Re: TC1410 MOSFET driver problems


<john@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:1173924856.137156.228480@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On 14 Mar, 03:53, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:
<j...@xxxxxxxxxxxxxxxxxxxxx> wrote in message

[...]

The test circuit in the data *** is a 100 kHz square wave. The rise
and
fall times of the IC are 25-40 nSec. While the circuit was working, the
driver stayed cold, and my simulation showed less than 100 mW. The
three-transistor driver has rise and fall times of about 200 nSec. The
Microchip App Note shows a 250 kHz example with 16 mW driver power.

I think there is something else wrong. I have not heard back yet from
Microchip. I have also requested samples of some TI parts that have an
output stage consisting of MOSFETs in parallel with bipolars, which
provides better drive through the transition area where the MOSFET
actually
switches.

Thanks for your comments.

Paul

Yes. I missed the 100kHz test diagram. Just spiced it myself though
and am seeing roughly 100ma rms gate drive but more interestingly (at
switch off) there's a 10V peak, delayed negative voltage pulse feeding
back to the driver (about 200nS wide). Seems the driver's reverse
limit is 5V.
john


My reply from Microchip:

Problem Resolution:
I suspect the problem is that you are driving too much current. Since we
don't know what the impedance of the mosfet is, we don't offer parts with
built in limiting, but you can do this as you obsereved by adding a
resistor.

You will need a driver with more current output. More current output
translates to more speed. Also, reducing the capacitance of the gate will
also help to decrease time, since there is less capacitor to charge.

To know what currents are being driven, put a scope probe on either side of
the series resistor into the mosfet gate and put the scope in differential
mode. You will then see the spike of current driven by the driver. If it
exceeds the maximum spec, you will need a larger resistor to properly
protect the mosfet.

You can figure out the worst case resistor size with ohms law. For example
16V into ground is a 16V difference, with a 500 mA drive, is 32 ohms. So
your 100 ohms is more than enough protection assuming these cases.
However, its probably overkill for this part. In either case, you said you
were migrating to a 3A part, so 1/6th the resistance should provide
adequate protection, or about 6 ohms.

==============================================================================

I ran a simulation with their TC1413 3 amp driver, and it seems that the
current is limited to 2.5 amps using 3 ohm driver resistance. The gate
resistance of the transistor seems to be only 0.8 ohms. Much depends on the
actual rise and fall times before being lowered by the capacitor as shown
on the test circuit. I used 20 nSec, but if I use 10 nSec the current peak
is 3.5 amps, again over the spec. I have a hard time believing a 10 nSec
transient 20% over specified peak would destroy the device. Also, I am
fairly sure I first put a 100 ohm limiter on the output and it still
failed.

I did not see the negative voltage pulse you describe. I am modeling the
gate very simply with a 9 nF inductance, and the 3800 pF capacitance. I
have a model for the HUF75645, which I will try. I think I will be safe
using the 3 A drivers and a smaller MOSFET with 350 pF gate, but I am leery
of the Microchip parts now, and I may try the TI TPS2817, which has a much
beefier output stage.

If you do see a negative spike that may cause the problem, I'd appreciate a
copy of your LTSpice ASC file.

Thanks,

Paul


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