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Old 29th August 2014, 05:46 AM   #1
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Unhappy Class D output filter design

can anybody help me in selecting corner frequency for a class D amplifier and L,C,Q Relations...
i have calculated it for a 2nd Order LPF for 4ohm Load.
L=R/(Q*2*pi*Fc)
Q=R*SQRT(C/L).
C=Q/(R*2*pi*Fc).
where am i Wrong.
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Old 29th August 2014, 07:05 AM   #2
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I unfortunately have the math skills of a 4th grader, so I use this tool when I need to calculate a filter....

RLC Low-Pass Filter Design Tool

Perhaps it can help you find your answer.
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Old 29th August 2014, 10:09 PM   #3
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I always use 1/(2 pi sqroot(LC))=f
to get resonant frequency.
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Old 30th August 2014, 03:22 AM   #4
xrk971 is online now xrk971  United States
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Get a copy of TINA spice program from TI and just simulate it - very easy to use, just draw the circuit, click on components to set value, then run AC frequency response analysis. Can use straight resistive load or a lumped element model of the driver for most accurate results of the filter - tuned to your specific driver.

SPICE-Based Analog Simulation Program - TINA-TI - TI Software Folder

For example, here is the filter for the TPA3116D2:

Click the image to open in full size.

Click the image to open in full size.
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Old 30th August 2014, 05:52 PM   #5
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Quote:
Originally Posted by xrk971 View Post
Get a copy of TINA spice program from TI and just simulate it...
Try out the online calculator link I posted. It gives a lot of useful info on filter performance, though you're limited to simple filters/loads.

Last edited by theAnonymous1; 30th August 2014 at 05:54 PM.
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Old 31st August 2014, 03:14 AM   #6
Eva is offline Eva  Spain
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Additionally, other interesting output filter design criteria is to move inductor value towards the inductance needed to achieve resonant switching when the amplifier is idle, this saves idle power.

Fsw = idle freq (hz)
Vrails = +/-rail voltage (V)
C = capacitance attached to the switching node (FET + snubbers) (F)
t = dead time (s) (minimum reliably allowed by circuit)

Current needed to charge C from -Vrail to +Vrail in t :

I = 2*Vrail*C/t


Ideal inductor value that would achieve this peak current just before switching node toggles:

L = Vrail*(.25/Fsw)/I


Gate resistors have to be chosen so that gate controlled turn-on dV/dt in switching node is not faster than the dV/dt created by the resonant current in the inductor. This suppresses switching losses for output currents from -I to +I.

I is also the peak output current that the amplifier will do before dead time distortion starts to show up. Higher I makes the low-power lowest-distortion zone wider.
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Old 31st August 2014, 08:28 PM   #7
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Quote:
Originally Posted by Eva View Post
Additionally, other interesting output filter design criteria is to move inductor value towards the inductance needed to achieve resonant switching when the amplifier is idle, this saves idle power.

Fsw = idle freq (hz)
Vrails = +/-rail voltage (V)
C = capacitance attached to the switching node (FET + snubbers) (F)
t = dead time (s) (minimum reliably allowed by circuit)

Current needed to charge C from -Vrail to +Vrail in t :

I = 2*Vrail*C/t


Ideal inductor value that would achieve this peak current just before switching node toggles:

L = Vrail*(.25/Fsw)/I


Gate resistors have to be chosen so that gate controlled turn-on dV/dt in switching node is not faster than the dV/dt created by the resonant current in the inductor. This suppresses switching losses for output currents from -I to +I.

I is also the peak output current that the amplifier will do before dead time distortion starts to show up. Higher I makes the low-power lowest-distortion zone wider.
Eva is that really you ? oh my word. where have you been ? soul searching? welcome back !
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Last edited by Reactance; 31st August 2014 at 08:32 PM.
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Old 3rd September 2014, 05:07 PM   #8
Dimonis is offline Dimonis  Russian Federation
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Quote:
Originally Posted by Eva View Post
Additionally, other interesting output filter design criteria is to move inductor value towards the inductance needed to achieve resonant switching when the amplifier is idle, this saves idle power.
Maybe , if the inductor is included in the feedback loop. But when not the criteria is a flat frequency response , look at Iraudamp7
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Old 3rd September 2014, 05:16 PM   #9
Dimonis is offline Dimonis  Russian Federation
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A simple programm for this - but in Russian , I think you'll understand who is who.
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