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Class D Switching Power Amplifiers and Power D/A conversion

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Old 11th October 2011, 02:25 PM   #11
acid_k2 is offline acid_k2  Italy
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Hi,
If MOS-fets in class-D amps are heating too much, the dead-time between the two control signals is too short. It causes a "shoot-through", a rapid short-circuit of the power supply at every commutation, when both MOS-fets are (fully or partially) switched on together. This results in very short but incredibly intense current pulses through both switching devices, so MOS-fets heat up (and then they fail).
The dead-time depends on the resistor R4 (in the schematic yuo posted), so tweaking the value from 220 ohm to 440 ohm increases the dead-time, so a MOSfet is fully turned off, when the opposite start to turn on.

Have you tried a +/-56V power supply on IRF540? A bit high voltage, IMMO. IRF540 are rated 100V. With 100v parts working, at 112v rail to rail there's a good chance they will fail (and there isn't safety margin).
And protect your speaker! Normal fuses at the output, and on power rails, will save you a lot of money.
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Old 12th October 2011, 12:58 AM   #12
stewin is offline stewin  Kenya
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thanks alot acid i had been wondering and puzzled GOD bless you big time. i had tried the irf540 at that voltage it works but fails . at that voltage i normaly use irf640 they work fine but if speaker fails the protection does not kick in to save the outputfets. i am not ignorant of using fets with low rds like irfb4227 but they are not there. The only available one is fb23n20 but when i used it with R4 as 220R it heated alot when used R4 as 440R it warmed the 540 and the 640 per
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Old 12th October 2011, 01:22 AM   #13
stewin is offline stewin  Kenya
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Could i be driving the fb23n20 wrongly cause at +/-56vlts was heating alot
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Old 12th October 2011, 02:21 PM   #14
acid_k2 is offline acid_k2  Italy
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I suggest you to start reading the Application Note AN-1070 (by International Rectifier) http://www.irf.com/technical-info/appnotes/an-1070.pdf
It's a technical paper about the losses (= heat) in MOSfet in class D amps, with a lot of formulas and calculation.


IRFB23N20 is a 200V fet, but has:
- more than 2 times Rds than IRF540
- about 2 times Diode Reverse Recovery Time than IRF540
- more Total Gate Charge than IRF540
- ect.

so it's more hard to switch on and off, and you have a lot more losses than IRF540.

About your question, -/+56V amp can deliver over 300W RMS on 4 ohm load. It's a lot of power for a single speaker. But if you use a +/- 35V power supply, you can use safetly IRF540 obtaining less power, but with less problems in driving MOSfets.

About the short-circuit protection, in the schematic you posted it's based on a 0,01 ohm resistor on positive rail, but the LTV816 has a forward voltage of about 1V, so the protection start at about 100A, if I'm not wrong. Probably too much, IMMO.
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Old 12th October 2011, 03:31 PM   #15
stewin is offline stewin  Kenya
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Thanks again acid . So if 23n20 cant be used can i use irfp250 or irfp260 like in the ucd 25-1200wts i have the application notes i will try to understand them . I am very poor when it comes to calculations and understanding formulas . But thanks alot i have atleast gotten something .
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Old 12th October 2011, 05:10 PM   #16
acid_k2 is offline acid_k2  Italy
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Quote:
Originally Posted by stewin View Post
So if 23n20 cant be used can i use irfp250 or irfp260 like in the ucd 25-1200wts .
Hi stewin, don't get me wrong.
IRFB23N20 is a good MOSfet, and you can use it, and you can use IRFP250 (or others) as well. They have similar data, and you can compare them searching the datasheets on the web.
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Old 12th October 2011, 11:51 PM   #17
stewin is offline stewin  Kenya
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Thanks man . But if i have to use the above fets what do i have to adjust the 440r resistor dealing with dead time adjustment and what else ? Thanking you in advance
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Old 13th October 2011, 12:36 PM   #18
acid_k2 is offline acid_k2  Italy
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stewin, you don't do your homeworks!
The answer is on Philips document UM10155 (link at post n.6), pag. 10
Your 440 resistor adjusts dead-time, lower value decreases dead-time and vice-versa.
You have to adjust that value in your circuit, reaching a compromise between:
- too long dead time: high THD
- too short dead time: shoot-through, heat, failure.

If MOSfets are harder to drive, so the drivers need more time to turn-on and turn-off the MOSfets (charge ad discharge the gates), you need a longer dead time to avoid shoot-through.
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Old 13th October 2011, 06:43 PM   #19
stewin is offline stewin  Kenya
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Thanks acid let me go through the philips notes and try to make this circuit work with irfp250 i hope i will make the irfp250 perfom cool and with high voltage +/-90vlts like when driven with ir2110 in ucd 25-1200wts .
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Old 19th October 2011, 03:09 PM   #20
stewin is offline stewin  Kenya
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hi acid i did part of my homework and i saw that Qg is a major determinant when choosing a fet for class d especially when switching at high speed thanks to tripath. looking at various datasheet of various fets i saw that irfp350 and irfp360 were superior to irfp250 also i saw the Qg of irfp260 was too high i think it cannot be used for classd irf640 and irf530 were superior. also noted that the Qg of the fet maters more than Rds

i was wondering how can i adjust the switching frequency in this schematic? thanks to you i found out how to adjust the dead time . reason for adjusting switching frequency is that i can use irfp250 like in the ucd-25-watts-1200-watts-using-2-mosfets
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File Type: zip MOSFET selection guide.zip (223.4 KB, 125 views)
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