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High-Side MOSFET gate driver voltage generation

W

wolti_At

Jan 1, 1970
0
Hello,

I am looking for some advice for controlling an N-channel MOSFET
(IPD64CN10N). Right now for testing I was using an IR2112 driver which
I already had available and a small isolation transformer. I do not
like this because this is a) quite expensive and b) a not so elegant
solution. My application is special in the case that the source of the
MOSFET does not stay on a constant potential - instead its source
might change between -24V and 24V referenced to ground. This voltage
is constant with respect to the switching times of the MOSFET(100kHz).
The input voltage available is +24V. I have investigated the following
solutions:

a) Using a charge pump would allow me to generate a voltage above the
positive supply rail to control the MOSFET. The main problem I see is
that at the lower end of the operating point I would end up with a to
high gate-source voltage and would destroy the MOSFET. Adding a zener
in series to the gate would solve this problem but would add an extra
and high load to the charge pump.
b) Use a small DC/DC converter with proper isolation. In this case
driving the mosfet is simple but the costs are high.
c) I can not use the standard bootstrap methods provided for example
by the IR2112 driver because I have no way to charge the bootstrap
capacitor when the MOSFET is turned off.

Thanks for all possible input,
Christian
 
L

legg

Jan 1, 1970
0
Hello,

I am looking for some advice for controlling an N-channel MOSFET
(IPD64CN10N). Right now for testing I was using an IR2112 driver which
I already had available and a small isolation transformer. I do not
like this because this is a) quite expensive and b) a not so elegant
solution. My application is special in the case that the source of the
MOSFET does not stay on a constant potential - instead its source
might change between -24V and 24V referenced to ground. This voltage
is constant with respect to the switching times of the MOSFET(100kHz).
The input voltage available is +24V. I have investigated the following
solutions:

a) Using a charge pump would allow me to generate a voltage above the
positive supply rail to control the MOSFET. The main problem I see is
that at the lower end of the operating point I would end up with a to
high gate-source voltage and would destroy the MOSFET. Adding a zener
in series to the gate would solve this problem but would add an extra
and high load to the charge pump.
b) Use a small DC/DC converter with proper isolation. In this case
driving the mosfet is simple but the costs are high.
c) I can not use the standard bootstrap methods provided for example
by the IR2112 driver because I have no way to charge the bootstrap
capacitor when the MOSFET is turned off.

Thanks for all possible input,
Christian

1) Don't float the source - use a p-channel mosfet in this location.

2) Include the floating supply in the +/- 24V source generation.

3) Re-examine your budget to put dollars and cents on your current
generalizations and prejudices re cost. Include volume considerations.
All other suggestions will have cost implications that you are
with-holding from the analysis - wasting our time, and yours ($).

4) Post a schematic. You may just be thinking yourself into a corner
unnecessarily.



RL
 
W

wolti_At

Jan 1, 1970
0
1) Don't float the source - use a p-channel mosfet in this location.

2) Include the floating supply in the +/- 24V source generation.

3) Re-examine your budget to put dollars and cents on your current
generalizations and prejudices re cost. Include volume considerations.
All other suggestions will have cost implications that you are
with-holding from the analysis - wasting our time, and yours ($).

4) Post a schematic. You may just be thinking yourself into a corner
unnecessarily.

Hello,

Sorry for my long reply - I have now decided to use a floating supply
with a FOD3181 0,5A optoisolated gate driver. One reason for this was
that the source of the n-channel MOSFET goes below zero volts and
almost all high side drivers for MOSFETs don't tolerate a negative
voltage with reference to their logic ground. Of course the source
also goes above zero volts because otherwise I would have not needed a
MOSFET at all. Posting a schematic is not possible for me right now
because part of my work will be presented in a paper at the
eurocon2009.

Thanks,
Christian
 
Hello,

Sorry for my long reply - I have now decided to use a floating supply
with a FOD3181 0,5A optoisolated gate driver. One reason for this was
that the source of the n-channel MOSFET goes below zero volts and
almost all high side drivers for MOSFETs don't tolerate a negative
voltage with reference to their logic ground. Of course the source
also goes above zero volts because otherwise I would have not needed a
MOSFET at all. Posting a schematic is not possible for me right now
because part of my work will be presented in a paper at the
eurocon2009.

Thanks,
Christian

how about something like adum6132, supply,isolation and driver in one
chip,
though only +/-200mA

-Lasse
 
L

legg

Jan 1, 1970
0
Hello,

Sorry for my long reply - I have now decided to use a floating supply
with a FOD3181 0,5A optoisolated gate driver. One reason for this was
that the source of the n-channel MOSFET goes below zero volts and
almost all high side drivers for MOSFETs don't tolerate a negative
voltage with reference to their logic ground. Of course the source
also goes above zero volts because otherwise I would have not needed a
MOSFET at all. Posting a schematic is not possible for me right now
because part of my work will be presented in a paper at the
eurocon2009.

You mean that the source goes below -24V, don't you? Most drivers are
configured to operate referenced to the most negative power rail, not
the 0V (system) reference.

If you are attempting to use control logic referenced to system 0V,
then this may be your trouble. Obviously, transformer-coupled or
optically coupled drivers don't care where the signal input reference
is, but lower-cost solutions will likely adopt the -rail driver
reference convention, if only to reduce the cost of driving one set of
switches; those switches tied to that rail.

If one of the switched nodes is intentionally designed to exceed a
supply rail, it may be of some advantage to configure the circuit so
that the excess occurs with respect to the + rail, when using
N-Channel or NPN switches.

Level shifting from the system's zero voltage rail is most simply
performed at signal logic impedance levels. These signals can then be
buffered to the drive power stage without inherently involving the act
of isolation. Combining the two functions may be a significant cost
aggravator.

RL
 
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