TempFET usage?

[Tim Dunne]

Hi folks

I'd like to use a tempFET bts115a in a PWM design, switchinga 12v fan.
Not fiddled with these devices before, and I'm confused about the use of the
zener protection. Are there any easy guides to using these devices?

The gate is to be switched by a PIC at 5v, but I'munsure about the use
of the zener diode recommended for protection.

TIA

Tim

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[Winfield Hill]

Tim Dunne wrote...

Hi folks

I'd like to use a tempFET bts115a in a PWM design, switchinga 12v fan.
Not fiddled with these devices before, and I'm confused about the use of
the zener protection. Are there any easy guides to using these devices?

The gate is to be switched by a PIC at 5v, but I'm unsure about the
use of the zener diode recommended for protection.

You'll need a series gate resistor, as recommended by Infineon, to
limit the tempFET's thyristor current if it triggers.

The bts115a is only rated for 50V, so if you expect an inductance
flyback beyond this the resulting avalanche condition has to be
considered. tempFET appnotes show a drain-to-gate zener-plus-diode
to re-activate a shutoff gate during an overvoltage, and clamp the
flyback at the zener-plus-gate-turnon voltage (this FET active-clamp
trick works well for low-voltage high-gate-capacitance FETs, but
should not be used for high-voltage FETs, which can break into RF
oscillation). Because the active-clamp trick has you connecting
stuff between high voltages on the FET's drain and its vulnerable
gate, they also suggest a gate-source zener diode to prevent the
voltage from ever momentarily going over the FET's 10V limit. For
this purpose, a 10V zener should work fine.

Infineon also suggests choosing a lower-voltage gate zener to clamp
the maximum gate voltage, in order to create a short-circuit current
limit for the FET. This idea is problematic to me, on its surface.
First, for any given drain current, there's a gate-voltage variation
due to drain temperature. Perhaps a counter to that concern is to
pick the gate voltage at high FET temperatures. Second, low-voltage
zener diodes, under 5V, have notoriously poor qualities, with poor
voltage tolerance and dynamic resistance, i.e. the zener voltage
varies dramatically with zener current. A counter to that concern
could be to use an active zener, e.g., a TL431 with two resistors.
This zener can be programmed to as low as 2.5V and is fairly fast.

You mention PWM control of your fan. Keep in mind the gate series
resistor required for tempFET operation, and the low gate voltages
required for using the current-limit concept, will both in the end
limit your PWM switching speed. There are several other ways to
protect a switching FET in the event of a load short circuit.


--
Thanks,
- Win