+ User Manual
Some example calculations to show the effect of large currents:
The dissipation (power lost in the FET) roughly depends on three factors: The current through
the FET, the switching frequency and the temperature of the FET.
Using the FET to switch (mainly resistive) heaters for a 3D-printer does not require high speed
switching. Switching between 1Hz and 100 Hz makes the switching losses marginal and only
the current through the FET and the FET's internal temperature are important.
Power dissipated in the FET is proportional with the current squared (
P=I
2
×R
):
When a 4A current flows through the FET, and the sum of Rdson and other resistive elements
in the FET is 50 milliOhm, a 200 mV voltage drop over the FET generates 0.8 W of dissipation
in the FET.
If we increase the current to 6A, assuming the FET is still kept at the same temperature, the
dissipation in the FET rises to 300 mV × 6 A, or 1.8 W !
At 8A, the dissipation reaches 3.2 W, four times the dissipation at double the current!
But another effect gets important: If the temperature of a FET rises, the Rdson increases too.
The Rdson will double if the FET's (internal) temperature rises from 25 degrees to 175 degrees
Celsius. This resistance increase of 100% will also increase the power that is dissipated in the
FET with 100%. The resulting temperature rise will increase the resistance further. Increasing
dissipation and temperature even further. This so-called thermal run-away must be prevented
by cooling the FET adequately.
If dissipation in a FET is kept below 1 W and ambient temperature is normal, no further
cooling of the FET is necessary. Above that, precautions have to be taken to keep the FET
cool.
The first step is to use a clip-on heat-sink. Both Aavid Thermaloy and Fisher have clip-on heat-
sinks. Aavid series 5768 and Fisher FK237 and FK240 will fit. These can handle around 2-4
Watts depending on type, and more with some forced air flow.
AAVID THERMALLOY - 576802B00000G. - HEAT SINK, FOR TO-220
FISCHER ELEKTRONIK - FK 237 SA 220 O - HEAT SINK, TO-220
version 1.4.9 (09/04/14)
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