3776 considerations
Posted by
beer@s...
on 2000-01-18 20:54:07 UTC
On 19 Jan, CAD_CAM_EDM_DRO@onelist.com wrote:
resistors, the gate to source breakdown voltage and the turnon voltage
of the FETs. If we assume a 36 volt supply and Z34 FETs ( Vgs max=20 ),
a 470 ohm ( to the 3776 ) and a 330 ohm ( gate to source ) resistor will
both turn the FETs on nicely and keep them within their limits.
( Be wary of using resistors too small. The 3776 has a maximum
dissipation spec and that spec is NOT conservative.)
If the resistor values were reversed, then a zener would be required.
However, this is a bad idea for two reasons - one, extra parts are
required, but two, the charge on the gate must exit this larger
gate-source resistor at turnoff, slowing turnoff time. Slower turnoff
means hotter FETs and lower switching speeds.
The Z24/34 FETs are fully on at 10 volts - there is little need to
supply too much higher a level than that. Pick 12 volts as an
"electronic" kinda number and work back from there.
Note that with a substantially lower motor supply voltage, the resistor
values shown (470/330) are a BAD choice. The FETs will then get less
than 10 volts at the gate, will not turn on fully, will live in the
linear region, will get hot, will get very hot, will then fail.
Alan
--
Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta
> Message: 2You have to consider the motor voltage, the ratio of the dividing
> Date: Tue, 18 Jan 2000 07:14:45 -0600
> From: Walter McCarthy <wmccarthy@...>
> Subject: Re: Digest Number 315
>
> On 3776 chips and troubles with fried drivers.
> On my stepper driver board using 3776, I found that the gate bypass diodes were
> essential to save the FET driver chips. The Ericcson book suggested they were
> optional. I found they were essential. Without the diode bypasses on the gates,
> the Fets fried in minutes. I lost several chips before sorting this glitch out.
resistors, the gate to source breakdown voltage and the turnon voltage
of the FETs. If we assume a 36 volt supply and Z34 FETs ( Vgs max=20 ),
a 470 ohm ( to the 3776 ) and a 330 ohm ( gate to source ) resistor will
both turn the FETs on nicely and keep them within their limits.
( Be wary of using resistors too small. The 3776 has a maximum
dissipation spec and that spec is NOT conservative.)
If the resistor values were reversed, then a zener would be required.
However, this is a bad idea for two reasons - one, extra parts are
required, but two, the charge on the gate must exit this larger
gate-source resistor at turnoff, slowing turnoff time. Slower turnoff
means hotter FETs and lower switching speeds.
The Z24/34 FETs are fully on at 10 volts - there is little need to
supply too much higher a level than that. Pick 12 volts as an
"electronic" kinda number and work back from there.
Note that with a substantially lower motor supply voltage, the resistor
values shown (470/330) are a BAD choice. The FETs will then get less
than 10 volts at the gate, will not turn on fully, will live in the
linear region, will get hot, will get very hot, will then fail.
Alan
--
Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta