As a high gain composite transistor structure, Darlington transistor is widely used in scenarios that require high current driving or high input impedance, such as relay driving, motor control, LED array driving, etc. Its packaging forms are diverse, mainly selected based on power level, heat dissipation requirements, space limitations, and installation methods (through-hole or surface mount). The following is a systematic review of common Darlington tube packaging:
1、 Through hole direct insertion packaging (suitable for prototyping, maintenance, and high-power applications)
TO92：
Description: The most common low-power plastic packaging. Small size and low cost.
Pin: Usually there are 3 pins (base, collector, emitter) arranged in a straight line.
Characteristics: Low power dissipation capacity (usually<1W), mainly used for signal amplification or driving small loads (such as low-power LEDs, small relays). Easy to manually weld.
Typical applications: low current switches, logic level conversion, sensor interface circuits.
TO126：
Description: Medium power plastic packaging. Larger than TO92, with a metal backplate (usually connected internally to the collector) for easy installation of small heat sinks.
Pin: 3 pins arranged in a straight line. Metal backplates are sometimes used as collector pins.
Features: Medium power dissipation capacity (several watts to over ten watts), better heat dissipation performance than TO92. It is a commonly used package for single tube power Darlington.
Typical applications: driving small and medium-sized relays, solenoids, small and medium power DC motors, and power LED strings.
TO220：
Description: A very popular medium to high power plastic packaging. There are protruding metal mounting plates (usually connected internally to the collector) used for mounting onto the heat sink.
Pin: 3 pins (base, collector, emitter) arranged in a straight line, with mounting holes on the metal sheet.
Features: Excellent heat dissipation capability (can reach tens of watts or even higher when combined with a heat sink), good mechanical strength, and easy installation of the heat sink. It is a standard package for single tube high-power Darlington.
Typical applications: driving large relays, solenoid valves, high-power DC/stepper motors, auxiliary power switches in switching power supplies, high current linear regulators.
TO3P / TO247：
Description: High power plastic packaging. The volume is larger than TO220, and the area of the metal backplate (collector) is larger.
Pin: 3 pins (base, collector, emitter) arranged in a straight line, with a large metal backplate at the bottom of the package.
Features: Extremely high power dissipation capacity (up to 100 watts when combined with a heat sink), extremely low thermal resistance. Used for single tube Darlington that requires the highest power output.
Typical applications: ultra-high current switch, industrial motor drive, high-power audio amplifier output stage (less common, IGBT/MOSFET more mainstream), welding machine control.
2、 Surface mount packaging (suitable for high-density automated production)
SOT23：
Description: Ultra small surface mount packaging. The volume is very small.
Pin: Usually 3 pins (base, collector, emitter), sometimes 4 pins (e.g. with independent protective diode).
Features: Low power dissipation capability (<0.5W), occupying minimal PCB area. Strong dependence on PCB heat dissipation design.
Typical applications: Small signal amplification with limited space, driving micro loads (such as individual LEDs), logic interfaces in portable devices.
SOT89：
Description: A small surface mount package with a large metal heat dissipation pad (usually connected to the collector electrode).
Pin: 3 pins (base, collector, emitter), with a large heat dissipation pad at the bottom.
Features: Slightly higher power than SOT23 (around 1W), heat dissipation mainly relies on the PCB copper foil area. The design of heat dissipation pads is crucial for performance.
Typical applications: small and medium power switches (such as small relays, buzzer drivers), power management auxiliary circuits that require miniaturization.
SOT223：
Description: Medium power surface mount packaging. Larger than SOT89, with a larger metal heat dissipation pad (collector).
Pin: Usually there are 4 pins, of which 3 are electrical signal pins (base, collector, emitter), and the fourth is a large heat dissipation/collector pad (sometimes 2 pins are short circuited as collectors). The heat dissipation pad is the core.
Features: Good power dissipation capability (12W or higher), heat dissipation relies on PCB design (large-area copper plating). It is a commonly used package for power Darlington in SMD.
Typical applications: driving small and medium-sized relays, LED arrays, small motors, and switching tubes in DCDC converters.
DPAK (TO252)：
Description: Medium power surface mount package, similar in appearance to a small TO220.
Pin: 3 pins (base, collector, emitter), with a large metal heat sink (collector) at the top, which is directly soldered onto the large-area copper foil of the PCB.
Features: The heat dissipation performance is superior to SOT223 (with a power of several watts to over ten watts), making it one of the mainstream packages for power applications in SMD. Careful design of PCB heat dissipation pads is required.
Typical applications: Similar to TO220, but used in space limited or automated production scenarios, such as power modules, motor drive modules, and high-power LED drivers.
D²PAK (TO263)：
Description: High power surface mount packaging, larger in size than DPAK.
Pin: 3 or more pins (possibly multiple collector or emitter pins to reduce inductance/resistance), with a very large metal heat sink (collector) at the top.
Features: The strongest heat dissipation capability in SMD packaging (with power up to tens of watts) and low thermal resistance. High requirements for PCB heat dissipation design and manufacturing process (reflow soldering).
Typical applications: high current switching power supplies, high-performance motor drive controllers, applications that require high power density and surface mount.
3、 Multi channel/array packaging (integrating multiple Darlington transistors)
DIP (Dual Inline Package)：
Description: Classic dual in-line plastic packaging.
Pin: Commonly available are 16 pin or 18 pin. Internally integrated with multiple (usually 7 or 8) Darlington diodes, and often with freewheeling diodes and input resistors.
Features: Convenient to use on breadboards or experimental boards, easy to manually weld. Medium power capability, typically capable of driving loads of several hundred mA per channel.
Typical applications: Drive multiple relay groups, stepper motors, multi position 7-segment digital tubes, and row/column drivers for LED dot matrix screens. Representative models: ULN2003A (7 channels), ULN2803A (8 channels).
SOP/SOIC (Small Outline Integrated Circuit)：
Description: Surface mount version of DIP.
Pin: Compatible with the corresponding DIP chip pin (such as 16 pin SOIC).
Features: Save PCB space, suitable for automated production. The function is the same as the multi-channel Darlington array packaged in DIP.
Typical application: Similar to DIP packaging applications, but used for compact product designs that require surface mount.
Summary of Selection Points
Power demand: is the primary factor in choosing packaging. Choose TO92/SOT23 for low power; Select TO126/SOT223/DPAK for medium power; Choose TO220/TO247/D ² PAK for high power.
Cooling conditions: Do we need an additional radiator? What is the heat dissipation capability of PCB? TO220/TO247/TO3P must be equipped with a radiator; SOT223/DPAK/D ² PAK relies on PCB heat dissipation; TO92/SOT23 mainly relies on air convection.
Installation method: through hole (THT) or surface mount (SMD)? Prototype/maintenance/high power tendency THT; Mass production/high-density design tends towards SMD.
Space limitation: The space is extremely compact and SOT23/SOT89 is selected; There is a certain space to choose SOT223/DPAK; Choose TO series for ample space or high power.
Integration: When driving multiple loads, a multi-channel array (DIP/SOP) is more space and cost efficient than using multiple single tubes.
Cost: Generally, the larger the package, the stronger the heat dissipation capability, and the more pins there are, the higher the cost. Balance according to application requirements.
In short, the packaging selection of Darlington transistors is a process of balancing power, heat dissipation, space, cost, and manufacturing process. Understanding the characteristics and applicable scenarios of various packaging is crucial for designing reliable and efficient electronic systems. Please refer to the data manual of the corresponding device for the optimal packaging of the specific model.