MOSFET Selection LTC4364

Bubbles27 said:

Is there a way to make the turn-on time faster?

Speeding up start-up time and increasing inrush is not the answer to blown FETs.
Once you select the inrush current and assume max Vin, check if the HGATE FET has the Safe Operating Area (SOA) performance to survive start-up.

Bubbles27 said:

Which MOSFET would you recommend if we were to replace the SQJQ184E? We are very space-constrained. Should we look at a MOSFET with smaller input capacitance?

Small Ciss isn't important in hotswap applications, since we intentionally add Cgate for inrush control.
You want FETs optimized for SOA performance.
Here's an example of such a FET: PSMN3R7-100BSE

Bubbles27 said:

• The maximum expected current through the channel is 60 A, this will only occur once the MOSFET is switched completely on.
• The maximum turn-on current is 20 A.

Make sure the load is off until Cout is charged (Inrush=0A).
Otherwise, the load current is added to the SOA stress.
Use the ENOUT pin to signal the downstream load that start-up is complete.

Thank you for the response.

• The SOA of the PSMN3R7-100BSE does not look better than the SQJQ184E
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• Ashapiro said:

  Make sure the load is off until Cout is charged (Inrush=0A).
  Otherwise, the load current is added to the SOA stress.
Yes that is what I feared. We were hoping to save on having to protect an output to a DAL-B device which we expect not to fail but having the possibility that it short circuits causes this problem.
• Ashapiro said:

  Use the ENOUT pin to signal the downstream load that start-up is complete.
Yes, we will reconsider our power architecture.

Regarding the SOA graphs, I'll refer you to this article about the Spirito Effect from the great Dan Eddleman.
https://www.analog.com/en/resources/technical-articles/mosfet-safe-operating-area-and-hot-swap-circuits.html

I'm skeptical because the SQJQ184E-T1_GE3 SOA curves are perfectly straight and in parallel.
There's no Spirito region, so I wouldn't trust its advertised SOA performance.

Thank you Ashapiro, this is really helpful. I will look into this article. Which MOSFET manufacturer would you say is trustworthy?

Nexperia
Alpha & Omega Semiconductor

Good day,

We did further testing and have determined that the LTC4364 does goes into a regulation scheme for about 300 us before switching off.

We have a 15 A load connected to the output of this circuit. The shunt resistor is 1 mOhm which sets the current limit to 50 A.

During a short-circuit we expect the LTC4364 to turn off the HGATE MOSFET. In the graph below we have:

• Yellow: Input Voltage of 28.6 V
• Blue:            Gate Voltage at 40 V
• Magenta: Source Voltage of 28.6 V
• Green: Current through MOSFET

We see that the difference between Gate and Source Voltage reduces below 3 V within the first 15 us of the short-circuit being applied. Thereafter the LTC 4364 starts to regulate the HGATE and the current at around 50 A for over 200 us even though it is a hard short to ground on the output and the output voltage is near zero. Why does the LTC 4364 regulate the current for this period?

It results in the MOSFET heating up past its 175 °C  and self-destructing.

We have noted that for load currents of less than 10 A that the MOSFET survives a short-circuit on the output.

Bubbles27 said:

During a short-circuit we expect the LTC4364 to turn off the HGATE MOSFET.

Bubbles27 said:

Why does the LTC 4364 regulate the current for this period?

The LTC4364 has active current limiting.
While current is being regulated, the timer cap is charged up to 1.25V, and then shuts of the powerpath.
It is up to the designer to size the TMR cap within the SOA of the HGATE FET for the following situations:

1. Startup (charging Cout)
2. Hard short (~0Ω, foldback current limit)
3. Soft short (resistive short, no foldback)
4. Input Surge

If you want instant timeout and a more simplified part, consider the LTC4368.