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Class D Switching Power Amplifiers and Power D/A conversion

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Old 20th March 2005, 05:57 AM   #11
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Hi, Subwo1,

Yes, it already produces sound (in low level), I guess this is a good start.
Anything weird in my schematic (besides the buffers you mentioned)? I've got no simulator, I make experiment by W2W connection.

Some questions :
1. Is it OK to use Elkos (elektrolit capacitor) for LC filter? Or MUST use non-elektrolit type (like WIMA or Mylar capacitor)?
2. I cannot use low valued L and low valued C (below 100uH and below 100uF). Where can I learn more about the design of LC filter of classD power amp?

My elkos and cores are heating very quickly in some values (low values). Something must be wrong in my experiment, since others here can use L<50uH + C<2.2uF without heat or current draw or other complain at all.
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Old 20th March 2005, 07:13 AM   #12
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Hi lumanauw, I am going to guess that with low loss output filter capacitors like polypropylene, the filter Q is higher than the noise tolerance of your circuit can handle. The electrolytic ones are lossy and provide damping to the filter. Electrolytic capacitors commonly get hot at high frequencies.

Your cores heating at low inductance values indicates to me they are being subjected to too much magnetic flux and maybe even saturating. Lower switching frequencies let the flux on the core build up longer. Try the modified gate driver buffer first to see if it will let you raise the switching frequency so that you can try a lower valued inductor. You are right that your goal will be something on the order of a 40uH choke and a 1uF capacitor.

You may also try to adjust the positioning of your parts and incorporate anti-interference techniques like twisting together parallel runs of wires, especially if they constitute a closed circuit path. So, a "hot" and a return wire should be twisted when possible. Keep high frequency circuit paths short and tight. Place a poly .22uF or so decoupling capacitor directly from the drain of the upper MOSFET to the source of the lower. They should be very close to each other. Keep all component lead lengths as short as possible. I recommend using 1/8w resistors as the optimal size for low voltage p2p. You may like to try to to keep the circuits in tight sections like your own versions of integrated circuits. Component bodies may be touching when voltages present permit.
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Old 20th March 2005, 07:13 AM   #13
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Here is your new simulator:

http://ltspice.linear.com/software/swcadiii.exe

It's free, full version, perfectly legal, is a decent program, sooner you get comfy with it the better, it will allow you to do all kinds of experimentation quickly and easily.
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Old 21st March 2005, 04:21 PM   #14
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Funny, i am working with LT step-up regulators for a specific application here....thaaaank you for this link !!
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Old 21st March 2005, 04:44 PM   #15
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Hi David

At least the combinations with a large cap make for a low corner frequency and therefore a low switching frequency as well. But if we have a combination of 20 uH and more than 1 uF of capacity we do not only have a low cutoff frequency - we also have a highish Q and a very low inductance for an amp switching at low frequencies, giving quite a high idle current through the coil.

Do you have a scope to see how fast it actually switches or do you have access to one ?

Regards

Charles
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Old 22nd March 2005, 12:45 AM   #16
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Hi, Subwo1,

Quote:
I'd say the turn-on time is too fast, and there may also be a lot of gate ringing. The turn-on portion of each MOSFET driver buffer needs some resistance. I'd try separating the emitters, placing a 10 to 20 ohm or so resistor between them. Connect the intersecting point between the PNP emitter and the resistor to the gate.
Your suggestions seems improving my experiment amp. I put R=47ohm in the totem pole (after emitor of NPN), and it improves the max level that I can play this amp. Before putting this R, in medium level it already wrecked sound, but after putting this R, in the same volume it doesn't wrecked.
I've been using basing totem pole (without R) for years. I don't know if putting R in totem pole emitors is a must
The gate pulse are changing shape a bit. Before R it is full square, but after putting R, the left top edge is rounding.

Subwo1, what is the proper totem pole for classD should looks like? (driving parrareled mosfets with small capacity chip like IR2111).

Hi, Charles,

The switching frequency of my current setup is only 24khz. I can raise it to more than 100khz, but the gate pulses are ringing (blurry in scope), and the edges are ringing.

My switching frequency is only 24khz. It works with L=200uH and C=220uF. I've put 12V zeners to protect the G-S. But when I put small inductance (50uH) or small capacitor (2.2uF), all upper and down mosfets are blown instantly. What is making all the mosfets blown? The zeners are OK after blown, it means the destructive thing is not from the gate, but from somewhere else. But where does it comes from? Too much Idrain? The mosfets are not burning. They just becomes shorted G-D-S. I'm confused what is ruining the mosfets.

I dare not raising the frequency because I still feel something is wrong right now. The value of LC seems to have small connection to the switching frequency. It is determined more by feedback arrangements, global feedback+local feedback around opamp in my experiment amp.
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Old 22nd March 2005, 01:01 AM   #17
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Default About IRF amp........................

In the iraudamp1.pdf, in page 6, in the "Self Oscilating PWM Modulator" topic, it is writen that IRF amp uses "Sigma Delta" modulation that makes all error in audible frequency is shifted to higher upper frequency.

How come the errors are converted to upper higher frequency, doesn't this means higher order distortion (in analog amp, higher order distortion are not wanted, because they make not pleasant sound)?

Pat Allen, you have built this IRF amp (feedback before LC). By any chance, have you compared it to feedback after LC (like UCD)? Or, how does IRF classD sounds?

What is "Sigma Delta" modulation? I've read this term in DAC. Is it a way to convert square wave to sinusoidal? Is this making the IRF amp taking feedback before LC? But in that chapter it writes "2nd order integrator with U1, C17, C18 converts rectangular wave form from classD switching stage and outputs a quadratic oscilatory waveform as a carrier signal". It is not converting square to sinusoidal, but square to what form?

If it is converting square to sinusoidal, then for me taking the feedback before the LC makes more sense than after LC, cause the square is already converted to analog (to be feedback to front end and compared with analog input signal) before LC.

If it works like that, then in IRF classD amp, even without LC at all the amp should work without destroying the mosfets? (unlike selfoscilating classD with feedback after LC, like my experiment amp. Very sensitive to LC value even to destruct the mosfets).
Destroying the tweeters maybe?

LC filter in selfoscilating classD that takes feedback before LC (like IRF) and after LC (UCD) has differen't purpose? In feedback before LC, LC just becomes a low pass filter, but in feedback after LC, the LC becomes energy storage?
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Old 22nd March 2005, 01:36 AM   #18
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Hi lumanauw,
I am glad about the improvement from the modified gate drivers. I cannot say what a totem pole should look like for paralleled MOSFETs since it varies depending on the circuit. But the MOSFETs turning off need enough time to get "off" before the other ones turn on, and it is common practice for the sake of efficiency to use the energy stored in the choke (or transformer) to carry the voltage away from the source of the device turning off to the other rail before the other device turns on. Best Regards.

PS. It is a general no-no in high frequency circuits to connect the cathode of a zener diode directly N-channel MOSFET gates. It definitely can cause ringing on the gate terminal, and MOSFET destruction. Reliance on the power supply voltage as the limiting factor in setting the maximum gate drive voltage is the accepted method.
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Old 22nd March 2005, 04:25 AM   #19
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Hi lumanauw,

Could you repost your schematic or point me to it and any information about your layout.

Thanks -- analogspiceman
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Old 22nd March 2005, 05:18 AM   #20
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Have you measured how fast your "hand-sewn" comparator and level-shifter works ? I don't know how the IR 2110's input logic reacts to slowly changing input signals. Maybe you should increase the current and decrease the resistors in said part of the circuit.

Regards

Charles
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