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Voltage Regulator - Practical Power Dissipation

Many people wonder why their Arduino boards get hot when they use Vin = 12V, and in fact, this issue comes up about once or twice a week on the Arduino Forum.

The short answer is that most Arduino boards use small surface-mount (smt) voltage regulators, which get hot even at moderate load currents, and especially when larger Vin values are used. Many Arduino boards are advertised as having "1A" voltage regulators, but in the real world, you cannot run load currents anywheres close to this level, because of overheating. See Table 2 below.

The real problem is that power dissipation and heating is related to both the voltage-drop across a device and the current through it, so the result is a "multiplicative" effect of the two factors, and not related to just the Vin or load current value alone. Ie,

       Pd = Vdrop * Iload --> heat generated

Once this relationship is grasped, the overheating issue is easily understood.


Table 1 - Thermal Ratings

Power dissipation for voltage regulators is based upon package size (TO-220 vs smt DPAK, SOT-223 or SOT-23), plus amount of heatsinking. The amount of "temperature rise", above ambient, versus power dissipation for these devices is as follows:

devicetemp risesize device area (to scale)
  • TO-220
  • 50 - 60 C/W 17mm x 9mm
    153 sq.mm
  • DPAK
  • 67 C/W 9mm x 7mm
    153 sq.mm
  • tiny SOT-223
  • 160 - 174 C/W 6.5mm x 5.3mm
    34.4 sq.mm
  • teensey SOT-23
  • 206 C/W 3mm x 1.75mm
    5.25 sq.mm

    (click to enlarge)
    Arduino Board Supplies

    The first two regulators are the style used in OT-Hobbies boards, and the latter in many Arduino boards. The smaller ones are clearly 3X-4X worse than the larger, in regards heat buildup.

    For the TO-220 device, a 20mm x 18mm pcb copper heatsink area under the regulator will reduce the temperature rise by approx 40%, relative to the value shown above (cf, Table 1 in LM1117 datasheet), and use of a metal heatsink under the device will further reduce heat buildup.

    For the DPAK and SOT-223 devices, the values shown assume minimum pad size (ie, pcb copper heatsinking area) of 5mm x 5mm, 2-oz copper (cf, fig 21,22 in NCP1117 datasheet). And if the copper area is increased by 400% (to 10mm x 10mm), then the temperatue rises are approx 20% and 40% smaller, respectively.

    Note also - that "typical" pcbs use 1-oz rather than 2-oz copper, so copper area heatsinking will be about 33% less effective than the values given above (cf, fig 6 in LM2940 datasheet).

    Thermal Shutdown. Voltage regulators are generally rated for thermal shutdown at 150-175 C, however, for reasons such as not overheating the pcbs and not burning one's fingers, maximum operating temperatures of 80-100 C are much more practical. After all, 100 C is already the temperature of boiling water!


    Table 2 - Temperature Rises

    The following table shows the power dissipation (Pd) and estimated temperature rise, above ambient, for different 5V regulators at various load currents, with Vin = 12V and 7V. The first temperature rise column indicates the result of using a metal heatsink with 50% effectivity under the TO-220 device.

    Voltage
    Drop (V)
    Load
    Current
    (mA)
    Pd (W) temperature rise (C)
    TO-220
    (heatsink)
    TO-220
    54 C/W
    DPAK
    67 C/W
    SOT-223 *
    167 C/W
    SOT-23 *
    206 C/W
    7V
    (Vin=12V,
    Vout=5V)
    50
    100
    250
    500
    1000
    0.35
    0.70
    1.75
    3.50
    7.00
    10
    19
    48
    95
    189
    19
    38
    95
    189
    378
    23
    46
    117
    234
    446
    58
    117
    292
    584
    1169
    72
    144
    361
    722
    1444
    2V
    (Vin=7V,
    Vout=5V)
    50
    100
    250
    500
    1000
    0.1
    0.2
    0.5
    1.0
    2.0
    2
    5
    13
    27
    54
    5
    11
    27
    54
    108
    7
    13
    34
    67
    134
    17
    33
    84
    167
    334
    21
    41
    104
    206
    412
    * = SOT-223 and SOT-23 regulators mounted on many Arduino boards.

    Temperature goes up fast, even with the larger regulators, and for the smaller smt regulators, the practical operating range is very restricted. It's clear that all devices run significantly cooler when Vin is only a few volts above the output voltage for the regulator. Vin = 12V is always going to be a problem.

    The tiny smt devices are clearly only good for use with low voltage drops (less than 4-5V) and small load currents (up to maybe 250 mA), no matter what their actual current ratings are. Boards with larger pcb copper areas under the regulators will provide some additional heatsinking capability, but don't expect miracles.


    Final note. On many boards, the 3.3V regulator bootstraps off of the 5V regulator, so it's voltage drop will only be 1.7V, and it will run cooler than if the full Vin is applied. In this case, the 3.3V device will handle load currents similar to the Vdrop = 2V levels in Table 2. For the tiniest regulators, this will still not be a very large current, and nowheres near 1A.

    In addition, this also means the heat dissipation will now be transferred to the 5V regulator, since it provides both 5V and 3.3V load currents, so it will run correspondingly hotter and can more easily overheat at larger Vin values.


    Reference Documents

  • LM2940 datasheet - (fig 6, Natl Semiconductor DS008822, 2000).
  • LM1117 datasheet - (Table 1, Natl Semiconductor DS100919, 2002).
  • NCP1117 datasheet - (fig 22,22, ON Semiconductor NCP1117/D, 2012).

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    © OT-Hobbies, May 2013