By Mitchell Belser, PE
I worked for a company that made overhead projectors. The company had just come out with a new line. The new projectors were significantly smaller than the previous models. I was contacted by a member of the product safety group. The new projectors were having reliability problems in the field. One of the returned units had some over heating problems. The projector contained a switching power supply made by a German company. This was the source of the power dissipation problem.
Apparently the power transistor was exceeding the safe operating area and a current sense resistor was also experiencing some problems as evidenced by the discolored region on the printed circuit board where the resistor was located. The power transistor issue was somewhat easy to figure out. The chassis was used as the heat sink for the power transistor. Unfortunately the chassis had been painted black. Painting the heat sink significantly increased the thermal resistance between the transistor case and the heat sink. There was also no heat sink compound to reduce the thermal resistance.
The overheating resistor was a little less obvious. The product safety engineer had used a digital multimeter to measure the voltage drop across the resistor to calculate the power dissipation. With the DMM set to measure DC voltage the power dissipation didn’t appear to be an issue. However, this was a switching power supply. The current in the sense resistor was not static but contained current spikes due to the switching of the power transistor. Using a DMM and measuring the DC voltage did not paint an accurate picture of what was really going on. I used a clamp-on ammeter and observed the current waveform on an oscilloscope. The peak current was about ten times the average value. Presented with this information the product safety engineer was able to calculate the true power dissipation.
Mitchell Belser is a registered professional engineer, has a MSEE from Auburn University and 20 years experience designing analog circuits, analog/mixed signal ICs, and product development. He is currently teaching in the computer engineering department at Jackson State University.