I think you will need primary coil divided into two sections. One for
85V-265VAC and one for 24VDC. Maybe you could select the extra long coil via some zener + transistor. Ie if the voltage is over a certain level the zener will conduct to the transistor wich will activate etc.. Just a quick brainstorm..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
-- \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\ \\\\ email: wolti@sil.at \\ Friends don\'t let friends drink and su(1) web: www.freemodbus.org \\ -- Kevin Harris
"Christian Walter"
** Yes you bloody well do !!!You need the 24 volts DC to be REGULATED against a 85V to 265V AC input range.
Only one way to get that at low cost.
A SMPS !!!!!!!!!!!
........ Phil
Phil Allison schrieb:
Hello,
Maybe I was not clear. I mean I don't need a very exact regulation and I do not care if it is 20V or 28V because the input range of the second DC/DC converters will be 16V - 30V. I think it also does not matter if there is a certain ripple on that internal powerrail because this will be taken care of by the second regulation and the frequency of the ripple is much lower than the switching frequency of the DC/DC converters. Using a line frequency power supply means I would buy a decent good transformer, add a full wave bridge rectifier and only have to use a low cap filter capacitor. So maybe that the output is around 18V and accepting 4Vpp ripple.
Regards, Christian
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.
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.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro Pefhany schrieb:
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.
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) 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
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
I think you'll have trouble getting this high over the input range.
You need to check how to size transformers when used with this kind of load. 100VA is about right.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Sounds like it may just require a single isolated flyback with dual outputs. The flyback primary is powered by both the rectified line AC and the 24VDC input. The primary coil will be tapped with separate feeds for the two DC inputs. You can use a standard IEC AC receptacle with integral DPST switch that opens the 24VDC feed when a plug is inserted. It will be important to open the GND connection of the 24VDC input when the AC is applied to prevent shorting the rectifier.
That sounds like the simplest way to do it, but he still needs an IEC receptacle with integral switch to cut out the 24VDC feed to the output of the Meanwell when the plug is inserted, break before make style. The
24VDC then drives two little cookbook buck regulators.265/85 = 3.12
It is not very hard to make a DC-DC converter work with in input that varies over that much range. If your quantities are low, you won't want a custom transformer in the mains to 24V section but lets assume you are free to pick the tranformer you want.
Many parts max out at 35V so lets assume that 270V gives you just about 30V DC This makes the secondary have
(30V+0.7) / sqrt(2) = 21.7VAC
At 85VAC you would have:
(21.7 * 85/270) * sqrt(2) - 0.7 = 9.66VDC
I have left out the losses in the resistance of the windings at this point.
Now you need to make a first cut on the design of the DC-DC converters. I assume they don't have to be isolated so a simple bucker can be used for the 5V. The 13.8 needs something like a SEPIC converter.
Once you have the first cut on the DC-DC design you need a good estimate of its efficiency at the 9.66V end of the span. You then will know how much current the circuit will draw at that end and can figure out a corrected value for the 9.66V and see if you are in trouble.
"MooseFET"
( snip load of putrid smelling, verbal diarrhoea )
** More very smelly Mooooooosseee droppings !!Splaaaaaaattttttttttt .......
...... Phil
Maybe combine the two inputs with a dual Schottky, which gives polarity reversal protection on the 24VDC input for free.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Don't be modest fred. Anything you asked on SEB would still make you an imbecile.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
No, he needs a pair of high current Schottky diodes from each power source to isolate the two possible power sources. Do it simple, and do it right. Switches on power connectors cause LOTS of field failures. It will also allow a battery backup system, as long as the output of the switcher is slightly higher than the 24 VDC source. I did this for an 'All channel emergency alert system' in a CATV headend 25 years ago.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
BTW: On a lot of HP equipment, there was a switch to switch to DC operation.
If the mains to 24VDC is a fairly simple minded one, no switch is needed. The output side can be held positive when the input side is unpowered. All that is really needed is a diode to keep the supply from powering the DC input.
With a few more parts, you can also make it work as a "float charger" to keep the batteries up when the mains supply is there.
We buy devides made in France and most likely others do it also that accepts 24V up to 260 V AC/DC on the same input. No switching of selections required.
-- "I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
The widest range I've ever done was 10V to over 300V DC. It makes the DC-DC components a lot bigger when you do it.
It's not unheard of for industrial timers and such like, since line voltage AC and 24VDC are the most common there's a strong incentive to make just one device that can do it all.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
That is interesting. Can you give me the name of the company selling these devices? In any case I will perform some research in this direction because better than rolling your own is always buying one. At least if the quantity is reasonably low (
-- \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\ \\\\ email: wolti@sil.at \\ Friends don\'t let friends drink and su(1) web: www.freemodbus.org \\ -- Kevin Harris
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