fet gate drive traces differential vs ground plane

[Jamie Morken]

Hi,

I am routing a fet's gate drive trace, from a fet driver IC. The
switching frequency is 100kHz and I would like switch times of 20ns.
Is it better to run the gate drive trace "differentially" so that there
is a wire coming back from the fets drain very close to the gate drive
trace, back to the fet driver IC, or is it better to use a ground plane
for the fet's drain (for a low side fet) and run the gate drive signal
on the opposite side of the board above the plane? Which solution has
lower gate drive trace inductance and which would give faster switching
with less ringing?

cheers,
Jamie

 

[Tim Williams]

I always run a twisted pair out. Doesn't quite apply as I'm always wiring
things point to point, but a similar approach on PCB will minimize
inductance and stray currents (you wouldn't want to connect the gate drive
to somewhere upstream in series with the source current, even if it's on a
ground plane).

If I were drawing a PCB, I think I'd run traces on one side out from the
gate and source. If the board is thin, running them on opposite sides would
work, too.

Tim

 

[James Arthur]

I always run a twisted pair out.

Tim, you're a beast! If your runs were <1cm (as
they should be), you won't need twisted anything.

When are you going to start using pcbs and
groundplanes? ;-) I'd ship you some FR-4 gratis...

Cheers,
James Arthur

Doesn't quite apply as I'm always wiring

 

[Tim Williams]

http://webpages.charter.net/dawill/tmoranwms/Elec_Sample2.jpg ;-)

I often breadboard gate drives, or put them on seperate PCBs, so I use a bit
of twisted pair to run to the bridge:
http://webpages.charter.net/dawill/tmoranwms/Elec_Ind7_4.jpg
I don't have any PCB that I think is the right dimensions to do a really
good bridge, so I build it in space with copper strap and loads of bypass.
I do actually have some pretty stocky PCB, but it's 'only' 0.015" copper
(around 10oz.?), and pretty widely spaced (0.2" I think). I don't think I
can bypass that as well, at least not in the canonical parts-inserted-in-PCB
method.
http://webpages.charter.net/dawill/Images/Induction806.jpg
Seems to work well, although I need it quite a bit slower than the OP in
this thread wants (in fact, full speed (80ns) edges seem to be detrimental
to it..).

Tim

 

[James Arthur]

http://webpages.charter.net/dawill/tmoranwms/Elec_Sample2.jpg ;-)

Okay, that's decent.

I often breadboard gate drives, or put them on seperate PCBs, so I use a bit
of twisted pair to run to the bridge:
http://webpages.charter.net/dawill/tmoranwms/Elec_Ind7_4.jpg

Hmmm, FETs or IGBTs? A little squirrely for FETs I'd think, but maybe
not as bad for a compound device like an IGBT (what's their Crss like?).

I don't have any PCB that I think is the right dimensions to do a really
good bridge, so I build it in space with copper strap and loads of bypass.
I do actually have some pretty stocky PCB, but it's 'only' 0.015" copper
(around 10oz.?), and pretty widely spaced (0.2" I think). I don't think I
can bypass that as well, at least not in the canonical parts-inserted-in-PCB
method.
http://webpages.charter.net/dawill/Images/Induction806.jpg

Oh my! You need those bus-bars, yes, fine, and fat copper there,
but a separate, local, thin-foil PCB for the gate driver might
be nice. You know, if you ever re-do.

Those twisted pairs... Oh, I suppose you've tested it and
the local damping resistors save the day, plus the slow
switching speed... but it's awfully easy to make FETs sing
that way.

Seems to work well, although I need it quite a bit slower than the OP in
this thread wants (in fact, full speed (80ns) edges seem to be detrimental
to it..).

Tim

Yeah, alright. Grrr.

Cheers,
James Arthur

 

[James Arthur]

I don't have any PCB that I think is the right dimensions to do a really
good bridge, so I build it in space with copper strap and loads of bypass.

It's perfectly okay to beef up traces by soldering on bus bars.
I've seen some ecl stuff with 3/8 x 1/2" solid copper bar for
Vee, soldered on plain old FR-4. Works fine.

That might be an easy construction technique--you could augment
traces by soldering lengths of heavy bare solid copper wire on
top of 'em. Cheap, too.

Cheers,
James Arthur

 

[Tim Williams]

Hmmm, FETs or IGBTs? A little squirrely for FETs I'd think, but maybe
not as bad for a compound device like an IGBT (what's their Crss like?).

IRG4PC50UD is what I'm using. C's in the low nF range. I don't have the
datasheet at hand (I left it downstairs).

Oh my! You need those bus-bars, yes, fine, and fat copper there,
but a separate, local, thin-foil PCB for the gate driver might
be nice. You know, if you ever re-do.

The gate drives are printed, so-to-speak... pad-per-hole to be exact. I
certainly could shorten the wires from them, but that would restrict my
setup right now, which I still want to be flexible.

Those twisted pairs... Oh, I suppose you've tested it and
the local damping resistors save the day, plus the slow
switching speed... but it's awfully easy to make FETs sing
that way.

I don't have a clue if the resistors do anything, I was just told to put
them there. ;-) I'm sure on more than one occasion, they at least saved
the gate drives from destruction.

There was one occasion when I saw oscillation;
http://webpages.charter.net/dawill/Images/Induction802.jpg
Snubbing solved that, reducing dV/dt. I suspect capacitive coupling across
the gate drive coupling transformer, since a similar signal appeared at the
oscillator side. The period is approximately the propagation delay of the
gate drive.

The gate drive is pretty fast, around 200ns rise/fall time at the gate,
which is about right given the total gate charge and resistor. I think it's
50ns when unloaded.

Tim

 

[James Arthur]

The gate drives are printed, so-to-speak... pad-per-hole to be exact. I
certainly could shorten the wires from them, but that would restrict my
setup right now, which I still want to be flexible.

No, I meant you could integrate them all on a local board,
right at the IGBTs. Like this:


Q1 Q2

+-------+ +-------+
| O | | O |
|-------| |-------|
| | | |
| | | |
| | | |
+-------+ +-------+
| | | | | |
| | | | | |
+---------------------------------+
| |
| |
| Driver |
| Board |
| |
| |
| |
| |
+---------------------------------+
| | | | | |
| | | | | |
+-------+ +-------+
| | | |
| | | |
| | | |
|-------| |-------|
| O | | O |
+-------+ +-------+

Q3 Q4

Make a single-layer board, SMT [1], with the parts and
IGBT leads all soldered to the top. Then
it's easy to mod, change IGBTs, or even change driver
boards. Bottom side = GND.

[1] Old-style parts surface mount easily too, and it saves
drilling.

That's clean electrically and easy to work on.
You could still fly the heavy stuff with airborne copper,
or you could run PCB traces, bolstered with strap or bar
or such. That's what I meant.

For later. You know, when you start mass production.


Cheers,
James Arthur

 

[Joerg]

IRG4PC50UD is what I'm using. C's in the low nF range. I don't have the
datasheet at hand (I left it downstairs).


The gate drives are printed, so-to-speak... pad-per-hole to be exact. I
certainly could shorten the wires from them, but that would restrict my
setup right now, which I still want to be flexible.


I don't have a clue if the resistors do anything, I was just told to put
them there. ;-) I'm sure on more than one occasion, they at least saved
the gate drives from destruction.

There was one occasion when I saw oscillation;
http://webpages.charter.net/dawill/Images/Induction802.jpg
Snubbing solved that, reducing dV/dt. I suspect capacitive coupling across
the gate drive coupling transformer, since a similar signal appeared at the
oscillator side. The period is approximately the propagation delay of the
gate drive.

Could also be the leakage inductance of the transformer.

The gate drive is pretty fast, around 200ns rise/fall time at the gate,
which is about right given the total gate charge and resistor. I think it's
50ns when unloaded.

200nsec isn't "pretty fast" ;-)