isolated DC-DC convertor and mosfet driver

[shortbus=]

Hi, I know only enough to get myself in trouble. My question is about using an isolated DC-DC module instead of a boot strap circuit when using a half bridge driver to make a high side switch. I know I could use a second small mosfet on the low side of the half bridge to recharge the boot strap but why not the isolated convertor?

Any one ever done this or am I just dreaming?

 

[shortbus=]

Look at a typical half bridge driver diagram:

The high voltage driver diagrams are different. They show the high side completely isolated from the low side. And from what I understand the high and low can be used separately. https://www.digchip.com/datasheets/download_datasheet.php?id=3264484&part-number=FAN7390

 

[rjenkinsgb]

And since I'll only be using the high side, won't that eliminate any AC from the low side turning off, since it won't even be in the circuit?

If you are switching a load from a positive supply using an N channel device, the switched output is it's source terminal, which will be changing voltage as it switches.

The high side control voltage is referenced to the source, regardless of what else in the circuit or what it is controlling.

 

[shortbus=]

MGJ2D121515SC is an example.

Hi. When I asked for a schematic showing how it was done, I didn't mean a real schematic of one of your projects, but just a sketch of the connections and components needed to operate a high side gate. I've been looking and searching for sample circuits using an isolated DC-DC converter and gate drive. While I can find things saying it is and can be done, no samples of how to do it. None of the companies selling the converters are much help. Thank you for any help you can give.

 

[rjenkinsgb]

I've been looking and searching for sample circuits using an isolated DC-DC converter and gate drive.

The 555 based one I posted should work perfectly?

 

[Tony Stewart]

https://power.murata.com/pub/data/apnotes/gdan-01.pdf

 

[ronsimpson]

Hi Ron, that Murata module is the one I bought. Would it be possible for you to show me a schematic of how your using it with a gate driver?

I cannot see what circuit you want but: Here is an example. The IRS2186 has two Gate Drivers. The bottom one is attached to ground and lives from VCC. The top gate driver lives on the Source of the top MOWFET. In normal use, when the bottom FET is on current frows through the diode and charges up the capacitor that holds power for the top Gate Driver. This works well when you keep switching. If the duty cycle drops to zero the capacitor runs out of power.

In this case I want to have the top FET on for a very long time and I need to keep power to the top side. Here I added, in red, a 12V to 15V isolated supply.

 

[shortbus=]

In this case I want to have the top FET on for a very long time and I need to keep power to the top side. Here I added, in red, a 12V to 15V isolated supply.

Thank you for the drawing. I didn't know how to handle the isolated DC common/ground. For some reason I thought adding the isolated common to the rest of a circuit no longer made it isolated.

Like I have said I know enough about electronics to be dangerous, I can wire a house or a car but electronics aren't always the same. Again thank you for clearing it up for me.

 

[shortbus=]

https://power.murata.com/pub/data/apnotes/gdan-01.pdf

Thank you for the link! Was that from the Murata website? I don't know how I missed it, or why Murata didn't point me to the app note.

 

[shortbus=]

The 555 based one I posted should work perfectly?

I know it would, but it seemed too complicated and would mean making a larger board, if a isolated DC-DC converter would work the board could be smaller.