– MOSFETPOWER TRANSISTOR

Example of power MOSFET parameters;

Parameter 2N6757

2N6792

VDS(max) (V) 150 400

ID(max) (A) - @ T = 25C 8 2

PD (W) 75 20

– MOSFETPOWER TRANSISTOR

The superior characteristics of MOSFETs are;

• Faster switching time;

• No second breakdown;

• Stable gain and response time over a wide temperature range (Figure on the next slide).

– MOSFETPOWER TRANSISTOR

Transconductance versus drain current curves for various values of temperature

– less than the variation in BJT current gain.

– MOSFETPOWER TRANSISTOR

Transfer characteristics curves for various values of temperature.

– MOSFETPOWER TRANSISTORStructure

– MOSFETPOWER TRANSISTORStructure

DMOS process can be used to produce a large number of hexagonal cells on a single chip.

– MOSFETPOWER TRANSISTORStructure

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.

– MOSFETPOWER TRANSISTOR

The “ON” resistive path between drain and source (rds(on))

is an important parameter in power capability of MOSFET

– ComparisonPOWER 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 sinksPOWER 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 sinksPOWER TRANSISTOR

• Additionally, heat sinks can be used to remove the heat developed in the transistor:

Heat sinks (Extra)POWER TRANSISTOR

Electrical equivalent circuit of thermal-conduction process

JADAJ PTT

JT Temperature of transistor junction

AT Ambient temperature

AJ TT Temperature difference Voltage difference

JA Thermal resistance between the junction and ambient Electrical resistance

DP Thermal power through the element Electric current.

Heat sinks (Extra)POWER TRANSISTOR

Manufacturers’ data sheet for power devices generally give:

• maximum operating junction (device) temperature, TJmax;

• thermal resistance from the junction to the case, JC;

The temperature conduction process may be represented as follows:

POWER TRANSISTOR

The following equation can be used to describe the temperature conduction process:

ambdev TT amb-snksnkcasecasedev DP

If the heat sink is not used, then;

ambcasecasedevambdev DPTT

Heat sinks (Extra)

Heat sinks (Extra)POWER TRANSISTOR

A MOSFET has the following parameters;

C/W; 75.1casedev

Determine the maximum power dissipation in the transistor and determine the temperature of the transistor case and heat sink.

EXAMPLE 8.3

C/W; 1snk-case

C/W; 5amb-snk C/W; 50amb-case

C; 150devmaxTTJ C; 30amb

T

Heat sinks (Extra)POWER TRANSISTOREXAMPLE 8.3 – Solution

Maximum power (without heat sink)

W32.25075.1

30150

amb-casecase-dev

ambmaxmax

TT

P JD

Maximum power (with heat sink)

amb-snksnk-casecase-dev

ambmaxmax

TTP JD

W5.155175.1

30150

Heat sinks (Extra)POWER TRANSISTOREXAMPLE 8.3 – Solution (cont’d)

Heat sink temperature

amb-snkmaxambsnk DPTT

amb-snkmaxambsnk DPTT

C 5.10755.1530

Heat sinks (Extra)POWER TRANSISTOREXAMPLE 8.3 – Solution (cont’d)

Case temperature

amb-snksnk-casemaxambcase DPTT

amb-snksnk-casemaxambcase DPTT

C 123515.1530

Note: The use of heat sink allows more power to be dissipated in the device.

Heat sinks (Extra)POWER TRANSISTORPower derating curve

Manufacturer usually specifies:

• the maximum temperature TJmax;

• the maximum power dissipation PDmax, at a particular

ambient temperature TA0 (usually 25C); and

• the thermal resistance JA.In addition, a graph – power derating curve is provided.

Heat sinks (Extra)POWER TRANSISTORPower derating curve

For operation below TA0, the device can

safely dissipate the rated value of PD0

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.