e 2 PowerEdge Economic Energy Using Low VCEsat BJT Family

e2PowerEdgeTM
Economic Energy using Low VCEsat BJT’s
Steve Sheard
Marketing Engineer
ON Semiconductor
[email protected]
Introduction
ON Semiconductor’s e2PowerEdgeTM family of low VCEsat Bipolar Junction Transistors
(BJT) are miniature surface mount devices featuring ultra low saturation voltage VCEsat
and high current gain capability. These are designed for use in low voltage, high speed
switching applications where affordable efficient energy control is important. Typical
application are power management in any circuit that needs low losses.
In portable and battery powered products such as cellular and cordless phones, PDAs,
computers, printers, digital cameras, digital camcorders, DVD players and MP3 players.
The functions controlled in portable products are battery charging, battery management,
over voltage protection, low drop out regulation, LED backlight switching, Royer
converter for LCD Backlights, vibrator, disc drives, and peripheral power, such as cameras
and flash units.
Other applications are low voltage servo motor controls in mass storage products such as
disc drives and tape drives; controlling small motors as in electric shavers. In the
automotive industry they can be used in air bag deployment, controlling mirrors and fuel
pumps, and in circuits used for instrument clusters, steering, transmission, toll readers,
LED lighting and power inverters. In industrial applications they are ideal for circuits
providing control in smoke detectors, vending machines, In Focus projectors, gas meters,
Telecommunication SLIC and RF access boxes. Where high currents need to be controlled
at high frequencies the Low VCEsat BJT is the ideal driver for a highly efficient Trench
MOSFET. The Linear Gain (Beta) of Low VCEsat BJT makes them ideal components in
analog amplifiers.
Technology
The Low VCEsat BJT devices use a technology that was first developed over 20 years ago
and was primarily used to achieve similar performance in a smaller die (die shrink). This
technology is called a “Perforated Emitter” and today is being focused towards reducing
the forward saturation voltage to achieve very low forward resistance. The perforated
emitter is a method of extending the base electrical layer across the complete die to contact
multiple perforations through the emitter. Each of these perforations creates miniature
transistors within the device and thus allows the current to be distributed evenly and with
greater efficiency. Photograph 1.
Base Electrical
layer fingers and
perforations
Collector is
Substrate
Base Contact
Emitter Contact
Photograph 1
Some of the new Low VCEsat BJT’s are now available with a saturation voltage at 1.0 Amps
of well under 50 mV. This equates to a forward resistance of under 50 mΩ, and proves
very competitive against a higher cost MOSFET.
PMU with an external pass transistor
The majority of portable products are moving towards an integrated Power Management
Unit (PMU) circuit designed specifically to control the different functions within the
product. The circuits, for the control of currents under 100 mA, are typically all imbedded
within the PMU, including the final pass transistor. However, for the control of currents
from 100 mA to 5.0 A an external pass transistor (MOSFET) is the typical design of
choice. An alternative to the MOSFET is to use a lower cost Low VCEsat BJT. The new
family of Low VCEsat BJTs offer potential savings of 5 to 20 cents compared to designs
using MOSFETs. Low VCEsat BJTs perform the same function as a MOSFET at a lower
cost, and as an added bonus, in many cases provide for improved power consumption
resulting in improved battery life. In many designs the high current gain allows Low
VCEsat BJT devices to be driven directly from the PMU’s control outputs.
Design Considerations
The Low VCEsat BJT is a current driven device, compared to the MOSFET which is a
voltage driven device. For this reason the designer needs to understand the current
limitations of the PMU control circuits being used, to determine the specific circuit
requirements when designing with a Low VCEsat BJT. For example, if the Low VCEsat BJT
is to control a current of 1.0 Amp and it has a worst case gain (hFE) of 100 then the base
current will need to be a minimum of 10 mA (IB) to ensure the Low VCEsat BJT goes into
saturation. The control pin of the PMU must be able to supply the 10 mA for the Low
VCEsat BJT to be driven directly; otherwise an additional drive stage would be required.
The designer also has to consider the power rating of the package for the Low VCEsat BJT.
For Example; the On Semiconductor Low VCEsat BJT NSS12600CF8T1G is mounted on a
FR4 printed circuit board 100 mm2 pads. The input voltage to be switched is 5.0 V and the
maximum constant current is 6.0 A. Ambient temperature is 250 C. The Power rating (PD)
with the specified pad is 1.0 W.
The typical Vce-sat for the NSS12600CF8T1G at 1.0 A is 45 mV. This equates to a power
dissipation of 45 mW. The Minimum Gain (hFE) at 1.0 A is 250. Thus the drive current (IB)
would need to be a little over 4.0 mA. The maximum limit on Vce-sat at 1.0 A is 80 mV
(from Data Sheet with beta 100), this equating to 80 mW, well below the 1.0 W rating for
the package at 250 C.
Derating the device for temperature. The Thermal Resistance for de-rating with minimum
pads (RθJA) is 1250 C/W (From Data Sheet). The formula for de-rating is PD = (Tj-max –
Tamb) / RθJA
For an application where Tj-max = 750 C, The Maximum allowable Power dissipation
would become PD = (75 – 25) / 125 = 400 mW.
The maximum calculated power of 80 mW still falls below the adjusted power when the
device is de-rated for a higher temperature of 750 C.
Charging Circuit
A review of charging circuit (Figure 1) in a portable product shows the pass transistor Q1
(Power MOSFET 2.0 Amp, 20 V, TSOP6 package) and the blocking Schottky Diode D1
can be replaced by a Low VCEsat BJT and a resistor. In this example the Low VCEsat BJT
saved $0.10 from the typical MOSFET cost and 316 mW lower power loss.
All the control for charging of the Lithium Ion battery is imbedded in a PMU. The PMU
control pin changes to high to turn on the external pass transistor Q1 and the charging
current is set at 1 Amp. The series Schottky Diode D1 is required to block any reverse
current from the battery.
The typical power dissipated through the pass transistor Q1 and the reverse blocking diode
D1 was calculated as:
Q1 Power = I2 x R, 1.0 Amp2 x RDS(ON) (60 mΩ) = 60 mW
D1 Power = I x VF, 1.0 Amp x VF Schottky (360 mV) = 360 mW
Total Power dissipated through Q1 and D1 = 420mW
The typical high volume cost of the MOSFET and Schottky Diode is $0.175
MOSFET
5.0 Vin
Q1
7 Vcc
Schottky (1 Amp)
R_sens
4.2
D1
OUT 8
V_BAT
BOM ASP
Q1 MOSFET
D1 Schottky Diode
Total:
$0.100
$0.075
$0.175
ISNS 1
PMU
CHG_Indicator
6 CFLG
3 COMP 4 GND
VSNS 5
Power Dissipated through Pass
Elements
ISEL 2
CHG_Control
Rcomp
60K
Q1: 1.0 Amp, 60 mΩ
D1: 1.0 Amp, 360 mV
Power Dissipated
Ccomp
Figure 1
The charging circuit (Figure 2) can be configured using a Low VCEsat BJT to replace the
MOSFET and the Schottky Diode. The Schottky Diode is not required because the Low
VCEsat BJT has this function inherent to its design. The control pin on the PMU is able to
= 60 mW
= 360 mW
= 420 mW
provide a maximum of 20 mA. The PMU would initiate a fast charge with the battery
voltage of 3.0V. With Q2 in saturation both the collector and emitter will be at
approximately 3.0 V, thus the base would be 2.3 v. The base current required to drive the
ON Semiconductor NSS12600CF8T1G Low VCEsat BJT, which has a minimum gain of
250, into saturation needs to be a little over 4.0 mA for a 1.0 Amp charging current.
Selecting a standard resistor value of 200 Ω for the base resistor will ensure the Low VCEsat
BJT is in saturation and that the limit for the drive pin is not exceeded.
The typical power dissipated through the pass transistor Q2 and bias resistor R1 was
calculated as:
Q2 Power = I x V, 1.0 Amp x Vce-sat (1.0 Amp, Beta 100 = 80 mV) = 80 mW
R1 Power = I2 x R, 0.011 Amp2 x 200 Ω = 24 mW
Total Power dissipated through Q2 and R1 = 104 mW
The typical high volume cost of the Low VCEsat BJT and Resistor is $0.10
5.0
NSS12600CF8T1G
Vin
R_sens
V_BAT
BOM ASP
Q2
R1
200Ω
7 Vcc
OUT 8
PMU
CHG_Indicator
6 CFLG
CHG_Control
3 COMP 4 GND
Rcomp
4.2
NSS12600CF8T1G $0.090
R1
$0.010
Total:
$0.100
ISNS 1
VSNS 5
Power Dissipated through Pass
Element
ISEL 2
Q2: 1.0 amp, Beta 100, VCEsat 135 mV = 80 mW
R1: 11mA, 200 Ω
= 24 mW
Power Dissipated
= 104 mW
60K
Ccomp
Figure 2
Charging Circuit Savings
The savings resulting from exchanging the MOSFET bypass transistor and Schottky Diode
with a Low VCEsat BJT and bias resistor were $0.075 per unit.
The exchange also resulted in a power dissipation savings of 316 mW making the thermal
considerations of the portable product much simpler.
More Complex Circuits
Integrated Circuits designed specifically with an external bypass MOSFET may not have
the ability to supply the required current to drive the Low VCEsat BJT into saturation
directly. In these circuits an extra digital transistor or small general purpose MOSFET (Q4)
can be used as illustrated in Figure 3.
5.0
NSS12600CF8T1G
R_sens V_BAT
Q3
Vin
4.2
Q4
7 Vcc
OUT 8
6 CFLG
CHG_Control
$0.090
$0.010
$0.020
$0.120
Power Dissipated through Pass Element
VSNS 5
3 COMP 4 GND
Q3 NSS12600CF8T1G
R2
Q4 (General purpose MOSFET)
Total:
ISNS 1
PMU
CHG_Indicator
BOM ASP
Q3: 1.0 amp, Beta 100, VCEsat 80 mV
Q4: 11 mA, 100 mΩ
R2: 11 mA, 200 Ω
Power Dissipated
ISEL 2
Rcomp
60K
R2
200Ω
= 80 mW
= 0.1 mW
= 24 mW
= 104 mW
Ccomp
Figure 3
The results are not quite as significant as the charging example. The cost savings is still
$0.055 per unit. Power saving is significant also – 316 mW less.
Bi-Directional Current Control
Figure 4 is an illustration of a battery management application with a dual MOSFET
configuration. By connecting the MOSFETs with their drains together one eliminates the
requirement for a blocking schottky diode and it also allows for the control of current in
either direction. i.e. Charging current in to the battery, power out to support USB.
The disadvantage of having the two MOSFETs in series is the doubling of the resistance
through the pass elements and thus doubling the power loss. It is a better solution
compared to the use of a blocking schottky diode, but it does cost significantly more.
Q6
Q5
5.0 Vin
R_sens
V_BAT
4.2
7 Vcc
OUT 8
BOM ASP
Q5
Q6
Total:
$0.160
$0.160
$0.320
ISNS 1
PMU
CHG_Indicator
6 CFLG
3 COMP 4 GND
VSNS 5
Power Dissipated through Pass
Elements
ISEL 2
CHG_Control
Rcomp
60K
Ccomp
Full Charge Ireg = 1.0 A
Rds(on) MOSFET Q5, 80 mΩ = 80 mW
Rds(on) MOSFET Q6, 80 mΩ = 80 mW
Power Dissipated
= 160 mW
Figure 4
Figure 5 is a similar battery management application using two Low VCEsat BJT. In this
design the Low VCEsat BJTs are connected in parallel and only one is turned on at a time;
Q7 for charging the battery, Q8 to allow power out to a peripheral. As only one device is
turned on at a time we only have to consider the resistance and power loss through one
device. There are also significant savings in the cost of two Low VCEsat BJT compared to
two MOSFETs.
External
Connect
Q8
Battery
R_sens
BOM ASP
Q7
R3
Q7/8 NSS12600CF8T1G x 2 $0.180
R3 & R4
$0.020
Total:
$0.200
R4
7 Vcc
OUT 8
ISNS 1
PMU
CHG_Indicator
6 CFLG
CHG_Control
3 COMP 4 GND
Rcomp
Power Dissipated through Pass Element
Q6 or Q7:
1.0 amp, Beta 100, VCEsat 80 mV = 80 mW
R3 or R4: 11 mA, 200 Ω
= 24 mW
Power Dissipated
= 104 mW
VSNS 5
ISEL 2
60K
Ccomp
Figure 5
Load Switch - Vibrator Control in Cellular Phones
A Low VCEsat BJTs is an ideal switch for controlling functions within a portable product
that are only on for a short duration. The vibrator in a cellular phone is good example;
Figure 6 is an illustration of the use of a Low VCEsat BJTs, being controlled with a Digital
Transistor, to turn the vibrator on and off.
A MOSFET approach may be more efficient and the power loss less but considering the
short time the vibrator is on, the lower cost of the Low VCEsat BJTs is very attractive.
VBAT
Vibrator ON
Vibrator
NSS404000CF8
Figure 6
Load Switch – Back Light Control in a cellular phone
Cellular phones often use multiple arrays of LED for illumination of keypads. Figure 7 is
an illustration using a Low VCEsat BJT to control the LED backlights.
VBA
LED
NSS30301L
Backlight
Figure 7
LED Driver Circuits
Pass Element in Buck / Boost converter
The MC34063 Step Up / Down / Inverting Switching Regulator can be configured to drive
LEDs. The Low VCEsat BJT, NSS40600CF8T1G is an ideal pass element, Q1 as shown in
Figure 8.
Figure 8
Q1 NSS20600CF8T1G
5
2.7
6.8K
Q2 NSS20200LT1G
MMSZ5V
680
Over Voltage Protection
NCP3063 Buck reference designs
Load
Additional Advantages of using a Low VCEsat BJT
The Low VCEsat BJT is less susceptible to ESD damage compared to the MOSFET and thus
a savings can be found in not having to provide extra ESD protection.
The Low VCEsat BJT has a lower turn on voltage (0.7v typical) compared to a MOSFET
(typically 4.0v – 10.0v) and is thus very attractive for low voltage circuits and for
situations where a controlled power down is required as the battery voltage drops. The low
turn on voltage would also eliminate the need for an oscillator and charge pump, normally
needed for a MOSFET.
The Low VCEsat BJT blocks voltage in both directions, eliminating the need for a blocking
schottky diode which is sometimes required when using a MOSFET.
The Low VCEsat BJT typically have a better temperature coefficient compared to a
MOSFET which provides for higher efficiency when operating at high temperatures
resulting in less temperature elevation in the portable product..
Feature
BJT
MOSFET
Low Vce-sat / Rds-on
Excellent in saturation
Needs current drive
Bi-directional
Needs high voltage Gate Drive to
get 100% enhancement
Mono-directional, needs schottky
diode
Good per Si Density
2v to 10v depending on design
Low
High
Blocking Capability
Pulse Current
Drive Voltage
Drive Current
Switching Speed
ESD Sensitivity
Cost Rce /mm2
High Current Switching
High per Si Density
Less than 1V
Moderate
Saturated – Low
Linear - High
Excellent
Excellent
Moderate
High Voltage Switching
Low Voltage Switching
Excellent
Excellent
Sensitive
Moderate
Excellent
Excellent
Poor
Application
Feature
Benefit
Pulsed Mode Battery
Charging
Low Vce-sat
hFE > 200
Low Rce /mm2
Small size – 4.0 mm2
Low profile – 0.75 mm
PNP transistor
High power dissipation / mm2
hFE > 200
Low Rce /mm2
Small size – 4.0 mm2
Low profile – 0.75 mm
PNP transistor
High Pulse Current
High Frequency
hFE > 200
Low Rce /mm2
Small size - 4.0 mm2
Low profile – 0.75 mm
PNP / NPN transistor
Low Vce-sat
High Frequency
hFE > 200
Low Rce /mm2
Small size - 4.0 mm2
Low profile – 0.75 mm
PNP / NPN transistor
Low Vce-sat
High power dissipation / mm2
hFE > 200
Low Rce /mm2
Small size - 4.0 mm2
Low profile – 0.75 mm
PNP / NPN transistor
High efficiency
High gain
Low cost vs MOSFET
Less board space
More compact design
High side control, Bi-directional voltage blocking
Efficient charging time
High gain
Low cost vs MOSFET
Less board space
More compact design
Bi-directional voltage blocking
Fast switching time
Fast switching time
High current gain
Low cost vs MOSFET
Less board space
More compact design
High / Low switch
High efficiency
Fast switching time
High current gain
Low cost vs MOSFET
Less board space
More compact design
Design flexibility, Bi-directional voltage blocking
High efficiency
High current control
High gain
Low cost vs MOSFET
Less board space
More compact design
High or Low side control, Bi-directional voltage
blocking
High / Low Bridge, Bi-directional voltage blocking
High efficiency
Low switching Losses
High current gain – lower control current
Low cost vs MOSFET
Less board space
Design flexibility
High / Low Bridge, Bi-directional voltage blocking
High efficiency
High current control
Low switching Losses
High current gain – lower control current
Low cost vs MOSFET
Less board space
Design flexibility
Linear Mode Battery
Charging
MosFET Gate Drive
Royer Converter for LCD
Backlight
Low Drop Out (LDO)
Regulator
Servo Motor Drive
Over voltage protection
PNP / NPN transistor
Low Vce-sat
High Frequency
hFE > 200
Low Rce /mm2
Small size - 4.0 mm2
Low profile – 0.75 mm
PNP / NPN transistor
Low Vce-sat
High power dissipation / mm2
High Frequency
hFE > 200
Low Rce /mm2
Small size - 4.0 mm2
Low profile – 0.75 mm
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