Design Con 2015

Ensure long lifetimes from electrolytic capacitors: A case study in LED light bulbs

-April 06, 2013

Hot LEDs and short-lived electrolytic capacitors

Several years ago I worked on a few designs for LED light bulbs. Very early on it became clear that the temperatures of components in such light bulbs can get quite high. I personally measured component temperatures as high as +130°C in light bulbs purchased at local retail stores. Now admittedly, these were early LED bulb designs. Manufacturers now understand that, even though these LED bulbs consume substantially lower power than those they would replace, they still must have good thermal engineering. This is the only way to get the lifetime of the electronics to match the lifetime of the LEDs themselves.

I found it disturbing that many of these hot designs contained electrolytic capacitors which are notorious for a short lifetime at elevated temperatures. I expected that the lifetimes of these capacitors would severely compromise the lifetime of the products, and not allow them to reach the 30,000 to 50,000 hour capability of the LEDs themselves. With common electrolytics rated at 2,000 to 5,000 hours at +85°C, I vowed not to use an electrolytic in any LED bulb designs.

In talking to LED bulb manufacturers at that time I found that many did not understand the limitations of electrolytic capacitors. It was common for them to point out, for example, that they had used a part rated for +105°C and had measured the temperature of this part to be +100°C. Surprise was often their reaction when I pointed out that their +105°C-rated capacitor was only guaranteed to meet spec after 2000 hours at +105°C.

 

Temperature and the endurance spec for electrolytic caps

It is important to understand both the “endurance” spec of electrolytic capacitors and how temperature affects it. There is always a time of operation associated with the temperature rating, typically between 1000 and 10,000 hours. The maximum temperature ratings are typically +85°C or +105°C. It is a common and largely valid rule of thumb that capacitor lifetime increases by a factor of 2 for every +10°C reduction in temperature.

Conversely, lifetime decreases by a factor of 2 for every +10°C increase in temperature. Therefore, at +100°C, the endurance of the 2,000 hour, +105°C-rated capacitor mentioned above would be 21/2 x 2000, or about 2800 hours. Even if you assume that, on average, the capacitor operates at +95°C, its lifetime only increases to 4000 hours. This is hardly the 30,000 to 50,000 hours desired for LED light bulbs.

It is no surprise, therefore, that many of these early LED bulbs had overly optimistic lifetime ratings due to the manufacturers’ misunderstanding of the ratings of their electrolytic capacitors. I took apart several such bulbs that died after less than 1000 hours of operation. Nearly all of them had a failed electrolytic capacitor. 

 

Power factor and ripple: a trade-off

The more cost-effective LED light bulb designs back in those years all utilized single stages. All of the single-stage architectures of which I am aware produce either good power factor or low-output 120Hz (100HZ) ripple. Not both…unless you use at least one large-valued capacitor. To get capacitor values large enough to allow a design to deliver both, electrolytics must be used.

At that time many manufacturers accepted products with substantial 120Hz ripple, as long as the power factor was high. I did several designs without electrolytics that had substantial 120Hz ripple in the LED current.

 


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