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Old 5th February 2008, 11:47 AM   #1
ranga is offline ranga  Romania
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Question Bulding My first class D amp.

Hello forum users! I am quite new to this forum but not that new to electronics. During high school the electronics microbe infected me. Since then (i am 26 now) i have made a lot of diy.

This how ever is my first Class D amplifier project! I have chosen this schematics, found in attach. I can not relocate the url

So, first of all the components i am going to use are: lm311 as comparator and LF356M as OpAmp. This is not because i think it is better but because these are the ones i have.
I have also found a mistake in the Integrator schematics: pin 3 of IC2A goes to gnd. and the entire net that connects to it.
I am going to use smd tehnology: 0603 for resistors and 1206 for capacitors in the input stages and normal components in rest. I have designed a pcb (not final version yet).
Please have a look and if any-one spots a mistake please comment. Also, if this schematic has been commented here can someone point me to, because i did not found anything for myself
Thank-you.
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File Type: zip scheme.zip (38.5 KB, 427 views)
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Old 5th February 2008, 12:26 PM   #2
ranga is offline ranga  Romania
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Default the pcb

I can not attach all the pcb layouts so i am using a hosting:
here it is:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

I have a splash spill on both sides that represents the gnd but i did not show it in the pictures. The gnd rail is highlighted on the copper side in some of the shots.
If any body would like to have the kicad .brd and .sch please contact me!
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Old 5th February 2008, 03:52 PM   #3
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You should read this first: http://www.irf.com/technical-info/appnotes/an-1135.pdf
Current loop is large enough to radiate a lot of EMI; decoupling is also very poor...
About schematic: try to add a 10u tantal bootstrap capacitor. Feedback before LC is old design

Nice try for first time class D...
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Old 6th February 2008, 11:32 AM   #4
ranga is offline ranga  Romania
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Default Thank you

I have read the pdf! Very good info in it.

All capacitors that are not smd or electrolytic are tantalum capacitors. The driver schematics is the one recommended by International Rectifiers for IR2110. Old design may it be but i would like to go with it because it is my first class D and so it is more to learn out of than to actually use it.
The trouble to change the schematics is this one: I have already purchase all the components (resistors, capacitors, semiconductors... even a 3kva toroid transformer and the electrolytic capacitor for the power supply (about 40 x 330uF/200v)) and i have to go around with them.

I am currently working to minimize the high current loops on the pcb. I don't think that i will be able to do much good in this section except to try to isolate them from the rest of the tracks. Updated pcb coming up soon...
One question though... i am missing only few components :the 74ac04 in smd package and the toroid core for the output inductor. Can i use yellow toroids for it, cause i have a lot of those (pwm frequency is 400-500 kHz) ? I saw the post's made by Eva regarding this topic but i am sure I didn't saw: "Don't use yellow cores for Class D output filter" anywhere. I also have a lot of "E+I" ferrite cores used in computer power supply, but i reckon that they are worse.
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Old 6th February 2008, 12:19 PM   #5
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You can use 3f3 gapped toroidal core (by hand) - white (tips: search metrafo dot ro), or you can order a samples from micrometals.com (like i did) - T200-2 red/clear.
Take care at max voltage of your fet's. Read on this forum why bootstrap technique is causing trouble at clip condition. You should note that this design is not very scalable (like you want to try).
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Old 6th February 2008, 12:30 PM   #6
Eva is offline Eva  Spain
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I'm sorry to say this but you have very much to improve in that layout. Ordering this PCB as shown would be a waste of money.

You can attempt to use yellow/white cores (micrometals #26 iron powder and equivalents) but core losses are likely to be too high resulting in a "burning hot" working temperature and very short life of the core and anything around it. This is unless you use a really big amount of turns to keep peak magnetic flux low, but then you are likely to end up with too high an inductance for full range usage. For a first try you can use the #26 cores without problem (as long as you turn power off before they melt )

Search a bit the forum and you will find info on inductor flux, turns, saturation and losses calculations, and on web sources for cores suitable for class D.
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Old 6th February 2008, 01:03 PM   #7
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Quote:
Originally posted by Eva
I'm sorry to say this but you have very much to improve in that layout. Ordering this PCB as shown would be a waste of money.

You can attempt to use yellow/white cores (micrometals #26 iron powder and equivalents) but core losses are likely to be too high resulting in a "burning hot" working temperature and very short life of the core and anything around it. This is unless you use a really big amount of turns to keep peak magnetic flux low, but then you are likely to end up with too high an inductance for full range usage. For a first try you can use the #26 cores without problem (as long as you turn power off before they melt )

Search a bit the forum and you will find info on inductor flux, turns, saturation and losses calculations, and on web sources for cores suitable for class D.
eva,

what's the difference with yellow/white cores and light green/yellow cores? from what I can tell, they are also powdered metal. and while we're at it, what about light green/red? I have lots of those over here.
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Old 6th February 2008, 01:20 PM   #8
Eva is offline Eva  Spain
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Go to www.micrometals.com to the materials tab and find out
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Old 6th February 2008, 06:03 PM   #9
djQUAN is offline djQUAN  Philippines
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found it.

thanks.
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Old 6th February 2008, 06:44 PM   #10
ranga is offline ranga  Romania
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http://i32.tinypic.com/21jx3zb.png
http://i30.tinypic.com/30kwwus.png

I have shortened the high current loops, and by moving the components closer together i think i can shorten them some more. I placed the capacitors as close as possible to the power MOSFETs . I have separated the high current section from the low current section as best as i could. I still have some limitations in design because i have to keep room for heat radiators and IR made their mos-fet with the G terminal so close to the D and S terminal

Seriously now: i will try to improve the layout but i am not sure how much will be possible considering the limitations that i have(the shear size of the electrolytic capacitors rated at 200V for example) . I will separate the low current zone by making a screen out of aluminum or copper foil that i will ground. I also made a star like ground for low level current side and another for the high. i will connect these 2 in a single point at the power supply. Also i will splash the gnd and -Vcc as described in the pdf posted upper. Also studying the bootstrap 10uF capacitor suggestion right now.

@ eva: I am not ordering anything. I am making it using photo method. Drill it with my CNC and manually construct vias.

Regarding the inductor core... i will make tests using what i have and supply with +/- 25 V on the rails. After that i will order some high quality cores. If i finish before this weekend the design part than on Monday i will have the pcb corroded and some stages already on it.
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