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RE: [802.3af] quick questions

Yair -

More comments on the inrush issue:

At 05:10 AM 6/16/2001 +0200, Yair Darshan wrote:
>         In type A, the isolating switch is working as a simple switch which
>turn on at around 32V with out hesterhisis.
>         The switch is turn on within few mSec. The switch is turn on by
>passing through the switch (Mosfet) linear region, which represents a
>resistor of 50-100 ohms.
>         It means that it limits the current to a value that is greater than
>0.5A (more close to 1A) and since the PSE inrush current limit is set to
>0.5A, this effect in the PD is not relevant.
>         The PD power supply controller contains UVLO that is set to turn on
>at around 40V and turn of at around 33V. it is located after the big
>cap(part of PWM controller)

Correct me if I'm wrong, but doesn't this type A circuit effectively split 
the dissipation between the PSE current limit FET and the PD UVLO FET? It 
will regulate the line voltage at turn-on to the UVLO level, and the rest 
of the dissipation will be in the PD. This means the PD switch needs to 
consider power dissipation as well as the PSE switch - it's not necessarily 
a tiny FET any more.

Type B is obviously OK since it has no isolating switch. We can't use it 
because it won't allow detection.

I'm most worried about a "type C", with a hysteretic turn-on in the PD. 
This scheme minimizes dissipation in the PD, allowing the smallest 
isolation FET. It can also oscillate at turn on with a current-limited PSE.

The isolating switch is the big headache here, since it's highly 
non-linear. Even type A can oscillate at startup if conditions are wrong - 
it is a common source amplifier with gain and phase shift (and a capacitive 
load!), and the PSE current-limited output also has loop behavior. Today's 
circuits don't oscillate, but as FET/IC technology advances, other 
solutions will become cost effective - and they may oscillate. We haven't 
even entered the realm of power supply behavior yet - the PD supply is 
still in UVLO, and the large PSE supply is well below its current limit. 
The oscillator is composed of the PSE inrush (outrush?) limiter and the 
isolation switch circuit in the PD.

I like the PD-inrush circuit because all of the possible oscillating 
elements are in the same box (the phone), and all are supplied by the same 
vendor. If the vendor does his job right, the phone won't oscillate. If he 
screws it up and it oscillates, he's a bad PD designer - don't buy his 
phones. The PSE current limit circuit is open loop during power-up, so it 
can't contribute. Even if we have a bad phone that oscillates internally, 
the voltage on the wire won't oscillate, and data disturbance is minimized.

In the PSE-inrush case, half of the loop (the PSE current limit) is made by 
one vendor, and half of the loop (the PD iso switch) is made by another. No 
vendor can test his PSE against future PD designs, nor can a PD designer 
test against future PSEs. Unless we specify impedances, phase margin, 
frequency bands, etc., we can't guarantee it won't oscillate with some 
future yet-to-be-designed PD, and a PD designer wishing to test his phone 
can't be sure he won't oscillate unless he buys every PSE available...

I'm guessing that this cross-brand interoperability issue hasn't really 
been tested in the field, since most current systems (that I'm aware of) 
are proprietary and only work with their own phones. True?

> > Your 350uF number is fine for dissipation at turn-on, and it's marginal
> > but
> > probably OK (57V*400mA*50ms=1.14J)
>         [Yair Darshan]  I think you have mistake here. The calculation is
>57*400mA*50mSec/2 = 0.504Joul. So you have 100% margin. You need to divide
>by 2 since it is triangle shape for a constant current charging a cap.
>Please check me again.

For power up, you are correct. For a shorted line, there is no triangle 
waveform, and the /2 factor disappears - although thermal limit will still 
save us.

Like you say, the PSE inrush solution works in the field today. I believe 
this is true. I fear it won't always be - and I'll sleep better if we 
mandate PD inrush control. We agreed in St. Louis to accept up to TBD cap 
without PD inrush, and I'm prepared to supply chips that will work 
thermally with 350uF. But I can't guarantee they won't oscillate!