Thermal design for handheld devices with plastic enclosures

You'll want to know the average worst-case power consumption in any ~5minute period, and the surface area of the outer casing, to start to evaluate the potential thermal stress at the hotspot. If there is an antenna, it can often be used as a dissiparor for the least efficient section ie the transmitter.

You should also evaluate the likelihood that an external ambient will actualy be 55C . Hand-held devices that are required to operate while being held, will see the 'hand' as a heat-regulating influence and a impose a further regulatory restriction to the actual permissible surface temperature rise, depending on the material properties of the casing.

RL

"veeresh" skrev i en meddelelse news: snipped-for-privacy@s15g2000prm.googlegroups.com...

Pee on it. That should keep it close to 37 deg. celcius. I.O.W: no.

Worst case power dissipation is 2.7W. there is an exposed ground plane on this chipset.. But this is facing towards the plastic body and there is no clearance left.. Also, i've provided thermal vias below this module(nearer to the shielding which cover the ICs)..

here, the antenna is internal PCB antenna.. I'll check once again if this can be used..

The unit may not be kept at 55, but because of other components and modules o the PCB, the ambient inside the box may rise to 55 Hand-held devices that are required to operate while

yes sir, as the external casing is made of plastic, i guess the heat can not be dissipated to the external environment.

If the duty cycle is sufficiently low, use a thick aluminum plate to quickly absorb the heat and to slowly dissipate it to the outside through the plastic wall of the box, using plenty of thermal grease and as much surface area as possible.

For example, 1 cm^3 of Al weighs 1.6 g and has a specific heat of .91 J/g-K, so it can absorb roughly 1.5 Joules per degree K of temperature rise.

-- Joe

This 2.7W is a continuous static power dissipation? I thought I heard or read 'packet transmission' somewhere - this suggests pulsations that occur on prompt - with possible interruption for other functions that may not require 2.7W dissipation. Is the input a human interface?

What kind of power source is used - does it contribute to the internal dissipation (what is it's efficiency)? How long is the power source expected (or proven) to last under this average worst-case power condition? This can often be a more practical indication of consuption/dissipation. Not a lot actually goes into free space as RF.

What is the proposed surface area of the plastic housing? What is the proposed plastic material type and thickness?

Getting the internal hotspot temperature nearer to the casing skin temperature involves reducing impedance between the hotspot and the skin. This means getting as much of the hardware in direct contact with the internal skin wall as is possible.

The contact will cause the external skin temperature to vary from the average, producing hotspots if the contact area is small.

Without contact, The internal skin wall temperature will be determined by that of the internal ambient air. This then determines the temperature of the electronics, then the hot spot. The characteristic gradients throughout the electronics in air can be determined outside of the case, by inspection.

You need to be more specific about the external operating ambient, as this will determine internal temperature rises above external ambient.

It won't be dissipated anywhere else, unless it's hand-held or mounted into some other absorbtive media.

This surface temperature limit will indicate permissible power dissipation limits of the package contents, as a preliminary check of design viability.

The question is whether the surface temperature will exceed acceptable limits. What standards are you subject to? This should list the permissible limits. Your sales department may have other ideas, written on paper somewhere.

Plastic is generally a poor thermal conductor, so higher safety-limited temperatures are often permitted than in those instances where hand-held casings are metal, as the hand can cool the plastic surface fairly quickly, ie without injury. ..............................................................