Power transistor
With the power transistor is in the electronics , a transistor for switching or controlling large voltages , currents or services designated.
A boundary between transistors for signal processing and power transistors is not defined uniformly, but transistors are usually referred to as power transistors if they have one or more of the following characteristics:
- maximum collector current or drain current over 1 A.
- minimum collector-emitter voltage or drain-source voltage above 50 V.
- maximum power dissipation over 2 W.
- Thermal resistance of junction housing below 15 K / W ( Pro-Electron definition)
Power transistors are mainly produced in housings that allow them to be mounted on heat sinks , as otherwise it is not possible to dissipate the power loss, which can amount to a few kilowatts in some types and applications. The comparatively low power loss in switching regulators , however, also enables the use of power transistors in SMD design, in which appropriately designed printed circuit boards are used for cooling.
The safe working range of a power transistor is shown in a SOAR diagram .
In analog electronic circuits - especially power amplifiers - the transistors are mostly used in the form of push-pull output stages.
species
- Bipolar power transistors are used, for example, as line output stages , in electronic ballasts and electronic low-voltage halogen lamp transformers. Another area of application is audio amplifier output stages. To reduce the high control currents, they are often designed as a Darlington circuit for slow switching applications , but are then slower and have a higher saturation voltage. Power Darlington transistors optimized for switching operation were used in drive converters and UPS systems until around 1995. However, these were quickly replaced by the newly developed IGBT . The parallel connection of bipolar transistors requires emitter resistors for current distribution, since their current gain has a positive temperature coefficient.
- In contrast to bipolar transistors, power MOSFETs can be controlled with almost no power at low switching frequencies , since pulsed control currents are only required to recharge the gate capacitance. The resistor R DS remaining when the transistor is switched on causes a power loss and has a positive temperature coefficient, which is why MOSFETs can be connected in parallel. MOSFETs can withstand lower chip temperatures (125 to 150 ° C) than bipolar transistors (150 to 180 ° C). Therefore, MOSFETs have to be cooled better than bipolar transistors with the same power loss. Power MOSFETs are used at low voltages of up to a few 100 volts and have the lowest static and dynamic losses of all power transistors at low voltages.
- Insulated gate bipolar transistors (IGBT) are (in the equivalent circuit diagram) like a bipolar pnp transistor that is controlled by a MOSFET, which combines the advantages of both transistor types, low control power and relatively low power loss. IGBTs are useful for switching voltages from a few 100 volts, as they basically have a voltage drop of around 2 volts when switched on. IGBTs have been used increasingly in power electronics since the mid-1990s and have a switching capacity for currents of up to 3600 A and voltages of up to 6500 V per IGBT module. IGBTs switch less quickly than bipolar transistors or MOSFETs.
cooling
The amount of currents and voltages that can be switched by semiconductors is limited by the options for dissipating the heat loss. Chips of power semiconductors are therefore bonded to surfaces of the housing that dissipate heat well . This can be the housing itself or a cooling flag made of copper, which usually also forms one of the electrical connections. If the power loss that occurs cannot be emitted via the surface by radiation or convection, such power transistors are mounted on a heat sink . They usually consist of materials with good thermal conductivity such as aluminum or copper. Primarily in mass products, the circuit board often only serves as cooling and has a larger copper surface on which, for example, the cooling tab of the drain connection is soldered. In addition, there can be several vias to the rear of the circuit board ( thermal vias ) for heat dissipation.
If an electrically insulated installation on heat sinks is required, thermal pads are used. Alternatively, there are transistor types with fully insulated plastic housings. The use of heat conducting paste is advantageous , since it fills the unevenness of the two contact surfaces and thus reduces the heat transfer resistance from the transistor housing to the heat sink. In the case of an isolating assembly, the type of isolator must also be taken into account; Depending on its mechanical properties, thermal grease has different benefits.
The cooling parameters are the following thermal resistances (unit Kelvin per watt):
- component-specific thermal resistance between the active area of the chip (junction) and the outer surface of the housing (case) or cooling flange (R thJC )
- component-specific thermal resistance between the active area of the chip (junction) and the environment (ambient) without heat sink (R thJA )
- Mounting and housing-specific thermal resistance between component cooling surface and heat sink surface
- heatsink-specific thermal resistance of the heatsink against the environment or the cooling medium; often given as a diagram as a function of the flow velocity of the air or the cooling liquid.
Assembly
Power transistors are available with soldered or screw connections. Large IGBTs are manufactured as disks with lines leading out to the side for control. The discs are clamped in a stack.
The electrical connection of the source of MOSFET and also of the emitter of bipolar and IGB transistors must be particularly low-induction or double, since it also carries the load current and is the reference potential for the control voltage. MOSFET and IGBT therefore often have two emitter or source connections in order to separate the two circuits for control and load current right into the housing.
