RE: PSE vs. PD power dissipation again
See my comments bellow.
> -----Original Message-----
> From: Dave Dwelley [SMTP:ddwelley@xxxxxxxxxx]
> Sent: ד מרץ 21 2001 4:43
> To: stds-802-3-pwrviamdi@xxxxxxxx
> Subject: PSE vs. PD power dissipation again
> Group -
> In lieu of a dedicated power ad-hoc reflector, I'm posting this to the
> general list. Is there a power reflector in the works?
> I'm assuming that we'd eventually like to integrate the power switches
> a PSE chip, and that PSE designers will tend to want to service multiple
> channels with a single chip: 4, 8, or more.
[Yair Darshan] (1) Yes we would like it, if it will not increase
system cost and complexity (PSE and PD).
> Also, I assume that we'd like
> to be able to power up many PDs simultaneously (when the wiring closet
> power comes back on after a California shutdown) - this isn't critical,
> it's desirable.
[Yair Darshan] (2) I don't think it is an issue at all, since after
break down of long time, it is not important if a PD will get service
right now or within few seconds.
> To do this, we need a scheme that keeps the power dissipation out of the
> PSE end.
[Yair Darshan] Agree. Remember that we have two cases of power
dissipation. Case 1: during startup. Case 2: During normal operation.
In Case 1: We indeed can reduce the power loss in PSE switch by
setting the PD current limit lower than the PSE, we are not "kill" the
problem completely, we just moving it to the PD with some ability of
reducing it by increasing PD cost.
Case 2: During normal operation we need to support 350mA avg, with
57V output. It means a 80-100V, 0.6-1A mosfet which its die size is function
of the Rdson. At 350mA average current the power loss on the FET will be
In low cost plastic package you can dissipate between 1-1.5W thus to
meet this number you will need low Rdson mosfet
which will have large die size.
We didn't mention yet what we will need with current peaks above the
average which is normal situation at some loads.
To summarize this: I am not sure that integrating the Mosfets in the
chip will get us important advantages.
Info received from chip vendors shows that:
a- They have the technology to implement Mosfet (HV technology )
with the chip (Low voltage technology)
b- Integrating the Mosfet into the chip will not save footprint
since the die size is large which will increase the package size.
c- The cost of integrated chip+Mosfet is greater than the cost of
low power low voltage small package chip with external Mosfet.
In light of the above what is the incentive to integrate the Mosfets
into the chip?
> Rick and Dieter have both shown that if the PD limits inrush
> current to some value lower than the PSE current limit (eg., 350mA for the
> PD, 500mA for the PSE), dissipation in the PSE is near zero.
[Yair Darshan] Dissipation on PSE switch is very low and
dissipation on PD switch is very high.
> This one of
> several options allowed by the draft standard as it reads now - others
> share the dissipation between the two ends (the Avaya resistor divider/FET
> scheme), or put all the dissipation in the PSE (the UVLO/latch-on scheme
> that Micrel showed at the meeting).
> If we allow any PD to push any dissipation back into the PSE, we force the
> PSE to be able to handle the worst case - all channels powering
> simultaneously, with all the power in the PSE.
[Yair Darshan] All channels are not required to startup
simultaneously. After detection we can turn on each channel
at atime since we have control on it according to management
> To do this, the PSE needs
> some accommodation: heat sinks,
[Yair Darshan] No heat sink required. With D2PACK, we can support
easily 500mA peak for 100mSec.
Now, if we reduce the time from 100mSec to
40-50mS it is even better.
> external FETs,
[Yair Darshan] Agreed.
> sequential power up algorithms (which lengthen average detect time), or
> low current limits at startup. None of these are desirable.
[Yair Darshan] - Sequential power up algorithm is easy to get with
no cost. We will need some intelligence in any case.
this requirement is part of it. It will help reducing power supply
size and many other good advantages.
[Yair Darshan] - It will not affect detection time, it will affect
the time that the PD is turning on, However we agreed that it is not an
issue, since boot up time can be long ( Laptop, PC etc...)
> There is the issue of line capacitance, which will put the PSE into its
> 500mA limit briefly (<74us) - but this short time duration won't
> significantly heat the PSE. We could also see a short on the wire - in
> case, the PSE could shut off quickly (<1ms) or incorporate foldback to
> limit dissipation, like Micrel showed.
[Yair Darshan] As stated before, if we can support 500mS for TBD
ms, the above is not an issue.
> I propose that we mandate that the PD limit the inrush current, say to
> 350mA +/-50mA, and mandate that the PSE limit at say 500mA +/-50mA. By
> forcing the PD to do this, we allow a multi-channel PSE chip with FETs on
> board. Otherwise we can't do it.
[Yair Darshan] I do not agree to this conclusion from the reasons
mentioned above, and from the reasons described
in my presentation regarding "Where to
locate the inrush current limit".
If you do not agree to my conclusions and
data presented there, lets discuss it and crack it,
until we will have the best understanding of
what is the optimum solution for us.
In my opinion, setting the PSE to 500mA for TBD msec. and not
forcing inrush current limiter in the PD is the desirable solution
in terms of performance/cost ratio.
It does not mean that the PD will not have current protection. It is
part of the PD power supply after the big cap.
We are discussing only on the inrush current limiter that should be
located before the PD big cap.
> This does make a bare-bones PD more complicated. In the short run, it
> probably requires a low-cost op amp and a sense resistor to implement - or
> a 150 ohm/~1W series resistor and a FET to short it out when the switcher
> input cap voltage approaches the line voltage
[Yair Darshan] All the above functions and more you have already in
the PSE. Why duplicate it also in PD?
> Going forward, the PD
> function (with power device, current limit, UVLO, the works) can be
> integrated - and since PDs generally don't need multiple channels, the
> power in the single switch is tolerable (as Dieter showed at the meeting).
[Yair Darshan] The power in the switch is tolerable also if it is
on PSE when external FET is used.
> The "30 watt" PD would conceivably need a dual - we'll use a bigger
> or some other trick to deal with the heat in that case.
> How much is it worth to integrate a multi-channel PSE chip?
[Yair Darshan] From the data that I have today: It is not worth the
effort. To many problems compared to trivial solution.
[Yair Darshan] To summarize the above, I think that we need to
answer the following questions:
1. Do we have a space problem that integration the Mosfet in the
chip can help us?
2. Do we have power loss problem when the fet is not integrated?
3. Do we have power loss problem when the fet is integrated?
4. How chip cost affected by integrating the Mosfet compared to chip
+ discrete Mosfet?
5. Cost of multi channel chip with integrated Mosfets compared to
multi channel chip with external Mosfets
6. Foot print of multi channel chip with integrated Mosfets
compared to multi channel chip with external Mosfets
7. Can we support many applications with low cost solution when the
PD contains the inrush current limit function?
7.1. What it does to PD cost
7.2. What it does to System cost
7.3. How it complicate PD design
7.4. How it affect PSE-PD inter-operate
Yair Darshan/ PowerDsine
> Dave Dwelley
> Linear Technology