Sunday, September 17, 2017

Failure Analysis

I received a Gen 3 regulator from the field which had been working well, and then stopped providing any drive the Alternators field.  Over the past couple of days I have been assessing what  failed.  This is my findings:

The FET driver chip (U7).  It is able to properly create the needed gate voltage levels, but it is unable to support any load placed on it.

The FETs used are a massive overkill for this application, on purpose:  I wanted to offer good efficiency (less wasted heat) and to be able to support large frame alternators without any concern of the ability to carry current.  And I used two of them.  (The connector is actually the limiting factor - 18A rating).  These large FETs do come with a rather large input capacitance which has to be driven by the FET driver chip.  Calculations show the driver chip is able to meet the FET requirements, and measurements confirm it, though there is indeed little margin.   Even so my suspension is the driver chip is bring pushed a too hard - causing the failure.

As a corrective action I will be increasing the gate series resistors from 10 ohms to 27 ohms.  This will reduce gate current significantly (about half) with little impact to the turn-on time and power (a gain of 25nS, and 20uW or so per FET).  Changes are:

R26, R29:  Was 10 ohm →   27 ohm

All regulators shipped after today will be assembled with these modifications.  If anyone wishes to refit their existing regulator the above parts may be sourced locally  (resistors are SM0603  --  1%), or I can send you the components, or you can mail the regulator back to me and I will refit and return it to them.   Please Email me if you are interested in me sending out the parts or reworking your existing regulator.

An alternative field modification may also be made to gain margin:  Remove R29. Be carefully not to damage the PCB if you wish to do the above mod at a later time.  Removing R29  will disable the redundant FET and reduce the driver load.  The remaining single FET is able to support 100's of amps peak, and is more than sufficient for any alternator application.

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