Diodes Incorporated ZTX557STOB

Overview

ZTX557STOB from Diodes Incorporated is a PNP silicon planar medium-power high-voltage transistor with V_CEO = −300 V, I_C = −0.5 A continuous, P_tot = 1.0 W, h_FE = 50–300 (at I_C = −10 mA), and f_T = 75 MHz - used in high-voltage switching and linear regulation stages of industrial power supplies and CRT deflection circuits.

For engineers reviewing the ZTX557STOB datasheet, ZTX557STOB pinout, ZTX557STOB application, or ZTX557STOB equivalent, key selection criteria include verified −300 V V_CEO rating, safe operating area under pulsed loads, h_FE consistency across I_C = −10 mA to −50 mA, and TO-92-compatible lead form factor for through-hole PCB integration.

Technical Context

This PNP bipolar junction transistor employs a planar epitaxial structure optimized for high-voltage blocking and stable DC current gain. Its −300 V V_CEO and −200 V V_CBO ratings enable use in flyback snubbers and relay drivers where collector swing exceeds −200 V.

With f_T = 75 MHz at I_C = −50 mA and V_CE = −10 V, it supports moderate-speed switching (t_r/t_f ≈ 1.0–4.0 µs) while maintaining low V_CE(sat) = −0.3 V at I_C = −50 mA / I_B = −5 mA - critical for minimizing conduction loss in quasi-resonant converters.

Key Specifications

Pinout & Package

Package: TO-92 compatible (3-lead plastic case, E-line configuration). Lead identification follows standard PNP TO-92 orientation: viewed from flat side with leads down, left-to-right = Emitter (E), Base (B), Collector (C).

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX557STOB, ZTX550STOB trades 100 V blocking headroom for identical footprint and cost, while MJD2955G offers vastly higher current but sacrifices voltage capability and requires mechanical redesign - making ZTX557STOB uniquely balanced for 0.5 A / −300 V through-hole switching.

Availability

ZTX557STOB is available at Aetrix Electronics and suitable for industrial power supply snubbers, CRT deflection circuits, relay driver stages, and high-voltage linear regulator pass elements requiring stable component supply and long-term obsolescence management.

Supply support for ZTX557STOB 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 centers across Asia and manufacturing in China, Taiwan, and the US.

ZTX557STOB belongs to the ZTX series of high-voltage bipolar transistors, developed specifically for reliability-critical industrial and legacy display applications demanding extended voltage standoff and thermal resilience.

FAQ

What is the maximum safe continuous collector current for ZTX557STOB at 70°C ambient?

At T_amb = 70°C, the maximum continuous collector current is −0.35 A. This is derived from thermal derating: P_tot = 1.0 W at 25°C, with θ_JA ≈ 200°C/W for TO-92 on minimal copper, resulting in ~0.5 W allowable at 70°C - limiting I_C to maintain V_CE(sat) × I_C ≤ 0.5 W.

Can ZTX557STOB be used in avalanche mode for snubberless flyback designs?

No - ZTX557STOB is not rated for repetitive avalanche operation. Its datasheet specifies only single-pulse SOA curves and does not guarantee controlled avalanche energy absorption. For snubberless topologies, purpose-designed avalanche-rated transistors like the FMMT718 should be selected instead.

Is the ZTX557STOB pinout compatible with the older ZTX557 in metal can (TO-5) package?

No - although both share identical electrical specifications and part number suffix, the ZTX557STOB uses TO-92 (E-B-C left-to-right, flat side facing), whereas the legacy TO-5 metal-can ZTX557 has C-B-E pinout. Mechanical and thermal interfaces differ significantly; PCB footprint and schematic symbol must be updated.

What is the typical V_BE(on) at I_C = −100 mA, and is it specified in the datasheet?

V_BE(on) is specified only at I_C = −50 mA (−1.0 V), not at −100 mA. At −100 mA, V_BE(on) typically rises to −1.15 V due to base resistance and emitter current crowding - but this is an extrapolated value from curve traces, not a guaranteed specification. Designers must verify actual V_BE under load using prototype testing.