The choice of Darlington transistor (usually referring to a composite structure composed of two bipolar junction transistors) is mainly based on its unique performance advantages to meet specific circuit requirements. The following are typical situations and considerations that need to be prioritized for using Darlington transistors:
Requires extremely high current gain:
Core advantage: The most essential advantage of Darlington transistors is their extremely high current gain (β), which can typically reach thousands or even tens of thousands of times (β _total ≈ β 1 β 2).
Application scenarios:
Driving high current loads: When small currents (such as from microcontroller GPIO pins, sensor outputs, logic gate outputs) are needed to control very large load currents (such as relay coils, DC motors, high-power LED light strings, solenoids), the gain of a single ordinary transistor is often insufficient. A Darlington transistor only requires a small base drive current to provide a sufficiently large collector current.
Simplified driver circuit: High gain means that the front-end driver circuit can be designed very simply (such as directly driven by microcontroller pins), without the need for additional pre drivers or amplification stages, reducing circuit complexity and cost
Requires high input impedance:
Principle: The input impedance of a Darlington transistor is mainly determined by the input impedance of the first transistor (whose base current is very small), resulting in a much higher input impedance than a single transistor.
Application scenarios:
Interface with high impedance signal source: When the internal resistance of the signal source is large or the output current capability is very weak (such as some sensors, capacitive microphone front-end, high resistance voltage divider networks), the high input impedance of the Darlington transistor can minimize the load effect on the signal source and avoid signal distortion or attenuation.
Directly driven by logic circuits: microcontrollers and CMOS/TTL logic gates have limited output driving capabilities and typically require high load impedance. The high input impedance of Darlington transistor matches well with it and can be directly connected without overloading the previous stage.
Application of medium and low speed switches:
Balance: The main drawbacks of Darlington transistors are high saturation voltage drop (usually 1V to several volts, depending on the current and specific device) and relatively slow switching speed (due to the charge storage of the first transistor requiring the second transistor to discharge, resulting in longer turn off time).
Applicable scenarios:
Relay drive: Relays do not require high switching speed (tens of Hz to hundreds of Hz), but they have requirements for driving current (tens of mA to hundreds of mA). The high gain and high input impedance of Darlington transistors make them an ideal choice, and their saturation voltage drop is usually acceptable at relay coil voltages (such as 5V, 12V, 24V). The common ULN2003/ULN2803 is the Darlington array IC.
LED array driver: Driving multiple parallel high-power LEDs or LED light strings requires a large current. Darlington tubes can operate at lower control currents and meet visual requirements for speed. Attention should be paid to the power consumption caused by saturation voltage drop.
Solenoid and small DC motor start stop control: Similar to relays, it does not require high speed and requires a certain driving capability.
Dimming/switching of incandescent lamps: does not require high speed (much lower than the frequency of the switching power supply).
Not applicable scenarios: high-frequency switches (such as switching power supplies, PWM high-speed motor control, high-frequency inverters), low-voltage applications that require extremely high efficiency (with a large proportion of saturation voltage drop). In these situations, MOSFETs (especially MOSFETs with low on resistance) are usually the preferred choice.
Multi channel integrated application:
Convenience: There are many array chips on the market that integrate multiple Darlington transistors (such as ULN2003: 7 channels, ULN2803: 8 channels). These chips typically contain freewheeling diodes (used to drive inductive loads) with a common emitter or common collector configuration.
Application scenario: It is necessary to drive multiple relays, stepper motor windings, multiple groups of LEDs, and other situations simultaneously. The use of integrated Darlington arrays can greatly simplify PCB layout, reduce component count, and improve reliability.
Cost sensitive and performance matching applications:
Economy: Under the premise of meeting the above performance requirements (high gain, high input impedance, medium low speed switch), Darlington transistors (especially discrete devices or standard arrays) usually have a cost advantage over designing complex discrete multi-stage amplifier circuits or using dedicated, possibly more expensive high/low side driver ICs.
Key considerations for selecting Darlington tubes:
When the control signal is very weak or the driving ability is extremely weak (high input impedance requirement), but a considerable load current needs to be controlled (high current gain requirement), Darlington transistor is a classic solution.
Load type and speed: The load refers to devices that are insensitive to switching speed (below a few hundred Hz), such as relays, small and medium power DC motors (start stop), solenoids, LED light strings, etc.
Adequate voltage margin: The system voltage is high enough to prevent the saturation voltage drop (Vce (sat)) of the Darlington transistor from consuming a significant proportion of the power supply voltage (for example, in 12V or 24V systems, a voltage drop of 1-2V is usually acceptable); In 3.3V or 5V low-voltage systems, power consumption needs to be carefully evaluated.
Simplify design: We hope to use the simplest circuit (usually a resistor Gallington transistor) to complete the driving task, reducing design complexity and BOM cost.
Multi channel drive convenience: When multiple similar loads need to be driven, the integrated Darlington array (ULN series) is very convenient and practical.
On the contrary, the use of Darlington tubes should be avoided in the following situations:
High frequency switch application: switch frequency exceeding tens of kHz (long turn off delay).
Low voltage and high current applications: The system voltage is very low (such as 3.3V), and the saturation voltage drop of the Darlington transistor causes the effective load voltage to be too low or the self power consumption to be too high, resulting in low efficiency.
Extremely sensitive to conduction voltage drop: requires extremely low conduction loss (such as power switches in high-efficiency DC-DC converters).
Need for extremely fast shutdown speed, such as high-speed switching power supply and precision PWM control.
In summary, the core logic behind choosing a Darlington transistor is to balance the advantages of driving simplification brought about by its ultra-high current gain and high input impedance with the disadvantages of higher saturation voltage drop and slower switching speed, ensuring that the target application scenario has a strong demand for the former while being insensitive or acceptable to the limitations of the latter.