PSE vs. PD power dissipation again
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. 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
To do this, we need a scheme that keeps the power dissipation out of the
PSE end. 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. 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. To do this, the PSE needs
some accommodation: heat sinks, external FETs, sequential power up
algorithms (which lengthen average detect time), or low current limits at
startup. None of these are desirable.
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.
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.
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. 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).
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?