EMT 112 / 4 ANALOGUE ELECTRONICS Self-Reading Power Transistor – BJT & MOSFET

POWER TRANSISTOR Transistor limitations Maximum rated current, Maximum rated voltage, Maximum rated power. The maximum rated power is related to the maximum allowable temperature of the transistor.

– BJT Large-area devices – the geometry and doping concentration are different from those of small-signal transistors Examples of BJT rating: Parameter Small-signal BJT (2N2222A) Power BJT (2N3055) Power BJT (2N6078) V CE (max) (V) I C (max) (A) P D (max) (W)  35 – 1005 – 2012 – 70 f T (MHz) POWER TRANSISTOR

Current gain depends on I C and is smaller in power BJT. The maximum rated collector current, I C(rated) may be related to the following: 1.maximum current that connecting wires can handle 2.The collector current at which the gain falls below a minimum specified value 3.current which leads to maximum power dissipation when the transistor is in saturation. – BJT POWER TRANSISTOR

Typical dc beta characteristics ( h FE versus I C ) for 2N3055 – BJT POWER TRANSISTOR

Current gain depends on I C and is smaller in power BJT. The maximum rated collector current, I C(rated) may be related to the following: 1.maximum current that connecting wires can handle 2.The collector current at which the gain falls below a minimum specified value 3.current which leads to maximum power dissipation when the transistor is in saturation. – BJT POWER TRANSISTOR

Current gain depends on I C and is smaller in power BJT. The maximum rated collector current, I C(rated) may be related to the following: 1.maximum current that connecting wires can handle 2.The collector current at which the gain falls below a minimum specified value 3.current which leads to maximum power dissipation when the transistor is in saturation. – BJT POWER TRANSISTOR

The maximum voltage limitation: Avalanche breakdown in the reverse-biased base- collector junction ( V CEO ); Second breakdown – nonuniformities in current density which inreases temperature in local regions in semiconductor. – BJT POWER TRANSISTOR

– BJT POWER TRANSISTOR I C – V CE characteristics showing breakdown effect

– BJT POWER TRANSISTOR The second term is usually small, hence; The average power over one cycle The instantaneous power dissipation in transistor

– BJT POWER TRANSISTOR The average power dissipated in a BJT must be kept below a specified maximum value to ensure that the temperature of the device does not exceed the maximum allowable value. If collector current and collector-emitter voltage are dc quantities, the maximum rated power, P T The power handling ability of a BJT is limited by two factors, i.e. junction temperature and second breakdown. SOA must be observed, i.e. do not exceed BJT power dissipation.

– BJT POWER TRANSISTOR The safe operating area (SOA) is bounded by I C(max) ; V CE(sus) and P T (Figure) SOA of a BJT (linear scale)

– BJT POWER TRANSISTOR SOA of a BJT (log scale)

– BJT POWER TRANSISTOR EXAMPLE I Determine the required ratings (current, voltage and power) of the BJT.

– BJT POWER TRANSISTOR EXAMPLE I – Solution For the maximum collector current; For the maximum collector- emitter voltage;

– BJT POWER TRANSISTOR EXAMPLE I – Solution The load line equation is; The load line must lie within the SOA The transistor power dissipation;

– BJT POWER TRANSISTOR EXAMPLE I – Solution The maximum power occurs when i.e. when or when At this point; and;

– BJT POWER TRANSISTOR EXAMPLE I – Solution Thus the transistor ratings are; In practice, a safety factor is normally used. The transistor with will be chosen.

– BJT POWER TRANSISTOR Physical structure; Large emitter area to handle large current Narrow emitter width to minimize parasitic base resistance May include small resistors (ballast resistor) in emitter leg to help maintain equal currents in each B–E junction.

– MOSFET POWER TRANSISTOR Example of power MOSFET parameters; Parameter2N N679 2 V DS(max) (V) I D(max) (A) T = 25  C 82 P D (W)7520

– MOSFET POWER TRANSISTOR The superior characteristics of MOSFETs are; Faster switching time; No second breakdown; Stable gain and response time over a wide temperature range (Figure in the next slide).

– MOSFET POWER TRANSISTOR Transconductance versus drain current curves for various values of temperature – less than the variation in BJT current gain.

– MOSFET POWER TRANSISTOR Transfer characteristics curves for various values of temperature.

– MOSFET POWER TRANSISTOR Structure

– MOSFET POWER TRANSISTOR Structure DMOS process can be used to produce a large number of hexagonal cells on a single chip.

– MOSFET POWER TRANSISTOR Structure These hexagonal cells can be paralleled to form large-area devices without the need of emitter ballast resistance. A single power MOSFET may contain as many as 25,000 parallel cells.

– MOSFET POWER TRANSISTOR The “ON” resistive path between drain and source ( r ds(on) ) is an important parameter in power capability of MOSFET

– Comparison POWER TRANSISTOR BJT Requires complex input circuitry because of high input current (current- controlled device) More sensitive to temperature variation – thermal runaway and problem of second breakdown. MOSFET Simple input circuitry because of low input current (voltage-controlled device). More immune to thermal runaway and second breakdown.

– Heat sinks POWER TRANSISTOR The power dissipated in a transistor can cause an internal temperature rise above ambient temperature. This heat, if not properly removed, may cause internal temperature above a safe limit and can cause permanent damage to transistor. Heat may be removed through proper packaging:

– Heat sinks POWER TRANSISTOR Additionally, heat sinks can be used to remove the heat developed in the transistor:

– Heat sinks POWER TRANSISTOR Electrical equivalent circuit of thermal- conduction process Temperature of transistor junction Ambient temperature Temperature difference  Voltage difference Thermal resistance between the junction and ambient  Electrical resistance Thermal power through the element  Electric current.

– Heat sinks POWER TRANSISTOR Manufacturers’ data sheet for power devices generally give: maximum operating junction (device) temperature, T Jmax ; thermal resistance from the junction to the case,  JC ; The temperature conduction process may be represented as follows:

– Heat sinks POWER TRANSISTOR The following equation can be used to describe the temperature conduction process: If the heat sink is not used, then;

– Heat sinks POWER TRANSISTOR A MOSFET has the following parameters; Determine the maximum power dissipation in the transistor and determine the temperature of the transistor case and heat sink. EXAMPLE II

– Heat sinks POWER TRANSISTOR EXAMPLE II – Solution Maximum power (without heat sink) Maximum power (with heat sink)

– Heat sinks POWER TRANSISTOR EXAMPLE II – Solution (cont’d) Heat sink temperature

– Heat sinks POWER TRANSISTOR EXAMPLE II – Solution (cont’d) Case temperature Note: The use of heat sink allows more power to be dissipated in the device.

– Heat sinks POWER TRANSISTOR Power derating curve Manufacturer usually specifies: the maximum temperature T Jmax ; the maximum power dissipation P Dmax, at a particular ambient temperature T A0 (usually 25  C); and the thermal resistance  JA. In addition, a graph – power derating curve is provided.

– Heat sinks POWER TRANSISTOR Power derating curve For operation below T A0, the device can safely dissipate the rated value of P D0 watts. If the device is to be operated at higher ambient temperature, the maximum allowable power dissipation must be derated according to the straight line.