MOSFET gate circuit for PWM drive

[pimpom]

Can anyone please explain (or suggest online reading that
explain) what factors have to be taken into consideration when
driving a power MOSFET with a PWM signal for power control such
as motor speed or power supplies, especially at the gate?

I understand that a MOSFET gate is essentially an open circuit at
low frequencies, but at higher frequencies, capacitive effects
modify the necessary drive power and switching times (and
possibly other things). Simple hobby-type circuits often drive
the gate with resistive coupling from an opamp or something with
a similar output, but more sophisticated circuits usually include
capacitors, diodes and inductors between driver and MOSFET.
Please help me understand the principles.

 

[Hammy]

On Mon, 6 Oct 2008 01:07:36 +0530, "pimpom" <pimpom@invalid.invalid>
wrote:

Can anyone please explain (or suggest online reading that
explain) what factors have to be taken into consideration when
driving a power MOSFET with a PWM signal for power control such
as motor speed or power supplies, especially at the gate?

I understand that a MOSFET gate is essentially an open circuit at
low frequencies, but at higher frequencies, capacitive effects
modify the necessary drive power and switching times (and
possibly other things). Simple hobby-type circuits often drive
the gate with resistive coupling from an opamp or something with
a similar output, but more sophisticated circuits usually include
capacitors, diodes and inductors between driver and MOSFET.
Please help me understand the principles.

Read through these.

http://www.irf.com/technical-info/appnotes.htm#acdc

AN-944: Use Gate Charge to Design the Gate Drive Circuit for Power
MOSFETs and IGBTs PDF

AN-947: Understanding HEXFET Switching Performance

Go through there archives tons of info!

 

[pimpom]

"Hammy" <spamme@hotmail.com> wrote in message
news:s97ie4l2nv9fdp9o288p6gkjrgl0k22t90@4ax.com...

On Mon, 6 Oct 2008 01:07:36 +0530, "pimpom"
pimpom@invalid.invalid
wrote:

Can anyone please explain (or suggest online reading that
explain) what factors have to be taken into consideration when
driving a power MOSFET with a PWM signal for power control such
as motor speed or power supplies, especially at the gate?

I understand that a MOSFET gate is essentially an open circuit
at
low frequencies, but at higher frequencies, capacitive effects
modify the necessary drive power and switching times (and
possibly other things). Simple hobby-type circuits often drive
the gate with resistive coupling from an opamp or something
with
a similar output, but more sophisticated circuits usually
include
capacitors, diodes and inductors between driver and MOSFET.
Please help me understand the principles.


Read through these.

http://www.irf.com/technical-info/appnotes.htm#acdc

AN-944: Use Gate Charge to Design the Gate Drive Circuit for
Power
MOSFETs and IGBTs PDF

AN-947: Understanding HEXFET Switching Performance

Go through there archives tons of info!

Thanks. I don't have time to go through everything right now, but
it appears to be a very good source. I've bokmarked it. Thanks
again.

 

[Jon Slaughter]

"pimpom" <pimpom@invalid.invalid> wrote in message
news:gcb52a$ig1$1@news.albasani.net...

Can anyone please explain (or suggest online reading that explain) what
factors have to be taken into consideration when driving a power MOSFET
with a PWM signal for power control such as motor speed or power supplies,
especially at the gate?

I understand that a MOSFET gate is essentially an open circuit at low
frequencies, but at higher frequencies, capacitive effects modify the
necessary drive power and switching times (and possibly other things).
Simple hobby-type circuits often drive the gate with resistive coupling
from an opamp or something with a similar output, but more sophisticated
circuits usually include capacitors, diodes and inductors between driver
and MOSFET. Please help me understand the principles.

The gate has capacitance. You must charge the capacitance. The faster you
charge it the more power you are using. The mofset also has it's own
internal resistance that dissipates power(R_DS(on)) along with power wasted
from cross conduction in an hbridge.

There are some good docs about this out there such as AN786 from Microchip
which gives equations for calculating the total power dissipation.

 

[pimpom]

"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:VQaGk.1684$Ei5.391@flpi143.ffdc.sbc.com...

"pimpom" <pimpom@invalid.invalid> wrote in message
news:gcb52a$ig1$1@news.albasani.net...
Can anyone please explain (or suggest online reading that
explain) what factors have to be taken into consideration when
driving a power MOSFET with a PWM signal for power control
such as motor speed or power supplies, especially at the gate?

I understand that a MOSFET gate is essentially an open circuit
at low frequencies, but at higher frequencies, capacitive
effects modify the necessary drive power and switching times
(and possibly other things). Simple hobby-type circuits often
drive the gate with resistive coupling from an opamp or
something with a similar output, but more sophisticated
circuits usually include capacitors, diodes and inductors
between driver and MOSFET. Please help me understand the
principles.


The gate has capacitance. You must charge the capacitance. The
faster you charge it the more power you are using. The mofset
also has it's own internal resistance that dissipates
power(R_DS(on)) along with power wasted from cross conduction
in an hbridge.

There are some good docs about this out there such as AN786
from Microchip which gives equations for calculating the total
power dissipation.

Thanks. I've downloaded AN786 and it also appears to be very

helpful. I'll study it in detail.