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Driving MOSFET gate from logic outputs

S

seanacais

Jan 1, 1970
0
Would a common base transistor configuration be a good way to drive
the gate of an N-MOSFET from a logic output?

The MOSFET is the high side of an H-Bridge sitting at about 40V. The
gate will be drawn to 48V which is much too high for logic outputs.
Since I want to run the bridge with high inputs it seems that a common
base configuration would give large voltage amplification in phase
with the inputs. The output of the logic gate would be fed into the
emitter and the collector would hook to the MOSFET gate (and +48V
through a current limiting resistor. I'm not positive but it seems as
if a resistor between the collector and the gate would be a good idea
as well.

In reading about this configuration several references stated that
it's normally used in RF circuits because of the large voltage
amplification. The bridge is to be PWM driven at possible tens of KHz
but nowhere near RF. Is there something I'm missing about this that
would make it unsuitable for this application? Insights, suggestions
and hints are all welcome!

Thanks,

Kevin
 
J

John Popelish

Jan 1, 1970
0
seanacais said:
Would a common base transistor configuration be a good way to drive
the gate of an N-MOSFET from a logic output?

The MOSFET is the high side of an H-Bridge sitting at about 40V. The
gate will be drawn to 48V which is much too high for logic outputs.
Since I want to run the bridge with high inputs it seems that a common
base configuration would give large voltage amplification in phase
with the inputs. The output of the logic gate would be fed into the
emitter and the collector would hook to the MOSFET gate (and +48V
through a current limiting resistor. I'm not positive but it seems as
if a resistor between the collector and the gate would be a good idea
as well.

In reading about this configuration several references stated that
it's normally used in RF circuits because of the large voltage
amplification. The bridge is to be PWM driven at possible tens of KHz
but nowhere near RF. Is there something I'm missing about this that
would make it unsuitable for this application? Insights, suggestions
and hints are all welcome!

The common base stage can be used to level shift and voltage
amplify the logic swing, but it provides little drive
current to quickly charge and discharge the gate
capacitance. You will probably need to add a complementary
emitter follower to provide this current capability.
 
D

D from BC

Jan 1, 1970
0
Would a common base transistor configuration be a good way to drive
the gate of an N-MOSFET from a logic output?

The MOSFET is the high side of an H-Bridge sitting at about 40V. The
gate will be drawn to 48V which is much too high for logic outputs.
Since I want to run the bridge with high inputs it seems that a common
base configuration would give large voltage amplification in phase
with the inputs. The output of the logic gate would be fed into the
emitter and the collector would hook to the MOSFET gate (and +48V
through a current limiting resistor. I'm not positive but it seems as
if a resistor between the collector and the gate would be a good idea
as well.

In reading about this configuration several references stated that
it's normally used in RF circuits because of the large voltage
amplification. The bridge is to be PWM driven at possible tens of KHz
but nowhere near RF. Is there something I'm missing about this that
would make it unsuitable for this application? Insights, suggestions
and hints are all welcome!

Thanks,

Kevin

I use mos driver IC's for low and high side switching.
D from BC
 
T

They call me Frenchy!!

Jan 1, 1970
0
Consider one of these???
http://www.st.com/stonline/products/literature/ds/7785/l6387.htm
It drives the high and low side of half the H-bridge. Two logic
inputs so you can control the dead time, etc. Bootstrap circuit
allows use of N-chan mosfets. They have a whole family of devices
that might be better for your particular application. I believe you
could run the whole H-bridge from one driver if you wanted. They even
have versions with the MOSFETs built in. Watch out for leakage on the
bootstrap cap @ 100% duty cycles.


-they call me frenchy.
 
M

Marra

Jan 1, 1970
0
Would a common base transistor configuration be a good way to drive
the gate of an N-MOSFET from a logic output?

The MOSFET is the high side of an H-Bridge sitting at about 40V. The
gate will be drawn to 48V which is much too high for logic outputs.
Since I want to run the bridge with high inputs it seems that a common
base configuration would give large voltage amplification in phase
with the inputs. The output of the logic gate would be fed into the
emitter and the collector would hook to the MOSFET gate (and +48V
through a current limiting resistor. I'm not positive but it seems as
if a resistor between the collector and the gate would be a good idea
as well.

In reading about this configuration several references stated that
it's normally used in RF circuits because of the large voltage
amplification. The bridge is to be PWM driven at possible tens of KHz
but nowhere near RF. Is there something I'm missing about this that
would make it unsuitable for this application? Insights, suggestions
and hints are all welcome!

Thanks,

Kevin

You could use opto isolators to drive the gates of the FETs.
 
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