Methodology of reliability enhancement for high power LED driver

The degradation of a linear-mode high power LED driver is studied. This driver is to provide constant current to LED regardless of its output voltage, but there is a minimum output voltage whereby the output current cannot be maintained as constant. Prolonged operation of the driver through relevant reliability test revealed that this minimum voltage will increase due to degradation of the output transistor in the driver, and when this minimum voltage exceeds the operating voltage of the driver, it no longer provides a constant current to the LEDs. Our investigation clearly reveals that the degradation mechanism is hot carrier injection as will be shown in this work, and hence it is inevitable. As high power LEDs have long life time, their drivers must also have equivalent lifetime so that the long life of LEDs can be leveraged in the entire luminary. One method to enhance its lifetime is to increase the hot carrier injection lifetime of the output transistor which may requires some fundamental change in the transistor structure and/or processing, and it is beyond the control of the driver designer and can be expensive also. A circuit approach is to use redundant circuit for enhancement which is common in the circuit world. Effective switch over to redundant circuit requires continuous health monitor of the primary circuit. In this work, the method to identify the health monitoring index, based on the degradation mechanism is shown, and the control circuit to detect and switch over to the redundant circuit is also designed and presented here.