LM5026: Which is the Best method to regulate multiple isolated outputs

Hi Aneesh

The requirement that all outputs be isolated from each other and a reasonably tight 1% regulation spec means that you will have to use a normal feedback loop to regulate one of the outputs. They are all more or less the same output power but I think I would regulate the 3.3V/9A output. You will have to use additional non-isolated DC/DC converters to regulate the other two outputs - a buck converter is probably best here. Of course the buck converter needs some headroom to operate - let's say its operating at 90% then the transformer secondary voltage will need to be 110% of the final output voltage - neglecting rectifer losses.

Regards
Colin

Hi Colin,
I have seen somebody using Magamp based regulators for each outputs. what is the advantages and disadvantages of magamp based regulator?.A buck regulator at or magamp regulator which will be a better choice for my application?

Regards
Aneesh

Hi Aneesh

I've used Magamps in the (distant) past. Yes, they work well but in my experience they are bulky and you really need a core material with a sharp saturation characteristic. I think I used a metallic glass core but you could probably use a ferrite core too. They aren't carefree, you still need a control loop and some form of resetting mechanism - especially if the duty cycle is high. If I were redesigning that application today I would probably choose a buck regulator mainly due to familiarity and the wider availability of suitable magnetics.

Regards
Colin

Hi Colin,
if you dont mind could you please share me the schematics of magamp based control what you have used?. I am interested in the magamp feedback loop circuitry.

Regards
Aneesh

Hi Aneesh

I'm sorry but I don't have the schematics. The designs were done many years ago when I was working with a different company and would in any case, be confidential.

Regards
Colin

Hi Colin,

Its ok. at least can you share me any application note or reference paper you have used for magamp?.

Regards
Aneesh

Hi Aneesh

Wikipedia (of course) has an interesting article on Mag Amps at en.wikipedia.org/.../Magnetic_amplifier
Bob Mammano's seminar topic at www.ti.com/.../slup129.pdf is probably more authorative and I'd use that as a good starting point.

Regards
Colin

Hi Colin,
Thanks for sharing the document. I have the following doubts by using buck regulator in place of magamp.

1. If we are using buck regulator in place of magamp there is one mosfet, one diode(freewheel) and one inductor required in series power path at output side. But if we are using magamp only the magamp core will come in series.

2. Extra output capacitors required before the buck stage(since it should be DC before feeding to buck converter) and after the buck stage. If we are using magamp only at output place capacitors required

3. No seperate PWM controller IC is required for magamp

The magamp based control is having the above 3 advantages but why still nobody preferring magamp?

Regards
Aneesh

Hi Aneesh

That's a good question - possibly the answer is just that people aren't very familiar with Magamps.

Regards
Colin

Hello Anesh,

The company Magnetics has a calculator tool for magnetic amplifiers.

Also attached is a document that clearly explains the design procedure

mag_amp.pdf

These square loop cores are difficult to source and are expensive but they do provide an elegant method of post regulation.

I think Hitachi also make similar parts.

Regards

John

Hi John,

Could you please answer me for the below points?

1. If we are using buck regulator in place of magamp there is one mosfet, one diode(freewheel) and one inductor required in series power path at output side. But if we are using magamp only the magamp core will come in series.

2. Extra output capacitors required before the buck stage(since it should be DC before feeding to buck converter) and after the buck stage. If we are using magamp only at output place capacitors required

3. No seperate PWM controller IC is required for magamp

The magamp based control is having the above 3 advantages but why still nobody preferring magamp?

Regards

Aneesh

Hello Anesh,
Yes you are right.
At first glance a saturating inductor (magamp) with an OPAMP, voltage reference and bipolar transistor is a simple solution to generate multiple outputs.
The problem is that the magamp needs to be built from a square loop magnetic material.
These are expensive parts built from materials such as cobalt or thin strips of metal such as nickel iron.
There are very few suppliers of these.
www.mag-inc.com/.../Tape-Wound-Cores
www.hitachimetals.com/.../magamp-square-loop-cores.php

They are mostly always used in military applications where cost is not an issue.
You should check the cost and availability of these parts and then ask your purchasing manager how he feels...........
You will find that multiple buck regulators are less expensive and easier to source parts.
Regards
John