TIP122G

The TIP122G from onsemi is an NPN Darlington power transistor containing two NPN transistors in a cascade (Darlington) configuration, plus built-in base-emitter shunt resistors for thermal stability. The Darlington connection multiplies the current gains of the two transistors: hFE(total) = hFE1 × hFE2, resulting in a minimum gain of 1000 and typical gain of 2500. This enormous gain means that only IB = IC/hFE = 5A/1000 = 5mA of base current is needed to drive a 5A load — well within the output capability of any microcontroller GPIO. The trade-off is a high saturation voltage: VCE(sat) can reach 4V at 5A, meaning 20W of power dissipation in the transistor at full load. This requires a substantial heatsink for continuous operation. The 100V rating makes the TIP122 suitable for 12V, 24V, and even 48V systems. The complementary PNP device is the TIP127G. The G suffix denotes the Pb-free TO-220 package.

The TIP122G uses a Darlington pair: the collector of the first (input) transistor Q1 connects to the base of the second (output) transistor Q2, and both collectors tie together. When base current flows into Q1’s base, Q1 amplifies it by hFE1 and feeds it to Q2’s base. Q2 then amplifies this by hFE2. The total current gain is hFE1 × hFE2 ≈ 1000-2500. The built-in shunt resistors (8kΩ across Q1’s B-E and 120Ω across Q2’s B-E) provide a leakage current path to prevent the Darlington from turning on due to collector leakage at high temperature. Without these resistors, Q1’s collector leakage would be amplified by Q2, potentially causing thermal runaway. The saturation voltage is high (2-4V) because Q2’s collector-emitter voltage can never fall below Q1’s VCE(sat) + Q2’s VBE ≈ 0.3V + 0.7V = 1.0V, and the shunt resistors divert some base current. The VBE(on) is also high (2.5V) because it’s the sum of Q1’s VBE + Q2’s VBE ≈ 0.7V + 0.7V = 1.4V plus the drop across Q1’s collector load. For switching applications, the base drive must supply IB = IC/hFE(min) × 2 (overdrive) = IC/500. For a 3A load: IB = 6mA. The base resistor: RB = (Vdrive − VBE(on)) / IB = (5V − 2.5V) / 6mA ≈ 400Ω.

TIP122G pin assignment (TO-220-3, front view, pins down): Pin 1 (left) = Base, Pin 2 (center) = Collector/Tab, Pin 3 (right) = Emitter. This is an NPN Darlington power transistor with minimum hFE of 1000, making it easy to drive from microcontroller GPIO. The tab is connected to Collector. Complementary PNP: TIP127G (-100V/-5A). For MOSFET alternative with lower VCE(sat), consider IRF540N (100V/33A logic-level MOSFET).

• Motor Driver: MCU GPIO → 220Ω → TIP122 base; collector to 12V motor; emitter GND; flyback diode across motor; IB≈10mA drives IC=3A
• Solenoid Control: 24V solenoid → TIP122 collector; emitter GND; IB=5mA from MCU; flyback diode essential
• Heater Element: 12V/3A heater → TIP122 collector; PWM on base for temperature control; heatsink on TIP122
• High-Power LED: 12V LED strip (2A) → TIP122 collector; PWM base drive for dimming; 4.7kΩ base resistor
• Audio Amp: Push-pull with TIP122 (NPN) + TIP127 (PNP); Class-AB; ±24V rails; 5A peak output

TIP122G is an NPN Darlington transistor 100V/5A from onsemi in TO-220. The complementary PNP is TIP127G (-100V/-5A). The TIP120G (60V) and TIP121G (80V) offer lower voltage ratings at identical current. The TIP142G provides 100V/10A for higher current needs. The BD679A from ST offers 80V/4A Darlington in SOT-93 for surface-mount. For MOSFET alternatives, IRLZ44N provides logic-level 55V/47A N-channel switching. The FQP30N06L offers 60V/30A with 35mΩ RDS(on) for more efficient switching.