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Class D Switching Power Amplifiers and Power D/A conversion

Using Class D for tracking power supply?
Using Class D for tracking power supply?
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Old 14th June 2018, 02:43 PM   #1
hellokitty123 is offline hellokitty123  United States
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Default Using Class D for tracking power supply?

There's a Class A amplifier I have a strong attraction to and I want to use it as a 50W amplifier but I will end up dissipating crazy amounts of heat at the idle current required. I was thinking the only way to keep the dissipation low is to have the power supply track the output signal using a class D amplifier that has a DC offset on it. I have never built a SMPS or class D amplifier, I need a bit of hand holding. For starters is class D properly suited to this task because as far as I know it has rather significant phase shifts at the output. Any thoughts? This is really the only way I am going to be able to use my Class A amplifier so I must solve this design problem.
 
Old 15th June 2018, 10:47 AM   #2
DF96 is offline DF96  England
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You will need to ensure that your Class A amp has excellent PSRR if you intend running it with a modulated PSU. If you can design a good enough Class D to do the job then maybe you don't need the Class A at all?
 
Old 15th June 2018, 11:23 AM   #3
hellokitty123 is offline hellokitty123  United States
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Wait, why would PSRR matter? The class D "power supply" will be following the output signal so it will effectively be acting as a signal cascode to the output devices so they only see DC voltage across their terminals.

Actually what I would do is not use the class D amp directly to power the amp, I would float a linear regulator within the modulated supply between the class D power supply and the amplifier so I'm not at the mercy of the "regulation" of the class D supply. This way I can keep the output devices @ only a few volts DC while allowing for huge voltage swings and high idle current.

Quote:
then maybe you don't need the Class A at all?
I don't like the sonic performance of class D amps I doubt I can make it sound as good as a class A, especially the one that I have. The output phase shifts of class D make it a real pain to correct the flaws in class D.

Last edited by hellokitty123; 15th June 2018 at 11:26 AM.
 
Old 15th June 2018, 12:03 PM   #4
DF96 is offline DF96  England
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The Class D amp either correctly follows the signal or it doesn't. If it does then it is good enough to be the main amp. If it doesn't then the Class A amp will see a distorted signal on its supply rail so it will need excellent PSRR. In any case, many Class A amps will have their gain varied by the supply rail voltage so at the very least you risk having significant second-order distortion.

If you don't like Class D then you can't use Class D in this way.
 
Old 15th June 2018, 12:17 PM   #5
dmills is offline dmills  United Kingdom
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You can get most of the goodness with a class H design, and if the output stage is good enough for that, it will be good enough for a class D envelope tracker.

Note that the risetime on the class D output will be important, this needs to ramp up quickly enough to avoid running out of headroom on a transient, possibly easier if you have a little delay in play.

One neat approach is to have a linear regulator supplying the power via a current sense resistor and then rig up a current mode switcher to servo the current in that shunt to zero, all the speed of the linear pass device, with most of the efficiency of a switcher, servo the linear reg to maintain a few volts across the output device.

You might find some of the discussion of "Slow drain modulation" from the RF power world of interest.

Regards, Dan.
 
Old 15th June 2018, 12:23 PM   #6
hellokitty123 is offline hellokitty123  United States
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Quote:
Originally Posted by DF96 View Post
The Class D amp either correctly follows the signal or it doesn't. If it does then it is good enough to be the main amp. If it doesn't then the Class A amp will see a distorted signal on its supply rail so it will need excellent PSRR. In any case, many Class A amps will have their gain varied by the supply rail voltage so at the very least you risk having significant second-order distortion.

If you don't like Class D then you can't use Class D in this way.
You make a good point. I guess I could wrap the amp in a voltage cascode of its own as a layer of protection against the class D imperfections. Spice is showing that this provides extremely high psrr. It would double the power dissipation in the output stage but it would still be far lower dissipation than it would be otherwise.

So I guess that problem is solved. How extensive are the phase shift issues with class D?

Quote:
Originally Posted by dmills View Post
You can get most of the goodness with a class H design, and if the output stage is good enough for that, it will be good enough for a class D envelope tracker.

Note that the risetime on the class D output will be important, this needs to ramp up quickly enough to avoid running out of headroom on a transient, possibly easier if you have a little delay in play.

One neat approach is to have a linear regulator supplying the power via a current sense resistor and then rig up a current mode switcher to servo the current in that shunt to zero, all the speed of the linear pass device, with most of the efficiency of a switcher, servo the linear reg to maintain a few volts across the output device.

You might find some of the discussion of "Slow drain modulation" from the RF power world of interest.

Regards, Dan.
Hmmm. I need a bit to go over what you've said.

Last edited by hellokitty123; 15th June 2018 at 12:26 PM.
 
Old 15th June 2018, 12:36 PM   #7
hellokitty123 is offline hellokitty123  United States
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I actually didn't know about class H until you mentioned it. I'm not sure I like the transient switching aspect. Seems like it could cause potential distortion.

I don't quite catch your meaning with your linear reg suggestion. Do you mind elaborating?
 
Old 15th June 2018, 01:21 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by hellokitty123
How extensive are the phase shift issues with class D?
What phase shift issues? I am no expert on Class D, so you tell me. Of course you need an output filter and that will add phase shift but it should have only a small effect in the audio band.
 
Old 15th June 2018, 01:47 PM   #9
hellokitty123 is offline hellokitty123  United States
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I've read that class D can have as high as 90 degree phase shift @ 20khz. This would not fly in a tracking power supply. But like you I am far from an expert on class D. I've done almost nothing with switching designs and I typically avoid filters like the plague.
 
Old 22nd June 2018, 03:25 AM   #10
hellokitty123 is offline hellokitty123  United States
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Anyone?
 

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