Download EMT 112 Self Reading - Power Amplifiers

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.
POWER TRANSISTOR – BJT
Large-area devices – the geometry and doping
concentration are different from those of small-signal
transistors
Examples of BJT rating:
Parameter
VCE (max) (V)
IC (max) (A)
PD (max) (W)

fT (MHz)
Small-signal
BJT
(2N2222A)
40
0.8
1.2
35 – 100
300
Power BJT Power BJT
(2N3055)
(2N6078)
60
250
15
115
7
45
5 – 20
0.8
12 – 70
1
POWER TRANSISTOR – BJT
Current gain depends on IC and is smaller in power BJT.
The maximum rated collector current, IC(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.
POWER TRANSISTOR – BJT
Typical dc beta
characteristics
( hFE versus IC)
for 2N3055
POWER TRANSISTOR – BJT
Current gain depends on IC and is smaller in power BJT.
The maximum rated collector current, IC(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.
POWER TRANSISTOR – BJT
Current gain depends on IC and is smaller in power BJT.
The maximum rated collector current, IC(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.
POWER TRANSISTOR – BJT
The maximum voltage limitation:
• Avalanche breakdown in the reverse-biased basecollector junction (VCEO);
• Second breakdown – nonuniformities in current
density which inreases temperature in local regions in
semiconductor.
POWER TRANSISTOR – BJT
IC–VCE characteristics
showing breakdown
effect
POWER TRANSISTOR – BJT
The instantaneous power dissipation in transistor
pQ  vCE iC  vBE iB
The second term is usually small, hence;
pQ  vCE iC
The average power over one cycle
1
PQ 
T

T
0
vCE iC dt
POWER TRANSISTOR – BJT
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, PT
PT  VCE I C
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.
POWER TRANSISTOR – BJT
The safe operating area (SOA) is bounded by IC(max); VCE(sus)
and PT (Figure)
SOA of a BJT
(linear scale)
POWER TRANSISTOR – BJT
SOA of a BJT
(log scale)
POWER TRANSISTOR – BJT
EXAMPLE I
Determine the required ratings
(current, voltage and power) of
the BJT.
POWER TRANSISTOR – BJT
EXAMPLE I – Solution
For VCE  0 the maximum
collector current;
VCC 24
I C max  

3A
RL
8
For I C  0 the maximum collectoremitter voltage;
VCE max   VCC  24 V
POWER TRANSISTOR – BJT
EXAMPLE I – Solution
The load line equation
is;
VCE  VCC  I C RL
The load line must lie
within the SOA
The transistor power
dissipation;
PT  VCE I C  VCC  I C RL I C  VCC I C  I C2 RL
POWER TRANSISTOR – BJT
EXAMPLE I – Solution
dPT
0
The maximum power occurs when
dI C
i.e. when VCC  2 I C RL  0
or when I C  1.5 A
At this point; VCE  VCC  I C RL  12 V
and;
PT  VCE I C  18 W
POWER TRANSISTOR – BJT
EXAMPLE I – Solution
Thus the transistor ratings are;
I C max   3 A
VCE max   24 V
PT  18 W
In practice, a safety factor is normally used. The transistor
with I C max   3 A, VCE max   24 V, PT  18 W will be
chosen.
POWER TRANSISTOR – BJT
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.
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 in 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
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 – Heat sinks
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 – Heat sinks
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 
POWER TRANSISTOR – Heat sinks
EXAMPLE II
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
EXAMPLE II – 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
EXAMPLE II – 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
EXAMPLE II – 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
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
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.