trying to build a simple Class D circuit

It doesn't matter. The threshold just sets the duty cycle, and a triangle or sawtooth will work fine to do that, as long as you don't have sync problems or anything, but for audio frequencies, I don't think that'd be a concern.

Have Fun! Rich

In theory with either a perfect triangle wave or a perfect sawtooth wave it does not matter which one you use.

With non-perfect waves there are some differences. If the slopes of the wave are not straight lines then the output of the amplifier will be distorted.

With a triangle wave, then both the rising and falling slopes need to be as straight as possible.

With a sawtooth wave, the quickness of the transition at the end of the wave reduces the effect of any non-linearity in this slope. Thus more effort can placed on producing a straight ramp.

Dan

If you're using stereo, the sawtooth will result in switching transients on both channels at the same time; I'd prefer the triangle.

More important, remember that the input currents are going to be high frequency, and an order of magnitude (or more) higher than the amplifier output currents; the power supply filtering will be a large part of your design problem.

Ok, from what I understand, I will need a sinusoidal audio input fed to the (+) side of a high-speed comparator (no ordinary op-amp), and a triangle generator (frequency at least 20x the max of the highest audio frequency, or 20x 20 kHz) connected to the (-) side of the comparator. The comparator output will be a square wave, varying from

Tough parts seem to be:

1. Feeding the resulting square wave into a half-bridge push-pull N- channel and P-channel mosfet pair, without allowing shorting of the power supply due to the mosfet requiring some time to turn off (OFF delay).

1. Activating the N-channel mosfet's gate, since it wants a higher voltage than +V.

Am I right so far?

Would (1) be partly solved by using XOR gates?

I'm looking at this:

and I'm wondering how the XOR gates can help here, since they are before the MOSFET driver itself...

Michael

the audio input should be analogue audio, needn't be sinusoidal

no, you could use a single supply and a H-bridge output stage or series capacitor

you could put an inductor in series with the supply, ot just use a mosfet driver chip....

no.

the 555 circuit looks wrong, I think trig and thrs should be connected to the wiper of R3 not the end.

I suspect the business with the xor ates was inserted to compensate for that.

two of the XOR gatres act as buffers two as inverters.

r2-c2 is the delay for 'dead time' this will put both drivers in same state for a short period.

I'd be interested in seeing a scope traces on the thrs pin of the 555.

th

nk the

Yep.

Yes, an H-bridge output stage would be nice... I do not understand though how a series capacitor will help. (Remember I am a beginner at electronics!) The analog input will be positive and negative... should I simply tie the Vs- end of the comparator to ground to get pulses between Vsupply and 0?

N-

Good idea! This H-bridge mosfet driver looks interesting... 250+ kHz switching frequencies...

Ah ok. I had a feeling a 555 wasn't a good choice anyway... not such a great comparator at high frequencies?

=C7=9D=C9=B9=E2=85=81

Greetings from the Northern Hemisphere

Michael

It's good to around 1MHz, but 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. Not 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. Second circuit:

ch

Bunch of circuits here:

It's elementary.

"Elementary Electronics" that is.

They ran an article in the mid-seventies about a Class-D amplifier using a

I can't remember the layout.

But chances are good this derives from that, if it's not the same schematic.

Michael

turns out the drawing is wrong. In his simulation he has 555 TRIG+THRS connected to R3+C1 instead of to R3+D2 and that makes a lot more sense.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or so from a 12V supply that's a real 20W continuous sinewave not some marketing hype 20W.

I can't vouch for the theoretical fidelity of this amplifier. The ramp used by comparitor is not linear and the switching frequency is modulated by the audio input, the non-linear ramp is likely to cause distortion the FM could cause problems too.

for the purists there is the 'zen' mosfet amplifier (and space heater*). said to be very good in sound quality. (*gets hot, needs a really big heatsink)

ther second circuit deals with the PWM frequency problem but not the linearity problem. this pone does that.

substitute it for the 555 part of the power amplifier.

Still there's probably going to problems with noise from the powersupply etc... these circuits are good as a learning exercise but in the real world they have problems.

It turns sout his schematic was wrong.

not such

posed

TC4451? Which circuit is this in?

Good outlook on life!

"I remember reading in Harpers magazine (at least I believe it was Harpers) a list of the top problems facing high schools back in the '50s. At the top of the list were stomach-wrenching problems that not even the world's most capable community organizer could hope to solve, such as chewing gum and being in the hallways without a pass. Today=E2=80= =99s list boasts much more tractable problems, as befits the hugely greater investment we make in education today and the '50s lack of a United States Department of Education (created in 1979), problems such as murder, rape, arson, drugs, illiteracy=E2=80=A6 Ah, indeed, those days of o= ld when tubes glowed bold."

Class A as a space-heater... ah, now I get it! Perfect for Russia or Norway I guess, but it gets HOT in my neck of the woods... was in the

sy-with-555-timer-ic/

al

=C7=9D=C9=B9=E2=85=81

The one with the XOR gate. this is the corrected version:

The author's discussion here.

Ah! I see you posted to his blog and he corrected it the next day.

Thanks!

Michael

Say... is it normal to run the TC4451 mosfet driver without any mosfets to drive?

Here's a datasheet:

It appears to have the following specs: Continuous output current =3D 2.6A (not sure why this is in the "Min" column on page 3... typo?)

Just wondering where you got the 20W continuous power range from.

Thanks,

Michael

version:

here.http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/

About as normal as using a 555 for an audio amplifier. (in other words, "not really") Whilst TC4451s are not designed to drive loudspeakers they can do it.

my estimate of 20W might actually be a bit on the high side,

t would

and

or

=C7=9D=C9=B9=E2=85=81

Ok I understand now. 2.6 A x 12V / sqrt(2) ~ 20A.

Thanks for all your help!

Michael

su=

supposed

circuit:

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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