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Old 21st May 2004, 08:41 AM   #1
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Join Date: Nov 2003
Location: England
Default Switchmode converters

Hi guys,

I'm going to have to compress this post down really tightly to keep it somewhere within the realms of being readable.

This converter is to power a form of moderately low fedelity signal generator for some experiments.

Since it closely resembles an audio amplifier, I thought it fitting to ask about it here. Simply, this device will be similar to an amplifier but I'm not looking for something that is exceptionally pleasing to play music through.

I am new to switchmode converters, which is why I'm not totally sure with them.

Ni-MH battery pack -
6 triple A cells. Fully drained, approaching 4.8Vdc.

Required -
50Vdc output @ 160 - 200mA. Must be isolated onto four seperate channels, giving ~40 - 50mA of isolated output per channel.

Each of these channels will form the DS lines for a MOSFET. And so, the MOSFETs will be controlling the current passing through the each channel - all but for a series resistor to prevent any surges over the peak current.

Problems -
Converter must be small and not too expensive. I want the whole end unit, including the converter, to fit in the palm of my hand.

I began designing such a unit around the Maxim 1523 boost controller. I have produce a design to supply the correct voltage and current ratings, it is also very small and relatively cheap.

However, I now have one channel providing all the current, not four isolated channels.

My futher thinking
Add an SMT transformer at the output to isolate the four channels.

The problems with this idea... it means the use of more than one coil, using up board space, adding weight and complexity. It also means that frequency response curves of the transformer may drastically alter the output fedelity - while it needn't be incredibly high, it can't be terrible either.

So my next thought was, a transformer based converter. I had initially been driven away from these after reading that they were the bulky, costly and sizable alternative.

However, this method would seem to suggest that I could drop the inductor.

My lack of understanding with these converters means that I have not been able to look deeply at their suitability. For instance, pulsing them with high frequency DC, reforming an AC waveform for them and then having to rerectify it on the other side, using step up transformers... and so on.

My last idea, and hope, is that I am unaware of some, far simpler, method of providing isolation.

What am I asking basically?
Is there any other, small, affordable, lightweight way of providing isolation?

What kind of topologies and methods would I use to begin using a transformer based converter?

Do any manufacturer's produce useful PDF sheet for their controllers than might help explain this method?

I initially chose the Maxim 1523 as Maxim provided a very in depth explanation of how to optimise your converter to run correctly... explaining what each element did. Whereas other manufacturers gave much more of a such it and see walk through, suggesting rough estimates for components.

Does anyone have any experience with capacitor based boost converters?

Again, I'm sorry for the way this is laid out and worded, but otherwise I end up with pages of explanation.

I would greatly appreciate any help with this!
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Old 21st May 2004, 10:44 AM   #2
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Join Date: Nov 2003
Location: England
After some futher reading, I have realised that the forward converter is the best choice for this problem.

Since the editing time has expired, I can not delete my original thread. And so, I welcome any futher suggestions. Such as, where to buy good quality, SMT transformers for forward converters.
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Old 21st May 2004, 11:17 AM   #3
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Join Date: Dec 2002
Location: Slovenia
Hi,

it is not very clear vhat are you trying to achieve. Do you want 4 stable 50V outputs? If this is right, then your best choice is flyback converter, since it has best cross regulation of simple type converters. I suggest you go to Power Integrations site and read their application notes. They are excellent for beginners.

For ready made transformers you may want to look at Coilcraft.

Best regards,

Jaka Racman
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