inductor

DC-DC Converter: Splitting up the project

I was originally thinking of one monolithic board which would have everything it needed.

There was provision for the lot, including a separate ATTiny24A so that you could omit all but one of the MOSFETs, swap the remaining MOSFET for a P-channel, drop the MOSFET drivers, one of the INA219s, and the ATTiny861, and you’d have just a monitoring board with a (low-speed) PWMable switch.  It’d plug into the same place and use the same host interface.  The one board could be made into just a boost, or just a buck.  Flexibility.

There was just one snag.  That’ll work for small power supplies with maybe up to 5A capability (~100W) but not for the 50A version.  The MOSFETs will fit, but the tracks will need to be huge, the board will be hideously expensive to make, and they don’t make inductors big enough.

Looking around for inductors, I did see these.  They’re not massive like the 10uH one I saw, and they’re not expensive.  The downside is they’re about 10% of what I really need.  I guess I’ll just make do.  They’re also not PCB-mount (mind you, a 40kg inductor doesn’t PCB-mount either).

Thus, it may be more sensible to separate the MOSFETs and high-power stuff from the controller.  Now here’s the rub.  We’re dealing with sub-15ns pulses.  PWM.

Years ago for my industrial experience, I did work on an electric harvester platform.  The system ran 48V.  The motors were rated at 20kW, and were made in house using windings wound from 5mm diameter enamelled copper wire and neodymium magnets.

We had loads of issues with MOSFETs blowing.  The MOSFET driver was mounted close to the MOSFETs, as I’m proposing to do here, but between DSP and driver, was a long-ish run of ribbon cable. @Bil Herd posted this article covering the challenges involving inductance on PCB layout.  That same problem applies to “long” cable runs too.

10 years ago when we were working on this project, I remember asking about if we had considered maybe using coax cable instead of a ribbon cable.  The idea was rubbished at the time.  Given we were PWMing 400A, I think there might’ve been something in that suggestion.

That ribbon (10~20cm of it) would have had a lovely inductance all of its own, and while I have no idea what frequency the PWM was running at (I might have the code somewhere but I can’t be stuffed digging it up), and we were fundamentally driving a single-ended signal over a fairly long distance.  Yes, ground was close, but probably not enough, a twisted pair would have been better, but even then not perfect.  We blew many MOSFETs on that project.  Big TO-263s!

An earlier article on differential signalling got me thinking: why not use LVDS for the PWM?  A quick search has revealed this receiver and transmitter (Mouser says two receivers on it, but I think that’s a typo).  The idea being that I send the PWM down a differential pair using LVDS.  155Mbps should be plenty fast enough (the ATTiny861 can only do 64MHz) and these parts will run at the 5V needed for fast switching.  In fact they require it.

Using twisted pairs, the inductance should cancel.  I’ll make a MOSFET board that just has these signal pairs:

  • +12V (for the MOSFET driver) and 0V
  • +5V (for the LVDS receiver) and 0V
  • High side PWM + and –
  • Low side PWM + and –

There’s a ground-loop I need to be wary of between the 12V and 5V rails, really it’s the same 0V rail for both.  I suspect they’ll still need to be connected at both ends.  Add in more of those screw terminals to take the input and output power off-board, and I think we should be set.

Similarly, the INA219 should probably be a separate board, with scope for having a chassis-mounted current shunt.  The connection to the current shunt’s sense output is a low-power connection, so no issue there.  You want to keep it short for accuracy reasons, but a simple twisted pair will work fine.

DC-DC Converter: PCB routing fun: something doesn’t add up

So, I was busy routing a board having come up with a basic schematic.  I wasn’t going to order the board yet, I wanted to just play around with the design, see how compact I could make this.

One thing that was niggling in the back of my mind, was how the traces would cope with the current.  I use 6AWG cable from the solar panels, and 8AWG from the batteries.  How wide should I make the traces? One calculator reckoned I should make them about 7cm wide!  Another option was to use heavier gauge traces, maybe 3oz copper.  A 5cm×5cm 2-layer board would cost a staggering AU$263 for just the PCB!

Okay, so I can work around this by fiddling the solder mask in Kicad and just solder some copper wire along the trace.  Not a show stopper.  I’ll just make wide traces so I know where to lay the wire and have plenty of area to solder it.

I was making the traces as wide as Kicad would let me, but something didn’t seem right.  The inductor, just seemed so, small…

When I did the search on Mouser, their interface allows you to pick a value, then hit the ≥ button to select everything “greater than”.  What I missed, is the option right down the bottom:

The “-” option, better known as “we couldn’t be stuffed looking up what the real value is”, is seen as “greater than everything else”.

A check of the datasheet itself, revealed the truth.

In short, there is no way that little tiddler is going to manage the current I was contemplating throwing at it!

What’s the biggest I can get that will handle that current?  Well if I take the “-” option out of the equation, they suggest this monster .  It’s 10uH instead of 33, so my ripple voltage will increase.  At $837.84, it is also a rare exception to the free shipping over $60 offer.

I might need to go play with some numbers to see what I can get away with.  The good news is that discontinuous output is not a show stopper for a battery charger.  I might have to make do with nanohenries of inductance instead of microhenries.