Power Supply Preference

[jlind54]

I'm going for a more diy approach than off-the-shelf. It will be awhile before I start tinkering with anything and I was just asking a few questions to head in the correct direction for research. From what I've read thus far PFC is a plus although it adds some complexity.

[MarianB]

Most of the ATX units ( PC supply ) are half bridge, ofline push pull would need power switches with voltage ratings of 800V minimum, and with increasing ID/IC current theyr price flyes up.

Choosing the proper config is done by first stating the exact working conditions, voltage and current ratings on the output, where they get theyr power from, and why not, the buget.

[jlind54]

Output I'm hoping something around +-35 with ability to supply 20 amps. Power would be supplied 115volts ac at 60hz on a 15amp breaker. The budget has not yet been determine but if I were to put a figure on it I would like to stay under 200USD.

[MarianB]

That would mean 1,4KW of output power, and accepting an efficiency of let say 80% that would require about 1,7KW power from mains, and about 15A at 115Vac, that is quite allot, and at the upper limmit of the breaker, maybe you should change it with a higher current rating one.

But anyway why do you need that much current?

[AndrewT]

Originally Posted by MarianB

....................why do you need that much current?

something in the range of 1000 to 1500 watts out of a class d amp for subwoofer duty

[MarianB]

Ok i appologise for not seeing the first post in time.

Anyway i would think that for 35V per rail 20A is quite large current isn't it? i didn't work with class D amp before nore will i do in the near future but i doubt it would require that large current for just 35V rails... tell me please @AndrewT am i wrong in believing this?

[palstanturhin]

He might have a 1ohm subwoofer?

[AndrewT]

You avoided my earlier question, before it was removed, but I consider it very important to this Thread, so I will repeat the question and give the answer.

Where does the amplifier get the current from to meet the demands of the speaker?

The transient current comes from the capacitors feeding the Power Rails. These capacitors are generally split into HF, MF decoupling and LF smoothing, when considering linear PSUs, (transformer+rectifier+smoothing).
The smoothing capacitance is also fitted to the SMPS, but it is generally of low value due to taking advantage of the high frequency of the supply into the SMPS smoothing capacitance, (some would refer to this as the output filter).

If the speaker is 2ohms and the supply is +-35Vdc then the maximum transient demand from the speaker can be around 35/2 * [fast transient factor]. This fast transient factor can approach 3times and has been measured at values slightly exceeding 5times what the nominal resistor value would demand.

Applying just 3times the peak transient current from the capacitors will be ~35/2*3 ~ 52.5Apk
A 56mF capacitance supplying that 52.5Apk for 1ms will discharge by ~0.94V
That 0.94V is ~2.7% of the original no load 35Vdc supply.
i.e. the 3times current factor has dropped the smoothing capacitance voltage from 100% of full charge to 97.3% of full charge in the 1ms of the transient demand.

Now what would a SMPS that has maybe just 100uF of filtering capacitance have to do to meet that 1ms transient demand of 52.5Apk?

BTW,
this is not an extreme example, 35Vdc & 2ohms speaker loading is only a 612.5W amplifier.
We are not into the kW range yet.

[magnoman]

Andrew
Do these 3-5 multipliers apply as well to sub's with very limited bw?
The converters bw could potentially also keep up with these slower signals (limiting droop but would have to be correspondingly rated for the peak power).

Thanks
-Antonio

[jlind54]

I've been looking around a bit and I've run across Connexelectronic. More specifically "TA3020 Audio Amplifier v4b". How could this be possible? I believe the psu I'm looking to build for such a similar task is going to be loads larger than the one built into this amp.