Download Power Amp (II)

POWER TRANSISTOR – MOSFET
Example of power MOSFET parameters;
Parameter
VDS(max) (V)
ID(max) (A) - @ T = 25C
PD (W)
2N675
7
150
2N679
2
400
8
2
75
20
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).
POWER TRANSISTOR – MOSFET
Transconductance
versus drain current
curves for various
values of temperature
– less than the
variation in BJT
current gain.
POWER TRANSISTOR – MOSFET
Transfer
characteristics curves
for various values of
temperature.
POWER TRANSISTOR – MOSFET
Structure
POWER TRANSISTOR – MOSFET
Structure
DMOS process can
be used to produce a
large number of
hexagonal cells on a
single chip.
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.
POWER TRANSISTOR – MOSFET
The “ON” resistive
path between drain
and source (rds(on))
is an important
parameter in
power capability of
MOSFET
POWER TRANSISTOR – Comparison
BJT
MOSFET
Requires complex input
circuitry because of high
input current (currentcontrolled device)
Simple input circuitry
because of low input
current (voltage-controlled
device).
More sensitive to
temperature variation –
thermal runaway and
problem of second
breakdown.
More immune to thermal
runaway and second
breakdown.
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:
POWER TRANSISTOR – Heat sinks
• Additionally, heat sinks can be used to remove the heat
developed in the transistor:
POWER TRANSISTOR
Heat sinks (Extra)
TJ  TA  PD JA
TJ
Temperature of transistor
junction
TA Ambient temperature
TJ  TA Temperature difference
 Voltage difference
Electrical equivalent
circuit of thermalconduction process
 JA Thermal resistance between the junction and ambient
 Electrical resistance
PD Thermal power through the element
 Electric current.
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:
Heat sinks (Extra)
POWER TRANSISTOR
The following equation can be
used to describe the
temperature conduction
process:
Tdev  Tamb
 PD  devcase   casesnk  snk-amb 
If the heat sink is not used, then;
Tdev  Tamb  PD devcase  caseamb 
Heat sinks (Extra)
POWER TRANSISTOR
Heat sinks (Extra)
EXAMPLE 8.3
A MOSFET has the following parameters;
 devcase  1.75 C/W;
 case-snk  1 C/W;
snk-amb  5 C/W;
 case-amb  50 C/W;
TJmax  Tdev  150 C;
Tamb  30 C;

Determine the maximum power dissipation in the
transistor and determine the temperature of the transistor
case and heat sink.
POWER TRANSISTOR
Heat sinks (Extra)
EXAMPLE 8.3 – Solution
Maximum power (without heat sink)
PDmax
TJ max  Tamb
150  30


 2.32 W
 dev-case   case-amb 1.75  50
Maximum power (with heat sink)
PDmax
TJ max  Tamb

 dev-case   case-snk   snk-amb
150  30

 15.5 W
1.75  1  5
POWER TRANSISTOR
Heat sinks (Extra)
EXAMPLE 8.3 – Solution (cont’d)
Heat sink temperature
Tsnk  Tamb  PDmax snk-amb
Tsnk  Tamb  PDmax snk-amb
 30  15.5  5  107.5 C
POWER TRANSISTOR
Heat sinks (Extra)
EXAMPLE 8.3 – Solution (cont’d)
Case temperature
Tcase  Tamb  PDmax case-snk  snk-amb 
Tcase  Tamb  PDmax case-snk  snk-amb 
 30  15.51  5  123 C
Note: The use of heat sink allows
more power to be dissipated in the
device.
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.
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.