetition frequency of 
the control signal can assume several values within a prescribed frequency 
range, the means for determining the integer being adapted to generate an 
integer for which it holds that a division of the trigger repetition 
frequency by the integer provides a repetition frequency of the control 
moments within this prescribed frequency range. 
It is a fourth aspect of the invention to provide a device for 
synchronizing a power supply to a line addressing repetition frequency of 
a line addressing circuit comprising means for generating trigger moments 
having a trigger repetition frequency equal to the line addressing 
repetition frequency, means for determining an integer larger than zero in 
dependence upon the trigger repetition frequency, and means for generating 
control moments in response to the trigger moments for determining a 
switching frequency of the power supply, the control moments having a 
repetition frequency being the trigger repetition frequency divided by the 
integer, characterized in that the repetition frequency of the control 
moments can assume several values within a prescribed frequency range, and 
in that the means for determining the integer results in an integer for 
which it holds that a division of the trigger repetition frequency by the 
integer provides a repetition frequency of the control moments within this 
prescribed frequency range. 
A display system comprises a picture display device, a multiple rate line 
addressing circuit for displaying pictures with different scan rates, and 
a switching power supply. The line addressing circuit causes scanning of 
lines of positions on the picture display device. The lines have a line 
addressing repetition frequency (further referred to as line frequency) 
related to the different scan rates. The line addressing circuit can be a 
line deflection circuit as used to scan a conventional picture tube, or 
row/dot-selection circuitry as used in matrix displays. Trigger moments 
are generated having a trigger repetition frequency equal to the line 
frequency. A switching frequency of the power supply is determined by 
control moments having a repetition frequency being the repetition 
frequency of the trigger moments divided by an integer larger than zero. 
The integer is determined such that the repetition frequency of the 
control moments occurs within a prescribed frequency range. In this way, 
the power supply is operated synchronized to the trigger moments and at a 
frequency lying within the prescribed frequency range. The power supply is 
designed such that it operates without problems within the prescribed 
range of switching frequencies. 
If hardware circuits are used to generate the trigger and control moments, 
these moments could be linked to moments on which a signal changes level. 
If a micro computer generated the trigger and control moments, this could 
as well be signals changing level as values representing a certain moment. 
In an embodiment of a method in accordance with the invention, which is 
characterized in that the step of determining the integer is adapted to 
cause the integer to be equal to one for all trigger repetition 
frequencies within the prescribed frequency range, the integer is 
generated to be one if the line frequency occurs within the prescribed 
frequency range of power supply switching frequencies. In this case a 
first advantage is that the switching frequency of the power supply is 
equal to the line frequency and thus switching of components of the power 
supply occurs only one time during each line period. If the switching 
frequency of the power supply were an integer multiple of the line 
frequency this would cause the disturbances to occur more often on the 
display device. This is especially the case if the power supply is used to 
generate an anode voltage of a picture tube. A second advantage is that 
the line frequency is not divided by an integer larger than one if this 
would result in a switching frequency of the power supply still occurring 
within the prescribed frequency range. As an example, if the prescribed 
frequency range includes frequencies from 15 kHz up to 60 kHz, and the 
line frequency is 45 kHz, the power supply will be switched on 45 kHz and 
not on 30 kHz or 15 kHz. In this way the power supply will operate as 
efficient as possible and will generate as stable output voltages as 
possible. 
In an embodiment of a method in accordance with the invention, which is 
characterized in that the step of determining the integer is adapted to 
generate the integer having a value causing the repetition frequency of 
the control signal to be as close as possible to a highest frequency 
within the prescribed frequency range but not being higher than the 
highest frequency, if the trigger repetition frequency is higher than the 
highest frequency, the line frequency is higher than a highest frequency 
within the prescribed frequency range. The integer is generated to obtain 
an as high as possible switching frequency of the power supply occurring 
within the prescribed frequency range. This has the advantage that the 
power supply will operate as efficient as possible and will generate as 
stable output voltages as possible. 
A preferred embodiment of a method in accordance with the invention is 
characterized that the step of determining the integer comprises the steps 
of: generating a disable period with a certain duration in response to the 
trigger signal, and generating a control signal in response to the trigger 
signal if this trigger signal does not occur during a preceding disable 
period, wherein the certain duration corresponds to the highest frequency. 
If the trigger moments do not occur during an active preceding disable 
period, the trigger moments generate a disable period with a certain 
duration and the trigger moments generate related control moments for 
switching the power supply. The certain duration corresponds to the 
highest frequency within the prescribed frequency range. If trigger 
moments have a repetition frequency within the prescribed frequency range, 
every trigger moment will trigger a disable period and the disable period 
will be inactive before a next trigger moment occurs. At every trigger 
moment, a control moment is generated to obtain a switching frequency of 
the power supply equal to the line frequency. If the trigger moments have 
a repetition frequency higher than the highest frequency within the 
prescribed frequency range, it will last until the first trigger moment 
occurring after the certain duration of a disable period to trigger a next 
disable period and a next control moment. In this way, the switching 
frequency of the power supply occurs within the prescribed frequency range 
and is as high as possible. 
A preferred embodiment of a display system in accordance with the 
invention, is characterized in that the means for determining the integer 
comprises a non-retriggerable multivibrator having an input coupled to 
receive the trigger signal and an output coupled to supply the control 
signal, the trigger signal causes the multivibrator to change to a disable 
state having a certain duration if the trigger signal does not occur 
during a preceding disable state, and the output stays in the disable 
state if the trigger signal occurs during this disable state. A 
non-retriggerable multivibrator is triggered by a trigger signal to change 
into a disable state with a certain duration. A trigger moment, which is a 
trigger signal, has no effect if the multivibrator is in the disable 
state. An output of the multivibrator generates a signal indicating the 
disable state. The control moment which is a control signal is, or is 
related to, this output signal. The certain duration again corresponds to 
the highest frequency within the prescribed frequency range. If a trigger 
signal has a repetition frequency within the prescribed frequency range, 
every trigger signal will trigger a disable period and the disable period 
will be inactive before a next trigger signal occurs. At every trigger 
signal, a control signal is generated. If the trigger signal has a 
repetition frequency higher than the highest frequency within the 
prescribed frequency range, it will last until the first trigger signal 
occurring after the certain duration to trigger a next disable period and 
a next control signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an embodiment of a display system according to the invention. 
A synchronization circuit 5 receives a picture signal Pi and supplies line 
and frame synchronizing signals Hs, Vs to an addressing circuit 7. The 
addressing circuit 7 comprises a line addressing circuit for generating a 
line deflection current through a line deflection coil Lh in response to 
the line synchronizing signal Hs, and a frame addressing circuit for 
generating a frame deflection current through a frame deflection coil Lv 
in response to the frame synchronizing signal Vs. The line addressing 
circuit generates a trigger signal St having a repetition frequency equal 
to a repetition frequency of the line deflection current or equal to the 
repetition frequency of the line synchronizing signal Hs. A first power 
supply 1 receives a non stabilized input voltage Vac (for example, an AC 
line voltage) and supplies a stabilized output voltage Vb to a second 
power supply 2 which is an EHT-supply for supplying an anode voltage Va to 
a picture tube 4. The first power supply 1 and the EHT-supply 2 are 
controlled by control signals Sc1 and Sc2, respectively. The control 
signals Sc1, Sc2 determine a switching frequency of the respective 
supplies 1, 2. A control circuit 6 receives the trigger signal St, and 
supplies the control signals Sc1, Sc2. The control circuit 6 comprises two 
non-retriggerable monostable multivibrators (for example, the MM74HC221A 
of National Semiconductors) 60, 61. Both multivibrators 60, 61 have a 
negative transition triggered input A connected to a low level L, a 
positive transition triggered input B to receive the trigger signal St, 
and a clear input CLR connected to a high level H. The first multivibrator 
60 has an output Q1 which supplies the first control signal Sc1. The 
second multivibrator 61 has an output Q2 which supplies the second control 
signal Sc2. A positive going edge of the trigger signal St causes the 
multivibrators 60, 61 to assume a disable state during a disable period. 
Each of the disable periods has a duration corresponding to a highest 
frequency on which the corresponding supply 1, 2 is allowed to operate. As 
both multivibrators 60, 61 operate in the same way the operation of only 
the first multivibrator 60 is elucidated. The trigger signal St occurring 
during a disable period has no effect as the first multivibrator 60 is 
non-retriggerable. The output signal Q1 is active high during the disable 
period. The rising edge of the output signal Q1 contains the repetition 
frequency information for controlling the first power supply 1. 
The operation of the multivibrator 60 is further elucidated with reference 
to FIGS. 2a-2d. For example, the highest switching frequency of the first 
power supply 1 is chosen to be 60 kHz, so that the duration of the disable 
period Td needs to be 16.7 .mu.s. FIGS. 2a, 2b, 2c, and 2d show the 
operation of the multivibrator 60 at a trigger signal St with repetition 
frequencies of 15 kHz, 50 kHz, 70 kHz, and 200 kHz, respectively. Only the 
relevant active moment of the trigger signal St is shown. The control 
signal Sc, determining the switching frequency of the first power supply 
1, is related to the rising edge of the output signal Q1. 
In FIGS. 2a and 2b, an active trigger signal St at moment t0 triggers the 
multivibrator 60 into the disable state in which the output signal Q1 has 
a high level during the disable period Td until moment t1. As the next 
trigger signal St occurs at a moment t2,t2' after moment t1 (the trigger 
signal St has a repetition frequency within the prescribed frequency 
range), each trigger signal St will trigger a disable period. At every 
trigger of a disable period, a control signal Sc is generated to obtain a 
switching frequency of the first power supply 1 equal to the line 
frequency, in this example being 15 kHz and 50 kHz, respectively. 
Also in FIGS. 2c and 2d, an active trigger signal St at moment t0 triggers 
the multivibrator 60 into the disable state in which the output signal Q1 
has a high level during the disable period Td until moment t1. All trigger 
signals St occurring before moment t1 are ignored. The first trigger 
signal St occurring after moment t1 on moments t2" and t2'" triggers a 
next disable state. So, if the trigger signal St has a repetition 
frequency higher than the highest frequency (60 kHz) within the prescribed 
frequency range, the first trigger signal St occurring after the certain 
duration of a disable period will trigger a next disable period and a next 
control signal Sc. In this way, the switching frequency of the first power 
supply 1 occurs within the prescribed frequency range, is equal to the 
repetition frequency of the trigger signal divided by an integer larger 
than one, and is as high as possible but not higher than the highest 
frequency within the prescribed frequency range. 
It will be evident that variations of the embodiment described above are 
possible within the scope of the invention. Although the embodiment shows 
two synchronized power supplies 1, 2 each synchronized by their own 
control signal Sc1, Sc2 it is also possible to synchronize both power 
supplies 1, 2 with one control signal having a repetition frequency in a 
range suitable for both power supplies. It is also possible to synchronize 
only one power supply. Instead of driving a picture tube 4 with deflection 
currents, it is alternatively possible to drive a matrix display with row 
and column selection circuits. The control circuit 6 can alternatively be 
a microcomputer programmed in a suitable way. This microcomputer detects 
moments of occurrence of the trigger signal St and generates moments of 
occurrence of the control signal Sc. The microcomputer could also perform 
the functions of the synchronization circuit. Detected horizontal 
synchronization moments could be represented in values. Moments of 
occurrence of the control signal Sc are calculated based on these values. 
It is also possible that one of the power supplies 1, 2 is synchronized to 
the control signal Sc2, Sc1 or a signal related thereto of the other power 
supply 2, 1 instead of being synchronized to the trigger signal St. More 
specifically, the control signal Sc1 of the first power supply 1 (being a 
line voltage power supply) can be obtained by dividing via a multivibrator 
60, a flyback pulse generated by the second power supply 2 (being the EHT 
supply). The multivibrators 60, 61 can be replaced by other circuits which 
divide the frequency in an appropriate manner. 
A preferred embodiment of determining the integer according to the 
invention is summarized below. 
The integer is one if the repetition frequency of the addressing of the 
lines occurs within a prescribed frequency range of the power supply. 
The integer is determined such that the switching frequency of the power 
supply (1, 2) occurs within the prescribed frequency range as close as 
possible to a highest frequency within the prescribed frequency range, if 
the repetition frequency is higher than the highest frequency.