Power supply

[Electronics4you]

Hi there

I need a 18V DC power supply from a 28V/7.5A AC toroidal transformer. The current output should be so close to the 7.5A as possible. What should I use to step down the power - transformers, lin. regulators...?

 

[Sceadwian]

Depends on how much that 28volts will sag when the output is at the full 7.5 amps. Have you measured 28 volts at the full load? If so what's the no load voltage?

 

[Electronics4you]

I haven't measured anything yet. I have just found a toroidal transformer with a 28V output. The transformer says just 28V/7.5A. Isn't that the max current at 28V?

 

[cadstarsucks]

I haven't measured anything yet. I have just found a toroidal transformer with a 28V output. The transformer says just 28V/7.5A. Isn't that the max current at 28V?

That would be the RMS voltage when supplying 7.5A to a resistive load. It would be horrendously inefficient to use a linear as you would be seeing to 40V peak plus line variation or most likely 35V average.

(35-18V)*7A=120W that you be dissipating and would have to supply a heat sink for. You need a switcher.

Dan

 

[Sceadwian]

You could always re-wire the transformer.

 

[RODALCO]

Or reduce the primary voltage by adding a suitable inductor in series with the incoming mains.
This can be calculated or done on a trial and error basis,

Of course be safe when working with mains voltages.

 

[ericgibbs]

hi,

The best thing to do would be to take off a few secondary turns. [28v winding]

A rule of thumb is 6 turns/volt per square inch of core. So a core area of 2 square inch would be 3 turns/volt.

As its 28Vrms at 7.5Arms and you want 18Vdc at 7.5Adc, I would give the linear regulator transistors at least
2 to 3 Volts overhead.[ V c to e]

Assuming fullwave rectification use a 10A FWB, rated about 120V.

Ripple capacitor of 10,000uF at 35Vdc.

If you reduced your winding turns to give say 20Vrms, this would give you a Vpk of 28V at no load and about 21V on load.

Connect up the transformer, measure the secondary Vac off load.
Disconnect the transformer from the supply, remove 10 turns from the secondary, reconnect and remeasure the secondary Vac.

This will give you a turns/volt figure, so calc how many more turns need to be taken off.

Do you mind if I ask what you are going to use the transformer for ?

EricG

 

[Hero999]

Why are you using a 28V transformer.

An 18V transformer should suffice, assuming a 3V loss in the regulator, 1.8V in the rectifiers and 1V ripple.

 

[ericgibbs]

hi Hero,

I think the OP has a 28V transformer on 'the shelf' that he wants to use for the 18Vdc psu?

Hope he lets us know what he going to use it for, we may be able to make
an alternative suggestion on how to use it.

Eric

 

[eblc1388]

Why are you using a 28V transformer.

An 18V transformer should suffice, assuming a 3V loss in the regulator, 1.8V in the rectifiers and 1V ripple.

The OP requires 18V DC at 7.5A. A 18V transformer will not do.

To be picky, a 7.5A transformer secondary will not do either. It needs to be 10.6A or more.

 

[Hero999]

Why would you want any more than 18V?

I suppose it depends on the size of the capacitor.

I agree a 7.5A transformer won't be enough, 10.6A sounds about right but that too depends on the amount of ripple which will depend on the capacitor size.

I find questions like this interesting, is it cheaper to use a larger transformer and therefore a larger heat sink or a larger capacitor and smaller transformer and heat sink?

 

[eblc1388]

Why would you want any more than 18V?

Why?

Because a 18V transformer cannot provide a regulated DC 18V output at 7.5A. You have mentioned correctly all the voltage drops but I don't understand why you would then went on to suggest a 18V transformer instead of 22~24V.

 

[Hero999]

[latex]V_{peak} = 18 \times sqrt{2} -3-1.8-1=19.66V[/latex]

If the transformer has 5% regulation (which isn't unreasonable for a torroidal transformer of this size) then the output will be 24.18V which is still higher than the minimum for acceptable regulation. Also, voltage regulators tend to increase their dropout voltage with temperature and therefore a lower average input voltage will reduce the 3V worst case prediction in our favour. Anyway isn't the voltage rating transformers normally specified at the maximum load current?

The only time I can really see this becoming a problem is if the mains voltage is on the low side (which doesn't normally happen here in the UK).

 

[eblc1388]

[latex]V_{peak} = 18 \times sqrt{2} -3-1.8-1=19.66V[/latex]

If the transformer has 5% regulation (which isn't unreasonable for a torroidal transformer of this size) then the output will be 24.18V which is still higher than the minimum for acceptable regulation.

I see that you have added 5% in your calculation of final voltage 24.18V. Why?

The transformer voltage regulation does not mean that the voltage is 5% higher when at full load.

At best the transformer secondary is 18V at full load, with 5% voltage rise without load. In some cases, it could be even a bit lower than 18V at full load because of losses.

Remember also that now the winding ac current is 10.6A and there are more I2R drop too.

 

[Hero999]

I see that you have added 5% in your calculation of final voltage 24.18V. Why

Sorry perhaps I didn't explain it well enough, I subtracted not added, the pea

The transformer voltage regulation does not mean that the voltage is 5% higher when at full load.

I know, it means the voltage will be 5% higher off load. I make this mistake of assuming it would be 5% lower at full load.

At best the transformer secondary is 18V at full load, with 5% voltage rise without load. In some cases, it could be even a bit lower than 18V at full load because of losses.

I would hope the manfucturer has factored in the losses into the regulation which is why it's 5% higher without a load.

emember also that now the winding ac current is 10.6A and there are more I2R drop too.

I suppose the peak voltage will be slightly lower as it's drawn in pulses rather than a continious waveform.

 

[eblc1388]

If anyone is interested in high current(over a few amps) power supply, then I can recommend the following link.

The author explains clearly why he need to use a 19V secondary transformer for a 13.8V 20A DC regulated supply.

13.8V 20A linear power supply

 

[sam2]

Thanks, thats a good site. I like the SWR meter.
sam

 

[Hero999]

I don't completely agree with that article, I agree that people do tend to undersize things (I sometimes do on the bases that it not only saves money and I probably won't use it at full power very often) but there's over sizing things to the point where it becomes unnecessarily expensive and bulky. I would choose a 15V transformer minimum and no higher than 18V at this current.

A 60000uF capacitor, used at 20A, and discharging during almost a half cycle at 50Hz (10ms), will drop the voltage by almost 3.3V

LTSpice says 2.55V of ripple at 15Vrms with a 20A constant current source on the output; this is assuming the transformer is perfect which I know is not true.

We need to calculate the equivalent series resistance to model this correctly. Suppose the 15V 30A transformer regulation is 5%, the off load voltage will be 15.75V but if it's loaded with a resistive load that draws 30A then the voltage will drop to 15V, the equivalent series resistance will have 0.75V across it at 30A, therefore ohms says it will be 0.75/30 = 25mhm:.

Accounting for the series resistance and the rectifier voltage drops, LTSpice says the ripple will be 2.2V, the peak will be 18V and the valley is just over 16.6V, if the regulator drop is really just 2V then we have 0.8V to spare. I do agree that if the mains voltage is on the low side it could still be a problem but this normally isn't the case in the UK. I don't think I've ever measured a voltage below 228V, it's normally 235V to 240V.

On the other hand, if you use a regulator with a lower drop, and/or a larger filter capacitor, then you can slightly ease the transformer voltage requirements. This can be very useful to keep the filter capacitor voltage rating requirement at 25V, since otherwise you would be forced to use a 35V capacitor, which is much larger and more expensive. A lower transformer voltage is also an advantage from the efficiency point of view. After all, the complete excess voltage has to be burned off by the regulator, causing a huge power loss and requiring a large heat sink!

This is precisely my argument, and is also why I would use N-channel MOSFETs rather than bjts. Also a 120,000:mu:F capacitor 25V is about the same price as a 35V 60,000:mu:F capacitor and you save money on the heat sink, transformer and rectifier.

Another issue is what kind of pass elements to use for the regulator. MOSFETs are not a good choice, because they are much more expensive than bipolar transistors for a given minimum voltage drop and power dissipation. So, almost every power supply uses bipolar transistors. NPN transistors are usually preferred over PNP ones, because they are cheaper for a given performance, and there is wider selection. So far, so good. But most designers place their pass transistors on the positive side, in emitter-follower fashion, adding a Darlington driver (or two Darlington stages). This is a very bad choice for several reasons: One, each transistor connected in that way produces a minimum voltage drop of 0.6 to 0.7V. A three-stage arrangement, as is often needed, would have a minimum drop of around 2V, plus the drop caused by any equalizing resistors! Also, the transistor collectors, which are connected to the cases, are at the unregulated positive voltage, and thus require insulation from the heat sink and power supply case. The necessary mica insulators add a huge amount of thermal resistance, making it much harder to cool the transistors properly.

I don't know when this was written but today MOSFETs and bjts are around the same price; not only that but MOSFETs have the following advantages.

• Lower dropout voltage.
• Better transient response.
• They don't need any drive current.
• You safely parallel MOSFETs without series resistors thus saving additional parts and voltage losses.

The only real disadvantage is they can be destroyed but high gate voltages but that's pretty easy to fix by adding a zener across the gate and source.

I do agree that it's better to use negative devices, PNP bjts and especially P-channel MOSFETs are more expensive for the same rating then their N-channel/NPN counterparts.

https://ludens.cl/Electron/Ps20/ps20.gif
What's going on with the :mu:A741? I would use a real op-amp like the TL081 that will give half decent transient response and would consider using a proper little regulator IC like the LM723.

I have built MOSFET regulators before with dropout voltages well below 100mV. A good quality MOSFET design should give good performance at this current even with a 14V transformer and a low mains input voltage.

 

[Electronics4you]

Sorry for my late answer

Yes, that's right, it's a shelf transformer, so I won't buy a 25.5Vrms (18V) transformer. I'm using it to power my motorcontrol for an electric car motor. It uses approximally 6-7A at 18V, which is the decired voltage for my purpose. But I can see, that a 7.5Arms transformer won't give 7.5A (my mistake), but 5.3A can be enough.

Otherwise if I want more direct current I can parallel two transformers to give me 10.6A current.

Now to the voltage question. The full wave bridge rectifier has a nominel dropout voltage of ~1.42, but at higher temperatures (the hign current) it can increase to the before mentioned almost 3V.

But my 28Vrms transformer will give 19.8V - ~3V = 16.8Vdc when coming out of the bridge - right? That's not enough to be 18V, but the 3V are also in worst case scenario. Using two 18.000µF in parallel (so the max ripple is 2.5%) the only thing left is to regulated the power supply, but how? Using an expensive switcher?

Also a 120.000µF capacitor 25V is about the same price as a 35V 60.000µF capacitor and you save money on the heat sink, transformer and rectifier.

How can a bigger capacitor give the circuit a lower thermal resistance <=> smaller heat sink?

 

[Electronics4you]

Another question:

The datasheet for the toroidal transformar says:
Pri.: 0V - JOINT - 220V-240V
Black-yellow/yellow-green-brown
Sec.: Blue-blue: 22V-4A
Yellow-yellow: 28V-7,5A
Red-red: 19V-2,6A

How to wire the primary wires?