Conference netcad::hub_mgnt

>  concerning a DEChub900MS with 3 power supplies my customer is asking for
>    
>    	- max. input power consumption [VA] 
>    	- max. heat dissipation [W] or [Btu/hr].
    
The numbers I could find were for a hub with 4 power supplies and a
maximum DC load (420W) - in short, the worst case.

        Input Power Consumption: 1100 VA 

        Heat Dissipation: 730 W

    
    Each power supply has an maximum power available to the DEChub of 140
    [W], as mentioned in the DEChub owners manual.
    That means having 4 power supplies I have a maximum power of 560 [W]
    available to the system.
    Maximum input power consumption of 1100 [VA] means, that all power
    supplies together have a loss power of 540 [W], meaning that only 49,1%
    of the input power is available to the system ?
    Please help me clarify this.
    Thanks in advance
    Regards Bernd

    
    [Hmm... they noticed]
    
    First, the fourth supply is always redundant - the hub manager should
    never allocate power over that provided by three supplies.  This is
    because the AC input line filter is sized for that load.  So the
    actual maximum power available is 420W.  Originally, the rule was to be 
    that no module would be produced that we could not populate a full hub
    with and power with 3 supplies, but that rule has already been waived
    to build the DECbridge900MX (I believe that the maximum you can have is
    5 in a hub). 
    
    Yes, the overall efficiency of the power system is not good.  
    
    I'm told that the numbers are a little misleading - I'll forward your
    note to one of our hardware people and try to get a more informed
    explanation for you, but the short answer is that they are correct.

    
    Bernd,
    
    I'm the responsible hardware engineer for the ONEHUB power supply. 
    Scott Lawrence told me about your power consumption question and I
    provided him with the numbers mentioned in 717.1. Scott's correct 
    that the supply is not very efficient due to the two stages of con-
    version used in the design (AC-48V, and then 48V-5V and 15V), but
    you're confusing VA and watts. The four supplies are designed to 
    supply 420 watts maximum on the 5V and 15V outputs. When you work
    backwards to compute input power, you need to divide the 420 watts 
    by .76 to account for the efficiency loss of the two 48V to 5V and 
    15V converters. This gives you 552 watts. You then need to divide
    by .76 again to account for the losses in the AC-48V stage. This gives
    you 727 watts of input power. 
     
       In order to get VA (volt-amperes), you need to work in the power 
    factor of the input current. For a capacitive input switching power 
    supply, a typical power factor is .65. When you divide 727 watts by .65,
    you come up with 1118 VA. Keep in mind that the numbers used in these 
    calculations are pretty much worst case. If you're concerned with cooling
    a satellite room or something similar, then you should be using the 727 
    watt number. The efficiency for the supply then works out to be 420/727
    =.577 or approximately 58%.
    
       The new buyout version of the ONEHUB power supply (H7894-MA) which
    is being designed by Zytec, will have power factor correction built
    into the supply. The overall efficiency of the supply will be about the
    same as the H7890-MA, but the input VA will be much better because the    
    power factor will be in the .98 or .99 range. This lowers the input 
    current required by the system and takes some stress off the input line
    filter which has a maximum current rating of 12 amperes. We're also 
    working with Zytec to boost the output power of each supply so that we
    can load the HUB to somewhere in the 510 watt range. This will allow
    a greater number of bridge modules to be plugged into the HUB. Zytec
    has not committed to this new power level (yet) because we still
    haven't powered up the first prototypes (March).
    
       Hope this answers your question.
    
    Bob