Checking Hot Swap Fault Retry Timing Against MOSFET Heating

A hot swap MOSFET may survive one limited current pulse and overheat when an automatic retry repeats that pulse. The fault timer and restart behavior determine a thermal sequence, not just a single shutdown point. Review the complete sequence before choosing a controller variant or timer capacitor.

Identify the exact fault policy

The TI LM5069 data sheet distinguishes a latching version from an automatic restart version and describes how current or power limiting starts the fault timer. The part suffix therefore changes the behavior of a persistent fault.

Record the timer charging conditions, timeout, retry interval, and reset mechanism. Include whether partial recovery resets the timer immediately or allows residual timing state to affect the next event.

Measure the dissipated power waveform

Capture MOSFET drain-to-source voltage and current during startup and during the required fault. Multiply synchronized waveforms to estimate instantaneous power, then integrate to obtain energy per attempt.

A hypothetical 30 W dissipation sustained for 20 ms represents 0.6 J per pulse. Repeating that pulse once per second gives 0.6 W average from those pulses, but the average alone does not establish peak junction temperature or compliance with the MOSFET’s safe operating area.

Include thermal memory between attempts

Use the device’s transient thermal information and the actual mounting condition to assess repeated pulses. The next attempt may begin before the die and package have cooled to ambient. A single pulse curve evaluated from a cool starting point can therefore be misleading.

Check current and voltage against the applicable safe operating area throughout each pulse, including linear operation. A low on resistance rating does not establish robustness while the transistor drops a large voltage under current limit.

Verify startup and persistent fault separately

Confirm that the timer allows the maximum intended output capacitance and load to start without nuisance shutdown. Then test a persistent bounded fault on an engineering setup and observe several retry cycles until the thermal trend is understood.

Also examine intermittent faults that clear briefly. Their timing can differ from a continuous short and may repeatedly exercise high dissipation without following the expected long cooling interval.

Save the controller variant, timing components, measured power pulses, retry sequence, and thermal assumptions. If the required startup time conflicts with fault stress, revisit the pass device, limiting policy, or startup load rather than merely extending the timer. A defensible design demonstrates acceptable repeated stress as well as successful startup; surviving one fault pulse is not evidence that indefinite automatic retries are acceptable.