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Old 6th July 2013, 04:15 PM   #1
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Default Need help with Full Bridge UcD Feedback Loop

Hi Everyone,

I've started building a Full Bridge UcD based amplifier, but I'm having troubles getting the darn thing to oscillate...

I've searched the forums extensively and was able to find two useful threads, but I'm still not over the finish line.

UcD project, Subwoofer class d amp, full h-bridge, single supply +50 V

Single supply bridge UcD, referencing the signals

Control Systems Theory is definitely not my strong point so I haven't the slightest clue as to what I'm doing incorrectly.

Attached is a schematic - does anyone have any ideas?

Thanks in Advance!
Rachael
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Old 6th July 2013, 04:44 PM   #2
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Quote:
Originally Posted by Rachael Hamilton View Post
Hi Everyone,

I've started building a Full Bridge UcD based amplifier, but I'm having troubles getting the darn thing to oscillate...

I've searched the forums extensively and was able to find two useful threads, but I'm still not over the finish line.

UcD project, Subwoofer class d amp, full h-bridge, single supply +50 V

Single supply bridge UcD, referencing the signals

Control Systems Theory is definitely not my strong point so I haven't the slightest clue as to what I'm doing incorrectly.

Attached is a schematic - does anyone have any ideas?

Thanks in Advance!
Rachael

Why did you do a full bridge why didn't you start with a simple half bridge Just curious ???
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Old 6th July 2013, 05:31 PM   #3
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Reactance,

I really enjoy electronics and pushing myself to understand new things.

My first amplifier was a split supply UcD, my second amplifier was a triangle based amplifier based on the "Thunderball" design posted here:

Class-D amplifier


I wanted to try out a full-bridge UcD as I've read about the many advantages a self oscillator has over a carrier-based design.

Thanks!
Rachael
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Old 7th July 2013, 08:02 AM   #4
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At the very least you will need a 1k resistor from P2 of U8 to 0V to balance the NFB paths and to set the dc voltage at the comparator. Check also that your audio input is dc coupled, or else P3 of the LM311 will also have a dc bias problem.
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Old 7th July 2013, 01:19 PM   #5
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Quote:
Originally Posted by Rachael Hamilton View Post
Reactance,

I really enjoy electronics and pushing myself to understand new things.

My first amplifier was a split supply UcD, my second amplifier was a triangle based amplifier based on the "Thunderball" design posted here:

Class-D amplifier


I wanted to try out a full-bridge UcD as I've read about the many advantages a self oscillator has over a carrier-based design.

Thanks!
Rachael
I myself is no expert,

I admire your interest but your diagram is riddled with many many issues im not sure how you got to the calculated component values??, the missing input bias, HF compensation, Dead-time control, level shifting etc...

Here is some help. This is a rare hard to find article on the net (how self oscillating class-d amplifiers work its better than the published UCD papers as it actually explains why the lack of detail is deliberately minimalistic and actually has simulations stepping through and explaining each switching node)

To be honest no one here is going to give you a robust circuit its like a gold rush as majority of some members are trying to mine knowledge and sell kits on ebay and the likes. (with some even failing hopelessly and throwing a fit unable to solve and reach their goals )

Self Oscillating Class D
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Last edited by Reactance; 7th July 2013 at 01:32 PM.
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Old 7th July 2013, 02:18 PM   #6
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Jpg from data sheet.

With 15V supply the input threshold is 9.5V.

With 12V supply the input threshold is probably between 7v and 8V.

5V drive will probably not work.

Put some diode clamping in the input lines for protection.

Look at open collector (drain) drivers and pull-up to 12V.

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File Type: jpg IR2110.JPG (50.1 KB, 228 views)
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Old 8th July 2013, 03:42 PM   #7
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If you really want the amp to run from a single +ve rail, then using two IRS20124 gate drivers instead of the IR2110 gives you the advantage that you get programmable dead time and also standard 5v or 3.3V input logic thresholds. You really need a way to get ordinary ac-coupled inputs into your comparator stage. Using a 24V supply, your comparator inputs (if you add the 1k to ground as mentioned before) will self-bias at 0.52V if the amplifier is oscillating at idle, and you really need an input level shift stage to sum-in the nfb.
I have done this very succesfully some time ago using a PNP differential pair with a current source in the emitter line, feeding two resistive loads to 0V, with the collectors of the differential stage feeding the + and - inputs of the comparator.
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Old 10th July 2013, 01:39 AM   #8
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All of your help has been greatly appreciated. After taking the advice offered and thinking this through more clearly I have come up with the following circuit.
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Old 10th July 2013, 01:49 AM   #9
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Sorry for my previous partial post.


I've attempted to modify my circuit based on all the advice you have posted.

Right now I believe my biggest problem seems to be getting the DC bias to a level where I can couple an AC signal to the comparator.

Is the circuit I've attached going to work or am I still out of line ?

Thanks,
Rachael
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Old 10th July 2013, 02:27 AM   #10
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Change R10 to 10K so that the feedback thresholds within the range of the comparators.

Add a corresponding resistor to the other feedback path so they are symmetrical.

Then you will not need the diodes.

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