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316  Using Arduino / General Electronics / Re: H Bridge ICs other than LMD18200 & L298 on: January 11, 2013, 06:48:35 pm
Most of the "continuous" ratings assume mega-heatsinking - watch out for that.  Find the Rds(on) values for the bridge and
calculate the heat dissipation (remember two devices are on at once).

Alas there seems to be a big gulf between the performance of discrete MOSFETs and ones integrated into bridge ICs.  Lots of
bridge ICs quote 0.2 to 0.3ohms or so, discrete SOIC/Dpak/TO220 MOSFETs of 0.01ohm or better are easy to find.  This is
presumably because discrete FETs are vertical current flow.

I'm only really drawing ~ 2.5A, but I didn't want to be operating so close to the line so to speak...

I have found this too re the difference in discrete vs packaged resistances, the LMD18200T had RDSon of 0.33ohms per switch, ouch! However, the ST Micro VMJ2SP30 bridge has a proper RDSon value, 19mOhm - and yes, that device has vertical flow FETs in it like you say. A very neat IC indeed!
317  Using Arduino / General Electronics / Re: Reccomend me a pcb package on: January 11, 2013, 04:51:59 pm
What's wrong with eagle?
Its updated regulary, can do 80mmx100mm boards with free version, gets easier to use with practice.
Has lots of parts libraries, and can design your own parts.
Can get a lot of stuff on a 80x100mm board.

Couldn't agree more - I haven't found the free version remotely restrictive. I find EAGLE so incredibly intuitive...

EDIT - Saying that, I can't work out how to build heatsink polygons that will touch the pads of ICs, it seems to be creating spaces automatically... Hmm.
318  Using Arduino / General Electronics / Re: H Bridge ICs other than LMD18200 & L298 on: January 11, 2013, 02:36:46 pm
Look at the parts used by Pololu on the h-bridge boards.


...wow - 14A continuous through those tiny SMD parts! Thanks!
319  Using Arduino / General Electronics / H Bridge ICs other than LMD18200 & L298 on: January 11, 2013, 11:36:23 am
Hi all,

I've using an LMD18200T h-bridge in a design I'm working on, however - I'd like a little more headroom than the 3A continuous, and 6A 200ms peak capability offered. Plus, at .33ohm on resistance per switch, dissipating is a little high too.

I've had a look, but can anybody recommend, or know of an h-bridge ICs that will deliver around 5A continuous? I would build a bridge out of discrete, but I'm very tight on PCB space in this case...

Many thanks!

EDIT - I've just found this, a Freescale MC33887 - http://www.freescale.com/files/analog/doc/data_sheet/MC33887.pdf

Looks a little more promising, yet - in a little SOIC package!
320  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 10, 2013, 06:01:07 pm
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But you have to do what you have to do, and see how it goes.

I think that'll dictate a lot of things with this board!

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The further away from the connector that the h-bridge is, the more possible noise it's
traces can propagate to the rest of the pcb cktry.

Well! smiley-grin Take a look at the reworked version, first pic - both layers. Second, top layer. Third, bottom layer.

I did a quick autoroute, without changing any settings, and with no connections from the ICs to the headers. It ended up with about 300 vias - now, inexperience here, would that be because I've got so many components on a two sided PCB, or because the layout is far from ideal?

I know a lot of these are probably vague ideas, but it's good to be steered in the right direction still... and once again - thank you hugely for your help so far, everybody.





321  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 10, 2013, 03:39:33 pm
It looks like male into female headers are rated at 3A, but I'm not mating them with female headers - instead, they'll be soldered into holes on a small board at 90 degrees that will house the actual connectors, then the whole lot slots into an aluminium enclosure. So given the cross area of the headers, I'm sure they'll cope with more 3A @ ~ 14V.

I was thinking of moving the bridge up the where the power supply is too, but I was a little worried about sticking the bridge right next to the small ICs in the top right... They're the sensitive ones.

I'll have a jig about, see what I can come up with...

EDIT - Just thought, with the bridge near to the headers, I can also save on power trace width by running them on both sides from the same bridge pin, to the same header pin.
322  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 10, 2013, 02:09:22 pm
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I can see a potential problem with using global fill, in that it violates the star-grounding
idea.

What I proposed, is the left side of the board would be star grounded, whilst the right hand side - which is just control circuitry - would have planes due to the number of 5v and ground connections required.

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Is this diagram the same thing we've been talking about for the past few days?

Absolutely smiley

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I don't see anything about separate power input for the h-bridge, or main caps

On the 60 way connector to the board, I'll have two power in pins, and 2 ground pins. One 'set' will supply all the lower power devices, the other will supply the noisy devices and these will star back to their own ground pin.

C46 is the 220uF main cap, but I've just realised my 1uF h-bridge cap is a small 0603 cap, C7 - i'll change that to another can-type electrolytic.

I just wanted to show my main component layout to catch and potential problems early...
323  Using Arduino / General Electronics / Re: Questions and questions to make the jump from schematics to PCB on: January 10, 2013, 10:53:52 am
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1. When a trace must be connected to several components (e.g. ground) is it a good practice to put the solder point of one component in the path to another one rather than to create direct connections to each component ?

Watch this thread re point 1. http://arduino.cc/forum/index.php?PHPSESSID=2b7d9401741b53072561456f5c04f041&topic=141589.0
324  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 10, 2013, 06:58:58 am
Thanks for all the input so far, here are some screenshots of my layout - well, part placement at least.

So, the d-pak packages on the left hand side are power switching FETs / IGBTs. The gate drivers are the small ICs between the rows. The IGBTs subject to a large inductive spike are the top row, top and bottom layers. The h - bridge is the through hole 11 pin vertical package.

My power supply, i.e. regulator, fuses, tranzorb, caps etc is the is central where the two horizontal d-paks are. Underneath however is a general purpose low side driver chip to configure which external relays should be powered. These will only switch during power up / off really.

I wanted to move the power supply away from the IGBTs a little, but the ICs to the right of my power supply are conditioning ~10kHz AC signals, converting USB to UART, reading thermocouples etc.

Bottom right is a flash IC, latch & external RAM - I've tried to keep these as far away as possible as random memory corruptions are not something I'm keen on getting. The two ICs underneath the MCU are SPI devices, so I'm hoping there's not an issue there with noise? The jumpers are to select between external filters for these two ICs, and i've left room around there to route signals over to the left hand side gate drivers.

I've still got ~40 resistors, ~20 caps to pull on to the board from outside of the outline, but these are small 0603 parts, as are the ones present currently on the board.

Does anybody have any recommendations, comments, criticisms (constructive of course, this is the first PCB i've laid out, and my first venture into SMD - all my other projects have just been through hole on vero board)?

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I would use as much free area top & bottom for ground polygon as you can.

Earlier on in the thread however, I was told I should avoid doing this as it will help interference get in to the ground. Or would that only typically be the case if I had a power component on the other side of the board, and I put a ground plane on the other?

Top and bottom, together -



Top -



Bottom -
325  Using Arduino / General Electronics / Re: Gate drive requirements of IGBTs on: January 10, 2013, 05:04:55 am
I see - thanks for the gate resistor calculation info!

I'm still a little confused about whether or not I need to / don't need to clamp. All the performance curves talk about being valid only for V clamp < 430V. That seems to imply that I need to design a voltage clamp into the circuitry? ...but then Figure 1 and Figure 2 are on about 'Self Clamped Inductive Switching'.

But given there's no clamp voltage given, only self clamped inductive switching energy, instead of worrying about actual clamping, just calculate the energy delivered by the inductive spike, and always keep that within the avalanche energy spec.

I feel very lost having never used IGBTs, or really BJTs for that matter...

Next question, current limiting - how do I go about limiting the output current to a certain value - say I only want 8A out, I don't want to use an external ballast resistor. So I assume the easiest way to do this is use a ballast resistor on my board, in a TO220 package or something to limit the current, but can I not limit current by only supplying the gate with a certain voltage?

The resistor approach dosen't seem feasible, even at my max duty of 33% - I'd be dissipating nearly 20W in the ballast resistor!

Or actually, at dwell times of only around 3ms - given the inductance of the primary itself, I'd imagine the current dosen't rise all that fast and it may be safe to drive the primary directly without a ballast resistor? EDIT - Or of course, use only coils that do not require a ballast resistor......
326  Using Arduino / General Electronics / Re: Gate drive requirements of IGBTs on: January 09, 2013, 11:50:43 am
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Well the collector-emitter breakdown voltage is 400V - should be a clue...  You probably don't want to clamp the spike anyway

The devices are advertised as self clamping, is the breakdown voltage where the voltage at which the device clamps? "should be a clue" certainly seems to imply so, but then you say you probably don't want to clamp, so I think maybe not?

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Again depends on how fast you want to switch (faster reduces dissipation in the main circuit, but increases noise and RFI).

So say my gate driver is capable of delivering 5A peak gate drives, but I want only to deliver peak 1A to switch the gate at a speed I want (I've got a good app note from Texas outlining very well how to calculate gate currents, speeds etc), then I just size the series gate resistor to limit current to 1A? Given the switching speeds, would it be neccesary to size up this resistor physically? All my resistors on my board are currently 0603 SMD parts...

Cheers for your help so far!
327  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 09, 2013, 04:48:52 am
"One end of the PCB could be the power end, both sides - whilst the other end would be control, both sides. "

That's what I was saying in #9.

I had misunderstood what you meant, and thought you were implying that I should put all my components on one side, and use the other layer of the PCB for routing traces.

Thank you!

EDIT - I have tens and tens of 5v and ground connections on the control end of my board, given these are away from all the power ICs the other side of the board, would it be appropriate or acceptable to have some sort of plane system on the control end of my board?
328  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 08, 2013, 06:37:01 pm
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Yeah, I meant to mention earlier and forgot, as Bob says, there is no rule that says it's
necessary to keep the power/ground and signal traces on opposite sides of the board.

Ah, that gives me a thought.... One end of the PCB could be the power end, both sides - whilst the other end would be control, both sides. That way, traces and vias shall not intertwine HV/noisy signals etc, and all power grounds can star to a central common ground connection at a central edge right next to the connector for the PCB.

Are my 220 and 1uF caps across the bridges power pins not the 'main' caps however? The diagram on th page you linked certainly imply that?
329  Using Arduino / General Electronics / Re: What is "ground shift problems"? on: January 08, 2013, 04:15:17 pm
I moved a part a tenth and re-poured the ground plane to do it.

I keep reading / hearing the expression "re-pouring of a ground plane" - what exactly does this refer to, please?
330  Using Arduino / General Electronics / Re: Preventing PCB interference on: January 08, 2013, 03:57:21 pm
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1. just lay out the pcb with the normal microcontroller cktry on one side and the h-bridge
    cktry on the other side, and only connect them together at a single ground point. Some
    people will use an inductor [choke] or ferrite to make the connection.

Will do, but using star traces to that single ground point on the power side. Your mention of a ferrite, do you mean the small ferrite beads such as the following SMD bead type? http://tinyurl.com/aygg7hg

...and when you say make the connection, I assume I only need one? So I connect the control side of the PCB to that ground via, with a small SMD ferrite bead

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2. once this is in place, simply run the h-bridge control signals from the microcontroller
   across to the h-bridge. Always good to put series-Rs in these lines to help protect the
   uC from motor noise feeding back from the h-bridge.

Okay - I haven't tested with series resistance yet, I'll whack some 1k resistors onto the control lines tomorrow and check my waveforms. No problem with feeding these as visa through the board, won't long traces have quite some inductance?

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3. locate the main terminals for motor power on the h-bridge side of the board, so the
    motor currents never cross to the other side, and never flow through the single point
    ground connection of item #1. Add bypass [0.1 uF] and reservoir [100 uf or more] caps right at the same terminals.

My h-bridge is a dedicated IC, an LMD18200T - I have the 12V supply for this IC decoupled with .1uF and a 220uF reservoir as per application note, and the outputs have bootstrap caps back into the IC, so do I need these caps on the output? Or do you mean two caps across the two motor terminals? I was planning to do this already to help reduce noise.

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4. it's best to use 2 separate external power supplies, one for the logic part and another for
    the motors. If you can't do this, then the next best thing to do is connect a single supply
    to the motor side, and jumper "1" wire over externally to supply logic power. This way,
    the motor currents are always confined to the h-bridge side, and not flowing between
    the 2 sides. Don't make an external ground connection between the 2 sides, as then you will have a ground loop.

I've only got one external power supply, but have a 60 way connector on to my PCB and have a couple of spare pins - perhaps providing 12v to two pins from the same source, so each side of the PCB has its own 12V pin would serve a similar purpose?

When you say external ground connection, as in - the ONLY ground between each side should be the one ground via discussed under heading 1?

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5. if you have analog cktry on the pcb, confine it to a separate area from the digital logic
    cktry, and also on the opposite side of the pcb from the h-bridge cktry.

My only analog signals are analog voltages going in to the ADC of the MCU, so I can't really avoid running these from the pcb connector over to the MCU....

Thanks for your help so far, by the way :-)

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]Can you you arrange it so that insted of all components being one layer of the board and allk the rest the other layer, have one group be one end of the board and the other group be the other end?
Then any traces & vias needed to make the routing work can be with the similar signal levels, and the high voltage/high current traces & vias can be seperated away, not overlapping the more sensitive lower voltage/current parts.

I'm afraid not, space constraints dictate otherwise - both sides of an 74 x 100 pcb are packed tight with components, it's going to be a challenge to keep components on the signal side not underneath high voltage / noisy components on the other side.
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