No fans allowed in SMPS design...so need multiple paralleled SMPS's?

[zenerbjt]

Dear Engineers,

We are using a Vicor DCM3623T75H26C2T00 DCDC module for a spec of

Vin=42-59VDC, Vout = 24V 5.8A.

The Module sits on a PCB, which is itself in a large-ish, sealed enclosure. The module will dissipate 17.2W of power at full load. The DCDC module’s internal temperature must stay below 110degC.

No cooling fans are allowed, just a heatsink.

DCM3623T75H26C2T00 DCDC module datasheet:
http://www.vicorpower.com/documents/datasheets/DCM3623x75H26C2yzz_ds.pdf



The DCDC module’s Rth(junction-case) is 1.5degC/W.
With the module’s power dissipation of 17.2W, the ambient temperature inside the enclosure will be 67 degC, we know this from temperature tests.

The heatsink used is the “180AB” by abl-heatsinks (datasheet below). Dimension is 215mm x 170mm x 7mm (height). Heatsink Rth = 0.3degC/W.

Anyway, DCDC Module internal “junction” temperature will be 67degC + 1.5*17.2W + 0.3* 17.2W = 98degC.

Since we are not using a fan, we do need this very large heatsink. This is unfortunate. Would you agree with these calculations?

180AB heatsink datasheet:
https://www.abl-heatsinks.co.uk/index.php?page=extrudedproduct&product=170


We are starting to think that since we are doing it fan-less, and the heatsink size needed is so big, we would actually be better off doing this with four paralleled Flyback converters instead(..with one single feedback error amplifier so they all share output current equally). Then the heat is spread between the four flyback converters, and the overall solution size will be less. What are your thoughts?

[zenerbjt]

For our Vin=42-59v and vout = 12v, 11A spec….
Use DCM3623T75H13C2T00  DCDC module
Internal amb = 67degC
DCDC module = 1.92degC/W
DCDC module dissipation = 14.5W
Heatsink R(th) = 0.3degC/W
So DCDC module junction temperature =
67degC + 14.5 * 1.92 +14.5 * 0.3 = 105degC
…Again, this is risky, because the module shuts down at 125degC…and the heatsink R(th) of 0.3degC/W cannot be relied upon, since our enclosure will severely hamper the “free air convection” that is assumed in the declaration of the heatsink’s 0.3degC/W R(th).

As such, i think that since we have  this modular PSU trapped in an enclosure which limits free air convection, we will have to use multiple custom SMPS in parallel instead.
A custom SMPS wont shut down at 125degC.
-I would say four flybacks in parallel for us.

Would  you agree  that our enclosure means "free air convection" is hampered and therefore the heatsink datasheet's declaration of "free air R(th)" cant apply to us?
(Our enclosure is some 70cm by 25cm by 8cm high)

[David Hess]

Why not create a thermally conductive path between the module and the enclosure, like with a heat pipe?

[zenerbjt]

The Vicor VTM modules are about 95% efficient, but as you know, they tend to need another converter ahead of them.

Some of the other modules are ~95% efficient at certain conditions, ie on max power and nominal vout etc.......but its difficult to find a module whose max power is your applications exact max power. The vicor modules efficiency drops away at lighter loads....as i suppose do most SMPS.

As you know, all modules from any manufac have an inbuilt thermal trip of 125degc...this is the big disadvanatage compared to custom SMPS, especially when not using a fan

[tom66]

125C is far too hot anyway.  It's good that they trip at that point, you wouldn't want your design to have a margin that tight.

Often fans are added to designs completely unnecessarily.  If you can, get hold of a book called "Hot Air Rises and Heat Sinks".  There are so many misunderstandings when it comes to fans, they are often added unnecessarily to products creating audible and electrical noise problems, dust, and add a mechanically life limited part to potentially a lifetime system.  Not to mention you have to be very careful in implementing them.

[jbb]

I agree with Blueskull: 89% efficiency is pretty low.

I plugged your numbers into the Vicor Power System Designer. It suggested a two-brick solution with an overall efficiency of 92% (still not great). That’s 12.1W losses, which is a 30% improvement on heatsink requirements.

Additionally, the heat load is spread over 2 modules, which reduces the temperature rise between the silicon and the heat sink.

Do you really need a regulated 24V output? If you can relax that things could improve a lot.