90degC ambient with Alu poly Caps?

[cupoftea]

Hi,
We have a 100W PFCd Boost then Two Tran Forward converter (40vout) which runs in a hot room in our growth plantation. Its 90degC in there.
We need it to keep powering for 200ms after any power down.
So we need a big bank of Alu poly caps on the 40V rail.
Is there any problem for us with 125degC rated Alu Polys and 90degC ambient?
There's no fans, its just screwed to a big metal wall.

We believe Alu Elec is no use due to temperature, Film is just too big for us.
Are there any others, even if very expensive, that could do the job?

 

[std_match]

You forgot to mention how much the 40V is allowed to drop during the 200 ms. For a 100W load?

 

[cupoftea]

Thanks would accept the 40V going down to 30V.

 

[mtwieg]

Usually holdup time is satisfied by adding capacitance to the PFC bus, not the final output bus.

 

[cupoftea]

Thanks yes, but due to the 90degC spec we cant use electro caps......and the PFC output is 200V.....so due to temp degC we must use Alu Poly caps....then
there are very few Alu poly offerings at 200V rating.
On digikey i only find a few 250V Alu Poly caps and they are 100uF and 220uF at the biggest.....and not so physically small.

I reckon it could be best to do a flyback PFC instead, and then the combo of flyback pfc and then 40v holdup caps will be less space than BoostPFC->HoldUpCaps->2TranFwd.
With Alu Poly you seem to get much more choice of caps and package size the lower in voltage you go.

 

[KlausST]

Hi,

some math:
40V .. 30V .. so let´s say 35V in average.
100W / 35V = about 3A

C = I * t / V = 3A * 0.2s / (40V - 30V) = 0.06F = 60mF

When you down at 30V the capacitor´s charge is still 43%
This means if you use a step up circuit to boost the capacitor voltage .. then you may use a higher voltage span of the capacitor let´s say 40V down to 10V
(which leaves only 6% of energy in the capcitor) you may use smaller capacitors. .. so around 40mF.

I had a some similar problem. But with my application it was able to rather early detect the loss of input power. And then I immediately switched off all unnecessary loads.
This helped me to reduce the power drawn from the backup capacitors to about 5% only. This also means I needed only 5% of capacity.

I think I´ve seen high voltage (>5.5V) supercapacitors, maybe these are an option.

Klaus

 

[mtwieg]

Thanks yes, but due to the 90degC spec we cant use electro caps......and the PFC output is 200V.....so due to temp degC we must use Alu Poly caps....

Wet electrolytic caps don't suddenly stop functioning at any specific temperature. One can find many options with rated lifespans at 125C operating temperature. In fact, at a glance I see more options with wet electrolytic caps than solid ones. Not sure what your operating lifespan requirements are though.

there are very few Alu poly offerings at 200V rating.

So? Stack them in series.

On digikey i only find a few 250V Alu Poly caps and they are 100uF and 220uF at the biggest.....and not so physically small.

If physical size is a concern, then even more reason to put the holdup capacitance on the PFC bus. Let's do a simple comparison:
Option A: Holdup is met by capacitance on 40V output
Holdup energy is 20J, Vout is allowed to drop to 30V, therefore the minimum holdup capacitance is Cout*(40^2-30^2)/2=20, so Cout=0.057F. The max energy stored in Cout is (40^2)/2*0.057 = 46.5J, even though you only utilize 20J.

Option B: Holdup is met by PFC bus capacitance. You can design such things such that the nominal bus voltage is arbitrarily high, for example choose 400V, and design the DCDC stage such that it can operate at full power with a bus voltage ranging from 200V to 400V.
So the minimum bus capacitance is Cout*(400^2-200^2)/2=20, so Cout = 330uF. The max energy stored is (400^2)/2*0.000333 = 26.6J.

Assuming the physical size of the capacitors are determined by energy density, option A will require 175% the capacitor volume of option B. In practice when looking at real components I would expect option B to have an even greater advantage. For example, option A could be done with 18 of these in parallel, total volume is 190 cm^3. Option B would require one of these, total volume is 33.6 cm^3.

 

[cupoftea]

Thanks, sorry but we need caps operable down to -55degC
Sorry i thought id said, but just checked and i stupidly forgot.

 

[Magnethicc]

Thanks, sorry but we need caps operable down to -55degC
Sorry i thought id said, but just checked and i stupidly forgot.

-55degrees in a growth plantation?

 

[cupoftea]

Thanks yes, but the same product goes to a different , secret application where its -55degC.