20N50B MOSFET — Specifications & Equivalents

About the 20N50B

The 20N50B is a MOSFET with a drain–source voltage (V_DS) rating of 500 V and a continuous drain current (I_D) of 20 A. It can dissipate up to 280 W and is rated to a junction temperature of 150 °C. It is supplied in the TO-3PB package.

At a glance

• Type: MOSFET
• Max drain–source voltage (V_DS): 500 V
• Max continuous drain current (I_D): 20 A
• Power dissipation (P_D): 280 W
• Max junction temperature (T_J): 150 °C
• Package: TO-3PB

Typical applications

With its 500 V drain–source rating the 20N50B sits in the high-voltage (offline) class of switching MOSFETs, typically found in primary-side switching in offline SMPS (flyback, forward and quasi-resonant), active PFC boost stages, half-bridge and full-bridge inverters, induction heating and electronic ballasts and BLDC and AC motor drives. The rating leaves usable margin above the ~325 V DC bus obtained by rectifying 230 VAC mains, which is why this class dominates offline power supplies.

Gate drive and switching notes

Keep gate–source voltage within the ±30 V maximum; exceeding it punctures the gate oxide and destroys the device permanently.

Switching behaviour is characterised by an output capacitance (C_oss) of 355 pF, which together with the gate drive determines switching loss and the practical upper frequency limit.

Choosing a replacement for the 20N50B

A drop-in substitute for the 20N50B should match its polarity and offer a drain–source rating of at least 500 V and a continuous drain current rating of at least 20 A.

Mechanically, check the replacement against the TO-3PB footprint and confirm the pin order — parts sharing a package outline do not always share a pinout.

Where the application switches inductive loads or runs near the voltage limit, keep extra headroom rather than choosing a part that only just meets the original numbers.

Frequently asked questions

What is the 20N50B?

The 20N50B is a MOSFET (metal-oxide-semiconductor field-effect transistor). It is rated for 500 V drain-source and 20 A continuous drain current.

What is the maximum voltage of the 20N50B?

The 20N50B has a maximum drain-source voltage (Vds) of 500 V. Treat this as an absolute maximum and design with margin, especially when switching inductive loads.

How much current can the 20N50B handle?

It is rated for 20 A of continuous drain current, with power dissipation limited to 280 W. Real-world capability depends on your heatsinking and ambient temperature.

What package does the 20N50B come in?

It is listed in TO-3PB. Verify the pinout of the specific variant, as one outline can carry different pin assignments.

What can I use to replace the 20N50B?

This page lists 17 candidate equivalents matched on electrical ratings and package. Any replacement should meet or exceed the original voltage, current and dissipation figures and use a compatible drive level.