Practical Design of A USB Light

This chapter discusses USB‐powered reading light design. The first thing is to look up the USB specification. The next thing to do is to select the LED, because everything else depends on that selection. Having chosen the LED, the chapter begins designing the driver. The first thing is to figure out the drive current. There are two aspects to efficiency calculations. The chapter starts with the power conversion efficiency at full output power and then calculates minimum power. It is need to estimate the temperature of the light emitting diode (LED) to make sure to meet the lifetime. Both depend on the junction temperature of the LED. More significantly for the design, encasing the whole thing in plastic gives the LED very high thermal resistance to the ambient.