FSCTxxA-UH5

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2. Hysteresis control (mode OFF)

FSCT can be shut down by the ON signal. When, this signal is low, the OUT pin is at high level, i.e. the FAN

is turned off.

This mode enables to save the energy wasted by the FAN in case of operations at very low output power.

For a 12 V supply voltage, the shut-down of the FAN brings a 0.5 to 2.5 W power saving (for a 200 mA 12 V

DC motor).

During the mode OFF, the FSCT doesn't lose temperature control; indeed, in case of over-temperature,

the FAN is automatically switched on. This safety feature protects the power supply or the semiconductor

devices from unexpected over-temperature.

In order to keep the energy saving benefit, the FSCT turns back off the FAN when the temperature falls be-

In practice, three cases can appear for a constant applied heating power (for instance, the output power of

the power supply where the FSCT is used) (cf.

figure 8):

s

s

riodical cycle.

s

in that case, unless the heating power decreases.

For example, with a 200 W computer power supply, working with a FSCT17 device put on the power semi-

Case 2 could be reached for a 50 W power consumption. Then, thanks to the large Hysteresis value (30 °C

avoid too many FAN starts-up cycle per hour.

Case 3 could be reached for a 75W, or higher, power consumption. For 75W, the power supply ambient

temperature stabilizes itself around 42°C.

It should be noted that, for Case 3, such steady state points of operation, are allowed due to the fact that the

OUT voltage follows the same linear law as that in ON mode. Then, the Hysteresis control is smarter than a

simple ON/OFF control mode.

Mode OFF

Mode ON

1V

T (°C)

j

Fig. 7: OUT voltage versus junction temperature.