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CC-E
9
Jul

How to ensure reliable operation of DC-DC converters (cooling and derating)

Posted by Martin on July 9, 2020

Today’s manufacturing techniques have improved DC-DC reliability considerably. The use of surface-mount placement for 100% of the components, computerised optical inspection of solder joints, automated test and laser marking has eliminated most of the manual tasks. Production for TDK‑Lambda’s CC-E series of DC-DC converters has even recently deployed double-armed robots to form the converter leads, attach metal EMI shielding covers and package the final product into trays. Reflecting this, many DC-DC converter manufacturers offer five-year warranties.

Internally, ceramic capacitors have replaced electrolytic capacitors that age as their electrolyte dries out. Consequently, Mean Time Between Failure (MTBF) calculations are correspondingly higher. TDK-Lambda’s CCG15 series of 15W converters have calculated MTBF numbers, using the Telcordia reliability prediction procedure, of millions of hours. The number varies with temperature and output load, as component reliability reduces as temperature rises. See Figure 1.

Figure 1: MTBF versus ambient temperature for the CCG15-48-S12 converter

To ensure high reliability and long field life, the user must ensure that the DC-DC converter operates as cool as possible in the end equipment. Some initial work should be performed before the design of the printed circuit board starts.

Power budget and derating

Most engineers will develop a ‘power budget’, estimating how much current will be needed for their load. As an example, we will use a budget of 12V 0.8A = 9.6W.

It is strongly advised never to operate any DC-DC converter or power supply at its maximum ratings. Derating will increase the field life of the end equipment and provide some margin if the actual power estimate comes over budget. A figure of 50% to 75% is widely used in the electronics industry. With this in mind, we will select a 15W converter and operate it at 63% load. The CCG15-48-S12 can deliver 12V 1.3A with an input voltage of 18 to 76V and we will use this product in our example.

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 for example, 40°C will probably suffice.

We will then use the manufacturer’s datasheet to determine where we are on the derating curves, see Figure 2.

Figure 2: Derating information for the CCG15-48-S12 converter

In our 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.

Airflow within the enclosure

When ‘natural convection’ is indicated on a derating curve, it does not mean zero airflow. As heat rises, there will be some (natural) air movement which will cool the converter. During the design of the printed circuit board, avoid large components impeding that natural airflow.

Testing

When testing your prototype system or equipment, measure the case temperature of the DC-DC converter and the ambient temperature to confirm the initial estimates and / or calculations.

Manufacturers will state which points that should be measured and where. In our example, the CCG converters are measured in the middle of the case. Figure 3, and typically 76mm away from the side of the converter (Figure 4), to avoid measuring the hot air rising from the case.

Figure 3: Measurement location for case temperature
Figure 4: Measurement of ambient temperature

Looking at Figure 1 again, we can see the MTBF number will be between 5M and 6M hours.

Most manufacturers, like TDK-Lambda, provide detailed on-line information for their products, as demonstrated in this article. Support is also available from experienced staff members.

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