What Is a power transistor?
A power transistor is a transistor with an allowable power of 1 W or more in operation, and is used in electrical equipment that is driven by a large current. The main roles of power transistors are current amplification, switching, and rectification of alternating current. Because of the large current they handle, they generate a lot of heat during operation, and some products use heat-resistant metal for the case or have accompanying fins for heat dissipation. There are several types of power transistors, of which bipolar power transistors, MOSFETs, and IGBTs are typical examples.
Uses of Power Transistors
Power transistors are used for switching and amplifying current in electrical equipment that requires a large current to operate. Typical applications include home appliances such as air conditioners, refrigerators, and washing machines, solar power generation, and electric vehicles. When selecting power transistors, it is necessary to consider the allowable current, voltage, heat generated during operation, and size. Also, if the product requires high-precision operation, switching speed and other factors must be considered.
Principle of Power Transistors
The operating principle of power transistors is explained separately for bipolar transistors, MOSFETs, and IGBTs.
1. Bipolar Transistor
A bipolar transistor is a transistor with a three-layer structure consisting of N-type and P-type semiconductors. The semiconductors that make up the bipolar transistor have terminals coming out of each semiconductor, which are called the base, emitter, and collector. With voltage applied to the emitter and collector, a large current flows between the emitter and collector when current flows in the base.
MOSFET
A MOSFET has the same structure as a bipolar transistor, but the names of the terminals change to source, drain, and gate. When voltage is applied to the gate, current flows between the source and drain. Because these transistors can switch at high speeds, they are used in products that require quick control.
IGBT
IGBTs have a similar structure to the above two transistors, and use the same names for their terminals as the bipolar transistors introduced earlier. The basic principle of operation is also the same as that of bipolar transistors, but they feature fast switching like MOSFETs.
Types
Power transistors are broadly classified into bipolar transistors and electrolytic effect transistors. Bipolar transistors are current-controlled devices and have an NPN or PNP structure. They generally have three terminals, two paths for the output current (input and output), and one input that controls the output current.
The circuit commonly used in power transistors is an emitter ground circuit, with the input signal on the base (B), the + power supply on the collector (C), and the emitter (E) grounded.
The DC current amplification factor of a bipolar transistor is expressed as β or hFE and ranges from tens to 200. Power transistors with Darlington structure are also available to earn β (here β is in the hundreds to thousands).
Field-effect transistors, on the other hand, are voltage-controlled devices and are either N- or P-channel structures. They generally have three terminals, two paths for the output voltage (input and output), plus one input that controls the output voltage.
The circuit commonly used in power electrolytic effect transistors is a source ground circuit, where the gate (G) is the input signal, the drain (D) is the + supply, and the source (S) is ground.
The DC voltage amplification factor of the electrolytic effect transistor is expressed by the mutual conductance, gm. Electrolytic effect transistors are superior to power transistors in terms of switching characteristics, and are widely used as switching elements in switching power supplies.
The word “transistor” generally refers to a bipolar transistor.
Failure
When a power transistor fails, the output of the circuit is usually lost or unstable. This is because power transistors are often used in output or power supply sections.
There are many possible causes of failure, but it is easy to determine if a power transistor has failed with a tester. First, a transistor is represented by a combination of two diodes.
In the case of an NPN transistor (2SC or 2SD)
Base (B) positive, conducts to the collector (C), and the opposite direction is non-conductive
Base (B) positive, conductive to emitter (E), nonconductive in reverse direction
Collector (C) – emitter (E) are disconnected in either direction
If all three of these conditions hold, the transistor is okay.
In the case of a PNP transistor (2SA or 2SB), the opposite direction is true
collector (C) is positive and conducts to base (B), non-conductive in the opposite direction
Emitter (E) positive, conducts to base (B), not in the reverse direction
Collector (C) – emitter (E) is disconnected in both directions
If all three of these conditions are satisfied, the transistor is okay. Note that this test cannot be used for Darlington transistors.
When the tester is used in the continuity range, the red side is negative and the black side is positive. Pay attention to the polarity of the test voltage on the tester leads. Also, be sure to disconnect the power supply, input wires, output wires, etc. of the non-test object before testing.