What Are Parallel Transistors?
Parallel transistors involve connecting two or more transistors in a configuration where their collector terminals, base terminals, and emitter terminals are connected together. This arrangement allows the current handling capacity to be increased compared to using a single transistor.
Parallel transistors are commonly used in high-power applications where a single transistor cannot handle the required current. By connecting multiple transistors in parallel, the current is divided among them, allowing higher total current flow without exceeding the limits of the individual transistors.
Advantages of Parallel Transistors
There are several key advantages to using parallel transistors:
- Increased current handling capacity
- Improved power dissipation
- Redundancy in case of single transistor failure
- Potentially lower cost than a single high-power transistor
How to Connect Transistors in Parallel
To properly connect transistors in parallel, follow these steps:
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Select transistors with closely matched characteristics, ideally from the same batch. Key parameters to match include current gain (hFE), collector-emitter saturation voltage, and thermal characteristics.
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Connect the collector terminals of all transistors together. Use thick wire or PCB traces to handle the high current.
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Connect the emitter terminals of all transistors together, again using conductors suitable for the current.
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Provide a common base signal to all transistors, either directly or through individual small-value resistors (10-100 ohms) to help balance currents.
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Include small emitter resistors (0.1-0.5 ohms) in series with each transistor to further aid current sharing and prevent thermal runaway.
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Ensure adequate heat sinking of all transistors, as they will generate significant heat under high current conditions.
Here is a simplified schematic showing parallel transistors with emitter resistors:
+--------------+
| |
| +----| |
+---| T1 |-----+
+----| |
| |
+---R1--| |
| | | Vout
Vin | +----| |
-----+---| T2 |-----+
+----| |
| |
+---R2--| |
| | |
| | |
+-------+-----+
|
|
GND
Transistor Selection for Parallel Operation
Proper transistor selection is critical for successful parallel operation. Transistor parameters should be as closely matched as possible. Even small variations between transistors can lead to uneven current sharing, causing some transistors to conduct more heavily and potentially exceed their safe operating limits.
When possible, source paralleled transistors from the same manufacturing batch. Reputable semiconductor manufacturers often specify parameters for parallel operation, and may even provide “matched sets” of transistors specifically for this purpose.
If discrete transistors must be used, consider pre-testing and matching devices. Measure key DC parameters such as hFE and Vce(sat) and group devices with similar characteristics.
Emitter Resistors for Current Balancing
Including small value resistors in series with each transistor’s emitter lead is an important technique to ensure proper current sharing between devices. These resistors, typically in the 0.1 to 0.5 ohm range, introduce a small amount of local negative feedback that compensates for variations in transistor characteristics.
Without emitter resistors, minor differences in Vbe or hFE can cause currents to be unevenly distributed. The transistor conducting the most current will heat up more than the others, further increasing its share of the current. This thermal runaway effect can rapidly lead to device failure.
Emitter resistors help mitigate this issue by automatically reducing the current in a transistor as it begins to conduct more heavily. If one transistor’s current increases, the voltage drop across its emitter resistor will increase, reducing Vbe and thus limiting further current increase. Conversely, a transistor conducting less current will have a lower emitter voltage, allowing it to conduct more and balance the currents.
The value of the emitter resistors should be kept as low as practical to minimize their impact on overall circuit performance. Excessively large resistances will reduce efficiency and limit maximum output current. Values around 0.22 ohms are common, but the optimal resistance depends on the specific circuit conditions and transistor characteristics.
In addition to emitter resistors, it is also beneficial to provide a small amount of resistance between the transistor bases and the input signal. Base resistors in the 10-100 ohm range help equalize base currents and further improve current sharing. They can also protect the transistors in case of a short circuit condition by limiting maximum base current.
Proper Heat Sinking for Parallel Transistors
Even with proper current balancing techniques, parallel power transistors will generate significant heat that must be effectively dissipated to prevent damage. Adequate heat sinking is essential.
Ideally, mount all transistors to a common heat sink. This helps maintain equal case temperatures among devices. If separate heat sinks must be used, ensure they have similar thermal resistances so transistor temperatures track closely.
Use thermal grease or thermally conductive pads between transistor cases and the heat sink to minimize thermal resistance. Avoid insulating washers unless electrically required, as they can impede heat transfer.
In high power applications, it may be necessary to temperature-compensate the bias circuit to prevent thermal runaway. This can be achieved by including temperature-sensitive elements like thermistors or diodes that reduce bias as temperature increases.
Example Parallel Transistor Circuits
Simple DC Load Switch
A basic application of parallel transistors is a high-current DC load switch:
+----| |
+----| T1 |-----|
| +----| |
| | |
Control | +---R1-| |
-------+----| | |
| | | |
| | +----| |
| +--| T2 |-----|
| +----| |
| | |
+------+---R2-| |
| | |
| | |
+------+-----+
|
|
GND
In this circuit, two NPN transistors are connected in parallel to switch a high-current load. The control signal is applied to the bases through current limiting resistors R1 and R2. When the control signal is high, both transistors turn on, allowing current to flow through the load. When the control is low, the transistors are off, and the load is disconnected.
Emitter resistors are not shown here for simplicity, but would normally be included. The resistor values and Transistor Types would be selected based on the load current and control signal characteristics.
Audio Power Amplifier Output Stage
Parallel transistors are commonly used in the output stages of audio power amplifiers to increase current capability and power output. Here is a simplified schematic of a push-pull transistor output:
Vcc
|
|
|
|
+----| |------+
| | |
| C| E|
| T3 | |T4
| | |
| E| C|
| |-----|------+
| | |
Input | | |
-------+ | +---R1--| |
| | C | | | Audio
+---|-|T1 E | | Output
| | | |
+-|--+ +-----+
| | |
| E| C |
| T2 |--| |
| | | |
+----|--+ |
| R2 |
+--------|----+
|
GND
This circuit uses two pairs of complementary transistors (T1/T2 and T3/T4) to amplify the audio input signal. The NPN transistors T1 and T3 conduct during positive half-cycles, while the PNP transistors T2 and T4 conduct during negative half-cycles. The output signal is taken between the emitters of T3 and T4.
Emitter resistors R1 and R2 set the idle bias current and help stabilize the output stage. Their values are typically a few ohms or less. The transistors must be mounted on a substantial heat sink to dissipate the heat generated by the high currents involved.
Practical audio amplifier output stages are more complex, with additional circuitry for biasing, crossover distortion reduction, and protection against short circuits and overloads. However, the basic principle of using parallel transistors for increased output current remains the same.
Potential Issues and Pitfalls
While parallel transistors offer several benefits, there are also potential problems to be aware of:
Current Hogging
If paralleled transistors are not well-matched or properly current-balanced, one device may tend to conduct the majority of the current. This “current hogging” can lead to overheating and failure of the overloaded transistor, potentially taking out the entire circuit.
Careful device selection and the use of emitter resistors helps mitigate this issue, but it remains a concern especially at high current levels. Monitoring transistor case temperatures and implementing over-current protection is advisable in critical applications.
Increased Capacitance
Connecting transistors in parallel effectively multiplies their junction capacitances. This can limit high-frequency performance and cause issues in fast-switching applications.
If high speed operation is required, consider using a single larger transistor instead of parallel devices. Alternatively, look for transistors with lower capacitances, or consider RF power transistor modules which are optimized for high-frequency use.
Base Drive Requirements
The increased current handling of parallel transistors comes with a corresponding increase in required base drive current. Make sure the driver circuit can supply sufficient current to fully saturate all the transistors under worst-case conditions.
Pay attention to the DC current gain (hFE) of the transistors, as this determines the base current for a given collector current. If the transistors have low hFE, more base current will be needed. Driver circuits may need to be buffered or redesigned to provide adequate base current.
Thermal Runaway
Thermal runaway is a self-reinforcing failure mode where a transistor’s temperature rises, causing it to conduct more current, which further increases its temperature until destruction. It is a particular concern with parallel transistors due to their high power dissipation and the potential for uneven current sharing.
To prevent thermal runaway, use emitter resistors to balance currents, and ensure the transistors have adequate heat sinking. Over-temperature protection, such as thermal switches or foldback current limiting, may be necessary in high-reliability applications.
FAQ
Q: Can I parallel transistors of different types or from different manufacturers?
A: It is generally not recommended to parallel transistors of different types or from different manufacturers. Transistor characteristics can vary significantly between types and brands, making it difficult to ensure proper current sharing. Whenever possible, use identical transistors from the same production batch.
Q: How closely matched do parallel transistors need to be?
A: The closer the match, the better. Ideally, paralleled transistors should have nearly identical DC current gain (hFE), collector-emitter saturation voltage, and junction capacitances. However, perfect matching is not always practical. Using emitter resistors and individual base resistors helps compensate for moderate variations between devices.
Q: Can parallel transistors be used for high-frequency applications?
A: Parallel transistors can be used at high frequencies, but performance may be limited due to increased junction capacitance. For best high-frequency results, consider RF power transistor modules or LDMOS devices which are optimized for high-speed operation.
Q: Is it necessary to use emitter resistors if the transistors are well-matched?
A: While closely matched transistors may share current adequately without emitter resistors under ideal conditions, including the resistors is still good design practice. Emitter resistors help compensate for variations in device characteristics that may arise due to temperature changes, aging, or other factors. They provide an extra margin of safety and reliability.
Q: What happens if one transistor in a parallel set fails open or short?
A: If a transistor fails open, the remaining devices will have to handle the full current, which may cause them to also fail if they are not adequately rated. If a transistor shorts, it can draw excessive current and potentially damage the power supply or other Circuit Components. Including fuses or other over-current protection is advisable to mitigate damage from single-device failures.
Conclusion
Parallel transistors are a powerful technique for increasing current handling capability in high-power applications. By properly selecting and matching devices, using emitter resistors for current balancing, and ensuring adequate heat sinking, designers can create robust and reliable power circuits.
However, parallel operation is not without challenges. Careful attention must be paid to transistor characteristics, base drive requirements, and thermal management to avoid issues like current hogging and thermal runaway.
By understanding the principles and potential pitfalls of parallel transistors, and by following best design practices, engineers can take advantage of this technique to push the boundaries of power electronics. With the proper implementation, parallel transistors can enable high-performance power systems for a wide range of applications, from audio amplifiers to industrial motor drives to renewable energy systems.
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