24VDC transformer power supply selection
24VDC transformer power supply selection
(OP)
Hi,
I building a 24vdc power supply at 7A ouput with four 2200uf capacitor for a total of 8800. I have some trouble selecting the right transformer for my application. Simulation give me a high peak current in the transformer but I'm not sure if this will make the transformer overheat and if I have to select a transformer with a lot more VA then the output power. I would like to know how to select the VA rating on the transformer that I need ? Thanks
I building a 24vdc power supply at 7A ouput with four 2200uf capacitor for a total of 8800. I have some trouble selecting the right transformer for my application. Simulation give me a high peak current in the transformer but I'm not sure if this will make the transformer overheat and if I have to select a transformer with a lot more VA then the output power. I would like to know how to select the VA rating on the transformer that I need ? Thanks





RE: 24VDC transformer power supply selection
2000 uF per amp (to have a reasonable low voltage ripple).
Transformer VA = 1.5 x the output power (to hopefully not saturate the transformer iron).
RE: 24VDC transformer power supply selection
Yes you will need a larger transformer because it will otherwise saturate every time its brief opportunity to supply current comes by.
Half wave no filter:
DC output voltage = 0.45V x Vac
Peak inverse voltage across diode = 1.4 x Vac
Diode current = I(load)
Diode peak current = 3.14 x I(load)
Transformer current = 1.57 x I(load)
##########################################
Full wave no filter:
DC output voltage = 0.9V x Vac
Peak inverse voltage across diode = 2.8 x Vac
Diode current = 0.5 x I(load)
Diode peak current = 1.57 x I(load)
Transformer current = 0.785 x I(load)
#########################################
Bridge no filter:
DC output voltage = 0.9V x Vac
Peak inverse voltage across diode = 1.4 x Vac
Diode current = 0.5 x I(load)
Diode peak current = 1.57 x I(load)
Transformer current = 1.11 x I(load)
#########################################
Bridge C(only) filter:
DC output voltage = 1.4V(unloaded) x Vac
Peak inverse voltage across diode = 1.4 x Vac
Diode current = 0.5 x I(load)
Diode peak current = (4 to 12) x Diode current
Transformer current = (2 to 3) x Diode current
Don't forget you can toast the rectifiers if you don't limit inrush to those caps which will look like a dead short without some input impedance between the transformer and the rectifiers.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 24VDC transformer power supply selection
RE: 24VDC transformer power supply selection
Thanks for all the information. But I still have a question about the inrush current in the caps. Is there a simple way to limit the current to the cap? A resistor seem like a bad idea because of the 7A current. I was thinking of just choosing a bridge that can support more amps. I know adding a smoothing inductor would be a good idea but I wanted to keep the cost down. Thanks
RE: 24VDC transformer power supply selection
Failing that you might need to consider and active soft start circuit with a big FET or BJT transistor.
RE: 24VDC transformer power supply selection
Looking at the transformer tech data can sometimes help. Sometimes you get the ratings when supplying a rectifier and capacitor filter when that is the most likely use for the transformer.
RE: 24VDC transformer power supply selection
We had a previous thread where we discussed the in-and-outs of various methods, including using a relay to switch out a series resistor once the secondary DC voltage has risen to a reasonable value (indicating that the inrush event is over).
RE: 24VDC transformer power supply selection
All the guys suggestions will do the job.
Supplies that size often use resistors. The resistor isn't that big a problem because remember you are only interested in limiting the initial inrush to something reasonable.
You want to limit the current thru the rectifiers to something reasonable for the rectifiers that you choose.
Consider.. You have a 10W load on this dropping resistor when drawing 7A.
This would be P=I2 x R. 10/I2 = R
R = 0.2 Ohms.
This would limit the inrush to:
I = V/R
I = 28V / 0.2 Ohms
I = 140A
You can iterate these calcs until you have a trade off you and your diodes can live with.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 24VDC transformer power supply selection
bklauba
RE: 24VDC transformer power supply selection
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I don't suffer from insanity. I enjoy it...
RE: 24VDC transformer power supply selection
Try:
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BK
RE: 24VDC transformer power supply selection
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 24VDC transformer power supply selection
htt
Scroll down halfway.
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I don't suffer from insanity. I enjoy it...
RE: 24VDC transformer power supply selection
BK
RE: 24VDC transformer power supply selection
In triplicate.
Keep the golden-rod submit the white and the green.
It will be taken up in committee 13F on April 15, 3041. You will be notified promptly on the status.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 24VDC transformer power supply selection
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I don't suffer from insanity. I enjoy it...
RE: 24VDC transformer power supply selection
EI cores have an unintentional air gap inherent in the design. Small, but it makes them softer in the squareness of the B-H loop. Torroids don't have this, so they're less tolerant of high line voltage. And since there's a lot of thermal mass involved, it can take several hours of operation to find this out.
RE: 24VDC transformer power supply selection
Worth mentioning that the tolerances on the capacitance value for high-value electrolytic capacitors are often quite wide and sometimes non-symmetrical around zero (as much as +100% / -50%). If your design is marginal in any way, you might want to examine the tolerances.