trying to build a simple Class D circuit

[Jamie]

transmogrifox wrote:

On Oct 15, 2:16=A0pm, Michael <mrdarr...@gmail.com> wrote:

On Oct 15, 12:53=A0pm, Jasen Betts <ja...@xnet.co.nz> wrote:


On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok. =A0I had a feeling a 555 wasn't a good choice anyway... not

su=

ch

a great comparator at high frequencies?

It's good to around 1MHz, =A0but I'm mystified about how it's

supposed

to work in this circuit

He's got a bunch of circuits here:

Thing is, they seem to be headphone amps. =A0Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

He seemed surprised that a basic transistor sounded better than his
second 555. =A0Second

circuit:http://geekcircuits.com/2010/01/class-d-amp=

-made-easy-with-555-timer-ic/


Bunch of circuits here: http://geekcircuits.com/category/class-d-amp/
still half asleep...



For something simple, I found a work-around to the shoot-through problem:
You can use 4 N-channel MOSFETs in an H-bridge. The low side MOSFETS are
switched at a high frequency. The High Side MOSFETs are switched according
to the input signal polarity. This gives you a push-pull amplifier
topology.

On positive signal polarity, quadrants 1 and 3 are off, quadrant 2 is full
on, and quadrant 4 is switching at a high frequency. The opposite is true
on a signal negative polarity. Now the speed of the high-side switching
transition can be tuned to the amount of crossover distortion you can
tolerate. You may even be able to reduce the shoot-through trouble by
placing an inductor in the source of the high-side MOSFETs, thus greatly
reducing your crossover margin. That way it will only source average
currents through the load, but will not respond to the fast pulse when the
signal is near zero, or silence.

The nice thing is this relaxes design requirements for high frequency
switching of the high side, and reduces power handling requirements on
transistors driving those gates (though for a 1W audio amp, you don't even
need gate drivers). Everything I can think about this method works in
favor of the designer: You don't have as much power loss due to power
dissipated in the gate driver circuits, shoot-through is minimal and can be
tuned away, at low signal levels this can be tuned to a slight DC offset to
eliminate switching current...

Just an idea. I wish I had a schematic up as that would make it more
clear. I haven't been able to find this mode of operation on the web...
and probably for good reasons. The concept is so old and outdated that
people have forgotten that hobbyists may be interested in trying this. The
thing that is popular today are the switching schemes that effectively
double the switching frequency (like Crown's "Class I"), or other things
you have been looking at.

A really simple ramp generator can be made with a comparator, capacitor,
diodes and a few other parts. It's a classic relaxation oscillator. The
key to making a linear ramp is to use a transistor as a constant current
source charging the capacitor where most schematics only show a resistor.
I have made a triangle oscillator by triggering a constant current sink to
discharge the capacitor instead of simply dumping it all at once...it seems
it was only good at audio frequencies though. Depending on the comparator,
this can go to several MHz as a ramp.



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Ok, To add to that..

Keep in mind that having a little overlap in the output of the bridge
as it switches direction or even all the time maybe can be a good idea!

why you say? by maintaining a minimum current in the output of the
bridge, you can help reduce the ill effects of EMV and harmonics by
keeping a minimum DC current in the bridge.

If you were driving purely resistive loads, you wouldn't need to
worry about this how ever, most of the time this isn't the case.

This made cause the bridge to dissipate more so then with out how
ever, better to warm a little tiny bit than keep replacing the bridge.