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Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Hey ,
Well I went and tried something new for me , My previous coil used FM to achieve a audio output from the secondary , but this required the driver to be pulled back and forth out of resonance from the secondary .
I have moved over to a simple class D design with a 555 Timer generating a triangle *ish* wave and that feeds a LF356 Op Amp , The non inverting input is biased to half the supply voltage ( 12 Volts ) and then a AC single of around 800mV from my ipod is capacitor coupled to the pin .
The result is a modulated square wave output , this is feed into a Hex inverter to drive 2 opt couplers that then drive the gate driver ICs to direct drive the mosfets on the class D stage
Im abit stumped on the LC filter stage . My switching frequency for the D amp stage is 30 KHz , I looked online and calculated for a 20 KHz cutoff on the filter , with around 1 ohm impendence I got the values of 12uF for the capacitor and 12uH for the inductor , But I would guess the impedance of the SSTC inverter is higher and the mismatch would cause nasty problems ?
Now I used the Capacitor value , and well for just a test made a inductor to try out the coil and it worked ! , I get a clean and loud output with nice audio :)
My main Question would be does the 12uF and 12uH for the LC seems a correct value , This is my first dive into this type of amp , especial for a SSTC I have looked around and tried working it out, but though a second opinion would help :)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
You need to know the maximum output current from the step-down, then you design the inductor based on current ripple. Usually around 20% ripple is acceptable for an iron powder core. Use the basic equation V = L*di/dt (V is half your output voltage and dt is half the switching period - the highest ripple for 50/50 duty).
As for the capacitor, the resonant frequency of the filter should be several times lower than the switching frequency, there are some equations to calculate the capacitance based on voltage ripple but you can also go with the amplitude transfer characteristics of an LC filter.
Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Sorry to double post ,
Found that the class D stage L = ( RL*√2 ) / Wo .
Where Wo is angular frequency that is worked out by Fc*2*Pi , So I worked out that for a Cut off of 20Khz , 20000*2*pi = 125663.7 . So then L = ( 46*√2 ) / 125663.7 = 0.000517 H , So the inductor is 517 uH ( microhenrys )
The capacitor is 1/( Wo*√2*RL) for this I worked out 1/(125663.7*√2*46) = 1.22e-7 So 122 nF capacitor .
So I think these are the values I need for the Low pass fitter on my Class D amp , or I screwed up someplace :P
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
You can not have a 20 kHz cut off for some 40 kHz switching frequency, it must be lower. And, you must add the capacitance of the snubber capacitor on the IGBT bridge to the class-D output capacitance, so you must have at least some 2.2 uF on the output.
Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Dr. Dark Current wrote ...
You can not have a 20 kHz cut off for some 40 kHz switching frequency, it must be lower. And, you must add the capacitance of the snubber capacitor on the IGBT bridge to the class-D output capacitance, so you must have at least some 2.2 uF on the output.
What would you suggest as a Cut off frequency for a switching speed of 40 KHz ? .
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
How I'd do it: Firstly calculate the inductance based on current ripple (like I indicated above; if you're not sure I can do the calculations for you). Then, calculate the resonant frequency of the L you just got and the capacitance of your snubber cap (if you're not sure, you can go with the 2.2 uF figure, though higher capacitance might be essential - depends on actual parameters). If the resulting frequency is something like 5-10 kHz, it sounds right. You do not need to add extra capacitance, just use the snubber caps as the output capacitance.
If you need this for audio modulation, you must have the cut off frequency in mind - the trebles will be somewhat muffled. But I think there is no simple way to make this better.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Solving for L: L = V*dt/di. Assuming 50 kHz switching. 0-300V output (what is the supply voltage for the step down?) and 20% current ripple, we get L = 150V * (20us/2)/1A = 1,5 mH. The resonant frequency with 2.2 uF capacitor is 2.7 kHz. This is kind of low, for a such low current draw from a SSTC, you can probably go with 1 uF or 0.47 uF snubber. Note that this would be unusable for a pulsed DRSSTC. If you feel the inductance is too high, you can decrease it probably up to a few times, but then I would suggest to realise the inductor on a gapped ferrite, not iron powder (because iron powder doesn't like a too large current ripple).
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Class D AM SSTC , LC Help
