1. POWER TRANSISTOR – MOSFET Parameter 2N6757 2N6792 VDS(max) (V) 150 400
Example of power MOSFET parameters;
Parameter
2N6757
2N6792
VDS(max) (V)
150
400
ID(max) (A) T = 25C 8 2
PD (W) 75 20

2. POWER TRANSISTOR – MOSFET 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).

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

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

5. POWER TRANSISTOR
– MOSFET
Structure

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

7. POWER TRANSISTOR – MOSFET 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.

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

9. POWER TRANSISTOR – Comparison 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.

10. POWER TRANSISTOR – Heat sinks
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:

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

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

13. POWER TRANSISTOR Heat sinks (Extra)
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:

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

15. POWER TRANSISTOR Heat sinks (Extra) EXAMPLE 8.3
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.

16. POWER TRANSISTOR Heat sinks (Extra) EXAMPLE 8.3 – Solution
Maximum power (without heat sink)
Maximum power (with heat sink)

17. POWER TRANSISTOR Heat sinks (Extra) EXAMPLE 8.3 – Solution (cont’d)
Heat sink temperature

18. POWER TRANSISTOR Heat sinks (Extra) EXAMPLE 8.3 – Solution (cont’d)
Case temperature
Note: The use of heat sink allows more power to be dissipated in the device.

19. POWER TRANSISTOR Heat sinks (Extra) Power derating curve
Manufacturer usually specifies:
the maximum temperature TJmax;
the maximum power dissipation PDmax, at a particular ambient temperature TA0 (usually 25C); and
the thermal resistance JA.
In addition, a graph – power derating curve is provided.

20. POWER TRANSISTOR Heat sinks (Extra) Power 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.