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Class D Switching Power Amplifiers and Power D/A conversion

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Old 1st December 2012, 05:06 PM   #91
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Quote:
Originally Posted by nigelwright7557 View Post
IR do a 1.7KW amplifier project using IRs2092 and gate buffers to up the gate drive.
Hi Nigel,
thanks for pointing to IRaud9.
Nevertheless I am of a similar opinion like workhorse.
- Pre filter feedback
- Triple paralleled MosFets
- Pure marketing numbers regarding power. They specify 1.7kW from +/-75V rails, which is poddible only with a perfectly regulated PSU - no sagging. And even then they have to go into clipping to achieve the power.

...hoping that I will not experience bad drawbacks during the real build and end up similar as IR...
Up to now I am heading for higher performance (faster and mostly load independend step response) and realistic power spec.
I am targeting 1kW/2R or bridged 2kW/4 from +/-80V rails, this allows to derive the specified power with realistic rail sagging.
Theoretically with hypex supplies it should end up in 1200W/2R or 2400W/4R bridged.
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Old 3rd December 2012, 07:13 AM   #92
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Quote:
Originally Posted by Workhorse View Post
Brilliant, the threshold could be set very easily thru symmetric current sources/mirrors.
Here is a TINA SPICE sim of an opamp with symmetric voltage controlled current sources. The Zener diodes clamp the VCVS so they only change the output current when the output signal approaches the rail. With suitable component values, something similar to this might be able to provide the upper and lower threshold voltages for my 'Window Comparator plus S-R latch' acting as the PWM modulator.
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File Type: jpg OUTPUT TO RAIL LEVEL.JPG (33.7 KB, 496 views)
File Type: jpg Output to rail transient.jpg (49.9 KB, 489 views)
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Old 3rd December 2012, 08:03 AM   #93
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The op-amp is only there to provide a suitable sine-wave signal to demonstrate the concept, but I think that a similar pair of voltage controlled current sources could monitor the output of a class-D amp to provide the threshold voltages for the comparator.
If the window comparator technique can work with only changing one of the threshold voltages at a time (according to if the output waveform was approaching the +ve or -ve rail), then the voltage outputs from my simple circuit above might be able to be used directly.
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Old 3rd December 2012, 04:19 PM   #94
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Originally Posted by Ouroboros View Post
.... but I think that a similar pair of voltage controlled current sources could monitor the output of a class-D amp to provide the threshold voltages for the comparator.
Nice

... have a look to Q9 and Q10 in posting #3.
They form a pair of current sources, which are monitoring the output of the class D amp.
Main difference is that I had chosen to run the BJTs in common base connection, because I do not need high gain in these controlled current sources, but appreciate linearity and speed.... and for my method of directly controlling the modulator thresholds with Q7 and Q8 also the signal polarity was fortunate.
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Old 4th December 2012, 07:31 AM   #95
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Quote:
Originally Posted by Ouroboros View Post

Quote:
Originally Posted by Workhorse View Post
Brilliant, the threshold could be set very easily thru symmetric current sources/mirrors.
Here is a TINA SPICE sim of an opamp with symmetric voltage controlled current sources. The Zener diodes clamp the VCVS so they only change the output current when the output signal approaches the rail. With suitable component values, something similar to this might be able to provide the upper and lower threshold voltages for my 'Window Comparator plus S-R latch' acting as the PWM modulator.
Exactly....
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Old 4th December 2012, 07:33 AM   #96
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Quote:
Originally Posted by ChocoHolic View Post
Nice

... have a look to Q9 and Q10 in posting #3.
They form a pair of current sources, which are monitoring the output of the class D amp.
Main difference is that I had chosen to run the BJTs in common base connection, because I do not need high gain in these controlled current sources, but appreciate linearity and speed.... and for my method of directly controlling the modulator thresholds with Q7 and Q8 also the signal polarity was fortunate.
Yup, could be done with CB//FC also.
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Old 4th December 2012, 06:01 PM   #97
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For PCB layout I am using KiCad.
Attached the schematics, with the status of today.
Typically they will evolve a little bit further during layout work.
...and of course later again, when reality strikes back..

Besides the protections of the IRS, the design contains overvoltage protection and overtemperature protection.
Speaker protection is not included. There are tons of circuits around and personal taste is pretty different (traditional relay vs MosFet relay..).

Discussion on schematic is opened...
Attached Files
File Type: pdf SystemD_2kW_Supply.pdf (66.8 KB, 326 views)
File Type: pdf SystemD_2kW_Power.pdf (115.5 KB, 368 views)
File Type: pdf SystemD_2kW_Modulator.pdf (92.9 KB, 344 views)
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Old 4th December 2012, 06:48 PM   #98
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Hello choco, I can see that you are using IRS20957. What is the advantage over IRS2011S? Are you sure that you do not want to use blocking diodes for the IRFP4668 stage? I think UCD2K has blocking diodes....
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Old 4th December 2012, 08:04 PM   #99
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Hi Tom,
I see four advantages of the IRS20957 vs IRS2011:

-Floating input stage, which allows direct drive from the comparator without level shifter inbetween.
-No need of inverted drive signals
-Convinient dead time adjustment
-Overcurrent sensing

But there are also downsides:
-Imperfections in the layout and snubbering can easily cause oversensitive shut downs (...let's hope that I'll get that right also for the fat TO-247 and the high power...)
-Range of possible dead time is not really wide


Blocking diodes:
You mean a shottky in series with the MosFet (for blocking currents through the body diode) plus a fast freewheeling diode across MosFet+schottky?
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Old 4th December 2012, 08:14 PM   #100
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"You mean a shottky in series with the MosFet (for blocking currents through the body diode) plus a fast freewheeling diode across MosFet+schottky?"

Exactly.
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