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Old 27th November 2011, 01:24 AM   #1
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Default SMPS units for smaller amplifiers

Hi Cristi, I'm working on a few lowish power amplifiers which want supplies in the 5 to 15V range with currents from about 1A peak up to 6A or so---mostly dual between +/-7.5V at 2A and +/-15V at 6A. The powers are low enough the inefficiency of operating from a linear supply is manageable. But I'm interested in evaluating their performance with an SMPS and quasiresonant or resonnt topologies seem the most attractive. From a power delivery standpoint the SMPS180QR is more than sufficient but it seems the 18V dual version of the SMPS300R is Connexelectronic/PCBStuff's the point of closest approach to what I'm looking for---higher rails are more of an issue for me than extra power or obtaining minimum cost.

Am I correct the +/-18V SMPS300V is currently the lowest dual rail voltage offered? If so, am I also correct in inferring it'd be capable of around 8.3A RMS and 11.1A peak with roughly 20mV of ripple at 1A? And what would be the adjustment range of the auxiliary supplies? While the supply would fry most of the amps I'm working on I do have one design where an 18V rail falls within the parts' typical operating region.
 
Old 29th November 2011, 09:11 AM   #2
Cristi is offline Cristi  Europe
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Theoretical, the output voltage of any SMPS can be as low as you need, even few volts or less. the only practical limitation is the reduced efficiency when it has to deliver low voltage and high current. the voltage drop on the rectifier diodes become comparable with the output voltage, and unless synchronous rectification is used, the efficiency goal is lost.
SMPS300R for example, can deliver minimum +-5V with severe derating, which will bring the total output power to max. 150W and a heatsink must be used for the secondary side diodes.
The main reason why i haven't developed and produce lower power SMPS, in the range of few tens of watts, up to 150W is because they are very cost inefficient, if compared with the commercial general purpose few bucks or less, adapters.
Unless there are special requirements, such as higher efficiency, lower size, lower EMI, lower ripple, better output voltage regulation or customs features, most of the ppls still prefer the wide available ones.
I have in plan to build one later, but have't decided yet about the power range and output voltages. i only know that i want to be much more thinner than what is available more silent and efficient. .
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Old 29th November 2011, 06:13 PM   #3
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I know; I ended up looking at your stuff because I couldn't build anything competitive with the prices. I see three drawbacks to the general purpose units; cost rises noticeably at 1A and above, specs are usually minimal, and the few dual supply units available almost never offer as much current on the negative rail as on the positive rail. I found myself rejecting just about everything I found for want of a good negative rail, but even if I was AC coupling the amplifier inputs to enable single supply operation I'd still end up rejecting just about everything due to no information being available on what the topology, ripple, and switching frequency were. Perhaps I'm just not looking in the right places, but it seems there is something to be said for supplies designed with audio in mind.

Good to know the SMPS300R is adaptable down to +-5V and that there might be a smaller supply sometime in the future. I'm guessing the secondary side diodes for the SMPS300R's primary supply are located just above the quick connects in the datasheet photo and the aux diodes just above the primaries. Doesn't look particularly amenable to heatsinking the standard parts or changing the board over to a GBPC type bridge. At what output voltage would heat sinking be required? From a parts selection standpoint there's a significant win to dropping the supply from +-18V to +-15V. Below +-15 it's mainly about improving overall efficiency to ease the thermals; while the switcher becomes less efficient it's still going to be more efficient than the output devices of a class AB amplifier or linear regulation.
 

 


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