LT4320-1 Low load (controller minimum threshold voltage not reached)

Question

Hello, 
 I am reaching out as I haven't seen the question answered in other topics. 
 I am working on an rectifier device mounted on a generator, aiming at providing 28VDC/600Amps output. 
 
 I have 3 IPT007N06N MOSFET in // for each switching element with a Rdson=0.75mOhm. (the aim is to replace a diode bridge so I have to use the lowest Rdson available, and avoid heavy heatsinks). 
 It is a 12-pulse rectifier as below:

With an output load of 600Amps, each MOS sees a voltage drop of around 40mV which is within the range of the controller. 
 
 However, at low output loads (60Amps), the voltage drop (Vds) across each MOS is down 5mV, which is under the specified 26mV of the datasheet. 
 
 I tried to simulate the circuit using the LTSpice demonstration circuit provided with low Rdson MOSFET to spot that issue in the simulation, but it appears to work fine. 
 
 Below are some signals of interest: 
 - In blue is the gate signal of one Top MOSFET 
 - In Green is the Vds voltage across a top MOSFET 
 - In red is the drain current across a top MOSFET

I don't really understand why the Gate signal is triggered as the Vds that the controller is monitoring (through the Outp - INn pins) has not even reached the threshold of -26mV. That simulation has exactly the same behavior as the one provided by LT: 
 
 Can someone advise if the controller will work below the 26mV voltage drop of the MOS as it appears to be the case on the simulation? And what triggers the gate signal? I assumed the controller triggered the Gate signal as soon as the threshold voltage of 26mV was reached, but this is not what I am seeing here... 
 
 Thanks for your help, 
 Max

ntenorio · Accepted Answer

Hi MaxBoot , 
 I have consulted product experts of this device to help you answer this query. The LT4320 will attempt to regulate the &ldquo;forward voltage&rdquo; across the top FET to be 40mV, typical. If the current x RDSON voltage is less than 40mV, then the gate should not be fully enhanced to maintain 40mV instead of some lower voltage. This will be near the threshold voltage, usually in the 2V to 4V above source (depending on FET). If current x RDSON is greater than 40mV, then the gate should be fully enhanced as the controller tries to reduce the &ldquo;forward voltage&rdquo; but runs into the limits of the MOSFET in use. The bottom FET acts more like a switch, either on or off instead of a regulated diode like the top FET. 
 For taking a closer look and thorough investigation on this, you may try reaching out to local FAE to provide support on the product by submitting a support ticket through this link: Technical Support Form | Analog Devices . 
 Or if you can give me your project information and estimated annual usage (EAU), I can help connect you to a local FAE. Thanks. 
 Regards, 
 Noel)

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LT4320-1 Low load (controller minimum threshold voltage not reached)