I've started playing with KiCAD and have created the screenshotted schematic, want to get that 'right' first, and I'll keep updating the screenshot and when 'good' I'll attach the file for KiCAD as well. Then I need to try and figure out how to create a PCB. Then I'll share that part as well and see if we can create a useful PCB for doing these Class D power amps for playing with different preamps with a lot lower cost (I know some here have bought pre made ones, but I think I'd like to create one we can reuse here as much as possible).
I'd love people to tell me what I'm doing 'way wrong' etc. This is copied from the infineon example circuit 100% so smarter folks than myself may knwo this is 'bad' for reasons... let me know them please
KiCadSchematic.png
Edit: updated due to typo on R14, and 12V battery was actually just VCC. Fixed that as well.
~Phil
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tristanc wrote: ↑Wed Oct 24, 2018 7:31 am
One thing I notice is your +/-35V entry points probably need pads / pins / some sort of connector instead of the power net symbol.
You can draw the connector linked to the power net somewhere else, of course, to keep the schematic making sense.
I thought pads/pins/connectors were only on pcb's not schematics. On many schematics all I've seen ever were what voltages came in at a specific point.
Actually, though, there was one thing that confused me, the way they showed the 12V source/battery. I don't get how the -35V would also s uddenly have 12V at that point, with all other things connected to it, wouldn't taht end up needing some kind of dropper resistor OR having some kind of separation from the -35V rail?
I don't get how the -35V would also s uddenly have 12V at that point, with all other things connected to it, wouldn't taht end up needing some kind of dropper resistor OR having some kind of separation from the -35V rail?
The schematic simply shows the negative terminal of a 12V battery connected to the -35V rail. This does not mean there is 12 volts at that point.
pompeiisneaks wrote: ↑Wed Oct 24, 2018 3:53 pmI thought pads/pins/connectors were only on pcb's not schematics. On many schematics all I've seen ever were what voltages came in at a specific point.
I think you need to include _everything_ you want on your PCB on the schematic. See the attached - you need to tell the package that, eg., you want 4 mounting holes, one of which is plated so you can make a connection between the copper pour ground plane and the chassis via a brass standoff. And that your +35 power supply will need to connect to the +35 net/port through a screw terminal.
My understanding is it that it goes: lay out components -> annotate with numbers -> associate parts to the components (ie, size of resistor, type of screw terminal) -> pass to PCB program -> arrange them all. If something is missing from the start it is difficult / annoying to add in later.
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pompeiisneaks wrote: ↑Wed Oct 24, 2018 3:53 pmI thought pads/pins/connectors were only on pcb's not schematics. On many schematics all I've seen ever were what voltages came in at a specific point.
I think you need to include _everything_ you want on your PCB on the schematic. See the attached - you need to tell the package that, eg., you want 4 mounting holes, one of which is plated so you can make a connection between the copper pour ground plane and the chassis via a brass standoff. And that your +35 power supply will need to connect to the +35 net/port through a screw terminal.
My understanding is it that it goes: lay out components -> annotate with numbers -> associate parts to the components (ie, size of resistor, type of screw terminal) -> pass to PCB program -> arrange them all. If something is missing from the start it is difficult / annoying to add in later.
I've definitely never used these types of apps before, so you're saying I have to put layout components on the schematic or the layout of the pcb will have problems? ugh. ok. Sounds more like you're saying you don't create a schematic, but a logical layout then convert to a physical one? Sorry seems I'm lost on the way this works having never done one of these schematic to pcb layout conversions before, seems counter-intuitive.
I whipped up a quick schematic just to demonstrate how I generally do the connectors. As far as the mounting holes go, as best as I understand it there really should be only one ground connection to the power supply / chassis. I'm not an expert on grounding for Class D, but in my experience that seems to work. So for mounting holes I just create circles on the edge cuts layer for the size screw I want to use when doing the layout. You can always scrape off the solder mask around the screw hole if it turns out it needs it, since the bottom layer of the board should be completely covered by the ground plane, but its much more of a pain in the ass the other way around.
Keep in mind this is just a quick schematic I threw together in about an hour at 2am so their may be some errors and it's definitely not as neat as it could be but I hope it helps. I might be able to create a layout at some point if I have the time.
Also the output devices specified in this schematic are IR's DirectFET's, they only come in a surface mount package and require reflow soldering to install. You can do it with a heat gun but its a bit tricky, so it's probably advisable to select some different devices unless you're comfortable with that or have a small toaster oven you can dedicate to doing the reflow components.
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Thanks! Yeah I didn't look that up for the mosfets, but the class d chip doesn't seem to come in a standard pin type package anymore. I was going to try and hand solder just that guy on. EEVBlog has some videos on drag soldering smd stuff, so I'd hoped that wouldn't be too hard.
It shouldn't be too bad. It's an SOIC16 package which has a lead pitch of about 1.2mm. You could probably do it without magnification depending on how good your vision is, with a standard tip. Though you'd probably want a 5x loupe for inspection of the joints. They can be had pretty cheaply. Just use lots of flux and thin solder and you'll be fine. I recently bought one of these for doing SMD stuff. Quite a good deal for the money:
A good pair of tweezers with fine tips are indispensable as well, here's some of the other stuff I bought recently for doing smd stuff that's quite handy:
As a note they absolutely suck, they lack tinning on the pads, and are about as cheaply built as you could possibly find. So if you can get quality results with them, you should have no problems at all.
So I've just built this schematic up, but may have toasted the thing already... I didn't find an easily available 35VDC supply but found 48VDC. I got appropriately available MosFets, that support that voltage, and have the other specs etc, but on power up it seems to have smoked the chip. I'm now thinking I didn't account for the VREF and OCSET pins here with the 1.2 and 8.2k voltage dividers, I'm guessing the point there was using the 35V for the voltage dividers to keep the voltage between the b- and Vref no higher than 5VDC as it will not support more than 5.4, and my math on those resistors with 48V seems to show more like 8 volts...
So... I'm wanting confirmation that I'd want to use a 1.5k and 15k resistor to set the difference between vref and ocset to beteen 4.8 and 5.4 volts? I seem to see about 43.2 if my math is right?
I'm not sure what else might have cooked it, all other voltage limits seem within the right range? (this is R3 and R5 on pints 7/8).
The application note for the part suggest that the sum of the two resistors should be ~10kOhms. The datasheet and app note also seem to suggest that the 0V reference for that particular portion of the circuit is the negative supply, so it would not be possible to apply more than the allowed voltage on that pin.
As the schematic above shows you have a current limit of ~20A, and as long as you are okay with that current limit then you should not need to adjust that.
What is your VCC supply coming from, and what is the voltage setting?
(I know some here have bought pre made ones, but I think I'd like to create one we can reuse here as much as possible). 