GB2177563A - Electrical driving circuits - Google Patents

Abstract

A circuit especially for driving fluorescent lamps has a rectifier circuit (1) to provide unsmoothed rectified DC which is applied to power the oscillating transistors, and a feedback circuit (5) to provide positive current feedback. It further has an automatic control circuit (6) adjusting the driving bias of the transistors to the safe region of dissipation so that the output is stable and its efficiency is maximized. The circuit is said to have efficiencies as high as 85-90% and a power factor as high as 95%. <IMAGE>

Description

SPECIFICATION Electrical driving circuits The present invention relates to electrical driving circuits.

The conventional electronic driving circuits, such as power supply circuit for fluorescent lamps, usually converts from AC to DC as the power source for an oscillator circuit, which then generates the voltage required to drive the lamp tu be(s). Such driving circuit has many drawbacks.

(a) Vcc (Collectorsupplyvoltage) and bias are sup- plied by DC and the output to the oscillator is of square wave. The harmonic frequencies and distortion of such square waves are high.

(b) The maximum efficiency of square wave output is about 70-80%, and it is impractical to increase its efficiency by adding a lot of compensating componentswhich resulted in higher cost.

(c) Poor circuit stability owing to the DC VCC, which fluctuates with the supply line voltage.

(d) Complicated and expensive circuit structure.

The present invention has a circuit to provide unfiltered DC voltage which is applied to drive the oscillating transistors, and then with a feedback circu it to provide positive current feedback to stabilize the circuit. Itfurther has an automatic control circuit adjusting the driving bias of the transistors to the safe region of dissipation so thatthe output is stable and its efficiency is maximized.

Embodiments ofthe invention are described by way of example, with reference to the drawings, in which: Figure lisa block diagram for the preferred embo dimentofthe present invention; Figure 2 is a circuit diagram for the preferred embo- dimentofthe present invention; Figure3is an application ofthe preferred embodiment in a driving circuitforfluorescent lamps; Figure4 is another application of the preferred embodiment in a driving circuitforfluorescent lamps.

Figure lisa block diagram for a preferred embodi ment ofthe present invention. It illustrates an unfiltered DC supply circuit, which is an improvement for the conventional power supply circuit. The circuit has a filter network to prevent interference to the supply line from the output of the oscillatorfrequency. With such an unfiltered waveform circuit (1), the present invention provides Vcc.a nd with a bias circuit (4), it provides a bias for a Pierce oscillator (2) so thatthe Pierce oscillator oscillates at a frequency of about 40 KHz. A feedback circu it (5) is used to provide feedback currentto stabilize the bias ofthe oscillatortransistors in order to prevent from unstable oscillation.An automatic control circu it to avoid the fluctuation of supply source which may over-drive the transistors, and provides output (3) with stable sinusoidal carrier wave.

Figure 2. is a circuit diagram for a preferred embo diment of the present invention, wherein the unfiltered waveform circuit is shown includes a VZsurge absorber, C2, C3 and L1, L2 in the circuit formed a low-pass filterto suppress the output of the oscillator frequency leaking to the supply line. Any leakage of the oscillatorfrequency may interfere other electrical devices using the same supply line, D1, D2, D3 and D4 composed a bridge rectifierto convert AC to halfwave output, in awaveform of unfiltered DC as shown in Figure 2. C5 is a small capacitorfor phase displacement of the half-wave form. L3 is an impedance matching coil which provides Vcc of unfiltered waveform to the oscillator circuit and in turn to give sinusoidal waveform output.The said unfiltered waveform circuit could be offullwave or half-wave rectification. D5 is a rectifier, C7, R5 and C8 composed a T-fi Ite r. D6 a n d R6 stabi lize the feedback voltage. Q1 and Q2 are oscillatortransistors, which, togetherwith L5 and C6fora stable oscillation circuit. R2, R3 and R4, atthe bases of the said oscillatortransistors, are bias control resistors. R2 and R3 are drawing powerfrom R1,and feeding back positive currentfrom R5.

Through an induction coil the oscillation circuit induces a negative voltage for automatic adjusting the bias of Q1 and Q2. It does not only prevent Q1 and Q2 from over-driven but also stabilize oscillation and output in order to promote working efficiency. The feed back voltage could be from either half-wave or fullwave rectification.

The transistors Q1 and Q2 in the present invention are used for the oscillator as well as output stage. The output is of carrier sinusoidal waveform as shown.

Therefore, its powerfactor is superiorthan that ofthe prior art.

FigureS shows application of the preferred embodimentforthe present invention in a driving ci rcuit of a fluorescent lamp. The circuit shown is substantially same as in Figure 2, and thus not to be described here.

L8 and C9 areto prevent the generation of interfer ence signal from thefluorescent lamp. C9 is a coupling capacitorforthefluorescent lamp to give adequate brightness and to be free from interference.

The present invention fits best for driving circuits of fluorescent lamps, neon lights and mercury-vapor lamps and provides good energy saving effects.

In the aforesaid preferred embodiment, The Vcc is from unfiltered positive half-wave, which may be changed to unfiltered negative half-wave by changing Q1 and Q2 to PNP type and reversing the polarity of D5 as shown in Figure 4, a circuitto replace is in Figure 3.

The present invention has the following functions and characteristics: (a) The output is in the form of carrier sinusoidal wave.

(b) Vcc is from unfiltered wave, which is different from that of DC in the prior art.

(c) Complete with feedback circuit and automatic bias compensation circuit to stabilize the current and voltage and protect the oscillatortransistors.

(d) Because Vcc is of unfiltered wave and the output is of sinusoidal carrier wave, its efficiency can be reached to 85-90% instead of 70-85% at the prior art.

(e) Power factor of the present invention can be as high as 95%.

(f) Very stable output.

(g) Simple circuit structure which uses less parts, and consequently, cost is less than the prior art; and (h) Sinusoidal wave output causes less interference to other electrical devices.

Claims (10)

1 An electrical driving circuit comprising: an unfiltered wave form circuitto give Vcc of unfiltered waveform; an AC driving oscillation circuit to cause transisters to oscillate; a feedback circuitto give positive current feedback to stabilize the output to maximise efficiency; and an automatic control circuitto give negative vol- tage for automatic adjusting the bias of the oscillator transistors.

2. A circuit as claimed in Claim 1 wherein the Vcc is of unfiltered waveform supplied bythe unfiltered waveform circuit.

3. A circuit as claimed in Claim 1 whereinthe unfiltered waveform circuit is composed of D1, D2, D3, C5 and L3, in which D1, D2, D3 and D4 are for rectification, C5 is for phase displacement, L3 is an impedance coil to supply voltage of unfiltered waveform to Q1 and Q2 for sinusoidal wave output.

4. A circuit as claimed in Claim 1 wherein the unfiltered waveform circuit includes a fullwave or half-wave rectifier.

5. A circuit as claimed in Claim 1 wherein the feedback voltage is a small positive voltage feedback to the bias circuit from the current induced by L, rectified and filtered by D5, C7, C8, and R5, and stabilized by D6 and R6.

6. A circuit as claimed in Claim 1 whereinthe automatic control circuit adjusts the bias of the oscillator circuit automatically by a negative bias from L.

7. A circuit as claimed in Claim 1 wherein the feedback circuit is a rectifier circuit of halfwave or fullwave rectification.

8. A circuit as claimed in Claim 1 wherein L8 installed to prevent interference and for interconnection through C9 when it is used forflourescent lamps, neon lights or mercury-vapor lamps.

9. A circuit substantially as herein described.

10. A circuit constructed and arranged substan tally as herein described with reference to any of the drawings.