Diodes Incorporated ZTX857STOB

Overview

ZTX857STOB from Diodes Incorporated is an NPN silicon planar medium-power high-current transistor with V_CEO = 300 V, I_C = 3 A continuous, P_tot = 1.2 W at T_amb = 25°C, V_CE(sat) ≤ 140 mV at I_C = 2 A / I_B = 200 mA, and h_FE ≥ 100 at I_C = 2 A - used in industrial switching power supplies and relay drivers.

For engineers reviewing the ZTX857STOB datasheet, ZTX857STOB pinout, ZTX857STOB application, or ZTX857STOB equivalent, key selection criteria include breakdown voltage margin (V_CEO = 300 V), low saturation voltage under high current, thermal resistance (R_th(j-amb) = 150 °C/W), and TO-92-compatible E-Line package compatibility with legacy PCB footprints.

Technical Context

This discrete NPN bipolar junction transistor operates in linear or saturated switching mode, supporting high-voltage DC-DC converter primary-side switching and solenoid/relay coil drive where fast turn-on (t_on = 100 ns) and robust safe operating area (SOA) up to 2 A at 50 V are required.

Its f_T = 80 MHz enables stable operation in moderate-frequency switching applications; C_obo = 11 pF at V_CB = 20 V supports predictable Miller effect behavior; and the specified V_BE(on) = 810–950 mV at I_C = 2 A ensures reliable base drive design with standard logic-level or microcontroller outputs.

Key Specifications

Pinout & Package

Package: TO-92-compatible E-Line (3-303), molded plastic case with leads formed to standard through-hole pitch. Mounting requires minimum 1 inch² copper area on PCB for rated power dissipation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN high-voltage switching transistor applications.

Compared with ZTX857STOB, ZTX851STOB trades voltage headroom for cost reduction in low-voltage systems, while MJD122G delivers higher current capacity at the expense of package size and voltage capability - making ZTX857STOB optimal for compact, 230 VAC-compatible medium-power switching.

Availability

ZTX857STOB is available at Aetrix Electronics and suitable for industrial relay drivers, offline SMPS primary switches, and solenoid actuator control requiring stable component supply across extended production cycles.

Supply support for ZTX857STOB includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Diodes Incorporated is a global manufacturer of discrete semiconductors and analog ICs, specializing in high-reliability components for industrial, computing, and consumer applications.

ZTX857STOB belongs to the ZTX series of high-voltage bipolar transistors designed specifically for cost-sensitive, medium-power switching in industrial controls and power conversion where TO-92 compatibility and 300 V capability are critical.

FAQ

What is the maximum safe operating voltage for ZTX857STOB in continuous DC applications?

The absolute maximum collector-emitter voltage (V_CEO) is 300 V DC. For reliable long-term operation, design should maintain ≤240 V across C–E under worst-case conditions including transients and temperature rise, per the SOA curve and derating guidelines in the datasheet.

Can ZTX857STOB be driven directly from a 3.3 V microcontroller GPIO?

Yes - with appropriate base resistor calculation. At I_C = 2 A and h_FE ≥ 100, required I_B ≈ 20 mA. A 100 Ω resistor from a 3.3 V GPIO yields ~23 mA base current, ensuring saturation while staying within GPIO limits.

Is ZTX857STOB suitable for switching inductive loads like relays or solenoids?

Yes - its specified SOA supports single-pulse currents up to 5 A and V_CE up to 50 V. When switching inductive loads, always use a reverse-biased flyback diode across the load to clamp turn-off voltage spikes and prevent secondary breakdown.

How does thermal performance change if less than 1 inch² copper is used on the PCB?

R_th(j-amb) increases significantly below 1 inch² copper: at 0.25 in², it rises to ~250 °C/W. This reduces maximum allowable power dissipation to ~0.7 W at 25°C ambient - requiring either reduced I_C, forced airflow, or derating above 40°C ambient.