Flyback transformer

Details Flyback transformer Flyback transformer

2001-08-13

2004-01-06

Mai, Anh (Department: 2832)

Inductor devices

Plural part core

C363S020000

Reexamination Certificate

active

06674356

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to flyback transformers and, more particularly, to a flyback transformer used in a PWM (Pulse Width Modulation) controlled voltage generator circuit for providing a high voltage to a cathode ray tube.
2. Description of the Related Art
FIG. 1
is a circuit diagram showing a high voltage generator circuit including a flyback transformer and which is the motivation for the development of the present invention. The anode of a diode
14
is connected to the primary winding of a flyback transformer
12
for use in a high voltage generator circuit
10
while the cathode of the diode
14
is connected to the drain of an FET (Field-Effect Transistor)
16
functioning as a switching element. The source of the FET
16
is connected to one end of a resistor
18
and the other end of the resistor
18
is grounded. A diode
20
is connected in parallel with the serial circuit of the diode
14
, the FET
16
, and the resistor
18
. The cathode of the diode
20
is connected to the anode of the diode
14
, while the anode of the diode
20
is grounded.
The serial connection of a resonance capacitor
22
and a diode
24
is connected in parallel with the diode
20
. One end of the resonance capacitor
22
is connected to the anode of the diode
14
, while the other end of the resonance capacitor
22
is connected to the cathode of the diode
24
. The anode of the diode
24
is grounded. The junction of the resonance capacitor
22
and the diode
24
is connected to the anode of another diode
26
, and the cathode of the diode
26
is connected to the primary winding of the flyback transformer
12
through a ringing suppressing circuit
28
. The ringing suppressing circuit
28
comprises a capacitor
30
, a resistor
32
, and an inductor
34
. A power supply +B is connected to the junction between the diode
26
and the ringing suppressing circuit
28
. The junction between the diode
26
and the ringing suppressing circuit
28
is grounded through a capacitor
36
and a capacitor
38
.
A signal for controlling the FET
16
in an on and off operation is fed to a gate of the FET
16
from a PWM (Pulse Width Modulation) control circuit
40
. The control PWM circuit
40
receives a voltage which is obtained by voltage dividing the output voltage across the secondary winding of the flyback transformer
12
. The control signal for controlling the FET
16
is formed based on the divided voltage and an input horizontal drive voltage. The junction of the FET
16
and the resistor
18
is connected to a protective circuit in the PWM
40
, and an excessive current flowing through the circuit is detected by the protective circuit.
FIG. 2
show waveforms of voltages at representative points of the high voltage generator circuit
10
. As shown, FIG.
2
(
a
) shows a waveform of a signal for controlling the FET
16
, and FIG.
2
(
b
) shows a waveform of the voltage at point A in FIG.
1
. FIG.
2
(
c
) shows a waveform of a current flowing in the primary winding of the flyback transformer
12
. When the FET
16
is turned on at t
0
, a current flows from the power supply +B through the primary winding of the flyback transformer
12
, the diode
14
, the FET
16
, and the resistor
18
. The primary winding of the flyback transformer
12
stores electromagnetic energy when this current flows.
When the FET
16
is turned off at time t
1
, a current flows from the primary winding of the flyback transformer
12
through the resonance capacitor
22
, and the diode
26
, and the primary winding of the flyback transformer
12
and the resonance capacitor
22
starts resonating, thereby generating a flyback pulse as shown in FIG.
2
(
b
). The flyback pulse is maximized at the moment the electromagnetic energy stored in the flyback transformer is entirely converted into electrostatic energy in the resonance capacitor
22
.
When the electromagnetic energy stored in the primary winding of the flyback transformer
12
is entirely transferred to the resonance capacitor
22
, a reverse currents flows through a path along the diode
24
, the resonance capacitor
22
, and the primary winding of the flyback transformer
12
. The electrostatic energy in the resonance capacitor
22
is transformed back into electrostatic energy in the primary winding of the flyback transformer
12
. Charge stored in a stray capacitance of the FET
16
is blocked by the diode
14
, and does not flow into the primary winding of the flyback transformer
12
.
The voltage at point A returns to zero at time t
2
when the flyback pulse ends. The diode
20
is turned on, permitting a current to flow from ground to the primary winding of the flyback transformer
12
. The voltage at point A rises when this current flows, and reaches the voltage of the power supply +B. At time t
3
, the diode
20
is turned off, and the current becomes zero. A current from the power supply +B attempts to flow into the resonance capacitor
22
, but a current prevention clamping circuit comprising the diodes
24
and
26
clamps the voltage across the resonance capacitor
22
to the voltage of the power supply +B. No current flows from the primary winding of the flyback transformer
12
to the resonance capacitor
22
. When the FET
16
is turned on at time t
4
, a current flows from the power supply +B to the primary winding of the flyback transformer
12
, and the state of the circuit is the same as that at time t
0
. The high voltage generator circuit functions repeating the above-referenced operation. The flyback transformer
12
raises the flyback pulse in voltage level, and provides a high voltage from the secondary winding thereof.
When the current drops to zero at time t
3
, resonance takes place between the primary winding of the flyback transformer
12
and capacitance of the circuit including the stray capacity present in the FET
16
. Ringing is thus generated from time t
3
to time t
4
. The ringing suppressing circuit
28
is used to control ringing.
In the high-tension generator circuit
10
, inductance Lp of the primary winding of the flyback transformer
12
satisfies the following condition:
Lp≦Eb×Ts/Ipp
where Eb is the voltage of the power supply +B, Ts is a duration of time from the end of the flyback pulse to the start of the next flyback pulse, and Ipp is an input current to the flyback transformer
12
. Under this condition, the input current Ipp must meet the permissible current of the FET
16
. The conventional flyback transformer
12
must satisfy these conditions and is designed to provide a required output voltage from the secondary winding thereof.
Magnetic flux density Bmax generated in the core of the flyback transformer
12
is Bmax=Lp×Ipp/N
1
×S, where N
1
is the number of the primary winding coils, and S is the cross-sectional area of the core of the primary winding. Given a constant voltage Eb of the power supply +B, the input current Ipp is maintained low, the core is reduced in size, and the flyback transformer
12
is made compact. A lower input current Ipp leads to a lower power consumption.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a compact and low-power consumption flyback transformer which reduces an input current flowing thereinto.
A flyback transformer of the present invention for use in a PWM controlled high voltage generator circuit, includes a layer-wound secondary winding having the number of layers equal to or larger than six.
In a preferred embodiment, a diode is configured with the cathode thereof connected to one end of the secondary winding and with the anode thereof grounded.
The flyback transformer is preferably used in a PWM controlled high voltage generator circuit having an operation frequency of 70 kHz or higher.
The number of turns of the secondary winding may be not more than 2500.
Given the same number of winding turns, an increase in the number of layers of layer-wound secondary winding narrows the width acros

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