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Help with inductor
Help with inductor
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Old 15th March 2019, 11:48 AM   #1
Dbnn is offline Dbnn  Greece
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Default Help with inductor

Hello to forum.
i've build one class D amp for small PA events and parties with TL494 PWM and i'm very happy with audio results.
So the problem is output inductor. i'm using inductor from Behringer eurolive PMH5000 powered console (console was fail to amplifiers and i'm converted in pathetic with xlr outputs).
So my case is, with 40vDC everything working fine but the inductor is getting warm. when i increase the voltage about 80-100vDC inductor getting HOT and the fets (irf640) is blown up.
without inductor and without load to output i can increase voltage up to 150vDC without ANY problem.

i'm posting the schematic and i'm try to find some software to calculate the inductor because i'm searching two weeks on several forums without result.tl494 class d.jpg

Can anyone help me??
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Old 15th March 2019, 11:58 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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I wonder what you expect as the nominal oscillation frequency is around 10kHZ, so OK for bass and sub. The 1mH choke is not right for such a low frequency!
The rise and fall time of the TL494 is up to 200nS which is a bit slow.
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Old 15th March 2019, 12:21 PM   #3
Dbnn is offline Dbnn  Greece
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yes, oscillation is about 10k, i know that the choke of schematic is wrong. i'm run it with behringers choke with 1uF cap at LC out filter and result was very good, in my ears with my fullrange speakers.

check the schematic of PMH5000, schematic says 20uH of L3 (that i used) and on software called coil64 i'm measuring 11uH... I'm not understand what and where is the mistake
coil.jpg
coil2.jpg

Last edited by Dbnn; 15th March 2019 at 12:24 PM.
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Old 15th March 2019, 12:34 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The problem is the Behringer runs at a pilot frequency of around 500kHZ and the 20uH choke will filter out the clock frequency leaving a path for the audio. The choke value, 20uH, allows the audio frequency to pass and 10kHZ is in the middle of the audio frequency.
That is the error.


Work out the induction required to block 10kHZ at the desired impedance and the usable frequency of the amplifier will reduce to below 1kHZ.
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Old 15th March 2019, 12:39 PM   #5
Dbnn is offline Dbnn  Greece
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ok i understand the error.
so if i keep the ferrite cores from behringer. how to calculate (with simple mathematics if is possible) the desired turns to make a suitable choke?
with Coil64 i must wind 60 turns to my ferrite to take 25uH inductance. Software is right or there is another way to calculate?
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