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Old 21st April 2005, 11:09 AM   #1
ted12 is offline ted12  Singapore
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Default BTL output filter

my class d design:
(1) triangle wave 300kHz (PWM)

(2)15V Vcc to the FULL H-bridge

(3)the output filter is butterworth 2nd order with bridge tied load configuration(BTL)



It is very strange.
When the freq of sin wave below 10kHz is applied as audio input , the output peak-peak sin wave is about 1V to 15v.
When the freq of sin wave increases from 10kHz, the output peak-peak voltage starts to increase.
As the input sin wave with freq 20Khz, the output peak-peak voltage becomes larger than 30V(about -10V to 30V).

I know that using BTL configuration, the output voltage will double.
However, in my design, why input signal below 10kHz cannot have double output voltage?
Why the output signal becomes larger as its freq starts to increase from10kHz?


Maybe I have wrong concept. Please help me. Thanks.


...very urgent...
thanks
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Old 21st April 2005, 11:16 AM   #2
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I guess you're not using feedback after the filter. It sounds as if your filter is at too low a frequency, and you're seeing the effects of resonance in the filter.
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Old 21st April 2005, 11:21 AM   #3
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Your filter will be self-resonant at about 25kHz, but if you're using a 4R load then then resonance will be fairly low-Q, so you shouldn't see to much of an increase. Perhaps I was wrong in my first reply, and it's not the filter, but something else wrong.
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Old 21st April 2005, 01:37 PM   #4
ted12 is offline ted12  Singapore
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more information of my design:
the feedback path is drawn before output filter
inductor: Toroidal Inductors
capacitor: C1 : WIMA MKP 1uF
C2,C3 : electrolytic 0.22uF
NMOS: BUZ11

More importantly:
the filter --- no problem in single-ended configuration, the flat magnitude response in audio freq band (below20Khz).
i.e. C1 is eliminated, C2,C3 :electrolytic 0.68uF

---urgent---
thanks
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Old 21st April 2005, 02:00 PM   #5
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Do you really mean electrolytic? I think this would be a bad idea as I doubt if a small electrolytic would cope with the ac switching current in them. They are only 0.22uF, you can easily fit film capacitors there.
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Old 22nd April 2005, 09:03 AM   #6
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Hi Ted

Are you measuring without load ? In this case the rising response will come from the filter's resonance which is just below 30 kHz.

Regards

Charles
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Old 22nd April 2005, 09:06 AM   #7
ted12 is offline ted12  Singapore
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I changed C2 and C3 as a monolithic 50V cap 0.22uF..instead of electrolytic cap.

But the same result,same waveform were obtained.(i.e. after input sin wave increased from 10khz, the amp of output waveform started to increase)

Could anyone have any futher suggestion?

---urgent---
thanks
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Old 22nd April 2005, 09:10 AM   #8
ted12 is offline ted12  Singapore
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Hi!Charles

I use 4ohm (Resistor - Variable ,Multi-turn) for the load.

Ted
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Old 22nd April 2005, 09:27 AM   #9
JohnW is offline JohnW  Hong Kong
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Ted,

I would bet that your multi-turn resistor is way too inductive - I run a quick simulation of the filter values you posted, and the filter response is pretty much correct when driving a 4 ohm load.

Find a non-inductive load for testing and use film caps in place of the electrolytics.

John

Edit, have you tried to measure the inductance of your load at 4 Ohms setting - I would not be surprised if it’s greater then your 15uH inductors...
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Old 22nd April 2005, 09:36 AM   #10
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Quote:
Find a non-inductive load for testing and use film caps in place of the electrolytics.

Good idea ! Additionally a Zobel might not be out of place !

Regards

Charles
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