BD679, TIP120, BDX54 Darlington Transistor Cross-Reference and Equivalents: Selection Guide

Bottom Line: BD679 (NPN, 80 V/4 A, SOT-32), TIP120 (NPN, 60 V/5 A, TO-220), and BDX54 (PNP, 80–100 V/8 A, TO-220) are complementary legacy Darlington transistors still widely designed into motor-drive, relay-drive, and solenoid circuits. When original part numbers are hard to source, engineers can substitute with pin-compatible equivalents—BD679A (ST, SOT-32), TIP120G (onsemi, TO-220), BDX54B/BDX54C/BDX54CG—without layout changes, provided they verify Vceo, Ic, and hFE against the application budget. All in-stock equivalents listed here are available for RFQ on FindMyChip from 200+ verified distributors.

Understanding Darlington Power Transistors: BD679, TIP120, and BDX54

Darlington transistors pair two bipolar junction transistors in a single package to achieve high current gain (hFE typically 750–20,000). A single logic-level base drive of a few milliamps can switch several amperes through a motor coil, relay, or heater element. BD679, TIP120, and BDX54 cover three common use cases: SOT-32 medium-current NPN (BD679), TO-220 higher-current NPN (TIP120), and TO-220 PNP complement (BDX54). Their legacy footprints remain unchanged across decades of production, making drop-in substitution straightforward when the original part is allocated or discontinued.

What Is the BD679?

The BD679 is an NPN Darlington transistor from STMicroelectronics in a 3-pin SOT-32 (also called TO-126) package. Key ratings are Vceo = 80 V, Ic = 4 A continuous, and hFE ≥ 750 at Ic = 1.5 A. The smaller SOT-32 package suits board-mounted applications where the TO-220 tab would complicate layout. The BD679A is a pin-compatible variant with guaranteed hFE ≥ 750 across the full temperature range and is the preferred in-stock equivalent.

What Is the TIP120?

The TIP120 is an NPN Darlington power transistor in TO-220AB, rated Vceo = 60 V, Ic = 5 A, and hFE ≥ 1000 at Ic = 3 A. ST and onsemi both manufacture it. Its 60 V ceiling is lower than the BD679 and BDX54 series, so it targets lower-voltage DC loads such as 12–48 V motor drivers, PWM-controlled solenoids, and LED arrays. The built-in base-emitter shunt resistors on the onsemi TIP120G variant suppress leakage current when driven from high-impedance logic. VCE(sat) is typically ≤ 2 V at Ic = 5 A, which determines the thermal budget at full load.

What Is the BDX54?

The BDX54 family provides the PNP complement to the BD679/TIP120 NPN devices, enabling complementary push-pull output stages. All variants share TO-220 packaging and Ic = 8 A continuous, making them suitable as high-side switches. Three suffix grades differ in Vceo: BDX54B = 80 V, BDX54C = 100 V, BDX54CG (onsemi) = 100 V. For new designs the 100 V variants offer better voltage headroom at only marginal cost increase. Pin assignments match the standard TO-220 CBE (Collector-Base-Emitter) order.

Cross-Reference Table: BD679, TIP120, BDX54 and Equivalents

MPN	Type	Vceo	Ic	hFE (min)	Package	Manufacturer
BD679	NPN Darlington	80 V	4 A	750	SOT-32	STMicroelectronics
BD679A	NPN Darlington	80 V	4 A	750	SOT-32	STMicroelectronics
TIP120	NPN Darlington	60 V	5 A	1000	TO-220AB	STMicroelectronics
TIP120G	NPN Darlington	60 V	5 A	1000	TO-220AB	ON Semiconductor
BDX54B	PNP Darlington	80 V	8 A	750	TO-220AB	STMicroelectronics
BDX54C	PNP Darlington	100 V	8 A	750	TO-220AB	STMicroelectronics
BDX54CG	PNP Darlington	100 V	8 A	750	TO-220AB	ON Semiconductor

Key Selection Parameters

1. Vceo — Collector-Emitter Breakdown Voltage

Vceo must exceed the maximum supply rail voltage plus any inductive kickback spike. For a 24 V relay-drive circuit, use a device rated ≥ 60 V (derating factor 2.5×). TIP120's 60 V rating is adequate for 12–24 V systems with a fast flyback diode. BD679 and BDX54B at 80 V suit 36–48 V industrial loads. BDX54C/BDX54CG at 100 V add extra margin for 60 V supply rails common in servo amplifiers.

2. Ic — Continuous Collector Current

Match the device's Ic rating to peak load current with at least 30% headroom. BD679 at 4 A handles motors drawing up to ~2.8 A continuously. TIP120 at 5 A is preferable for 3–4 A loads. BDX54 family at 8 A suits high-side switches in brushed DC motor H-bridges and solenoid valves. Exceeding Ic in pulsed operation is allowed only within the Safe Operating Area (SOA) curves in the datasheet.

3. hFE — DC Current Gain

Darlington pairs combine two transistors, so hFE is the product of both stages—typically 750 to 20,000. A higher hFE means a smaller base drive current is needed, which is critical for 3.3 V or 5 V GPIO drive. The onsemi TIP120G specifies VCE(sat) ≤ 2 V at Ic = 3 A, Ib = 12 mA, ensuring that a 220 Ω base resistor from a 5 V MCU pin is sufficient to fully saturate the transistor.

4. Package — SOT-32 vs TO-220

BD679/BD679A come in SOT-32 (TO-126), which mounts directly on PCB without a mounting hole, suitable for currents ≤ 4 A with a small clip heatsink. TIP120, BDX54, and their variants use TO-220AB, which accepts standard extrusion heatsinks and chassis mounting. Thermal resistance junction-to-case (RθJC) is typically 3.1 °C/W for TO-220 and 6.25 °C/W for SOT-32; total power dissipation = (Ic × VCE(sat)) must stay within the device's total power rating with adequate heatsinking.

5. NPN vs PNP — High-Side vs Low-Side Switching

NPN devices (BD679, TIP120) switch on the low side: the collector connects to the load, the emitter to ground. PNP devices (BDX54) switch on the high side: the emitter connects to the positive supply. Complementary NPN/PNP pairs—for example TIP120 + BDX54C—form the output stage of class-AB audio amplifiers and DC motor H-bridges without level-shift circuitry. Confirm the base drive polarity before substituting across polarities.

6. VCE(sat) — Saturation Voltage and Power Loss

VCE(sat) directly determines conduction losses: P = Ic × VCE(sat). At Ic = 5 A, VCE(sat) ≤ 2 V (typical for Darlington pairs) dissipates 10 W, requiring an appropriate heatsink. Darlington pairs inherently have higher VCE(sat) than single-transistor designs. For applications where 2 V drop is unacceptable, consider a logic-level MOSFET such as IRL540 instead, but for high-base-drive-impedance circuits Darlington transistors remain the lower-BOM-cost option.

7. Lead Time and Multi-Source Availability

BD679, TIP120, and BDX54 are legacy parts manufactured by multiple vendors (ST, onsemi/Fairchild). Single-source dependency is a supply-chain risk; specifying both ST and onsemi part numbers in the BOM—such as TIP120 + TIP120G or BDX54C + BDX54CG—creates immediate dual-source flexibility. Search FindMyChip to compare real-time stock and pricing across 200+ verified distributors before placing volume orders.

Selection Decision Guide

Use this logic to choose the right Darlington transistor:

1. Is the load on the high side (positive rail)? → Use a PNP device from the BDX54 family.
2. Is the load on the low side (ground-referenced)? → Use an NPN device (BD679 or TIP120).
3. Supply rail ≤ 48 V and load current ≤ 4 A? → BD679 or BD679A in SOT-32.
4. Supply rail ≤ 60 V and load current ≤ 5 A? → TIP120 or TIP120G in TO-220.
5. Need PNP complement at 80 V? → BDX54B.
6. Need PNP complement at 100 V or best voltage headroom? → BDX54C (ST) or BDX54CG (onsemi).
7. Is original part allocated or out of stock? → Submit an RFQ on FindMyChip—our team sources equivalents from 200+ verified distributors with 5-point authentication.

Frequently Asked Questions

What is the equivalent of BD679?

The nearest equivalent is the BD679A from STMicroelectronics—same SOT-32 package, same 80 V/4 A ratings, and guaranteed hFE ≥ 750. Both are direct pin and electrical replacements. For TO-220 form factor, the TIP121 (80 V/5 A NPN Darlington) is a functional substitute with higher current headroom. Confirm the package footprint before substituting SOT-32 with TO-220.

Can TIP120 replace BD679?

TIP120 is electrically similar (NPN Darlington, high hFE) but uses a TO-220 package versus BD679's SOT-32, so a layout change is required. Electrically, TIP120 has a lower Vceo (60 V vs 80 V) and higher Ic (5 A vs 4 A). If your supply voltage is ≤ 48 V and the board has room for a TO-220 footprint, TIP120 or TIP120G can replace BD679 with slight derating.

What is the BDX54C equivalent?

The BDX54CG from ON Semiconductor is a direct functional and pin-compatible equivalent to the BDX54C from STMicroelectronics—both are PNP Darlington, 100 V, 8 A, TO-220. The "G" suffix in onsemi's naming convention denotes a Pb-free, green package. The BDX54B is a lower-voltage (80 V) variant and is not a direct substitute for BDX54C in 60 V+ supply rail designs.

How do I drive a Darlington transistor from a 3.3 V MCU GPIO?

Connect a base resistor between the GPIO and the base pin sized to supply Ib = Ic / hFE_min. For TIP120G at Ic = 3 A, hFE_min = 1000, so Ib_min = 3 mA. With a 3.3 V GPIO and VBE ≈ 1.4 V (two junctions), a 620 Ω resistor provides (3.3 − 1.4) / 620 Ω ≈ 3 mA. Add a flyback diode (e.g., 1N4148) across inductive loads to suppress kickback spikes above Vceo.

Is TIP120 still in production?

Yes. TIP120 is actively produced by STMicroelectronics and ON Semiconductor (as TIP120G). Both manufacturers maintain the TO-220 package with AEC-Q101 qualification available on select date codes. Stock levels fluctuate; check real-time inventory on FindMyChip or request a quote for volume pricing across authorized and qualified distributors.

Conclusion

BD679, TIP120, and BDX54 remain practical choices for motor-drive, relay-drive, and solenoid-switching designs where logic-level base drive is a constraint. BD679/BD679A cover the SOT-32 medium-current NPN niche; TIP120/TIP120G address higher-current TO-220 NPN loads up to 60 V; BDX54B, BDX54C, and BDX54CG handle PNP high-side switching up to 100 V/8 A. Multi-sourcing across ST and onsemi part numbers eliminates single-vendor supply risk. To find in-stock equivalents, verify pricing, and request samples, submit an RFQ on FindMyChip or search for Darlington transistors across our 200+ verified global distributors.