1500W peak pulse — the surge rating that decides the fit
The 1.5KE400C is a 1500W (1.5kW) peak pulse power TVS diode from Littelfuse's 1.5KE series, using a Zener breakdown mechanism to clamp transients. The 1500W rating at 10/1000µs waveform defines its energy-handling capacity — a surge above this level risks device failure, so the clamping voltage (575.4V max at 2.8A peak pulse current) must be below the downstream circuit's absolute maximum rating to provide protection.
342V standoff, 360V breakdown — the protection window
Reverse standoff voltage is 342V typical — the device does not conduct at this DC bias, so it sits across the rail without loading the circuit. Breakdown starts at 360V min, and clamping occurs at 575.4V max at the rated 2.8A peak pulse current (10/1000µs). The gap between standoff and breakdown defines the transient threshold: a surge that raises the line above 360V triggers the Zener avalanche, but the circuit must survive the full clamping voltage until the surge energy dissipates. This simplifies PCB layout and BOM count for AC lines or bipolar DC rails where polarity reversal is a risk.
DO-201AA axial — through-hole mounting and thermal path
The DO-201AA (also known as DO-27) axial-lead package is a through-hole form factor with a 0.052-inch diameter body. The leads are tin-plated copper; the junction-to-lead thermal path is short, so the PCB copper area at the solder joint — not the package itself — sets the steady-state power dissipation. For pulse duty, the 1500W rating is achievable with standard 1 oz copper pads and adequate lead length for strain relief. The 175°C TJ max allows the device to absorb repetitive surges without derating the peak pulse power, provided the average power stays within the thermal limit set by the board layout.
Obsolete — sourcing through independent channels
New production has ceased; remaining inventory moves through independent surplus and broker channels. No pin-compatible replacement exists from Littelfuse or a second-source vendor. A BOM revision to a different TVS family (same package, similar standoff voltage) requires re-qualification of the clamping voltage and surge energy. The 1.5KE400C is a drop-in fit only when the original design specified this exact order code — any substitution must be validated against the circuit's transient immunity requirements.