Column: DC/DC converters
Cooling and derating DC-DC converters for reliable operation
By Lars Foerster, DC/DC Business Development Manager EMEA, TDK-Lambda T
oday’s manufacturing techniques have considerably improved DC-DC reliability. The use of surface- mount placement for
all components, computerised optical inspection of solder joints, automated test and laser marking have eliminated most manual tasks in the production process. In making its CC-E series of DC-DC converters, TDK Lambda even uses double-armed robots to form the converter leads, attach metal EMI shielding covers and package the final product into trays. Increased product reliability has led many DC-DC converter manufacturers to offer five- year warranties. Internally, ceramic capacitors have
replaced electrolytics that age as their electrolyte dries out. Consequently, mean time between failure (MTBF) calculations are correspondingly higher. Based on the Telcordia reliability prediction procedure, TDK-Lambda’s CCG15 series of 15W converters have calculated MTBFs of millions of hours. Naturally, this number varies
with temperature and output load, as component reliability decreases as temperature rises; see Figure 1.
Power budget and derating To ensure high reliability and long service life, designers must ensure that the DC-DC converter operates as coolly as possible in the end equipment. Thus, some initial work must be performed before the design of the printed circuit board starts. Most engineers will develop a ‘power
budget’, estimating the current needed for their load. As an example, we will use a budget of 12V at 0.8A = 9.6W. It’s important never to operate any
DC-DC converter or power supply at its maximum rating. Derating will increase the field life of the end equipment and provide some margin if the actual power estimate goes over budget. Figures between 50 and 75% are
widely used in the electronics industry. With this in mind, we will select a 15W converter and operate it at 63% load in our example. The CCG15-48-S12 can deliver 12V at 1.3A with an input voltage of 18-76V.
Figure 1: MTBF versus ambient temperature for the CCG15-48-S12 converter
Ambient temperature The ambient temperature inside the system or enclosure can be calculated, estimated or measured. If the application is a convection-cooled, bench-mounted instrument with ventilation slots, 40°C ambient will probably suffice. We will then use the manufacturer’s datasheet to determine where we are on the derating curves; see Figure 2. In this example there is no derating
required with a convection-cooled 40°C ambient at 63% load. Even if the ambient temperature rose to 75°C due to an abnormal situation, there is sufficient margin. When “natural convection” is indicated
on a derating curve, it doesn’t mean zero airflow. As heat rises, some natural air movement will cool the converter. During the design of the printed circuit board, avoid large components impeding that natural airflow.
Testing When testing the prototype system or equipment, measure the case temperature of the DC-DC converter and the ambient temperature, to confirm
Figure 2: Derating information for the CCG15-48-S12 converter
14 February 2021
www.electronicsworld.co.uk
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