Description
The BC640 is a medium-power PNP bipolar junction transistor (BJT). It serves as a more robust version of the low-power transistors (like the 2N3906), offering a higher current rating of 1 Ampere. This makes it an excellent “medium-power” choice for driving small motors, relays, or serving as a driver for even larger power transistors.
Key Specifications
| Parameter | Symbol | Rating |
| Collector-Emitter Voltage | $V_{CEO}$ | -80V |
| Continuous Collector Current | $I_C$ | -1A |
| Peak Collector Current | $I_{CM}$ | -1.5A |
| Power Dissipation | $P_D$ | 625mW (at 25°C) |
| DC Current Gain | $h_{FE}$ | 40–160 (at 150mA) |
| Transition Frequency | $f_T$ | 50MHz |
Key Characteristics & Applications
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Higher Current Capacity: While signal transistors like the 2N3906 are limited to 200mA, the BC640 handles up to 1A. This allows it to switch small solenoids, indicator lamps, or moderate-sized relays directly.
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Medium-Power PNP Switching: It is frequently used for “high-side” switching. Because it is a PNP device, it connects the positive supply rail to the load when the base is pulled low.
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Complementary Pair: The BC640 is the PNP complement to the BC639 (NPN). Using them together is a standard practice for creating push-pull stages in small audio amplifiers or H-bridge motor drivers.
Design Considerations
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Thermal Constraints: Despite its 1A rating, the BC640 is usually housed in a TO-92 plastic package, which has limited heat dissipation. If you are switching a load near 0.8A–1A, it will get very hot. For sustained high-current operation, use a small clip-on heatsink or limit the duty cycle.
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Base Drive: To keep the BC640 fully saturated at 1A, you need to provide a substantial base current. If the load is close to 1A, a base current of roughly $50mA – 100mA$ is required. A direct microcontroller pin might struggle to provide this, so you may need a small “pre-driver” transistor if you are switching maximum currents.
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Application Note: When switching inductive loads (like a relay coil), always include a flyback diode. Because this transistor can handle up to 80V, it is very well-suited for 12V, 24V, and 48V systems.
Comparison
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BC640 vs. 2N3906: The BC640 is more powerful (1A vs 200mA) and handles higher voltage (80V vs 40V), making it much better for driving peripheral components.
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BC640 vs. TIP125/127: The BC640 is a single-stage transistor, whereas the TIP series are Darlington pairs. The BC640 is better for lower current applications (under 1A) where you want to minimize the voltage drop ($V_{CE(sat)}$) and avoid the extra heat generated by the Darlington configuration.
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