1. Field of the Invention
The present invention relates generally to a horizontal drive circuit for a CRT (cathode ray tube) display, and more particularly, to a horizontal drive circuit which uses a switch-mode power supply operating in a boost topology to generate an appropriate supply voltage needed by the horizontal drive circuit.
2. Background of the Invention
The most commonly used method to provide power to a horizontal drive circuit, with voltage adjustment capability, is to use the RegB+ of the horizontal output stage (usually around 130 VDC) in series with power resistors to reduce the 130 VDC to approximately 65 VDC. FIG. 1 illustrates this conventional horizontal drive circuit 100 which includes capacitor 108, a flyback transformer 112, and a horizontal drive transistor 116 being driven by a horizontal drive IC (integrated circuit) 114. The RegB+ voltage is applied to the circuit 100 at connection 102 and supplies a proper voltage level at connection 110 by reducing the voltage through power resistors 104, 106.
By changing the value of the resistors 104, 106, one can adjust the horizontal drive supply voltage 110. For example, typically two (2) 5W resistors (either placed in series or parallel with a typical equivalent value of 1.5 Kohm to 4.7 Kohm) are used to drop a supply voltage to the horizontal driver circuit to an acceptable/useable voltage. The wattage of the resistors needs to be large enough to handle a fault condition of a shorted driver transistor 116. This method is costly, requires large power resistors (due to fault conditions) which waste printed circuit board (PCB) space, and is not efficient (power is wasted in dropping resistors).
A possible solution would be to have a separate Buck-Boost Switch-mode Supply generating the appropriate horizontal drive voltage which uses its own control IC (integrated circuit) and switching device. However, this solution would require more printed circuit board area and would also have higher costs.
Therefore, a horizontal drive circuit is needed that advantageously combines the separate functions of a boost power supply with a horizontal drive circuit that occupies less printed circuit board area and achieves higher efficiency at lower cost.
It is therefore an object of the present invention to provide a horizontal drive circuit incorporating a boost switch-mode power supply.
It is another object of the present invention to provide a horizontal drive circuit incorporating a boost switch-mode power supply that occupies less printed circuit board area than the conventional combination of a horizontal drive circuit and associated power supply.
It is a further object of the present invention to provide a horizontal drive circuit incorporating a boost switch-mode power supply that achieves higher efficiency over many different horizontal frequencies.
Another object of the present invention is to provide a horizontal drive circuit incorporating a boost switch-mode power supply that can handle a fault condition of a shorted drive transistor without the use of large power resistors.
To achieve the above objects, a horizontal drive circuit employed in CRT displays which uses a switch-mode power supply operating in a boost topology to generate the appropriate supply voltage needed by the horizontal drive circuit is provided. The boost power supply is integrated into the horizontal drive circuit which allows for low printed circuit board area, low cost, and good performance over many different horizontal frequencies. A means of voltage adjustment and fault protection are also provided.
According to an aspect of the present invention, a horizontal drive circuit includes a flyback transformer having a first end of a primary coil connected in parallel to a capacitor and a second end of the primary coil connected to a horizontal drive transistor, and a boost switch-mode power supply. The boost switch-mode power supply includes an input voltage; a power bus including an inductor for receiving the input voltage and a first diode connected in series; and a switching means connected between the second end of the primary coil and a junction between the inductor and the first diode. Preferably, the switching means takes the form of a second diode. The boost switch-mode power supply further provides a voltage adjustment means through the use of a resistor.