During a search for MOSFET drivers I came across a couple of interesting
tutorials by Julian Ilett using a dual complementary optoisolators to
switch the gate of a high side N-channel MOSFET between the source terminal
and a bootstrap capacitor. If you're interested here are some of them:

https://www.youtube.com/watch?v=3DtebprQvrqk4&t=3D425s

https://www.youtube.com/watch?v=3Duff1nzFo140

https://www.youtube.com/watch?v=3DiNYeww1Sjzk&t=3D61s

Each have an excellent explanation of both the use of a bootstrap capacitor
for the gate and the use of optoisolators in a complementary configuration.

Before moving on, personally I'm not interested in black box gate drivers
at the present time. I'm really interested in how those drivers work and
how they can be duplicated with junkbox discrete parts.

My real interest is how to adapt what is a trivially simple driver into
specifically some step down lithium battery charging projects I have near t=
he top
of my project list. However I'm running into some issues in adapting these.
Here are the main problems:

1. In Julian's examples the load (a lightbulb) is grounded. So when the
mosfet is off, the terminals of the bulb, along with the source terminal of
the mosfet are at ground potential. A battery as a load will never be
grounded at the source. So for example charging a 40V pack, the source of
the high side mosfet is always at 30V or higher.

2. In the same vein, unlike Julian's example where the top voltage in the
circuit is 12V, the main charging voltage will be about 50V. While 12V is
clearly good as Vgs, 50V clearly is not. I have some golf cart DC/DC
converters that will output 12V or so, but getting it switched is going to
be a bit of a challenge.

3. The optoisolators are the third issue. They are 6N139. Their CTR is
excellent, so they can drive the totem pole for the gate, However, they
have a maximum Vce of 18V, running into the same problem as #2.


So given these circumstances, I'm interested in the following:

1. How to switch the bootstrap cap negative terminal between the battery
voltage and ground to charge it?

2. How to protect the optos which may have up to 44V between the the
collector and the emitter?

3. How to limit the gate voltage to a standard Vgs of 15-20V?

Any suggestions welcome.

BAJ

--=20
Byron A. Jeff
Associate Professor: Department of Computer Science and Information Technol=
ogy
College of Information and Mathematical Sciences
Clayton State University
http://faculty.clayton.edu/bjeff
--=20
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