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Old 10th February 2009, 01:31 PM   #1
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Default New to Class D design, please help!

Hi everyone, I am new to the forum and I am trying to build a class D amplifier for a school project. I plan on using this chip from IR, the IRS2092.

I have a very good understanding of electronics, but I am an audio newbie. I feel confident that I can get this amplifier working, but I am having trouble finding a good starting point.

The specs of my amplifier will be:

half bridge, 2x50W into 2 4ohm load stereo
+/- 35V rail
Through hole design

I am using the 2092 chip and the recommended IRFI4212 dual MOSFET. I have been looking at the app notes and reference designs from IR, specifically the IRAUDAMP7. https://ec.irf.com/v6/en/US/adirect/...tID=IRAUDAMP7D

I like this chip because it is self oscillating and has self PWM, but I am a beginner and I am open to suggestions for any design change. My professor recommended this chip to me, but warned me that it is very delicate and can be somewhat hard to get working. Are there any better chips out there for a beginner to use, that would be more user friendly? I also plan on designing a PCB for this amplifier.

Does anyone have any extra tips or tricks in setting this amplifier up for a breadboard/PCB? I heard the current limiting needs to be turned off in order to get this amplifier to work....is there anything else?

I am stumped on where to begin with this amplifier, because out of all the resources that IR has for this chip, the recommended circuit varies greatly between them. I cannot find an explanation for some of the components, and how the values were calculated.

Any input would be greatly appreciated, thanks
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Old 11th February 2009, 01:52 AM   #2
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Quote:
Originally posted by Workhorse
Isn't your professor a little dumb item??

If he knows that the chip is delicate to handle for a newbee like ya, then why did he recommended it and that too without burning his own hands on it prior giving you this project.

They like to "challenge" us

I think my biggest problem with this chip is the typical application circuit shown on the 2092 datasheet has a few/lacks a few components compared to their reference design....and my logic is that if the reference design is in production (IRAUDAMP7) then it should work, whereas the typical operating circuit is theory... is this correct?

it also seems that the formulas given to calculate values in the app note and datasheet do not exactly match up with the reference design, i.e. they say to provide at least 0.5 mA of current on the Ref pin, and the reference design only provides 0.3mA, etc

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so would it be a better idea to go off the typical application or the reference design?
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Old 11th February 2009, 10:49 PM   #3
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The main problem i'm having is figuring out the VCC referenced to -B. in the iraudamp7 schematic, they show resistors R114A and R115A connected to ground, followed by a 10 ohm resistor. how does this create 12v referenced to -B?
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Old 11th February 2009, 11:03 PM   #4
DcibeL is offline DcibeL  Canada
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Quote:
Originally posted by cmhadley
The main problem i'm having is figuring out the VCC referenced to -B. in the iraudamp7 schematic, they show resistors R114A and R115A connected to ground, followed by a 10 ohm resistor. how does this create 12v referenced to -B?
Is that all? Did you not see the transistor and 15V Zener? It's a simple series regulator.
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Old 12th February 2009, 04:15 AM   #5
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Quote:
Originally posted by DcibeL

Is that all? Did you not see the transistor and 15V Zener? It's a simple series regulator.

the -B reference is throwing me off....

the "ground" is actually +35V, ...so the zener regulates the voltage to 15 volts, referenced to -B...

then the Vbe of the transistor is probably 0.7 volts...so the VCC would be 14.3ish V referenced to -B? Sorry if this is a really stupid question, but I have no experience with a negative reference ground, and I don't want to mess up the chip on power up.

and the resistors, R114 sets the current level and is used to calculate R115....

is this correct? Thanks
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Old 12th February 2009, 03:39 PM   #6
DcibeL is offline DcibeL  Canada
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Quote:
Originally posted by cmhadley
the -B reference is throwing me off....

the "ground" is actually +35V, ...so the zener regulates the voltage to 15 volts, referenced to -B...
Well...you have +/- 35V as +B and -B supplies. The ground is 0V, or +35V from -B.

When voltages are negative current flows the other way...

Quote:
Originally posted by cmhadley
I have a very good understanding of electronics
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Old 13th February 2009, 09:48 AM   #7
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I got the supply working, and I breadboarded one channel of the amp. I am going to try it later today.

I am not a complete idiot, sorry if I have come across as one. I have not worked with power electronics in a couple of years, so I apologize if there are a few things I don't remember how to do. I have just been very stressed out about this project and I'm just trying to make sure I do everything correctly and understand every single bit before I turn it on.

From the research I have done, this chip seems very robust and has very nice included features. Unfortunately, because it has everything built in, it needs to be configured just right or stuff will not work/blow up. This chip is all or nothing, which is good or bad. This was the main problem I was having while comparing schematics.

I have noticed that some designs use different methods for LPF, bootstrapping, current limiting, and voltage biasing on pins. This is what I was having a hard time understanding...as in which method is best.

I am not developing a commercial product, nor do I need this amplifier to be the best one ever built. I just need it to show functionality and be reliable. That was the main purpose of my thread.

I appreciate the suggestions/answers in the replies. There are many knowledgeable people on this forum and I have learned a lot from the short time I've been here.
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Old 13th February 2009, 12:31 PM   #8
acid_k2 is offline acid_k2  Italy
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@Workhorse: I agree with you with almost all you write in this thread, but (little critique) your posts didn't give any help.

@cmhadley
R114 sets the current level, and help lowering the voltage at TIP31C collector.
R115 is the bias resistor for zener, it keep some current flowing through the zener diode, giving better voltage stability.

The regulated voltage (about 14V at emitter of TIP31C, referred to B-) goes to:
- pin 12 (Vcc)
- through R20A and D3A diode, to the bootstrap cap CP6A, and so it gives a FLOATING power supply (cap charge) to the upper mosfet driver inside the chip (pin 15)
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Old 13th February 2009, 03:03 PM   #9
DcibeL is offline DcibeL  Canada
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Hi cmhadley,
Sorry if I came a cross as rude, but usually when I see post from college students who want "help" with their projects, the reality is that they just want someone to do their homework for them.

There is a lot to know about class-D amplifier design, a lot of it goes over my head even. However, in a school project the objective is usually not to make the best performing design, but just to show proof of concept that the ideas work in a real world situation.
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Old 13th February 2009, 03:25 PM   #10
Eva is offline Eva  Spain
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If you really have some experience, understanding an auxiliary 15V rail referenced to -B should not be a problem at all. Part of the circuit uses 0V as ground, and part of the circuit uses -B as ground. There is a linear regulator fed from 0V, grounded to -B and producing 15V relative to -B.
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