Patent Application
20080030147
The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of at least one embodiment of the present invention. In the drawings, like reference numerals designate corresponding parts throughout various views, and all the views are schematic.
Reference will now be made to the drawings to describe preferred and exemplary embodiments in detail.
Referring to
Referring also to
The first transistor 210 is typically a P-channel enhancement mode metal-oxide-semiconductor field-effect transistor, which includes a gate electrode 211 connected to the first pulse generator 230, a source electrode 212 connected to the power supply 260, and a drain electrode 213 connected to a first terminal 252 of the primary winding 251 of the transformer 250.
The second transistor 220 is typically an N-channel enhancement mode metal-oxide-semiconductor field-effect transistor, which includes a gate electrode 221 connected to the second pulse generator 240, a source electrode 222 connected to ground, and a drain electrode 223 connected to the first terminal 252 of the primary winding 251 of the transformer 250.
The first capacitor 290 includes a terminal (not labeled) connected to the power supply 260, and another terminal (not labeled) connected to a second terminal 253 of the primary winding 251 of the transformer 250.
The second capacitor 270 includes a terminal (not labeled) connected to ground, and another terminal (not labeled) connected to the power supply 260 for filtering low frequency interferences from the power supply 260. In the exemplary embodiment, the second capacitor 270 is an electrolytic capacitor, and a capacitance of the electrolytic capacitor is 220 μF (microfarads).
The third capacitor 280 includes a terminal (not labeled) connected to ground, and another terminal (not labeled) connected to the power supply 260 for filtering high frequency interferences from the power supply 260. In the exemplary embodiment, the third capacitor 280 is a multilayer ceramic capacitor (MLCC).
In the exemplary embodiment, the power supply 260 is an 18V power supply. An amplitude of the first pulse generator 230 is 18V, a working frequency of the first pulse generator 230 is 50 KHz, and a duty ratio of the first pulse generator 230 is 0.65. An amplitude of the second pulse generator 240 is 5V, a working frequency of the second pulse generator 240 is 50 KHz, and a duty ratio of the second pulse generator 240 is 0.35. The first and second transistors 210, 220 are typically AP4511GH transistors. The transformer 250 is typically an EEL19 transformer.
In operation of the backlight driving circuit 20, when pulse signals from the first and second pulse generators 230, 240 are both high level signals, the first transistor 210 is switched off, and the second transistor 220 is switched on. The power supply 260, the first capacitor 290, the primary winding 251 of the transformer 250, and the second transistor 220 cooperatively form a charging current path. The power supply 260 provides primary energy storage stored in the transformer 250 to increase a primary current of the transformer 250, and charges the first capacitor 290. When the first capacitor 290 and the primary winding 251 of the transformer 250 proceed to resonate in series, the primary current of the transformer 250 reaches a maximal value. That is, a primary energy storage of the transformer 250 reaches a saturated state. Then the transformer 250 continues to release the primary energy stored therein, the power supply 260 continues to charge the first capacitor 290, and the primary current of the transformer 250 progressively decreases. When the first capacitor 290 is charged to 18V, the stored primary energy of the transformer 250 is completely released, and the primary current of the transformer 250 is equal to 0.
When the pulse signals from the first and second pulse generators 230, 240 are both low level signals, the first transistor 210 is switched on, and the second transistor 220 is switched off. The first capacitor 290, the primary winding 251 of the transformer 250, and the first transistor 210 cooperatively form a discharging current path. The first capacitor 290 begins to discharge, the transformer 250 begins to store primary energy therein, and the primary current of the transformer 250 progressively increases. When the first capacitor 290 and the primary winding 251 of the transformer 250 proceed to resonate in series, the primary current of the transformer 250 reaches the maximal value. That is, the primary energy storage of the transformer 250 reaches the saturated state. Then the transformer 250 begins to release the primary energy stored therein, and the first capacitor 290 continues to discharge.
In summary, the backlight driving circuit 20 includes the discharging current path formed by the first capacitor 290, the primary winding 251 of the transformer 250, and the first transistor 210. Because the first capacitor 290 has a characteristic whereby it generally cannot be discharged completely, the current passing through the first transistor 210 is relatively low. Therefore the first transistor 210 has low power consumption, and correspondingly dissipates a low amount of the power consumed in the form of heat energy. Thus the first transistor 210 can reliably operate with a low working temperature. Moreover, the backlight driving circuit 20 further includes the charging current path formed by the power supply 260, the primary winding 251 of the transformer 250, and the second transistor 220. Because the first capacitor 290 has a characteristic whereby it generally can be charged completely, the current passing through the second transistor 220 is relatively high. However, the second transistor 220 is an N-MOSFET having relatively low essential resistance, which is typically about 0.01Ω. Therefore the second transistor 220 has low power consumption, and correspondingly dissipates a low amount of the power consumed in the form of heat energy. Thus the second transistor 220 can reliably operate with a low working temperature. These advantages mean that the reliability of the backlight driving circuit 20 and the LCD 2 are improved.
In an alternative embodiment, the first transistor 210 can be a P-channel depletion mode metal-oxide-semiconductor field-effect transistor. In another alternative embodiment, the second transistor 220 can be an N-channel depletion mode metal-oxide-semiconductor field-effect transistor.
It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit or scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 95128628 | Aug 2006 | TW | national |
