Patent Application
20070223907
The invention relates in general to the driving circuit, and more particularly to the photo-flash driving circuit. 2. Description of the Related Art
The photo-flash of the camera illuminates the subject to be photographed. The Xenon light bulb used in the photo-flash operates at a high voltage, about 300V to 330V. But the camera operates from, for example, two Alkaline batteries connected in series, which provides 3.3V voltage while the battery cells are new. Therefore, a converting circuit is needed to boost the battery voltage to about 300V for driving the photo-flash.
IC 110 includes a transistor Q1 and a control circuit 112 for turning transistor Q1 on and off alternatively. The control circuit 112 includes a start-up circuit 121, a Vo/Vin comparator 123, an OR gate 125, an AND gate 127, a peak current detector 129, a SR flip/flop 131, and a Vo detector 133.
After time t2, transistor Q1 turns off and its collector-emitter voltage Vce increases from low level (Vce saturation) to Vin+Vo/n, where n is the turns-ratio of transformer 160. As Vce exceeds the level of Vin+Vo/n, input current I1 at the primary coil commutates to the secondary coil as the output current, I2, and rectifier diode D2 turns on. During and after a very short transition period, output current I2 flows into capacitor C2 and gradually decreases from Ipk/n to zero by a rate of −Vo/Ls, where Ls is the secondary-side self-inductance of transformer 160.
Once output current I2 drops to zero, rectifier diode D2 turns off. Then Vce begins to drop via Lp. When Vce drops below the level of Vin, at time t3, comparator 123 issues a high signal to set SR flip/flop 131, such that transistor Q1 turns on again.
Transistor Q1 switches at a frequency ranging from 50 kHz to 300 kHz. As output voltage Vo increases, the conduction time of output current I2 gets shorter, and that the switching frequency of transistor Q1 increases. Capacitor C2 is charged to its rated level after many switching cycles. When Vo detector 133 detects C2 is fully charged, a photo-flash ready signal is issued.
Diode D3, connected in parallel with transistor Q1, provides a reverse current path to keep Vce from swinging too far below zero.
However, in a typical photoflash circuit, the input voltage, Vin, is provided by two Alkaline batteries. Vin is 3.3V while the batteries are new, and Vin may dwindle to 1.8V while the energy of the batteries is exhausted. In general, the charger IC will not work properly if the input voltage is too low. Take the LT3468 charger IC of Linear Technology Coporation for example, it specifies an operating input voltage range from 2.5V to 8V. It will not work properly if the input voltage is below 2.5V because there would not be a sufficient voltage to drive transistor Q1 into deep saturation (Vce <0.2V).
Alternatively, transistor Q1 can be replaced by a MOSFET. Again, Vin of 2.5V would not turn on the MOSFET completely. The Rds(on) of the MOSFET at Vgs=2.5V is several times larger than the Rds(on) at Vgs=5V. If the Vgs is 1.8V, the Rds(on) would be much worse.
Consequently, the efficiency would be very poor if the input voltage is low, and in some cases the efficiency may drop to well below 50%. In other word, up to half of the input power is dissipated as loss and won't reach the secondary side to charge the output capacitor.
It is therefore an object of the present invention to provide an improved photo-flash driving circuit that can operate properly and efficiently over a wide range of input voltage.
The invention achieves the above-identified object by providing a photo-flash driving circuit, which includes an input node, a transformer, a charger IC, a rectifier diode and a high-voltage capacitor. The input node receives a DC input voltage. The transformer has a primary coil, whose first end is connected to the input node, and a secondary coil. A power storage unit is connected to the second end of the primary coil for providing a supply voltage to the charger IC. The charger IC has a power pin for receiving the supply voltage, and a switch pin connected to the second end of the primary coil for providing a pulse series such that the transformer outputs an AC voltage from the first end of the secondary coil. The anode of the rectifier diode is connected to the first end of the secondary coil. The capacitor is connected to the cathode of the rectifier diode and charged for providing an output voltage to the photo-flash.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.
Charger IC 310 includes a transistor Q1 and a control circuit 312 for turning transistor Q1 on and off alternatively. The operation of charger IC 310 is similar to charger IC 110 as described and shown in
Power storage unit 310 includes a recovery diode D1 and a recovery capacitor C3 to store the leakage energy from transformer 330 during its switching operation for providing the supply voltage Vcc to charger IC 310.
Transistor Q1 can be either a bipolar transistor or a power MOSFET The energy stored in the primary-side leakage inductance is roughly 0.5*Lk*Ipk2. If Lk=0.2 uH, Ipk=1.0A, each switch-on cycle of the MOSFET creates a leakage energy of 0.1 ujoule. At a switching frequency of 150 kHz, the leakage power is 15 mW. Assuming the charger IC consumes 2 mA current at an average supply voltage of 4V, it consumes 8 mW power. Therefore, to charge a 1 uF recovery capacitor C1 from 1.5V to 6V, it takes about 26 ms and 18 uJoule. If the total-charging time of the photo-flash 350 is 5 seconds, only the initial 26 ms (0.52%) is required to boost the voltage source Vcc from 1.5V to 6V. In the initial 26 ms period, the MOSFET is driven with a Vgs of less than 6V, and the Rds(on) of the MOSFET may not be the lowest. Then, Vgs increases from 6V to about 12V, as capacitor C2 is charged to about 300V. Thus, Rds(on) is the lowest for the majority of the overall charging period.
The photo-flash driving circuit according to the preferred embodiment has the following advantages:
1. The driving circuit can operate over a wide input voltage range, as low as 1.8V;
2. No extra wire is needed to connect the power pin of the charger IC to an external supply voltage, such as a 5V source.
While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.
