600 V depletion-mode MOSFET in a SOT-223 — what it is for
The depletion-mode characteristic means the channel is normally on at zero gate voltage (Vgs=0V), and a negative gate voltage is required to turn it off — the opposite of an enhancement-mode MOSFET. This makes it the natural choice for normally-on switch applications, current-source circuits, and linear regulators where a depletion-mode device simplifies the control loop.
45 Ohm Rds(on) and 4.9 nC gate charge — switching and conduction trade-offs
Maximum on-resistance is 45 Ohm at 120 mA drain current with 10 V gate drive. That is a high Rds(on) by power MOSFET standards, but the 120 mA current rating keeps conduction losses manageable — at full current the I²R loss is about 0.65 W, within the 1.8 W package dissipation limit. The total gate charge is 4.9 nC at 5 V, so the gate drive energy per switching cycle is low; a simple gate-drive circuit from a logic-level output or a small-signal transistor works fine for low-frequency switching up to a few tens of kHz. Input capacitance is 146 pF at 25 V Vds, which means the gate-drive rise time is fast even with modest source impedance.
Depletion-mode operation — no negative gate voltage needed for turn-off?
The BSP135L6433 is a depletion-mode MOSFET, so at Vgs=0V the channel conducts. To turn the device off, the gate must be driven negative relative to the source — typically -5 V to -10 V for a hard cutoff. The maximum gate-source voltage rating is ±20 V, so a -10 V gate drive is within the safe operating area. This is a critical design consideration: in a circuit that expects an enhancement-mode part, the BSP135L6433 will conduct continuously unless the gate is actively pulled negative. The drive voltage for minimum Rds(on) is 10 V; for zero Rds(on) the drive voltage is 0 V (the on-condition).
PG-SOT223-4-21 package — footprint and thermal reality
The large tab on the package is the drain terminal and provides the primary thermal path. With 1.8 W maximum power dissipation at 25°C ambient, the junction-to-ambient thermal resistance is around 70-80 °C/W depending on PCB copper area. For continuous operation near the 150°C junction limit, a minimum of 1 square inch of copper on the drain tab is recommended.
It is a current-production part in the SIPMOS series with no announced end-of-life or last-time-buy schedule. For new designs, this means no near-term obsolescence risk.