Re: +48 precharge

On 29-03-2024 15:33, John Larkin wrote:

On Fri, 29 Mar 2024 11:54:02 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:
 

On 24-03-2024 23:16, John Larkin wrote:

On Sun, 24 Mar 2024 16:10:25 -0400, Joe Gwinn <joegwinn@comcast.net>
wrote:
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On Sun, 24 Mar 2024 12:14:57 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:
>

>
We have a board that tends to blow up.
>
It has a couple of isolated dc/dc converters, gate driver chips and
big mosfet full-bridges driving transformers. The gate drivers get
their inputs from an FPGA.
>
The probelm is that the +48 volts to the h-bridges comes up at power
turn-on, but the FPGA is configured some minutes later, after Linux
boots up. And I don't entirely trust the FPGA outputs meanwhile.
Possibly never.
>
After designing many complex fixes, a simple fix is to precharge the
module's +48 rail gently, and slam it on hard after everything is
verified stable.
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<https://www.dropbox.com/scl/fi/i7mgvnad9h1itxf8p0t76/P941_942_Precharge_1.jpg?rlkey=tv3rh3kzlw40th20oes6hlhr2&raw=1>
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The one-shot gets its I'M OK trigger from the FPGA, which can only
happen if the FPGA is working, I hope.

>
Can you require significant net charge transfer on a short period of
time from the FPGA before the one-shot will trigger?
>
Joe Gwinn

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The retriggerable one-shot is 1 microsecond, and the FPGA (once it's
alive and well) will clock it at 2 MHz. No clock, no hard +48.
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We'll have seconds available to charge the filter caps, before we make
the I'M_OK signal to enable stuff. It a Linux system!
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The FPGA will have a defined state during startup, and your gatedrivers
have UVLO, so during boot the FPGAs are defined as high-z, and you use a
pull-down resistor to pull the outputs low during boot. Normally you
would use a delay for the VDD for the gatedrivers, so both ramping VCC
and controlled VDD start makes sure there are no conflict.
>
I am currently working on a 3MW converter. We are controlling it
directly from a FPGA, like described above.
>
When I worked at Vestas doing 6MW wind turbine, we did it the same way.
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Cheers
>
Klaus

 We have blown up fets and drivers on this product, which is
embarassing, so we're doing everything reasonable to stop that.
 We have been using LTC4444 gate drivers, which are one possible
villain. We're switching to the UCC27712 next rev. But it sure sounds
prudent to apply +48 to the bridges last thing in the startup
sequence, as opposed to a kilowatt of +48 first thing at powerup...
before even the logic supplies are up. About 15 important things
happen between powerup an steady-state operation, and there could be
lots of gotchas lurking.
 I've thought about really high-power electronics, like the ends of a
megavolt DC transmission line. That's scary. Failures must be
spectaular. And expensive.
 I prefer products that won't break your foot if you drop one.
 

This app note from Lattice describes the state of the pins during startup:
https://www.latticesemi.com/~/media/8DFD1009E98E42C9B53C4BADE6D8B3E5.ashx
It exits UVLO at 2.5V, so you just need to have the UVLO of the gatedriver to be higher than that.
But, yes, your idea to apply the 48V last is a good idea. In a turbine that is really not an option.
The scary stuff about 1000V/2000A is that an error occurs, busbar can spray molten copper in all directions, and your literally toast. Safety is really important on high power stuff.
UCC27712 is a good choice, the Texas/Unitrode guys knows how to make good gatedrivers :-)