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Old 12th October 2011, 10:02 PM   #1
Derfpa is offline Derfpa  Sweden
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Default Educational class-D circuit

Hi folks!
I've been around this forum for a while now and just observing various circuits. But frankly I had lack of knowledge to understand many of the circuits completely until quite recently.

I recently begun my second year of my masters degree at Lund University where my main subject is electronics. And from what I've learned so far have made me eager to take the step into elaborate with amplifiers! I've always found class-d amplifiers especially interesting manly because the high efficiency but also the radically different design towards other amplifier classes.

So, since I'm quite new to laborating with electronics in general but is willing to overcome potential obstacles i want to create a straight forward amplifier that have many elements that you can find in theese kind of amplifiers. I thought that my amplifier will contain the following elements:

Triangle wave generator with comparator to create a PWM-signal.

Some kind of mosfet-driver to drive mosfets. (Sure, theres existing IC-s for this, but wheres the fun with that?) Maybe use the risetime of the comparator + schmitt trigger/another comparator to create a tiny delay to avoid momentary shortcut trough mosfets?

Mosfet in first step, halvbridge design, but later in full bridge design.

No feedback since i prefer to run on stabilized dual rail power supply. (Got two Xbox 360 powersupplies laying around. )

So what i ask for is resources of where i can find inspiration of creating these kind of elements or suggestions from you. Maybe there's something more i should add eventually to turn the educational level up a notch? The purpose is mainly not to sound great, but rather to overcome obstacles that should be overcomed to get a good result.

Last night i elaborated with:
Triangle Waveform Generator circuit with 555
and a LM311 (comparator) and created a PWM-module. Although I'm not very happy with the shape of the triangle wave shape and the rise time of the comparator weren't very satisfying either. One problem is that I currently lack of components, but there's a package soon arriving in my mailbox that will take care of that.

So first things first, what do you recommend me to use for my PWM-module? Any good triangle wave generators? Alternative comparators, or LM311 is maybe the best for this purpose? All inputs i can get is of great value!

(If you managed to read all the way down here you deserve yourself a pat on your shoulder!)
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Old 12th October 2011, 10:12 PM   #2
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Hi and welcome to diyAudio It sounds like you are hitting the road running I'm afraid I can't answer as I have no idea about class D amps but I'm sure plenty of people here who do!

Any intelligence I may appear to have is purely artificial!
Some of my photos
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Old 12th October 2011, 10:25 PM   #3
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I have built a very similar class d amp just for the exercise.

I used a triangle wave generator using a couple of op amps and then the audio in and triangle wave into a comparator. The output of the comparator was level translated for a IR2113 MOSFET driver.

I didnt use feedback but the supply wasnt regulated and so got a bit of hum.
I managed to minimise the hum with a DC offset pot on the input to the triangle generator.

It sounded OK.

The problems come with this type of circuit when you overload them and the MOSFET upper supply can discharge and the mosfet ends up in a linear region and burns out. So long as the amp isnt abused it works fine.

In the end I opted for an IRS2092 design although this was fussy about layout, decoupling and the output inductor. I ended up with a t106-2 inductor core and that worked fine.
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Old 12th October 2011, 11:23 PM   #4
Derfpa is offline Derfpa  Sweden
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Tony, well... I've been thinking about these kind of amplifiers for a few years now so i guess now is the time

Nigel, what do you mean with linear region? You mean if the charge/discharge is too slow and the mosfet is not switching?
I actually tried with a triangle wave generator from two opamps, but the wave looked awful at frequencies at about 20kHz and higher. :/ The 555 seems more stable at higher frequencies, so maybe i should experiment more with that one.
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Old 12th October 2011, 11:55 PM   #5
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If the upper mosfet power supply droops then the voltage into the mosfet wont be a full one or a full zero meaning the mosfet will sink a lot of power and probably fry.

I had similar problems with opamps and high frequencies, I also had trouble with some op amps inverting on higher level signals. I used a tl082 in the end and that was OK but struggled a little at the tips of the triangle wave.
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Old 13th October 2011, 12:24 AM   #6
jcx is offline jcx  United States
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reading everything you can find by Bruno Putzeys would be a good education in Class D and contemporary modulation schemes
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Old 13th October 2011, 11:35 AM   #7
Derfpa is offline Derfpa  Sweden
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Wow, I'm very amazed by Bruno Putzeys! Ive read some articles about him until now and ill definitely read more!

Nigel, if I use a stabilized power source, is that really a problem? Maybe I don't understand you completely :/
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Old 14th October 2011, 02:59 PM   #8
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The problem with the top MOSFET gate drive is that (if both of the half-bridge MOSFETs are N-ch) the gate drive voltage is of course with respect to the source of the device, so the gate and the source of the top MOSFET are swinging up and down by almost the full supply voltage at the switching frequency. Although you can make discrete drivers to allow for this, I find it is a lot better to use the IRS20124 (for single-supply applications) or the IRS20955 or 20957 (for dual symetrical supply rails).
The section of the gate driver which drives the gate of the top MOSFET has a 'bootstrapped' supply rail, using a charge pump driven from the switching output of the amp. I remember that International Rectifier had some app notes which gave a lot of useful information about this.
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Old 15th October 2011, 03:29 PM   #9
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Hi Derfpa,
a good starting point for a discrete solution is given in a Philips application note - even if you do not want the UcD principle.

You can use the entire section of the discrete level shifter, gate drive and power stage for inspiration.

It should be well possible to combine it with a triangle modulator.
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Old 16th October 2011, 05:17 PM   #10
Derfpa is offline Derfpa  Sweden
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As you say Ouroboros, its pretty essential to have some dead time if high voltages/high switching frequency is used.
My idea was to use the rise time of the comparator that gives the PWM-signal and use the risetime (maybe increase the risetime with a a capacitor?) and use a schmitt trigger to shorten the actual "on" time.

The thing is that there's much things going on inside the mosfetdriver that i dont know about/understand. The idea is to understand exactly whats going on, all tough the result would probably suffer.
I guess the pdf from International Rectifier is this?:
Maybe i should put my effort in understanding the driver instead which would allow me to use it.

ChocoHolic, since i started to look at materials from Bruno Putzeys i accepted that self oscillating amps might actually be a good idea. Even though its something id like to learn more about its more of a later project.
I actually stumbled upon the same pdf and as you say, its definitely worth the effort to dissect and use parts of it.

Now, i have two questions in my mind:
How much would the shape of the trianglewave affect the PWM? Its hard to get it linear, so is it really worth the effort to make it as good looking as possible? I mean, ill something like this:
make a bad PWM?

And, how much would the result differ in hooking stuff up on a bread bread board, soldering to a protoboard and soldering smd-components to a etched pcb?
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