Diodes Incorporated ZTX857QSTZ

Overview

ZTX857QSTZ 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 relay drivers and flyback switch-mode power supply (SMPS) output stages.

For engineers reviewing the ZTX857QSTZ datasheet, ZTX857QSTZ pinout, ZTX857QSTZ application, or ZTX857QSTZ equivalent, key selection criteria include breakdown voltage margin (V_CEO = 300 V), saturation voltage under high-current switching (≤140 mV @ 2 A), thermal resistance (R_th(j-amb) = 150 °C/W), safe operating area (SOA) compliance at 50 V, and TO-92-compatible lead form factor for through-hole PCB assembly.

Technical Context

This discrete NPN bipolar junction transistor operates in linear amplification or saturated switching mode, supporting DC and pulsed collector currents up to 5 A with guaranteed V_(BR)CEO ≥ 300 V and V_(BR)CBO ≥ 330 V. Its low V_CE(sat) and high h_FE (≥100 @ I_C = 2 A) enable efficient current control in medium-voltage, medium-current switching nodes.

The device features a maximum junction temperature of +200°C, R_th(j-case) = 50 °C/W, and SOA-limited operation under transient conditions - validated per single-pulse testing at T_amb = 25°C with pulse width ≤ 300 µs and duty cycle ≤ 2%.

Key Specifications

Pinout & Package

Package: TO-92 compatible (E-Line 3-303 outline), through-hole mounting, epoxy-molded plastic case with internal leadframe optimized for medium-power thermal performance.

Key Features

Applications

Equivalent & Alternatives

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

Compared with ZTX851, ZTX857QSTZ delivers +50 V breakdown margin and +1 A continuous current - critical for 230 VAC-derived rails. Versus MJD122, it trades package size and power handling for voltage capability and TO-92 compatibility in space-constrained designs.

Availability

ZTX857QSTZ is available at Aetrix Electronics and suitable for industrial relay drivers, flyback SMPS output stages, DC motor control circuits, and overvoltage protection circuits requiring stable component supply and long-term manufacturability.

Supply support for ZTX857QSTZ 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, automotive, and computing applications.

ZTX857QSTZ belongs to the ZTX series of high-voltage NPN transistors designed specifically for medium-power switching in industrial controls, power conversion, and protection circuits where voltage margin and thermal robustness are critical.

FAQ

What is the maximum safe operating voltage for ZTX857QSTZ in a common-emitter configuration?

The absolute maximum collector-emitter voltage is V_CEO = 300 V, verified under I_C = 10 mA test condition. For reliable long-term operation, design margins should limit steady-state V_CE to ≤240 V, especially under inductive switching or elevated temperature (T_j > 100°C), per the SOA curve in Figure 3-305.

Can ZTX857QSTZ be used in surface-mount designs?

No - ZTX857QSTZ uses a TO-92 through-hole package (E-Line 3-303 outline) with axial leads. It is not SMD-compatible. For surface-mount equivalents, consider Diodes Inc.'s DXT75100 (SOT-23) or MMBT5551 (SOT-23), though both have lower V_CEO (160 V and 160 V respectively) and current ratings.

What base drive current is required to fully saturate ZTX857QSTZ at 3 A collector current?

Per datasheet Fig. 3-304, V_CE(sat) ≤ 200 mV at I_C = 3 A requires I_B = 600 mA. With h_FE ≥ 15 at this condition, minimum forced β = 5 ensures saturation. A practical design uses I_B = 600–750 mA via a current-limited driver stage or appropriately sized base resistor.

How does thermal resistance affect ZTX857QSTZ's power handling at 70°C ambient?

With R_th(j-amb) = 150 °C/W, a 70°C ambient yields ΔT = 150 × P_diss. To hold T_j ≤ 150°C (derated from 200°C max), max allowable P_diss = (150 − 70) / 150 = 0.53 W. This is 44% of its 25°C-rated 1.2 W - confirming need for thermal derating or heatsinking in warm environments.