Diodes Incorporated ZTX851STZ

Overview

ZTX851STZ from Diodes Incorporated is an NPN silicon planar medium-power high-current transistor with V_CEO = 60 V, continuous I_C = 5 A, peak I_CM = 20 A, V_CE(sat) as low as 100 mV at I_C = 5 A / I_B = 200 mA, and P_tot = 1.2 W at T_amb = 25°C - used in emergency lighting circuits requiring robust switching under sustained load.

For engineers reviewing the ZTX851STZ datasheet, ZTX851STZ pinout, ZTX851STZ application, or ZTX851STZ equivalent, key selection criteria include verified saturation voltage at high current, thermal resistance (R_th(j-amb) = 150 °C/W), h_FE stability across 10 mA–5 A, TO-92 mechanical compatibility, and safe operating area (SOA) compliance for pulsed loads.

Technical Context

This discrete NPN bipolar junction transistor operates in linear or saturated switching mode, supporting DC and pulsed collector currents up to 20 A with guaranteed V_CE(sat) ≤ 200 mV at I_C = 5 A and I_B = 200 mA. Its f_T = 130 MHz enables moderate-speed switching in power control stages.

The device features a rugged E-Line TO-92 package with emitter-base breakdown voltage V_(BR)EBO ≥ 6 V and collector-emitter breakdown V_(BR)CEO ≥ 60 V, rated for operation from –55°C to +200°C junction temperature, with thermal resistance R_th(j-case) = 50 °C/W enabling effective PCB heat sinking via copper pad.

Key Specifications

Pinout & Package

E-Line TO-92 package (3-lead, plastic, through-hole); compatible with standard TO-92 footprints and mounting practices. Lead identification: Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector (viewed from flat side with leads down, left-to-right).

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX851STZ, the ZTX851 offers identical performance in bulk form but lacks production traceability, while MJD127G trades TO-92 convenience for higher voltage headroom and greater thermal inertia - making ZTX851STZ optimal for space-constrained, high-reliability 24–48 V emergency lighting designs.

Availability

ZTX851STZ is available at Aetrix Electronics and suitable for emergency lighting drivers, DC motor starters, redundant power OR-ing circuits, and overcurrent protection switches requiring stable component supply and long-term manufacturability.

Supply support for ZTX851STZ 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 power management, signal integrity, and protection solutions for industrial and automotive markets.

ZTX851STZ belongs to Diodes' E-Line high-current transistor family, designed specifically for cost-sensitive, thermally demanding switching applications where TO-92 form factor and proven ruggedness are critical.

FAQ

What is the maximum safe continuous collector current for ZTX851STZ at 70°C ambient?

At T_amb = 70°C, derating begins at 25°C per the R_th(j-amb) = 150 °C/W curve. With T_j(max) = 200°C, allowable ΔT = 130°C, so max P_tot = 130°C ÷ 150 °C/W ≈ 0.87 W. Using V_CE(sat) ≈ 150 mV at 3 A, I_C is limited to ~3.5 A continuous to stay within thermal limits.

Can ZTX851STZ replace ZTX850 in existing designs?

No - ZTX850 is a PNP transistor (complementary to ZTX851), with opposite polarity, different pinout (C-B-E vs E-B-C), and distinct biasing requirements. Substitution would invert circuit function and likely cause immediate failure without schematic revision.

Is ZTX851STZ suitable for PWM-driven inductive loads?

Yes - its SOA curve confirms safe single-pulse operation up to 20 A × 10 V × 100 µs (20 mJ), and measured t_on/t_off of 45 ns/1100 ns support PWM frequencies up to ~100 kHz when paired with appropriate base drive and flyback protection.

Does ZTX851STZ require a heatsink for 5 A continuous operation?

No - when mounted on ≥1 inch² of 2 oz copper on FR-4 PCB per datasheet condition, R_th(j-amb) = 150 °C/W allows 5 A operation at T_amb ≤ 45°C. Above that, forced air or localized copper pour extension is recommended to maintain T_j < 150°C.