Fonera (model 2100) overheating issue


About

The fonera is a cheap wireless access point. There are three versions of it: the 2100 model, the 2200 model (externally similar to the the 2100, but internally different) and the Fonera+, with two ethernet connectors.

The problem

The 2100 model has the problem of becoming very hot while operating. This issue is mostly solved in the other models. Since the one I bought is a 2100, I made some measurement to understand why it overheats, while the 2200 model, which is only slightly different does not. I opened it up, connected an amperometer to the power cable to verify its power consumption, and measued the temperature of various components using a digital thermometer.

 The two components that become hotter are the cpu and the power regulator, but surprisingly, the power regulator is actually hotter than the cpu!
The results of the test are shown here:

Test conditions:
 -ambient temperature  20°C
 -firmware 0.7.1-r1
 -cpu temperature measured on heatsink
 -the voltage regulator is the chip close to the power
  connector (5V to 3.3V regultor)
 -voltage regulator power dissipation is calculated with this formula:
  (5V-3.3V)*power_consumption
 -fonera case closed

IDLE TEST:
Conditions:
 -wifi active, not transmitting nor receiving
 -eth connected
 -logged on ssh
 -running "top" through ssh, cpu load <2%
Results:
 -power consumption                   0.65 A (3.25 W)
 -cpu temp                            53.9°C
 -voltage regulator temp              61.0°C
 -voltage regulator power dissipation 1.11 W

LOAD TEST:
Conditions:
 -wifi active, not transmitting nor receiving
 -eth connected
 -logged on ssh
 -runnin "yes >/dev/null" in background to simulate high cpu load
 -running "top" through ssh, cpu load ~ 100%
Results:
 -power consumption                   0.75 A (3.75 W)
 -cpu temp                            58.7°C
 -voltage regulator temp              69.5°C
 -voltage regulator power dissipation 1.28 W


As can be seen from the data, the power regulator takes up 1/3 of the total power consumption, while the other 2/3 is shared between:
so that's why it is the hottest component on the board. Searching on the web images of the 2200 model shows that the linear regulator is replaced with a switching one (which has a better power efficiency).

This is an image of the Fonera 2100 opened up. the power regulator is identified with an arrow.

Possible solutions

  1. Leave the fonera case open. Temperature drops to nicer values due to better air flow (simplest solution).
  2. Put an heatsink on the voltage regulator. Since it looks like most of the heat is generated by the power regulator and is then spread across the whole circuit through the metal lines on the PCB, an heatsink on this chip will not only reduce the temperature of the chip itself, but also the temperature of the other components.
  3. Put a small fan on the Fonera. Since the 220V to 5V power adapter is rated 2A, and the Fonera does not seem to require more than 1A, it is safe to power the fan with the Fonera power supply. (Usually fan require less than 0.2A)
  4. Replace the power regulator with a switching one. This is not easy because there is no drop-in replacement with the same pinout, so it is necessary to build a small circuit with all required components (switching regulator, inductor,...) remove the original chip and connect the pads on the PCB to the switching circuit with wires.