1. Field of the Invention
The present invention relates to a modified push-pull booster circuit, in particular to a modified circuit that can improve the operating efficiency of a transformer by reducing the turn ratio in the transformer and simplifying the circuit, which is capable of cutting down the production cost considerably.
2. Description of Related Arts
Though switching power supplies are vital for electronic products, the efficiency of a dc booster circuit is just as important to a switching power supply. With reference to FIG. 4, a conventional push-pull booster circuit for an ac/dc transformer is composed of a pair of alternately switching transistors (Q1, Q2) that allow electrical energy to be transferred from the primary winding to the secondary winding in a transformer. Since the current in the primary and secondary windings of a transformer (70) flows in opposite directions, degaussing takes place each time the two transistors (Q1, Q2) in the windings of the transformer (70) break over. The secondary winding of the transformer (70) is coupled to a bridge rectifier (71) so the output from the transformer can be inverted by full wave rectification. The rectified current is then passed to the charging circuit (72) formed by two capacitors (C1, C2) that generate positive and negative bus voltages for a load circuit.
Since the capacitance in the capacitors (C1, C2) is usually quite high, the peak current of the bus charging circuit can fluctuate widely. Adding inductors (L1, L2) on the positive and negative electrodes of the charging circuit (72) is necessary. The main purpose of adding the inductors (L1, L2) is to limit the rise in current in the secondary winding of the transformer (70), and to maintain a continuous flow even during cut off.
However, when the current abruptly changes direction, the inductors (L1, L2) in series generate a peak voltage, which increases the voltage stress on the rectifying diodes (D1, D2, D3, D4) in the bridge rectifier (71). Therefore, a snubber circuit (73) is normally added between the inductors (L1, L2). The snubber circuit (73) consists of two subcircuits with one subcircuit connected to each side of the bridge rectifier. Each subcircuit of the snubber circuit (73) is composed of a diode (D5, D6), a capacitor (C3, C4) and a resistor (R1, R2). Consequently, the peak voltage through the bridge rectifier (71) is reduced, which decreases the voltage stress on the rectifying diodes (D1, D2, D3, D4) and permits the use of components made to less stringent specifications.
The foregoing modification to the booster circuit uses a snubber circuit (73) that adds components and raises the cost of production. Also, the inherent power loss in the snubber circuit (73) will decrease the overall efficiency of the circuit, and the series inductors (L1, L2) will reflect the voltage when the current changes direction suddenly, thus partially offsetting the boosted voltage of the push-pull transformer (70). To offset the reflected voltage, the turns on the secondary winding of the transformer (70) must be increased to obtain the design bus voltage. However, the increased number of turns on the secondary winding has a negative effect on production costs, manufacturing complexity and operating efficiency of the transformer (70) as a whole.
Although good reasons exist for using inductors in series, a better solution is presented in the present invention.
The main objective of the present invention is to provide a modified push-pull booster circuit capable of simplifying the circuit design, lowering the production cost and improving the operating efficiency of the transformer.
The push-pull booster circuit in accordance with the present invention mainly includes two switch transistors; a bridge rectifier and a charging circuit. The two alternately switching transistors are connected to the primary winding of the transformer. The bridge rectifier and the charging circuit are connected sequentially to the secondary winding, and a pair of inductors with inductive coupling is connected between the bridge rectifier and the charging circuit, or between the secondary winding of the transformer and the bridge rectifier.
Since these two inductors are in series configuration, current passing through produces a degaussing effect, equivalent to leakage inductance. When a current suddenly increases through the switch transistors in the primary winding of the transformer or the secondary winding of the transformer, the induced voltage in both inductors will cancel each other, thereby preventing a peak voltage on the rectifying diodes of the bridge rectifier. Also, when current is induced in any one of the inductors, an induced voltage will appear on the other winding due to the inductive coupling, which will cause recharging of the capacitor and boosting the bus voltage.
The inductively coupled inductors are implemented by a second transformer with the same number of turns on the primary and secondary windings.
The features and structure of the present invention will be more clearly understood when taken in conjunction with the accompanying figures.