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Class D Switching Power Amplifiers and Power D/A conversion

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Old 25th January 2008, 10:03 AM   #151
luka is offline luka  Slovenia
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Hi

Yes, so I think you will have to think what would go where, coz that big current tracks... well even you won't need , you know what, I would put buffers on main, what much space I'm sure you will find
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Old 29th January 2008, 03:37 PM   #152
djQUAN is offline djQUAN  Philippines
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before rebuilding this thing, I decided to try the regulated gate drive small scale first.

I will design a powersupply that will drive a 750wrms mono class D amp. (amp section will be another story )

Eva,

provided I do everything right, do you think two pairs of RFP50N06 fets is enough for that amplifier? yes, it is for subwoofer duty and I expect it to run into clipping when pushed hard.

the heatsink I'm planning on using has enough room for eight fets if needed. I was originally thinking of using eight IRFZ44 fets but if you got 800watts out of a pair then I could do the same or more with two pairs?

I will be providing regulated gate drive and do everything right this time. plan everything carefully before going and building stuff. I just got lucky with the amps I did before that up to now are still working fine.
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Old 31st January 2008, 07:00 AM   #153
Eva is offline Eva  Spain
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IRFZ44N seem to be slightly better than RFP50N06.

I would use them at 70A peak per device or less. For two pairs this translates into 140A peak, or 1700W peak. What rails into what load impedance are you going to use in the end?

You don't need much heatsinking, what is required is very low leakage inductance and good gate drive.

You already know how I would lay it out, I showed a lot of my tricks in the push-pull supply of the 2x150W class-D car amp. Essentially I'm switching the two 2200uF 25V 0.028ohm-ESR low-ESL Panasonic capacitors into the windings though 0.012ohm IRFZ48V. Layout inductance is almost negligible because paths are very short and currents in opposite directions flow through opposite PCB planes. Excess transformer leakage inductance is negligible too, since the magnet wires enter the PCB in the same place where they leave the winding structure. Note that I got a two fold decrease in leakage inductance (and less tall spikes and cooler IRFZ48V) just by rewinding the toroid for better primary-to-primary coupling. 2x120W of music was piece of cake without any heatsink, despite the 60A peaks.

You could consider one toroid and two pairs of MOSFET for each supply rail. Each toroid should use dual secondaries and independent rectifiers. You may lay out the control circuit in a small daughter board and use a PIC for the protection schemes, or even a PIC16F684 with ADC and hardware PWM for everything...

In fact, next time I'm probably going to skip the old SG3525/TL494 stuff. I'm currently using PIC16F690 at 20Mhz in a commercial project and it is powerful enough for controlling a few concurrent processes and doing both high-precision high-frequency hardware PWM for main switching and additional low-precision low-frequency software PWM for non-critical tasks. The ADC is reasonably fast too, it can sample at 25Khz or so. You can even do software-assisted self-oscillating control with the help of the built-in comparators.
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Old 31st January 2008, 12:44 PM   #154
djQUAN is offline djQUAN  Philippines
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that's great news.

I'm thinking of squeezing as much power as I can out of as small as an amp I could make. goal is around 1000wrms into a 1ohm mono load but I was hoping for about atleast 750wrms if I can't meet the 1kw mark. entire amp case/heatsink is about 9" by 10.5"

I have several 25V 2200uF Panasonic FM (or was that FC? the ones with gold lettering and dark blue plastic sleeves) which I'm going to use for primary side filtering.

two toroids is a great idea as I don't have a big enough core that could fit in the case. have an amidon FT 240-77 core but it takes too much board space. I have a couple smaller ones that will fit nicely (about 1.5" dia) and works quite well in a push pull smps. only problem is I don't have available FR rectifiers. I only have one common anode, a common cathode and two doublers on hand and don't know where to get them locally. I also have many to-220 schottky's 90V 10A

so a possible option is one common cathode rectifier for one trafo and one common anode rectifier for another trafo?

or what about independent fets and primaries and paralleled secondaries?

with the power output goal, I'm looking at more than +/-50V rails
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Old 16th March 2008, 11:00 PM   #155
luka is offline luka  Slovenia
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Hi

Ramp during dead time rises 26v in 1.14us, is that ok? Or too slow?
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Old 17th March 2008, 12:16 PM   #156
Eva is offline Eva  Spain
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It's a bit slow but ok anyway.
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Old 17th March 2008, 12:23 PM   #157
luka is offline luka  Slovenia
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Hi

that is on primary side, drain to drain...but I don't like waveform when it is loaded, dead time is seen clearly, but it has spike same hight as input voltage is, right after ?turn-off? is present. Is that leackage inductance?
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Old 1st April 2008, 04:46 PM   #158
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since I wasn't able to save the old PCB, I made a new one with a much better layout.

still have the regulated gate supply etc etc as the old board except that this is much smaller. I only used six IRFZ44's per phase and only one trafo.

three aux windings supply gate drive, +/-15 and +5V outputs.

rail voltage is dropped to +/-55V output as not enough copper could fit in the core to minimize I2R losses.

gate drive is regulated to 17V peak and drops out of regulation at 11V input. gate drive is still at 12V peak at 8.2V input.

funny thing though... optimum operating freq is 15kHz only!!!! but drain waveforms are perfect square waves!!! absolutely no heating (except for the gate drive regulator getting warm).

power testing will be done as soon as I get caps that fit the PCB layout. the old 1000uF 100V caps are too big.
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Old 2nd April 2008, 03:06 PM   #159
djQUAN is offline djQUAN  Philippines
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did a bit of power testing right up to a couple of watts (without output caps, don't have any that fit) and everything stays cool.

there is no freq jittering issue that I had with the old layout so I guess this will work well.

time to find a heatsink that the SMPS and amp module can fit in.
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Old 17th June 2008, 12:59 PM   #160
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still couldn't find a heatsink with the shape that I wanted.

I wonder if I could use amp8 with +/-55V and 2R bridged load.
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