|Dec 27, 2013, 07:31 AM|
AMS1117 Regulator HeatSink Mod for mCP-X BL
AMS1117 Regulator Data Sheet
Background: AMS1117 PS regulator SOT-223
The AMS1117 series have internal power and thermal limiting
circuitry designed to protect the device under overload conditions.
However maximum junction temperature ratings of 125°C should
not be exceeded under continuous normal load conditions.
Careful consideration must be given to all sources of thermal
resistance from junction to ambient. For the surface mount
package SOT-223 additional heat sources mounted near the device
must be considered. The heat dissipation capability of the PC
board and its copper traces is used as a heat sink for the device.
The thermal resistance from the junction to the tab for the
AMS1117 is 15°C/W. Thermal resistance from tab to ambient can
be as low as 30°C/W.
The total thermal resistance from junction to ambient can be as
low as 45°C/W. This requires a reasonable sized PC board with at
least on layer of copper to spread the heat across the board and
couple it into the surrounding air.
Experiments have shown that the heat spreading copper layer does
not need to be electrically connected to the tab of the device. The
PC material can be very effective at transmitting heat between the
pad area, attached to the pad of the device, and a ground plane
layer either inside or on the opposite side of the board. Although
the actual thermal resistance of the PC material is high, the
Length/Area ratio of the thermal resistance between layers is
small. The data in Table 1, was taken using 1/16” FR-4 board with
1 oz. copper foil, and it can be used as a rough guideline for
estimating thermal resistance.
Maximum junction temperature must not exceed 125°C.
The Issue : Heat
All that to say....the AMS1117 uses the PCB's copper traces on the BL's board, and by default the connected components to those traces front, back & middle for cooling, and that it will ultimately shut off @~ 257ºF. We all know this component can get hot, especially with a fully charged lipo installed and if you cycle the servos in throttle hold, you can put your finger on it and feel how hot it gets. My issue with the design here is that although Im sure it passed spec, and has been thoroughly tested, it looks like Blade did a last min quick fix and double stacked these components maybe hoping for the best but expecting the worst on returns. I speculate this heat can cause issues with the servo's and 3in1 board reacting unpredictable on certain link-up's, which is why in some case and on some boards, no two link up's cause the BL to fly consistent from reports.
The AMS1117 chip burns off the 8.4v from a fully charged lipo and feeds the 3 servo's 4.2v and the main board (minus the 2 Brushless ESC's) this linear "regulated" power supply. The extra voltage is burned off as heat because they chose to use linear instead of switched BEC on board. Though the designers really did not have a choice because of cost and board real estate. The difference between a linear BEC (battery eliminator circuit) and switched BEC is that a linear has better regulation and less noise, but more heat where the switched has more noise but much less heat. Excess heat can cause electronics to act in an unpredictable way, and eventually fail.
A Solution; Installing an on chip heat sink
Installing a heat sink could put the AMS1117 in the pathway for more efficient air cooling, as it comes stock the BL has the chip located in the lower left side of the main board. The canopy mounting post, 2 tantalum caps, and the power wires block airflow to this chip. By installing a heat sink and extending it out more to hit blade down wash and in effect increasing surface area, more efficient cooling of the chip and therefore surrounding circuitry should occur. This could help with keeping heat to a minimum and within tolerances. The Invensense 3 axis gyro is located right behind and to the right of the AMS1117 and does change temp with however hot the regulator gets. Temps rise and fall quickly with this chip, as lipo voltage changes, so the goal is to get a more consistant temp overall and more heat off the board and into the air.
Installing the Heat sink
Things youll need;
1) Dremel tool with grinder/cut wheel attachment or equivalent.
2) Old CPU Heat Sink
3) 30 Min Slow Cure Epoxy
4) A few mg. of aluminum shavings from CPU*
5) Cut to fit Heat Sink for AMS1117
6) mCP-x BL 3in1 main Board
7) A steady hand and about 20 mins with 8 hours cure time.
A) Cut the Heat Sink to fit the top of the AMS1117. (make sure it does not interfere with servo)
B) Using a small container use a grinder wheel to gather the aluminum shavings.
C) Put the shavings into a small pile and mix into 2 equal parts epoxy, let sit and get to a firm consistency, plenty of time to work with this tope of epoxy
D) Dip Heat Sink into epoxy or use a tooth pick to apply it to the Heat Sink
E) Place Heat Sink on AMS1117
F) Place Board on flat level surface and let dry for 8 hours overnight.
Additional Notes; The reason I added Aluminum shavings to the epoxy is to better transfer heat, though you should be careful not to put so much epoxy on that you short out the 3 leads on the front of the AMS 1117 which are Ground/Adjust, Vin, Vout, respectively. The rear pad, with the big glob of solder is the chips "on board" heat sink and its ok to get the epoxy mix on that, as a matter of fact that will only help with heat transfer. The 30 min epoxy should handle the heat transfer with no problem, as long as cured at room temps and for an appropriate amount of time. Though they do sell an already made thermal epoxy adhesive compound for this, I couldn't justify buying a tube of it for this one small application so I made my own. If anyone would like to try this and see how it effects the BL inflight and if it helps with more consistent flights feel free to post outcomes.
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|Dec 28, 2013, 08:24 AM|
Thank you, I do appreciate the comment and hope it helps. The mCPx-BL gets a bad rap from some, but I'm finding it is a good flying micro heli, very powerful and fast for its size. mSR is another good Blade also.
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