Re: Bizzare behaviour from SG/UC3525

Pooh Bear wrote:
[snip]
Graham

Gidday Graham, 


Good day to you too Terry.


try this: lift OUTA and OUTB pins. "swap" them with little dangly wires. 


Ummm... guess what I was thinking of doing next ! It's called desperation !



If the OUTA pin suddenly starts working and OUTB goes funny, then its
something to do with the external connection(s) to OUTA


Yuh . That figures. But a 1~2 cm track - with careful attention to routeing ?

perhaps there is a solder short, or noise coupling into that pin. I read the 3525 data***, and its not clear from the block diagram how such an event might occur, but without looking at a real diagram theres no way to tell.




then pull the same trick with the IR2110 inputs. 


OK.


some gatedrive outputs get pissy when output inductance pulls them below
0V (or above Vcc). a SOT23 dual schottky on each output can eliminate
that potential problem.


Interesting.

there are some Unitrode app notes that talk about this, and IIRC bipolar output stages are most susceptible, FET output stages are pretty much immune. Some Unitrode datasheets even say "no output schottky diodes necessary" but alas I cant recall which ones.


I omitted to mention - for sake of simplicity- that I briefly decoupled the
3525 shutdown pin with a 10 nF cap to local gnd to avoid any noise pickup.
Frankly it shouldn't be a sensitive node but I'm really puzzled now.

That 'helped' the odd 'jitter' on the hi-side drive. I thought I was on the
home run - I selected the switch position that stops the 3525 outputs and
removed power. A couple of second later the IR2110 went *bang* - well fizzz to
be more accurate - (  no IGBTs danaged though ).

What's weird is that it was the low side driver that went short ! About 2 ohms
from Vs to  Lout after it went bust !



try loading them capacitively (perhaps 10-20pF) sans IR2110, see if the
waveforms are still OK.



waveforms are fine sans IR2110.

I'm driven bonkers how the pulse width on OutA and OutB is different whern
it's in circuit ( one presumes there's a clue here but I haven't yet been able
to make any sense of it  ). One might assume a load current issue but I have
only 33k from DCbus/2 to the switching point !


true, along with whatever stray capacitance (FETs, layout etc), which will cause nice fast current spikes and associated LdI/dt voltage spikes. the dV/dt wont change much at light load (and may even get worse), so capacitively coupled noise is still there.

does the magnitude of Vdc affect the crazy pulse?



set the DC bus to 0V, provide a separate high-side power supply and see
what happens - with no actual output switching, does the problem occur.



I admit - I'm tempted to provide a specific Vboot supply anyway - using a 555
for example.

But this ain't the issue


my SOP is to provide multiple well-isolated linear power supplies for every gatedrive, and use those exclusively, until my power stage can happily drive full load all day. Or at least until I'm happy the gatedrive really works.

doing this allows you to ignore the charge-pump power supply aspects, UVLO etc. and just concentrate on banging the gates around.



place a series resistor between the 3525 & 2110, perhaps 1k. 


I'll try that !


Cheers, Graham 

bung a lamp in series with the VDC supply, its cheaper than a FET.


Cheers
Terry

PS the PDF offer stills tands.
.

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