Electronics > Projects, Designs, and Technical Stuff
0-70V, 0-5A Lab Power Supply Design
Kleinstein:
Filtering the extra frequencies from a scr regulator is not so easy, as the frequencies are relatively low. It is possible but still not so simple.
Large filter caps can have the disadvantage of getting a even lower power factor, especially if there is no series inductor to improve the power factor.
A switched mode converter has the advantage that passive filtering is easier with the higher frequency noise. So the choice is not that clear.
For the current rating one could have peak current limits higher than the long time continuous rating. Especially transformers are relatively slow to react and can deliver more than nominal power for quite some time. This is especially sufficient for the audio output peak currents.
H713:
Alright, here's where I'm currently at:
I believe I have a way to do transformer tap switching without having to design and order a custom transformer. The Antek AN-3236 has a pair of windings on both the primary and the secondary. Assuming that I use one transformer for each power supply, that gives me three steps, which should be workable.
Yes, this PS will probably be digitally controlled using a microcontroller. In addition to the tap switching, it will also manage the speed of the cooling fans. This brings me to a question regarding microcontrollers:
If I want to digitally control both transformer tap switching and speed of the cooling fans (dependent upon heatsink temperature), would this require two separate microcontrollers or could one handle both processes?
Zero999:
My advice would be to not use a microcontroller to control the tap switching. Use comparator(s) on the output of the power supply to control the tap switching circuit. That way it will work irrespective of the microcontroller crashing and the same goes for the fans.
Neomys Sapiens:
I would refrain from digital control other than providing the set values via D/A converters.
I think that those PS that use multiple MosFets in a linear mode must have had the benefit of a builder who could ask everything from a supplier, like very very tight selections. The only appliance which I have personal experience with was a custom unit which was used to weld hermetic cases for hybrids. If one of the Mosfets did break ranks even a bit, mass extinction ensued. While it is certainly feasible, you would have to do the selection yourself, which could be costly. If you want to try anyway, APT, IH (now Renesas) and IR had appnotes on the topic.
Where I see problems is the 2HE requirement. Even assuming that you use fans with tunnel-type heatsinks, the heatsink has to have some capacity for heat which means mass, and therefore size.
I would also recommend to give that brute a line switch with holding coil/undervoltage trigger and wiring the failure alarm contacts from the fans into this circuit alung with the thermal switches in the transformer and those at the heatsink.
Concerning the preregulator, careful consideration should go into the control law. It is quite easy to produce instability here. Also, there are some alternatives to switching windings by relay or using phase control. Switching by triacs would be instantaneous and could be fast enough to make it work as a cycle-to-cycle switch. I have to concede that varying phase angle between the line voltage and current in the secondary could make this tricky.
Also, there is the magnetic amplifier (Transduktor), which introduces some noise by saturation, but certainly less than a phase control.
As you are doing audio power amplifiers, you might be already familiar with the McPherson circuit, where dissipation is limited by split rails. An example (using single transistors for a rather small and compact PS) was once published by Ian Hickman in the Electronis&Wireless World. My research led me to the original publication by McPherson, which was aimed at AF amplifiers. I unfortunately do not have it myself (IEEE transactions),
splin:
If it is one off (or only a few required) it should be fairly easy to add additional windings to a standard toroidal transformers (provided the centre isn't potted!).
For example, I have a 2x50V Vigortronix 625VA, VTX-146-625-250, which requires only 1.68 turns/volt. So an extra 12V secondary would only need 20 turns. The centre has a diameter of around 38mm so plenty of room to accomodate extra windings. Starting with a 2 x 15V transformer you would get the advantage of lower copper losses in the secondary at lower voltages, whilst adding the extra windings with progressively finer wire to make it a bit easier to wind.
Alternatively, you could carefully cut the existing secondary to add your own taps. In the case of my 625VA, the existing secondary is 2 x 84 turns of 1.25mm dia wire in one layer. This wouldn't be feasible for much smaller transformers of course with secondary windings having many more turns of finer wire.
A further example; my 225VA 2 x 50V Vigortronics VTX-146-225-255 has 2 x 150 turns of .82mm so 3 turns/volt (with 30mm inner diameter).
Or remove the existing secondaries and rewind. Some transformers have the primary winding on top of the secondary(s) but that isn't the case for my Vigortronix. Jerry Walker has some really well presented videos on transformers and has done lots of rewinds. This one is good where he designs a linear supply to power an RD6000 switching supply and rewinds a pair of toroids:
[EDIT] He actually rewinds the transformers in part 4. Search for 'RD6006 Linear Part4' (without the quotes).
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