Re: PSE vs. PD power dissipation again]
Yair,
see comments below:
Yair Darshan wrote:
> Dieter,
> I understand the problems you are presenting, please see my comments bellow.
> Thanks.
> Yair.
>
> > -----Original Message-----
> > From: Dieter Knollman [SMTP:djhk@xxxxxxxxxxx]
> > Sent: ה, מרץ 22, 2001 12:35 AM
> > To: 'Yair Darshan'
> > Cc: stds-802-3-pwrviamdi@xxxxxxxx
> > Subject: Re: PSE vs. PD power dissipation again]
> >
> > Yair,
> >
> > The main issue that I have is that the PSE is not allowed to trip
> > immediately.
> [Yair Darshan]
>
> Understood, lets see if we can solve it:
>
> We need to check if we can specify the following operating modes:
>
> 1. During normal operation: In case of short circuit the PSE will
> turn Off within TBD1 mSec/uSec (less than 1mSec) if the current is > TBD2 mA
> (600mA)
> 2. During normal operation: The PSE shall limit its output power to
> 350mA average max. (current peaks and timing should be discussed)
> 3. During startup : The PSE shall limit its output
> current to TBD mA (500mA) for TBD3 mSec. (10-100mSec)
>
> By this definitions we can differentiate between startup and short
> condition. What do you think about it?
[Dieter] You are describing a far more complicated current limit circuit. The
circuit requires 2 current thresholds and multiple timers. I was hoping to use
a simple transistor circuit.
This is a way to differentiate between startup and shorts. Your approach should
allow a slightly smaller external MOSFET, but at the cost of a precise current
limiting circuit.
>
>
> > It is required to deliver an overcurrent for some time. This overcurrent
> > must
> > be sustained by other devices connected to a powered up PSE.
> > One example is the PSE output diode. In case of a cross over between
> > PSEs, this
> > diode must handle the overcurrent for the specified time. Hence a big
> > diode for
> > the clamp.
> [Yair Darshan]
>
> You are correct, however, this components has to sustain 350mA
> continuos. The difference in requirement for a diodes needed to support
> 500mA for 10-100mSec
> is small or negligible, since in any case the nominal rating should
> be twice or so compared to the actual from reliability and stress reasons.
>
> > Another example is a breakdown fault. Here the circuit must sustain the
> > overcurrent for the specified time.
> > Allowing the PSE to trip rapidly may prevent some smoke.
> > Allowing the PSE to trip immediately is simply safer.
> [Yair Darshan]
>
> Yes you are right, Now use this reasoning in the PD side and you
> will see that you move this problem to the PD with some ability to reduce
> it, but you are not eliminate it.
>
> I understand your motivation to reduce package size/power loss in
> PSE switch in order to allow integration it in a chip to reduce system
> overall cost.
> But, the assumptions you are using to justify it are wrong to my
> opinion and I have shown in my presentation that system cost is increased...
[Dieter] I disagree. With PSE current limiting, the system needs two big FETs
plus precise current limiting.
With PD current limiting, no big FET plus simple current limiting for small
start up currents. Looks like a cost reduction for the PSE and the PD.
>
>
> If you do not agree to my reasoning and rationalization, lets
> discuss it one by one and crack it until we will get the best system
> definitions/specifications.
>
> I would like to summarize my position of the issue:
>
> 1. Setting PD inrush current limit to be lower than PSE inrush
> current limit will reduce PSE switch size - Correct
> 2. Setting PD inrush current limit to be lower than PSE inrush
> current limit will increase PD isolating switch size.
[Dieter] True only if you keep the large inrush current. If you reduce the
inrush current you can use a smaller isolation switch and save cost in the PD.
>
> 3. Reducing PSE switch size allows integrating the mosfet in the
> chip - allow lower cost solution. - Not accurate.
> - Chip vendors says that it will cost more due to
> the mixed technologies used (HV & LV)
> - Chip vendors says that to support 350mA/80V with
> low dissipation, large die size required, chip package increased, overall
> cost increase compared to
> small LV chip with external Mosfet.
[Dieter] This depends on technology. The goal is to achieve a multi port PSE
solution with internal switches.
The automotive industry has octal switches with build in protection. The
voltage is lower, but the current is higher.
Can a semiconductor vendor address this point? I've always been a fan of
dis-integration, but I'm trying to change.
>
> 4. The functions needed to get inrush current limit, are already
> exist in the PSE, duplicate it in the PD, costs more.
[Dieter] The PSE needs to have a simple current limiter. Your approach
requires a more precise circuit and multiple timers. Adding a simple current
limiter to the PD, cost a transistor plus resistor (see schematic in my
presentation). The cost is more than offset by the savings of the smaller
isolation switch. PD current limiting can be a lower cost if you reduce the
inrush current.
>
> 5. There are more.. but I think the above are the major ones.
>
>
> The problem you are raising is basically a function of the package
> size, and, I am trying to show that the package size is not a problem if the
> intention is to reduce cost by integrating the mosfet in the chip , than we
> didn't achieve this goal due to the fact that we increased PD cost.
>
> > The second issue is that even with the overcurrent for a limited time, the
> > PD
> > still needs a big FET.
> [Yair Darshan]
>
> Why the PD still need a big fet? please explain?
[Dieter] I am assuming PSE current limiting. At start up the PD should not
draw current until the line voltage is greater than the startup threshold (30
volts). The PD must not allow the line voltage to drop below this voltage.
Initially the PD big cap is discharged. Somewhere between the line and the cap,
there is 30 volts initially. This voltage will decrease as the cap charges.
The initial power dissipation is 30 * 0.5 = 15 watts. The best way to reduce
the power is to lower the current limit. This is the point that I was trying to
make in my talk.
>
> The isolating switch can be very small fet since it can be operated
> as a fast switch (it is the same mosfet size that you will need in the PSE
> if you use your concept...)
[Dieter] Your circuit used an isolation switch plus a current limiter. If you
combine the two, the above argument applies.
>
> The other mosfet is the power supply mosfet which we are not
> discussing it since it will be needed for any concept.
[Dieter] The power supply is not the problem, it's the input cap.
>
> If you want to integrate the isolating function with inrush current
> in the PD, than you will need to increase the mosfet size in the PD......
[Dieter] This is only true if you keep the inrush current at a large value. If
you reduce the current limit, you can use a smaller mosfet in the PD.
I'm trying to reduce the PD cost.
>
>
>
> > The only way to use a smaller FET is by reducing the
> > current.
> >
> > Dieter
> >
> > >
> > ------------------------------------------------------------------------
> > >
> > > Subject: RE: PSE vs. PD power dissipation again
> > > Date: Wed, 21 Mar 2001 10:12:03 -0500
> > > From: "Lynch, Brian" <brian_lynch@xxxxxx>
> > > To: "'Yair Darshan'" <YairD@xxxxxxxxxxxxxx>,
> > > "'Dave Dwelley'"
> > > <ddwelley@xxxxxxxxxx>,
> > > stds-802-3-pwrviamdi@xxxxxxxx
> > >
> > > Yair, Dave, all,
> > >
> > > I'll throw some more fuel to the fire below...
> > >
> > > Brian
> > >
> > > >-----Original Message-----
> > > >From: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
> > > >Sent: Wednesday, March 21, 2001 5:06 AM
> > > >To: 'Dave Dwelley'; stds-802-3-pwrviamdi@xxxxxxxx
> > > >Subject: RE: PSE vs. PD power dissipation again
> > > >
> > > >
> > > >
> > > >Dave,
> > > >See my comments bellow.
> > > >
> > > >Yair.
> > > >
> > > >> -----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
> > > >> into
> > > >> 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,
> > > >> but
> > > >> 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.
> > >
> > > [Brian Lynch] If it is OK for a PD to get service "within a few seconds"
> > > then
> > > why do we concentrate so much on detection/classification time? If, by
> > > adding
> > > a classification step to discovery, we add 200ms, is that a problem? How
> > > much time
> > > will it take for the 100th port to come alive in a large system?
> > >
> > > >> 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.
> > >
> > > [Brian Lynch] I agree that in startup the power has to be lost in either
> > > the PSE or the PD. That is physics. By putting the inrush current limit
> > > on the PD side, and setting the current limit to a value below the
> > current
> > > limit in the PSE, we get a number of advantages.
> > > - The PSE switch losses are always low. Turn ON losses
> > > can be limited to 300mw for the initial "spike" and continuous
> > > operation losses are in the 100mw region. (assuming a 1 ohm MOSFET).
> > > In a practical case, the RdsON will be much less. A SOT-223 FET or SO-8
> > with
> > >
> > > RdsON in the .02 to .05 range will dissipate much less.
> > > - In a fault condition (as a shorted wire) the reaction time may
> > be
> > > virtually instantaneous, keeping PSE switch losses low.
> > > - The PD designer has the freedom to choose the size and rate of
> > > charge of
> > > his PD's bulk capacitor, and thus the size of the MOSFET used. A low
> > cost PD
> > > can use a small MOSFET, and a higher end device may use a larger. The
> > point
> > > is that
> > > PD can be sized to the job, and not be limited by avoidable constraints.
> > >
> > > If the PSE limits the inrush current:
> > > - The PSE muse always be sized for the worst case, which is at
> > least
> > >
> > > 350ma for the maximum startup time allowed. So for large systems and
> > small,
> > > the PSE
> > > would need to have and expensive D2PAK MOSFET on every port.
> > > - Under a shorted wire condition, the switch must stay on for
> > and
> > > extended
> > > period of time. (A time greater than the longest startup time). If the
> > > current limit is
> > > set at 500ma, then the losses could be as high as 57*.5=28.5 watts for
> > up to
> > > 500ms.
> > > - The size of the PD bulk capacitor must be limited, and the
> > time to
> > > turn-on
> > > must be limited in the spec, constraining future design freedom.
> > > - To avoid startup issues, additional circuitry is needed in the
> > PD
> > > to keep
> > > the switch ON while the PSE voltage drops to zero and begins charging
> > the
> > > bulk capacitor.
> > > The added circuitry must have energy storage in it sufficient to keep
> > the PD
> > > switch alive for the
> > > time it takes to charge the capacitor....more cost and potential for
> > failure
> > > to start up.
> > >
> > > > 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
> > > >0.350^2xRdson.
> > > [Brian Lynch] 100mw/port or less.
> > >
> > > > 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.
> > > [Brian Lynch] The peak current requirements, no matter what they are,
> > will
> > > only the change the numbers, not their relative positioning.
> > >
> > > >
> > > > 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.
> > > >
> > > [Brian Lynch] The loss in the PD switch FET can be limited to any value,
> > > depending on the startup time allowed. It can be as high as 10watts for
> > > 100ms
> > > startup (into a 100uf capacitor charging to 57 volts) or as low as 200mw
> > for
> > > a
> > > longer period of time, then less than 100mw continuous. It is up to the
> > PD
> > > designer to
> > > decide the trade offs he is willing to make.
> > >
> > > >> 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
> > > >requirements.
> > > >
> > > [Brian Lynch] Will mid-span have the same intelligence for startup?
> > >
> > > >> 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.
> > > [Brian Lynch] Perhaps better for dissipation in the PSE switch, but
> > worse
> > > for a PD designer, since now EVERY PD must start in a certain time. Low
> > cost
> > > and
> > > high end.
> > >
> > > >> 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
> > > >> this
> > > >> 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.
> > >
> > > [Brian Lynch] I the 350+/-50ma and 500ma +/-50ma are reasonable numbers.
> > > I think putting the inrush control in the PD side provides a more robust
> > > solution that allows more design freedom and less opportunity for
> > > mis-behavior.
> > > It is relatively easy to specify, and allows designers to size their PDs
> > > according
> > > to their own needs, without impacting system behavior.
> > >
> > > Yair, I think the approach you describe can be made to work in a limited
> > > scope,
> > > but with so many vendors, so many designers, and so many unknown future
> > > designs,
> > > I think it is prone to difficulty, not only in specifying, but in
> > > implimentation.
> > >
> > > >
> > > > 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?
> > >
> > > [Brian Lynch] THere has to be some control for the switch in the PD.
> > Whether
> > > it is
> > > ON/OFF or a current limit circuit, there has to be something. A resistor
> > and
> > > an NPN
> > > would do it, too.
> > > >
> > > >> 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
> > > >> package
> > > >> 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
> > > >>
> > > > << File: Card for Dieter Knollman >>
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