Power Conditioning , Regulators

[strahd_von_zarovich]

Hi guys , my teacher asked me to design a power conditioning unit like below.

Power conditioning block diagram( Circuit Schematics are not mandatory, but as a result it will be evaluated as bonus point)
Description: Input Voltage: 28VDC
Output Voltage: according to 5 VDC, (Isolated) (10 points)
according to 3,3 VDC, (Isolated) (10 points)
according to 28 VDC (Isolated) (10 points)

I choosed 2 regulators from Digikey for 3V3 and 5V output . But i guess , i must use 28V directly without regulator for 28V output? Can i use 28V input voltage with some capacitors to eliminate noise for 28VDC output. . My last question , are they enough for power conditioning unit or something else is required and what does isolated stand for ? No other information about the design is given .

 

[d123]

Hi. If the 28VDC is regulated then I imagine a few capacitors wouldn't hurt and may help if it's noisier than acceptable for its purpose. I think isolated would refer to either isolated from the mains supply, and probably from your description isolated from the 28VDC input, which I've seen done with transformers in quite a few schematics, and there must be other ways to achieve this, like relays or solid state relays.
That's quite a big step-down from 28 to 5 or 3.3V, don't know the regulators you've chosen or the current they need to handle but from headaches I had with a power supply you may need to think about heat sinking the regulators and the calculations that go with seeing if the regulators will stay within their SOA/junction temperature. Sounds like a lot of reading the datasheets and doing the accompanying maths. Heat-sinking and airflow with a fan may or may not be a requirement for you.
Due to inexperience I found that a 24VDC down to 5VDC supply was pretty depressing to put together for 0.1A, and ended up using about 20VDC down to 5VDC as the Vin - Vout x I = Watts way beyond what I felt comfortable heat-sinking and just expected the regulator to burn up.
If you're not quite sure, have a little read/search the Internet for (what I imagine are) linear power supplies or regulators. Also, if you haven't already, you should look at SMPS regulators (not linear) on Digikey and avoid what I described above, apparently they are much noisier but will cope better with that step-down. Also, devices like buck-boost converters and so on may offer you a suitable option/method.

 

[BradtheRad]

'Isolated' generally implies no ability for mains AC to get into your circuit. Resistors and capacitors are not sufficiently isolating. Usually it is done by interposing a transformer.

I'm pretty sure the assignment is going to be a little trickier than putting voltage regulator IC's on the 28V supply and saying 'finished'.

No other information about the design is given .

Ampere levels are not given. Although you or I would say the specs are incomplete, the fact that there is no Ampere or Watt spec, suggests the grade is for a design that has isolation, rather than a particular power rating.

It is a safer approach for us to assume specs describe adverse situations, rather than easy situations.

28VDC to 28VDC *isolated* suggests a transformer.

 

[Warpspeed]

Power conditioning 28v dc to 28v dc suggests the output voltage may (?) need to be higher than the input voltage under some conditions.

As isolation is also a requirement, a lot more than just a couple of off the shelf linear regulators will be required.

 

[treez]

isolation means there is no direct connection with a conductor between the bits of circuitry that are isolated from each other......one gets "degrees" of isolation, ie, that the insulation in between the two isolated bits of circuitry will not break down and conduct with say several thousand volts of potential between them....this sounds funny, but eg mains voltage can go up to several kilovolts in magnitude.

 

[chuckey]

So its a 28V invertor with three separate windings each with its own rectifier and stabiliser. The reason for the isolation (and I have seen this) is for 28V equipment with a - earth having to be operated in a vehicle with a + earth.
Frank