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Class D Switching Power Amplifiers and Power D/A conversion

Ebay cheap TDA7498 boards
Ebay cheap TDA7498 boards
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Old 2nd January 2018, 07:31 PM   #451
chermann is offline chermann  Austria
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Quote:
Originally Posted by r_kalar_2 View Post
Chermann, I think that the diode is used to separate the input ground from the power supply ground. What I know is that sometimes a resistor(4r7) is used from the input ground to circuit ground to avoid ground loop. But a diode? I can only speculate.

thanks...i will do the next step tomorrow...

bye chris
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Old 2nd January 2018, 10:58 PM   #452
FauxFrench is offline FauxFrench  France
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My guess is that D8 serves a mechanical insulating purpose as a spacer. The rather large metallic heatsink is hanging not much above several SMD components. If the heatsink touches these components it may destroy the amplifier. And, the heatsink is riding on top of the small TDA7498E with only two elastic fixation points.
My suggestion is that D8 acts as an insulating spacer together with the two polarity diodes (that takes care of three corners of the heatsink) and one of the SMD components in the last corner.
Machine mounting of an extra diode costs nothing and D8 has a high (insulating) profile.

Last edited by FauxFrench; 2nd January 2018 at 11:01 PM.
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Old 3rd January 2018, 12:49 AM   #453
r_kalar_2 is offline r_kalar_2  Bangladesh
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Any one knows the voltage drop number of tda7498? Can't seem to find it in datasheet.
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Old 3rd January 2018, 06:58 AM   #454
FauxFrench is offline FauxFrench  France
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Voltage drop number?
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Old 3rd January 2018, 10:32 AM   #455
r_kalar_2 is offline r_kalar_2  Bangladesh
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Originally Posted by FauxFrench View Post
Voltage drop number?
Yes, voltage drop across the chip. Example:Typical voltage drop for lm3886 is 2 .5v.
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Old 3rd January 2018, 05:27 PM   #456
chermann is offline chermann  Austria
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Quote:
Originally Posted by r_kalar_2 View Post
Yes, voltage drop across the chip. Example:Typical voltage drop for lm3886 is 2 .5v.
hi
So you mentioned the point with the lowest voltage where the chip is not working?

If i remember right to my first try at this baord it must about 8V -9 Volts DC
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Old 3rd January 2018, 05:54 PM   #457
r_kalar_2 is offline r_kalar_2  Bangladesh
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Chermann, no I am talking about something completely different. Lm3886 has typical dropout voltage for positive side is 1.5 v and negative side is 2.5 v. So, if the supply voltage is say 24v, you will get maximum undistorted swing of 22.5v. From this value and with the help of total quiescent power supply current, you can calculate the possible power that will be drawn from the psu for undistorted output power. Then multiply it with 1.5 and you have your psu requirement for a given voltage and impedance load. source: neurochrome psu design article.
But I can't find the typical dropout voltage value for TDA7498. But I am guessing from the Trevor marshall's article that it's 3v.
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Old 3rd January 2018, 06:08 PM   #458
chermann is offline chermann  Austria
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Quote:
Originally Posted by r_kalar_2 View Post
Chermann, no I am talking about something completely different. Lm3886 has typical dropout voltage for positive side is 1.5 v and negative side is 2.5 v. So, if the supply voltage is say 24v, you will get maximum undistorted swing of 22.5v. From this value and with the help of total quiescent power supply current, you can calculate the possible power that will be drawn from the psu for undistorted output power. Then multiply it with 1.5 and you have your psu requirement for a given voltage and impedance load. source: neurochrome psu design article.
But I can't find the typical dropout voltage value for TDA7498. But I am guessing from the Trevor marshall's article that it's 3v.

Hi
Ahhh........ now i understand you better. ......
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Old 3rd January 2018, 06:39 PM   #459
FauxFrench is offline FauxFrench  France
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For a (linear) class AB amplifier there are limits for how close to the rails the output can go.
For a class D amplifier it is different because the output consist of two power switches driven into saturation. For the TDA7498, the typical resistance of each output switch is 0.2 Ohm. Thus, the output voltage swing from a TDA7498 is the full rail voltage MINUS the drop across each switch that the load current causes.

As a plain example: With a current of 3A (24V rail voltage and 8 Ohm) the drop across each switch is 3A*0.2Ohm=0.6V. Thus, in this case with 24V and 8 Ohm the output voltage swing is 0.6V to 23.4V.
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Old 4th January 2018, 09:04 AM   #460
r_kalar_2 is offline r_kalar_2  Bangladesh
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Only 0.6v? That's very low.
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