Prototyping of White LED Drivers

Posted on Feb 6, 2014

It lights on the high-brightness LED even at 0. 8V if a battery can feed the circuit 93mA at 0. 8V. The step-up efficiency from 0. 8V to 1. 5V is 75% in average and the peak efficiency (83%) is located at a voltage around 1. 1V. The circuit consists of very few components and it gives a step-up efficiency of 70% if the drive transistor is carefully chosen.

Click here to download the full size of the above Circuit.

Fig. -2 is the circuit for a practical application. It is not an intention to increase a number of components used but in the Fig. -2, a Schotkey diode D2 and a smoothing capacitor C1 are added between Q1 and a WLED. The reason is that there is a large ringing on the collector Q1 and it exceeds the absolute maximum forward voltage of the WLED and need to be smoothed by the additional components. Using a insulated wire of AWG30(wire diameter: 0. 2546mm) or similar wire, wind it 8 turns on the TDK`s axial inductor SP0406-101J (100 µH) and fix the windings by a glue. Photo-2 is the handmade transformer. The gold color band indicating an inductance tolerance is set for the polarity of primary winding and start to wind a wire from the side of brown color band as for the polarity of secondary winding. By the way, the WLED current doesn`t reach 20mA if R1 in Fig. -2 is 820 ohms. The reason is to stand by for a use of Panasonic`s Oxilide battery which has a initial voltage of 1. 7V and the current exceeds the absolute maximum current of WLED if 680 ohms resistor and the battery is used. It may be an idea to implement the current limiting circuit (inside the red dotted line) shown in Fig. -4. Note that don`t power up the circuit without connecting a WLED, otherwise a WLED is burned out because C1 is charged up to much higher voltage than the absolute maximum forward voltage of WLED. It may be an advisable to connect temporally D3...

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