Discrete, mixed, and integrated multi-port solutions
Yair:
Good questions, good observations. Thanks taking the time to reply. I'm
going to answer you by the numbers; I hope I don't make a confusing chain
for other people this way...
1. I consider that, with adequate protection, the MOSFET need be rated
for 400mA ~ 500mA maximum, possibly less (as you know, that's an ongoing
discussion). Implementing very rapid and rather sophisticated (e.g.,
dual-level or dual-rate) overcurrent protection on an integrated MOSFET is
not very hard these days. Good design practice also would thermally protect
the MOSFET(s) integrated onto a monolithic device. This gives the safeties
of slow overcurrent protection, short-circuit protection, and overpower
(overtemp) protection. The added cost for these functions id not zero, but
it gets lower every year, while remaining very elusive if not impractical on
a "standard" discrete MOSFET. I agree that 80V is a minimum number for
BV(dss).
2. Cost is a very different thing from either price or value. In some
cases, an external discrete MOSFET will yield the lowest-price, or better
still, highest-value solution. But in many cases, the design time, the
board space, and/or the simple issue of qualifying and inventorying
components from a number of different vendors makes the value - and
therefore the price - of even a slightly more expensive fully integrated
solution greater than any savings which might practically be realized in a
quasi-discrete circuit implementation. By way of crude comparison, I can
build a pretty good op amp with 5 transistors and 5 resistors, which comes
out to less than 10 cents in component cost. But each of those 10 parts
will cost 4 cents to place on a board, and the board area consumed will
vastly exceed that of a 20 cent SOT-23 op-amp, which I can get from any of a
number of companies. Which is the lower parts cost? Which is the better
value?
3. The functionality which an integrated device can subsume into
itself, and the degree of MOSFET monitoring and protection which it allows
at minimal extra effort, vastly exceeds anything which could be done in an
8-pin IC alone. That's one way of looking at it. A better analysis was
given by my colleague Bruce Inn, who is more aware of the process details
for IC fabrication than I am. I will say this, though: the digital
revolution has made almost all of us into "mixed-signal" houses. The
question is simply whether we take our principal value-added from analog or
digital expertise. If the function requires digital, digital will be found
that will do the function, at a reasonable cost (remember the first digital
watches? Pulsar. Using Integrated Injection Logic).
4. Having worked with the Taiwanese on board layout, I will
respectfully take issue with you on this point. The Taiwanese do not LIKE
surface-mount parts, as they would much rather be able to stand a device up
on a single-sided PCB. And, if possible, to run at least one trace between
each two leads of that device. The last refuge of the DIP package (Delta,
Lite-On, AcBel, PhiHong). We are looking at a "skim-the-cream" industry
today. But I anticipate that very soon the 802.3af standard will mostly be
accommodated by Taiwanese and PRC vendors, or we will be back to wall warts.
5. Good layout always involves tradeoffs. This may in some cases force
a quasi-discrete, or at least multi-chip solution, whether or not a single
chip multi-port device would otherwise be desirable in the application. No
contest on this point, my friend.
6. I hate to do this, but I refer back to my Statement #2. Cost,
Price, and Value. Density, Inventory, Vendor Qualification, Design Cycle
Time.
7. One additional point: A device carrying on-board MOSFETs intended
for the PSE market would have those MOSFETs purpose-built to fit the needs
of the application, no more, no less. This would facilitate having the
proper specs MOSFET specs out of the box for leakage at temperature, leakage
at voltage, R(on), and thermal resistance. I probably missed some, but you
get the idea.
So there's my opinions. Always makes me think, when we chat.
> Peter Schwartz
> Applications Engineer
> Micrel Semiconductor
> Phone: 408.435.2460
> FAX: 408.456.0490
> peter.schwartz@micrel.com <mailto:peter.schwartz@micrel.com>
[From Yair Darshan]
Hi Peter,
Thanks for your info, However now I am more confused., I got other info, and
in order to be sure that we are talking about the same parameters I have
some questions to you. See below.
1. Are you taking in account that the Mosfet need to be rated
to 1A/80V with low enough Rdson that can meet our requirements?
2. Are you saying that integrating the Mosfet into the chip
will cost less than using low voltage chip with external Mosfet?
3. Using low cost chip say 8pin if we are lucky with the above rated
Mosfet will not drive us to larger package? Please advise?
4. This examples shows my point, they are TO220 package with 3-5 pins,
and this is exactly my point you are not saving board space. It is about the
same or worse, especially for multi-port.
5. In addition, better layout in some application is achieved if the
Mosfet is located on Area A and the Control on Area B for controlling high
currents etc., especially in a multi-port environment.
6. Finally, if you check your examples for high quantities you will see
that Integrated solution will cost 1.2 - 1.5 times as much (Power
Integration and others) as the discrete solution will cost:
Controller + Mosfet = 0.4$ + 0.33$ = 0.73$
which is less...
Please let me know what is your opinion on the above comments.
Thanks-
Yair.
> -----Original Message-----
> From: Schwartz, Peter [SMTP:Peter.Schwartz@micrel.com]
<mailto:[SMTP:Peter.Schwartz@micrel.com]>
> Sent: March 22, 2001 10:11 PM
> To: Yair Darshan
> Cc: stds-802-3-pwrviamdi@ieee.org
<mailto:stds-802-3-pwrviamdi@ieee.org>
> Subject: PSE vs. PD power dissipation again
>
>
> Yair, Dieter, and all:
>
> Speaking both as an engineer having started with thermionics and
now
> working
> with integrated power MOSFETs, and as someone who has a direct
exposure to
> the semiconductor industry, I believe I can say the following with
both
> history and present knowledge to back me up:
>
> A discrete MOSFET in conjunction with a low-voltage controller
circuit for
> the PSE may be a lower parts-cost-per-port solution today, and for
a year
> or > two.
> That is TBD. But eventually (sooner rather than later), the
MOSFET
> will be integrated, as board space and circuit complexity has a
way of
> becoming more expensive than silicon - or conversely, silicon just
keeps
> getting cheaper.
> I suggest the examples of Power Integrations (the > "TopSwitch"
family) and the similar ST Micro "Viper" family of one-chip > off-line power
supplies.
> Open any 'ATX'-type computer power supply. It > can't be done
for the price, but by the miraculous alchemy our industry > works to turn
sand into gold, it is.
>
> Conclusion: Multiple-port PSE switches with integrated MOSFETs
will > become > an available, cost-effective, and viable alternative to >
multiple-component-per-port implementations.
> Thanks for letting me sound off here.
> Peter Schwartz
> Applications Engineer
> Micrel Semiconductor
> Phone: 408.435.2460
> FAX: 408.456.0490
> peter.schwartz@micrel.com <mailto:peter.schwartz@micrel.com>
>