Investigating LED driver output electrical parameter changes due to failing electronic components

Abstract

1. A theoretical model to explain how MOSFET degradation affects the average output voltage of an open-loop switched-mode dc-dc LED driver.

2. Identified the degradation process of the MOSFET and resulting Ron increase under thermal cycling and continuous high temperature conditions experienced in LED driver applications.

3. Demonstrated a method to distinguish capacitor degradation and the MOSFET degradation from the output voltage characteristics of an open-loop switched-mode dc-dc LED driver.

LED array, printed circuit board (PCB) and a driver are the key components of an LED lighting system. Therefore, failure of any one component can result in system failure. To enable end-of-life prediction capability in LED lighting systems, it requires real-time information regarding the health of the high failure probability components. Past studies have indicated that the LED driver failure is one of the main causes of lighting system failure in applications [2], [3], [12]-[14]. A thorough literature survey revealed that electrolytic capacitor and MOSFET are the two frequently failing components in LED drivers. Past studies have explored the electrolytic capacitor degradation and the resulting changes in output electrical parameters of LED drivers. Therefore, changes in the LED driver output electrical parameters due to degrading components, such as the electrolytic capacitor and the MOSFET, can be very useful for estimating the time to end-of-life. However, the author was not able to find any past studies on MOSFET degradation and failure and the resulting effects on the output electrical parameters of switched-mode LED drivers.

Even though there are number of methods to detect the degradation status of the MOSFETs from the device-level measurements, it is not easy to access the component inside the drive due to the potting material applied on the driver components to improve the heat dissipation, to make the measurements. Therefore, the goal of this dissertation is to investigate changes in output electrical parameters of the open-loop switched-mode dc-dc LED driver due to MOSFET degradation.

This dissertation further showed that MOSFET and electrolytic capacitor degradations can be detected by analyzing the average output voltage and the peak-to-peak ripple on the driver output voltage. These changes in the LED driver output characteristics can be useful for creating intelligent lighting systems with time to failure estimation.

Based on the findings from this dissertation study the following recommendations can be made to the LED lighting system manufactures.

1. Intelligent lighting system creation: By analyzing the average output voltage and the magnitude of the peak-to-peak ripple of the driver output voltage, it is feasible to estimate the degradation state of the MOSFET and electrolytic capacitor in an open-loop switched-mode LED driver. Also, these measurements can be useful for creating intelligent lighting systems with driver failure time estimation.

2. Driver lifetime improvement: By including thermal management solutions to the MOSFET and keeping its junction temperature low the driver lifetime can be improved.

The goal of this dissertation study is to investigate and identify measurable driver output parameters that indicate the drive health status due to degrading components. Intelligent LED lighting systems is one of the current trends in the lighting industry. The time to end-of-life estimator is a useful feature to have in intelligent lighting systems to allow the facility managers to schedule and perform the maintenance in a timely manner. However, presently available commercial LED lighting systems do not have such a feature, partially due to insufficient information available to create such a feature. This is one of the main reasons for selecting this dissertation topic and the knowledge gained in this study would contribute to the development of intelligent lighting systems.

Since electrolytic capacitor and MOSFET degradation contribute to more than 90% of the driver failures, investigating MOSFET degradation and its impact on output electrical parameters of switched-mode LED drivers can provide the necessary information to develop the time estimator to end-of-life in intelligent LED lighting systems. Therefore, this dissertation study focused on MOSFET degradation and its effect on LED driver output electrical parameters.

A theoretical model was developed to understand the effect on the driver output electrical parameters due to degrading MOSFET and it revealed that when a MOSFET degrades its on-state resistance, Ron, increases and the resulting average output voltage of an open-loop switched-mode dc-dc LED driver reduces. The theoretical model predictions were validated using SPICE simulations and a laboratory experiment. Typically, the components in an LED lighting system experience constant high temperature and thermal cycling stresses when they are switched on and off in applications. The device-level MOSFET degradation under thermal cycling and continuous exposure to high temperature conditions resulted in increased Ron. Thermal cycling creates dynamic stresses on the die attach solder due to the mismatch of the coefficient of thermal expansion (CTE) and consequently the die attach solder degrades with time and increases the on-state resistance. The experimental data showed strong correlation to the Coffin Manson model indicating die attach solder degradation and the activation energy calculated for this failure mechanism was 0.76 eV. The MOSFET samples subjected to continuous high temperature, also showed increased Ron with the stress duration. The calculated activation energy for this failure mechanism was 0.23 eV and the possible failure mechanisms for this degradation could be semiconductor sheet resistance and the metallized contact resistance increase. Driver-level thermal cycling experiments conducted in this dissertation study showed reduction in average output voltage for160 ⁰C condition during the study period, more test time is required to see similar trends for lower temperature conditions of 120 ⁰C and 140 ⁰C.

• Possible thermal management solutions to MOSFET include, the use of external heat sinks for the MOSFET, higher thermal conductive thermal interface material (TIM), that can result in reduced effective thermal resistance between the ambient and MOSFET junction.

• If the application permits, reduce the heat density of the LED driver printed circuit board (PCB) by increasing the area of the PCB and the driver envelope.

o Increase spacing between components to minimize the operating temperatures of the critical components such as electrolytic capacitors, MOSFETs, and diodes.

This dissertation study made the following new knowledge contributions to the fields of electronic devices and LED lighting systems: