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'Philips BUK100-50GL MosFET issue'
1999\10\24@111540 by White Horse Design

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face
<x-flowed>I bought a couple of MOSFETS, Philips BUK100-50GL, overkill I know (for my
current requirements) but easily available from the Farnell catalogue. I
haven't found data sheets or application notes yet, but they are driveable
from TTL which sounded attractive. ;-)

It's a TO220 package with the pins labelled (top view looking down) i/p, d, s.

I've experimented with a 0k, 10k and 100k resistor connected from source to
i/p, +5V on the source (source voltage) and drain going to target Vcc control.

I find I get 4.5V out when the i/p is floating or tied to Gnd or tied to
+5V, in all the above except with the 10k resistor where the output fell to
about 4.1V.

Anyone mind telling me what I'm doing "wrong"?!

Or any alternatives for controlling two 5V devices from digital TTL (or 5V
and 3.3V later) which take around 500mA (5V), and around 300mA (3V) - but
with very short 2A peaks (GSM when transmitting).
Regards

Adrian
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</x-flowed>

1999\10\24@171447 by Jinx

face picon face
If the peaks are very short, I've found the ZTX range of transistors very
good.
I'm most familiar with the ZTX649, 650 & 651 (npn) and ZTX749, 750 & 751
(pnp). Current rating is 2A (only a small E-line package). They switch power
very fast and have low saturation voltages. Very useful for IR transmitters
BTW,
much better than something like a BC337 or BC327 especially for uS pulses

Joe.
>
> Or any alternatives for controlling two 5V devices from digital TTL (or 5V
> and 3.3V later) which take around 500mA (5V), and around 300mA (3V) - but
> with very short 2A peaks (GSM when transmitting).
> Regards
>
> Adrian
> ---
>

1999\10\24@180730 by steve

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face
> I've experimented with a 0k, 10k and 100k resistor connected from source to
> i/p, +5V on the source (source voltage) and drain going to target Vcc control.
>
> I find I get 4.5V out when the i/p is floating or tied to Gnd or tied to
> +5V, in all the above except with the 10k resistor where the output fell to
> about 4.1V.
>
> Anyone mind telling me what I'm doing "wrong"?!

Sure. In that configuration you have an expensive diode.
Putting 5V on the source and your load on the drain forward biases
the drain/source diode. The device can do more things if you connect
those around the other way.

To turn on, the FET needs to see some volts on the gate (i/p)
relative to the source. If you are using this as a switch to power up
other circuits, you now have a problem that the gate (i/p) needs to
be higher than the input voltage for it to turn on.

You may find that a PNP transistor is more suitable. You can turn
it on by pulling the base down to ground with an open
collector o/p and let a resistor pull it up when you want it off.
You can use a P-channel FET but watch for the drain-source diode.

Steve.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: .....stevebKILLspamspam@spam@tla.co.nz      fax +64 9 820-1929
======================================================

1999\10\25@012903 by V sml

picon face
I think you want to do high-side switching with N-channel Mosfets.
BUK100-50GL is a N-channel type.  You can change to use a P-channel,
or
switch the load on the low-side if it is acceptable in your design.
You are not biasing VGS right from what you wrote.

High-side:

VCC ---MOSFET Switch---Load

Low-side:

VCC ---Load---MOSFET switch---Gnd

See the philips web-site datasheet on this device.

If you needs some App. note on MOSFET, I can email you from what I got
from Temic web-site.

Ling SM
i-Pal Communications

>I've experimented with a 0k, 10k and 100k resistor connected from
source to
i/p, +5V on the source (source voltage) and drain going to target Vcc
control.

I find I get 4.5V out when the i/p is floating or tied to Gnd or tied
to
+5V, in all the above except with the 10k resistor where the output
fell to
about 4.1V.

Anyone mind telling me what I'm doing "wrong"?!

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