# [802.3af] Startup and PD input cap

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Guys

I would like to have your comments for the following summary of the
calculation procedure for setting the max. PD input cap to be handled by the
PSE.

Target: 		To make it possible to define more than 50uF as the
point in which the responsibility for inrush current limiting is moved from
the PSE to the PD.
Incentive:           1.  Low cost PD power supply implementations works at
100KHZ. for 10-12W power supply, max. 470uF is needed. for 5W power supply
220uF-270uF
is needed.
Caps lower that 50uF requires high frequency
switching power supply (around 500KHZ) which costs much more.

1.1 50-60% of the applications are 5-8Watts. 30-35%
are 10-12Watts. It means that around 95% of the applications will need 220uF
to 470uF.
(Data based on PD power requirement survey done
during the last 6 month)

2.  In order to meets system stability criteria as
discussed over the reflector during the last 3 weeks, we need to keep low
L/C ratio at the PD power
supply input. Stability criteria requires that
L/(ESR*C)<< Zin, L inductance, C=Capacitance of the
EMI filter, ESR is the equivalent series
resistance of the Cap.(There are additional stability criteria, however this
one concerns the EMI filter
connected to negative resistance network) It
means that we need to allow low inductance for a given Cap size or Large cap
for a
given inductor size. In order to implement the
EMI filter we need the inductor to have 10-500uH (pending on topology,
switching frequency and EMI
requirements) therefor we need Larger Caps.
Although (2) can be achieved when the inrush current
limiting is in the PD, It will be cost effective to the system to allow
larger cap in the PD allowing
the PSE to be responsible to limit the inrush
current pending that it will allow the integrated chip in the PSE.

Vport= Port voltage
Iport =  Port current limit level
N= Number of active ports per device(active port=at startup mode).
Tc= The time that the port is in current limit situation = The PD capacitor
charging time.
Emax= Max energy aloud on the device.

1. Assuming Emax=0.5*N*Vport*Iport*Tc,   Energy dissipated on the device
during startup mode

For the following max. values:
Vport=57V max.
Ip=0.5A max
Emax=1Joule (as per Dave data)

Assuming that in 8 port device only one port is performing the startup mode
(we can control the timing) and there is a cooling time until the 2nd port
will be in startup mode.
N=1. (Remember that it is non repetitive operation.)

2. Tc max = Emax/(0.5*N*Vport*Iport) = 1Joule/(0.5x1*57V*0.5A)=70.16mSec

3. Ip*Tc=Cin*Vp

4. Cin max=Ip*Tc/Vp=0.5A*70.16mSec/57V = 615uF

From eq. 4 we can have 615uF instead of 50uF.

Since the above numbers are worst case calculation, we have the following
margin:

The PSE can be set to 0.4A min.   (The calculation in eq-2 was for 0.5A)
Tc max can be set to 50mSecmin  (The result of eq-2 was 70.16mS)

According to the above margin Cin max would be: Cin
max=0.4A*50mSec/57V=350uF

Therfore we have 615uF/350uF  ==> 75% margin.

In addition, the above calculations assumes repetitive operation which is
not the case for startup, hence much larger margin, with no effect on power
supply loss, cable loss etc.

We can utilize the numbers that we have used for the normal powering mode
and use them as a private case for the startup mode:

During startup, the PSE will limit its output current to:
1.	Ip min=0.4  Ipmax=0.5
2.	Time duration: 50mSec min. 70mSec max.
3.	Period: 1 sec min. ( To allow low average power in order to have
enough cooling time.  It is similar to the timings of the normal operating
mode)

PD spec.
Under the above numbers the PD will be specified as follows.
1.	Up to 350uF at PD input, PD designer have the following resources:
2.	Ip=0.4A min for 50mSec min.

For caps greater than 350uF, the PD designer will take care of limiting the
inrush current to be 0.4A max (i.e. < 0.4A)

Thanks

Yair.

Darshan Yair
Chief   Engineer
> PowerDsine Ltd.  -  Powering Converged Networks
> 1 Hanagar St., P.O. Box 7220
> Neve Ne'eman Industrial Zone
> Hod Hasharon 45421, Israel
Tel:  +972-9-775-5100, Cell: +972-54-893019
Fax: +972-9-775-5111
> E-mail: <mailto:yaird@xxxxxxxxxxxxxx>.
> http://www.powerdsine.com
>
>
>

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