Re: 85-265VAC and 24VDC combined power supply

On Fri, 10 Aug 2007 14:16:56 +0200, Christian Walter <wolti@xxxxxx>
wrote:

Spehro Pefhany schrieb:
On Fri, 10 Aug 2007 11:25:41 +0200, the renowned Christian Walter
<wolti@xxxxxx> wrote:

Hello,

I am currently working on a device where I am evaluating options for the
design of the power supply. On the secondary side we need two power
supply output with 5V/4A and 13,8V/2A (uncommon voltage). The input
voltages are the standard 85V-265VAC range and 24VDC.

We want every unit to be capable of being used in both environments.
Therefore the power supply should allow both inputs. Because of 24VDC
input I will need two DC/DC converters at the output side. One from
24VDC/5VDC and one from 24VDC/13,8VDC. These converters are easy to
design and can have a high efficiency (>90-95%).

What I am unsure about is how to handle the transformation from the
primary 85V-265V to the intermediate internal voltage of 24V. A
regulated switched mode power supply is certainly a overkill because I
don't need the 24V to be regulated. So I thought I could use a standard
50/60Hz input stage with a simple full bridge rectifier and a good
transformer. Size is not the problem but power efficiency and costs are.
Or are there any better options? I searched for quite some time in
goggle for possible AC input stages but did not come up with a good
solution.

Kind regards,
Christian Walter


Have you considered an off-the-shelf universal input line to 24VDC
module? You can get a cULus etc. approved Meanwell 65W open frame
supply for USD 18. 1-9, and as you note the DC-DC converters can be
cookbook designs.

Hello,

Thanks a lot for your detailed answer. I though about using an open
frame power supply but I am unsure about the mechanical aspects. One
requirement is that there are terminal connectors with L,N,PE,+24V (DC),
GND(DC) at one side of the PCB. Connectors are from phoenix contact and
the series is the MKDS/3 (the typical green ones used in industrial
applications). Using an open frame part means that I have to add some
internal wires and have to think about howto mount it. I have not found
any PCB mountable version of AC/DC converters with 40W and 24V output.

Topology is typically flyback for universal input off line switchers..
you could certainly design one. 50W or 60W isn't too bad. Another
option is to change the specifications to dual input voltage and use a
manually switched voltage doubler input stage as in PC power supplies
(the rail is always 400VDC whether the input voltage is 120V or 240V).
If you use a 50/60Hz transformer it will have to be ~200VA for FW
rectification (nominally 240VAC 50/60Hz to ~21VAC), so it will be
fairly large and heavy (and expensive). The transformer can be fairly
efficient but you'll lose some power in the rectifier.

I use a full-wave bridge rectifier and assume an average output current
of 2.8A at 16V on the DC rail (I want the DC/DC converters to work
optimal and so I try to have the input and output range to be near to
each other). I use a bridge rectifier with schottky diodes to keep the
voltage drop low. I used the following calculations:

Efficiency DC/DC converters (for 5V/4A and 13,8V/2A) = 90% (guessed)

I think you'll have trouble getting this high over the input range.

POut = 40W (by design because in total the output modules will not use
more than this. I mention this because 5*4 + 13,8*2 is greater than 40W).

Therefore I have to supply an average power of 40W/0.9 = 44.4W at the
internal power rail. Choosing 16V as the internal voltage when the AC
supply is used gives me an estimate for the transformer with

PTrans = 1.2 * 2.8A * 17 (2 * 0,5V for schottky) = 57,12VA

You need to check how to size transformers when used with this kind of
load. 100VA is about right.

Because the peak power will only be used for short times I though about
using a 50VA toroidal transformer. The one which came to my mind is form
talema, the 70083 (approx 20$ for a single unit). Or is this is to low I
would go with the next bigger size rated at 80VA with a diameter of 90mm.

Kind regards,
Christian



Best regards,
Spehro Pefhany

Best regards,
Spehro Pefhany
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