MJD50T4G

The MJD50T4G is an NPN Darlington power transistor from ON Semiconductor, designed for high-voltage, medium-current switching applications. The Darlington pair configuration connects two NPN transistors in a compound arrangement that multiplies their individual current gains, resulting in extremely high overall current gain (typically 15,000 at IC = 1A). This high gain means that only a very small base drive current (on the order of milliamps or less) is needed to control substantial collector currents, making it easy to drive directly from microcontroller I/O pins or low-power logic circuits. The ‘G’ suffix indicates the Pb-free version of the device. With a collector-emitter voltage rating of 300V and continuous collector current of 1.5A (4A peak), the MJD50T4G is well-suited for driving inductive loads such as relays, solenoids, and motors, as well as lamp loads and other power switching applications. The device incorporates internal base-emitter resistors on both transistors for improved stability and noise immunity. The DPAK (TO-252) surface-mount package provides good thermal performance for its size and is well-suited for automated PCB assembly. The device is RoHS compliant and halogen-free, meeting modern environmental standards. With an operating junction temperature range of -55°C to +150°C, it can operate reliably in a wide range of environmental conditions.

The MJD50T4G operates as a Darlington pair, which consists of two NPN bipolar junction transistors (BJTs) connected in a cascaded configuration where the emitter current of the first (driver) transistor provides the base current for the second (output) transistor. This configuration multiplies the current gains of the two transistors: hFE_total = hFE1 × hFE2. The result is an extremely high current gain – typically 15,000 for the MJD50T4G – allowing the device to be driven by very low base currents. When a small positive current flows into the base of the first transistor, its amplified emitter current flows into the base of the second transistor, which then amplifies it further to provide the output collector current. The integrated base-emitter resistors (one across each transistor’s base-emitter junction) help discharge the base-emitter capacitance during turn-off, improving switching speed and preventing accidental turn-on due to leakage current or noise. In the saturation (ON) state, both transistors are fully conducting, and the total saturation voltage is the sum of the two transistors’ VCE(sat) plus the VBE of the output transistor, typically around 2V – higher than a single BJT but still acceptable for most switching applications. For inductive load driving applications, an external freewheeling (flyback) diode should be connected across the load to provide a path for the inductor current when the transistor turns off, preventing damaging voltage spikes. The device includes thermal considerations: as power dissipation increases, the junction temperature rises, and the device should be derated according to the thermal characteristics to maintain junction temperature within safe limits.

• Relay drivers and solenoid valve control circuits
• Lamp drivers and LED display column/row drivers
• Low- to medium-power switching applications
• Motor control and stepper motor driver stages
• Industrial automation and control systems
• Consumer electronics power switching and AC line control