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Class D Switching Power Amplifiers and Power D/A conversion

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Old 22nd October 2016, 12:00 PM   #1
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Default Switching Range

Here is a sticky for us. Switching range is a measure for, how much Class-D a Class-D amplifier really is.

Class-D is Class-B with Pulse Width Modulation. Pulse width shall go with signal. Because actors are inert, minimum pulse width and so minimum PWM-transferred amplitude are limited. Between lo and hi signal limit lies Switching Range Fswitching (F for factor), which can be calculated as the inverse of the product of 2Pi, power actor time constant (which is time for output power to reach half of full power) and pulse width modulation frequency. With a modulation frequency of 300KHz and a power actor time constant of 5ns we obtain Fswitching=40dB.

Below switching range, the amplifier must try to work in unmodulated fashion. Actors tend to ignore small inputs, hence crossover distortion appears. But we can feed back. As loop gain at fmodulation is -1 (oscillation), feedback cannot be stronger than fmodulation divided by fsignal. We set closed loop gain to 1 and obtain a feedback range Ffeedback of 40dB for a signal of 3KHz.

Intermediate dynamic range Fintermediate is Fswitching+Ffeedback=80dB. Ncore is optimized for this.

Another means is idle current, size of which should be somewhere between signal currents at lo limits of Fintermediate and Fswitching. With a maximum current of 20A, we need an idle current inbetween 2mA to 200mA. Abletec does it like this.

An alternative is feedforward. Adding a lo voltage amplifier, which outputs virtual ground, on which the error of the Class-D one appears, we can also overcome distortion. Devialet use feedforward.

I hope, that this helps.

Last edited by Grasso789; 23rd October 2016 at 04:56 AM. Reason: Modifications of scope
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Old 22nd October 2016, 02:49 PM   #2
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Are you trying to tell us that you can detect crossover distortion at 400kHz+?
Class D is on or off. That is it is operating in class C. Class C is off until switched on. The crossover distortion you speak about is not distortion, it is dead time that is needed to avoid both output FETs to be on at the same time by allowing time for the Gate - Drain capacitances to discharge.
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Old 22nd October 2016, 03:09 PM   #3
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It is crossover distortion at 4KHz and below, what bothers me -- theoretically, as i have not conscientously heard it in a Class-D amplifier.

Class C is single-ended, used with narrow bandpasses for producing a single radio frequency. Class-B is symmetrical, say push-pull, as needed for at least moderately efficient generation of hi bandwidth signals.

Dead Time is practically the opposite of idle current. If one analyses good Class-D circuits of the feedforward-less flavour, one will see that they do not have any Dead Time, but rather both power actors do conduct a bit at the same time.

Last edited by Grasso789; 22nd October 2016 at 03:13 PM.
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Old 22nd October 2016, 03:42 PM   #4
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Quote:
Originally Posted by Grasso789 View Post

...

If one analyses good Class-D circuits of the feedforward-less flavour, one will see that they do not have any Dead Time, but rather both power actors do conduct a bit at the same time.
A large (although narrow) current spike?

Sounds inefficient.

Any examples we could look at?
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Old 22nd October 2016, 06:19 PM   #5
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Voltage at knot between both power actors does not equal current running thru them: In idle state, voltage shows a symmetrical full swing square, but current is few. Yet current or more precisely pulse length of current is, what determines output power.

Last edited by Grasso789; 22nd October 2016 at 06:23 PM.
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Old 22nd October 2016, 07:06 PM   #6
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Quote:
A large (although narrow) current spike?
I wrote "a bit", not "large". If power actor gates are fed by the raw (only Ugs- and Ig-limited) PWM signal, and some idle current runs thru them, then there is no Dead Time.
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Old 22nd October 2016, 11:57 PM   #7
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So for "idle current" do you mean the 1-100uA of D-S leakage or some larger value as a result of the device not being fully turned off?
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Old 23rd October 2016, 01:52 AM   #8
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The latter. Below switching range, output stages of Class-D and Class-A/B work the same way. In fact, one can run any Class-A/B amplifier as Class-D: Just add a PWModulator frontend, a lowpass backend and modify feedback. Depending on fmodulation it may run hot already in idle state, because the output stage is not up to the task, but there is no principal difference.
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Old 23rd October 2016, 07:50 AM   #9
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The whole thread is total junk. For "switching range" Grasso gives different definitions every time (in different topics and forums), but none of the definitions are correct and disambigous. At this time his mistic "power actor time constant" become "time for output power to reach half of full power". This is alone a complete mess, since the output power changing speed of a ClassD amp is not related to the switching device, but the filter. So "output power" must be a mistake. But what power could he really meant? I dont know, but very probably he dont know either. An other problem is in the time, since he only mentioned 1 point, the end, but what about the start?

There are many more similarly false and/or meaningless statements and bad terminologies in this topic, but correcting or falsifying them needs much time (simply because they are soo many), and I see zero valuable information here, so my advice is to close or abandon this mentally harmful thread, not to infect our minds.
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Old 23rd October 2016, 02:38 PM   #10
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Quote:
Originally Posted by DUG View Post
So for "idle current" do you mean the 1-100uA of D-S leakage or some larger value as a result of the device not being fully turned off?
Quote:
Originally Posted by Grasso789 View Post
The latter....
So if you don't turn the device fully off intentionally is it still considered Class D?

Wouldn't it be class A (by definition)?

Running a class A amplifier as a PWM source seems inefficient to me.

Any examples we could look at?
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Last edited by DUG; 23rd October 2016 at 02:48 PM.
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