Re: Trying to develop MOSFET intuition
- From: Jim Thompson <To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx>
- Date: Sat, 13 Oct 2007 08:24:00 -0700
On Sat, 13 Oct 2007 07:44:21 -0700, Paul Mathews <opto@xxxxxxxxxxx>
wrote:
On Oct 11, 7:19 am, "Peter S. May" <m...@xxxxxxxxx> wrote:
I'm trying to rebuild parts of the Microchip PICDEM FS USB board
schematic partly as an experiment but mostly as a learning experience.
There's a part of the circuit that appears to allow the device to
auto-select between USB bus power and wall-brick power, that looks sort
of like this:
NDS352P 1N5817
[VBUS>---+--+--[S G D]---->|--+-[+5V>
| | | |
| | | |
[VREG>---|--|-----|------->|--+
\ | \ 1N5817
100k / =.1uF / 33k
\ | \
| | |
+--+-----+--| GND
From what I read, NDS352P is a P-channel MOSFET. I also can't seem to
locate this part on Allied, and didn't have any better luck with a
couple of cross-references.
I'm thinking that this is an opportunity for me to learn about MOSFETs
since I've never used one before. I only recently really learned about
BJTs, and can only really keep it straight because, in general, only a
small selection of the figures are relevant: hFE (the current gain Ib
to Ic), which isn't crucial when used as a switch instead of an analog
amplifier, Vce (voltage drop collector to emitter, i.e., how much less
than Vcc is dropped across the output load), and max Ic (how much
current can be passed through the load).
What are the important (analogous?) figures for a MOSFET? What features
should I be looking for in a replacement for NDS352P?
Thanks
PSM
The Schottky diodes in the circuit allow whichever voltage is higher
to supply power to the circuit. It's not clear why anyone thought they
needed the PFET in this case. At most, it might reduce the magnitude
of a power up surge, since the 33K slows turn on slightly. The C is
simply for bypass. The 100K discharges C when VBUS goes away.
Simplest mental model of MOSFET: VOLTAGE Gate to source reduces
RESISTANCE between Drain and Source (Rdson) when Vgs is of correct
polarity for type of MOSFET. For Vgs > threshold, Rdson doesn't
decrease much further. A BODY DIODE shunts Drain and Source, such that
Drain and Source act like a diode for reverse polarity Vds. Dynamic
capacitances, gate spreading resistance, gate oxide breakdown, and
thermal characteristics complicate the model.
Paul Mathews
Posted originally in 2004....
http://analog-innovations.com/SED/PerfectDiodeForChargerIsolation.pdf
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
.
- Follow-Ups:
- Re: Trying to develop MOSFET intuition
- From: Allan Herriman
- Re: Trying to develop MOSFET intuition
- References:
- Trying to develop MOSFET intuition
- From: Peter S. May
- Re: Trying to develop MOSFET intuition
- From: Paul Mathews
- Trying to develop MOSFET intuition
- Prev by Date: Re: OT: Eudora a good alternative to Thunderbird?
- Next by Date: Re: Biasing MMIC amplifiers (GALI-5)
- Previous by thread: Re: Trying to develop MOSFET intuition
- Next by thread: Re: Trying to develop MOSFET intuition
- Index(es):
Relevant Pages
|
