1) MOSFETs have faster switching times and are more stable over a wide range of temperatures compared to BJTs. 2) Power MOSFETs can contain thousands of parallel hexagonal cells on a single chip to handle high currents without emitter ballast resistors. 3) The on-resistance of the MOSFET channel (RDS(on)) determines its power capability. Heat sinks are often required to dissipate heat from high power MOSFETs and prevent damage.

1. – MOSFETPOWER TRANSISTOR
Example of power MOSFET parameters;
Parameter 2N675
7
2N679
2
VDS(max) (V) 150 400
ID(max) (A) - @ T = 25C 8 2
PD (W) 75 20

2. – 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).

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

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

5. – MOSFETPOWER TRANSISTOR
Structure

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

7. – MOSFETPOWER 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.

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

9. – 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.

10. – 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:

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

12. 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.

13. 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:

14. 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)

15. 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;150devmax

 TTJ C;30amb

T

16. Heat sinks (Extra)POWER TRANSISTOR
EXAMPLE 8.3 – Solution
Maximum power (without heat sink)
W32.2
5075.1
30150
amb-casecase-dev
ambmax
max 







TT
P J
D
Maximum power (with heat sink)
amb-snksnk-casecase-dev
ambmax
max
 


TT
P J
D
W5.15
5175.1
30150





17. Heat sinks (Extra)POWER TRANSISTOR
EXAMPLE 8.3 – Solution (cont’d)
Heat sink temperature
amb-snkmaxambsnk DPTT 
amb-snkmaxambsnk DPTT 
C5.10755.1530 


18. Heat sinks (Extra)POWER TRANSISTOR
EXAMPLE 8.3 – Solution (cont’d)
Case temperature
 amb-snksnk-casemaxambcase   DPTT
 amb-snksnk-casemaxambcase   DPTT
  C123515.1530 

Note: The use of heat sink allows
more power to be dissipated in the
device.

19. Heat sinks (Extra)POWER TRANSISTOR
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. Heat sinks (Extra)POWER TRANSISTOR
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.