Thermal Management for Picor Cool-Power Isolated DC-DC Converters

May 4, 2012
Picor Cool-Power

Picor Cool-Power® DC-DC Converter

This is the second in a three-part series that discusses thermal management issues, and how to address them for different product types. In the first part, we looked at high power density DC- DC bricks. Here, we focus on the high power density, silicon-centric products, such as the new PI3101 Picor Cool-Power Isolated DC-DC converter. These products’ power densities of up to 400W / in ³ could give hardware designers some significant thermal management challenges, given that they provide up to three times greater power from less than half the space of conventional packages.

Although these devices can operate with an efficiency of up to 87%, as with all power systems, the maximum available output power will degrade with ambient temperature. Accordingly, effective thermal management must be used to realise the device’s full potential.

Systems designers can match their thermal management strategy to their target environment by using two key tools – appropriately profiled heatsinks, and forced air cooling. Sometimes either approach, or a mixture of both, can be used to maintain the ambient temperature within acceptable limits.

Even so, there are some environments that preclude the use of fan cooling, and others where heatsinks cannot be applied. For example, communications systems are one area where designers may seek to avoid heatsinks. This is because such systems typically comprise rack-mounted boards on a narrow pitch, with restricted or no clearance for heatsinks between adjacent boards. By contrast, fans are unwelcome in other applications, either because of their audible noise or because their use is considered to compromise system reliability.

Vicor has therefore published a set of three graphs for the PI3101 isolated DC-DC converter that provide a reference for power systems designers as they address its thermal management issues; these are shown below.

The first graph comprises a set of plots for the PI3101 used without a heatsink. It shows how the maximum available output power degrades as ambient temperature increases, for varying levels of cooling air flow, measured in Linear Feet per Minute, or LFM. Note that one option shown is for 0 LFM, so operation of the PI3101 with no heatsink and no forced air cooling is possible with moderate loading and ambient temperature. Under such circumstances this may be the best solution.

The other two graphs show how fitting a 6.3 mm heatsink and 11 mm heatsink respectively affect the maximum output power vs temperature plots for the same set of air flow values. Designers can work across all three graphs to find the airflow/heatsink trade-off that best satisfies their particular criteria. For example, a converter with an 11 mm heatsink relying on natural convection cooling could deliver 32 W at 55˚C ambient. If, however, the proximity of an adjacent board leaves insufficient clearance for a heatsink as mentioned above, then the heatsink can be dispensed with – provided the airflow can be increased to 400 LFM. If some clearance is available, another alternative would be to fit a 6.3 mm heatsink. If the ambient temperature remains at 55˚C, a reduced forced air flow of 200 LFM would support converter output power to over 44 W.

By contrast, another system may call for 55 W at 35˚C. Without a heatsink, forced air at a rate of 1000 LFM would be required, in turn leading to the use of fans that could be intrusively noisy. Instead, using a 11 mm heatsink would allow the PI3101 to deliver 55 W at 35˚C with a cooling air flow reduced to 200 LFM. Irrespective of the environment he is designing into, these three graphs allow the designer to find the optimum balance of airflow and heatsink size for his power requirement, prevailing ambient temperature and other related constraints.

PI3101 Power output vs Temperature vs Airflow – No heatsink

Fig.1: PI3101 Power output vs Temperature vs Airflow – No heatsink

PI3101 Power output vs Temperature vs Airflow – 6.3 mm heatsink

Fig.2: PI3101 Power output vs Temperature vs Airflow – 6.3 mm heatsink

PI3101 Power output vs Temperature vs Airflow – 11 mm heatsink

Fig.3: PI3101 Power output vs Temperature vs Airflow – 11 mm heatsink

Whichever thermal management solution they choose, designers can enhance its performance by using the PI3101’s on-chip features. The device’s ‘TM’ pin provides an external indication of its internal package temperature, which is within approximately ±5˚K of the hottest junction temperature. The pin’s output is a scaled, buffered analogue voltage which is proportional to the internal temperature in degrees Kelvin. This can be monitored by an external microcontroller or an adaptive fan speed controller, which can regulate air flow in the system. Also, if a thermal overload condition arises, the TM pin is pulled low. This thermal shutdown function is a fault feature, which interrupts power processing if a certain maximum temperature is exceeded.

The PI3101 is intended for designers seeking to achieve the maximum possible power efficiency from the smallest possible form factor. The set of three Power Out vs Temperature graphs and the device’s TM temperature indication pin are useful tools that help power designers support the device’s power performance with the most efficient and reliable thermal management design possible.

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