Diodes Incorporated FZT857TC

Overview

FZT857 from Diodes Incorporated is a 300V NPN medium-power transistor in SOT223 package, designed for high-voltage switching applications requiring robust breakdown voltage and low saturation resistance. It delivers 3.5A continuous collector current, 155mV V_CE(SAT) at 1A, and 87mΩ equivalent on-resistance, with AEC-Q101 qualification for automotive reliability.

For engineers reviewing the FZT857 datasheet, FZT857 pinout, FZT857 application, or FZT857 equivalent, key selection criteria include its 300V V_CEO, high h_FE up to 3A, thermal resistance of 42°C/W (52mm×52mm 2oz Cu), ESD HBM rating of 8kV, and complementary PNP pairing with FZT957.

Technical Context

This NPN bipolar junction transistor operates as a high-voltage, medium-current switch with specified gain (h_FE ≥15 at 3A) and low-saturation performance critical for flyback converters and industrial power supplies. Its 350V V_CBO, 300V V_CEO, and 7V V_EBO define safe operating limits under transient overvoltage conditions.

The device uses a planar epitaxial die structure with matte tin-plated leads and is qualified to AEC-Q101 for automotive-grade reliability. Thermal performance is defined for two PCB mounting configurations: 42°C/W (52mm×52mm 2oz Cu) and 78°C/W (25mm×25mm 1oz Cu), enabling accurate thermal design in constrained layouts.

Key Specifications

Pinout & Package

SOT223 package: surface-mount, 4-lead (3 active + tab), molded green compound, UL 94V-0 rated, 0.112g weight. Collector lead is internally connected to the exposed thermal pad (pin 3).

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX851 and MJD31C, FZT857 uniquely combines 300V rating, SOT223 thermal efficiency, and AEC-Q101 compliance-making it the only option for compact, high-reliability, universal-input power stages where voltage margin, thermal density, and automotive qualification intersect.

Availability

FZT857 is available at Aetrix Electronics and suitable for industrial power supplies, automotive lighting drivers, and AC-DC adapters requiring stable component supply, long-term lifecycle support, and RoHS-compliant green packaging.

Supply support for FZT857 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, headquartered in Plano, Texas, with design, manufacturing, and sales operations across Asia, Europe, and the Americas.

The FZT857 belongs to Diodes' high-voltage bipolar transistor product line, engineered specifically for robustness in offline power conversion, automotive lighting, and industrial control applications where reliability under thermal and electrical stress is critical.

FAQ

What is the maximum allowable junction temperature for FZT857?

The absolute maximum junction temperature is +150°C, with storage temperature ranging from –55°C to +150°C. Operation above 125°C requires derating per the published thermal curves-specifically, power dissipation must be reduced linearly beyond 25°C ambient using R_θJA = 42°C/W (52mm×52mm 2oz Cu) or 78°C/W (25mm×25mm 1oz Cu).

Can FZT857 replace a MOSFET in high-voltage switching circuits?

FZT857 can substitute for low-speed, high-voltage MOSFETs in applications below 200 kHz where gate drive simplicity and cost matter more than ultra-low Q_g. Unlike MOSFETs, it requires sustained base current (e.g., 600 mA at 3.5A I_C) and exhibits higher conduction loss at very low currents-but offers superior ruggedness against avalanche and dv/dt-induced turn-on.

Is the exposed thermal pad on the SOT223 package electrically connected?

Yes-the exposed pad (pin 3) is internally connected to the collector. It must be soldered to a dedicated PCB copper pour tied to the collector net for both thermal conduction and electrical integrity. Floating or grounded pads will cause functional failure or thermal runaway.

Does FZT857 support pulse-width modulation (PWM) at 100 kHz?

Yes-its f_T of 80 MHz and typical t_ON/t_OFF of 100 ns / 5.3 µs enable clean 100 kHz PWM switching in DCM flyback and forward converters. However, due to minority-carrier storage delay, minimum off-time should exceed 10 µs to ensure full carrier recombination and avoid secondary breakdown.