POWER
SUPPLEMENT
Low-VCE(sat) BJTs vs. MOSFETs:
Cost considerations
New low-VCE(sat) BJTs provide a viable low-cost alternative to
planar MOSFETs in 500-mA to 5.0-A apps
BY STEVE SHEARD
ON Semiconductor
Phoenix, AZ
http://onsemi.com
D
potential savings of $0.05 to $0.20
compared with designs using MOSFETs. Low-VCE(sat) BJTs perform the
same function as a MOSFET at a lower cost, and as an added bonus, in
many cases provide for improved
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-VCE(sat) BJT.
For example, if the BJT is
to control a current of 1 A
and has a worst-case gain of
100, then the base current
will need to be a minimum
of 10 mA to ensure that the
BJT goes into saturation.
The control pin must be
able to supply the 10 mA
for the BJT to be driven diFig. 1
rectly; otherwise an additional drive stage would be
required.
esigners of portable products,
such as cell phones,
digital cameras, digital
camcorders, DVD players,
MP3 players and PDAs, are under continuous pressure to reduce the cost of the bill of materials without any sacrifice in
performance. This is a real
challenge for the designer
who is under continuing pressure to add features and not
adversely affect battery life.
The majority of portable
products are moving toward an intepower consumption resulting in imCharging circuit example
grated power management unit
proved battery life.
A review of a typical charging circuit
(PMU) circuit designed specifically
Some of the new devices are now
X401ONSE0805
(see Fig. 1) in a portable product
to control the different functions
available with a saturation voltage at
shows the pass transistor Q1 (power
within the product. These may be
1 A of well under 100 mV. This
MOSFET 2 A, 20 V, TSOP6 package)
for the control of battery charging,
equates to a forward resistance of
and the blocking Schottky
battery management, overdiode D1.
voltage protection, backlight,
All the control for chargvibrator, disc drives, and peing of the lithium ion batripherals power, such as camtery is embedded in a PMU.
eras and flash units.
The PMU control pin
The circuits, for the control
changes to high to turn on
of currents under 500 mA, are
the external pass transistor
typically all embedded within
Q1 and the charging current
the PMU, including the final
is set at 1 A. The series
pass transistor. However, for
Fig. 2
Schottky diode D 1 is rethe control of currents from
quired to block any reverse
500 mA to 5 A, an external
current from the battery.
MOSFET pass transistor is the
The typical power dissipated
typical design of choice. An alternaunder 100 mV, and proves very
through the pass transistor Q1 and
tive to the MOSFET is to use a lowercompetitive against a higher-cost
the reverse blocking diode D1 was
cost low-VCE(sat) bipolar transistor,
MOSFET.
calculated as:
which may also provide a powerX402ONSE0805
saving.
Design considerations
Q1 power = I2 x R, 1 A2 x RDS(on)
The MOSFET is a voltage driven de(60 mV) = 60 mW
New low-VCE(sat) BJTs
vice, compared to the low-VCE(sat)
D1 power = I x VF, 1 A x VF
A new wave of low-VCE(sat) BJTs offer
BJT being a current driven device.
Reprinted from ELECTRONIC PRODUCTS POWER SUPPLEMENT 2005
P O W E R S U P P L E M E N T - LOW-VCE(SAT) BJTS
Schottky (360 mV) = 360 mW
Total power dissipated through
Q1 and D1 = 420 mW
The typical high-volume cost of the
MOSFET and Schottky diode is
$0.175
The charging circuit can be configured using a low-V CE(sat) BJT—
such as the ON Semiconductor
NSS35200 CF8T1G—to replace the
MOSFET and the Schottky diode
(see Fig. 2). The Schottky diode is
not required because the BJT has
this function inherent to its design.
The control pin on the PMU is
able to provide a maximum of 20
mA. The PMU would initiate a fast
charge with the battery voltage of
3.0 V. 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 BJT, which has a minimum
gain of 100, into saturation needs
to be 10 mA for a 1-A charging current. Selecting a standard resistor
value of 200 V for the base resistor
will ensure the 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, VCE(sat) (1 A,
Beta 100 = 135 mV) = 135 mW
R1 power = I2 x R, 0.011 A2 x 200
V = 24 mW
Total power dissipated through
Q2 and R1 = 159 mW
The typical high-volume cost of the
low-VCE(sat) BJT and resistor is $0.10
Charging circuit savings
The savings resulting from exchanging the MOSFET bypass transistor and Schottky diode with a
low-V CE(sat) BJT and bias resistor
were $0.075 per unit.
The exchange also resulted in a
power dissipation savings of 261
mW making the thermal considerations of the portable product much
simpler.
VS. MOSFETS:
COST
CONSIDERATIONS
More-complex
circuits
ICs designed specifically with an
external bypass MOSFET may not
have the ability to supply the required current to drive the lowVCE(sat) BJT into saturation directly.
In these circuits an extra digital
transistor or small general-purpose
MOSFET can be used.
The results are not quite as significant as the charging example. The
cost savings is still $0.055 per unit.
Power dissipation is the same.
Additional
advantages
The BJT is less susceptible to ESD
damage and thus a savings can be
found in not having to provide extra ESD protection. In addition, the
BJT has a lower turn-on voltage
(typically 0.7 V) and thus an oscillator and charge pump—which are
normally needed for a MOSFET—
may be eliminated. Finally, the BJT
is more efficient at switching medim
um currents.