Description
The TIP122 is a popular 100V, 5A NPN Darlington transistor. It serves as the high-voltage complement to the TIP120 (60V) and TIP121 (80V). Because it is an NPN device, it is primarily used for low-side switching, where the transistor sits between the load and the ground (GND) rail.
Key Specifications
| Parameter | Symbol | Rating |
| Collector-Emitter Voltage | $V_{CEO}$ | 100V |
| Continuous Collector Current | $I_C$ | 5A |
| Peak Collector Current | $I_{CM}$ | 8A |
| Collector-Emitter Saturation Voltage | $V_{CE(sat)}$ | 2.0V (at 3A) |
| DC Current Gain | $h_{FE}$ | 1,000 min (at 3A) |
| Power Dissipation | $P_D$ | 65W |
The TIP12x NPN Series: Quick Reference
The TIP12x series shares the same physical pinout and current capabilities, varying only by the maximum voltage they can block:
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TIP120: 60V max.
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TIP121: 80V max.
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TIP122: 100V max.
Essential Design Concepts
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Low-Side Switching: In this configuration, the load is connected directly to the positive voltage supply, and the TIP122 collector is connected to the other side of the load. When the base receives a positive signal, the transistor completes the path to ground.
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Darlington Advantage: Because of its high gain ($h_{FE} \ge 1000$), you can control a 5A load with only a few milliamperes of base current. This allows for direct, albeit cautious, interfacing with microcontrollers.
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Saturation and Heating: Like all Darlington pairs, the TIP122 has a higher internal voltage drop than a single standard BJT. At 5A, you can expect a ~2V drop across the device.
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Thermal Calculation: $P = I \times V_{drop}$. At 3A, you are dissipating $3A \times 2V = 6W$. A moderate heatsink is required for sustained operation.
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Best Practices for Implementation
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Base Resistor: Always use a base resistor (e.g., $1k\Omega$ to $2.2k\Omega$) to limit the current flowing from your control pin into the transistor’s base.
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Flyback Diode: When controlling inductive loads (motors, solenoids, or large relays), you must include a flyback diode (such as the 1N4007) wired in reverse-bias across the load. This prevents the high-voltage kickback, which occurs when switching off, from destroying the transistor.
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Heatsinking: If you are running the device near its 5A limit, the TO-220 package will heat up rapidly. Use a thermal pad and a metal heatsink to ensure the junction temperature stays within safe limits.
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