Why high voltage supply rails for class D amps?

[grigson]

Hi,

Why do Class D amplifier designers prefer high voltage supply rails?


Please find attached here two Full-Bridge , Class D amplifier simulations which run in the free simulator called LTspice by linear.com.

Each simulation is of a 4KHz sine wave which gives 100W of power in the speaker.

One of the Class D amplifiers has an 80V rail and the other has a 40V rail.

Each simulation gives the same output signal (i.e. the same current in the speaker)

……so why do engineers prefer higher voltage rails for Class D amplifiers?……….
……because these simulations show that you can get just the same speaker signal with a lower rail voltage…… and simply using a lower value of filter inductor.



(To run these simulations , you will need to make sure that the text files called “sin” and “sin1” are situated in the same folder as the LTspice simulation files.)

Also, do you know if my representation of the speaker as an 8 Ohm load is correct?…..Does it have an inductance associated with it?…….is this inductance likely to be poorly toleranced.?


The simulation files are in .txt format..........(i had to do this else they wouldnt upload.....anyway, all you need to do to run them in LTspice is save them as .asc files.)

..i cant tell if its uploaded the files or not , so here is where you can definetely get the files....

http://www.edaboard.com/thread248650.html

 

[OperaHouse]

smaller caps and cheaper semiconductors. Some day a tenth of a cent will mean a lot to you.

 

[ScottyUK]

Commercial amp designs typcally rectify the AC line directly and switch the rectified line voltage directly. They use an HF transformer which provides both isolation and level matching. They may well use an active PFC front end because it gives a relatively stable DC level of about 400V (from a 230V line) without requiring large DC link capacitors.

Why? It is generally cheaper to buy voltage rating than current rating at the level commonly encountered with rectified line voltages.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Since high voltage doesn't start until something more than 35kV, it is highly unlike there there is high voltage in any amplifier.

 

[ScottyUK]

In the IEC world there is only Low Voltage and High Voltage. The former is up to 1000V AC or 1500V DC, and the latter is anything greater than Low Voltage. 'Medium Voltage' officially doesn't exist anymore although the term lingers on in power distribution.

Point taken though.


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If we learn from our mistakes I'm getting a great education!

 

[MacGyverS2000]

Also, do you know if my representation of the speaker as an 8 Ohm load is correct?.....Does it have an inductance associated with it?.......is this inductance likely to be poorly toleranced.?

You're kidding us, right? It's a freakin' coil of wire! Go measure a speaker and tell us if you read 8 ohms from it...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[davidbeach]

Well, regardless of how it gets defined, 80V has never been anywhere near high voltage. When working with high voltage 80V is just noise. Why these electronics types think that high voltage starts at 24V is beyond me. So easy for them, they never have to deal with anything other than low voltage.

 

[geekEE]

A speaker is a really complex impedance that varies greatly by frequency. It really looks nothing at all like an 8-ohm resistor. In the impedance curves at this website, you can see that the enclosure affects the impedance.

http://www.churchsoundcheck.com/imp1.html

 

[OperaHouse]

I though I knew audio. Just what are "27.5 spot monitors"

 

[geekEE]

Hmm..I didn't see that when I first glanced at it. I'm guessing that the "27.5 spot monitors" was a bad joke referring to hooking up a bunch of monitors (i.e. more than 27) to a single amp.