SP6134H Datasheet, PDF (10/15 Pages) Sipex Corporation – High Voltage, 600 KHz Synchronous PWM Controller

SP6134H
High Voltage, 600kHz Synchronous PWM Step Down
Controller
MOSFET SELECTION
The losses associated with MOSFETs can be
divided into conduction and switching losses.
Conduction losses are related to the on
resistance of MOSFETs, and increase with the
load current. Switching losses occur on each
on/off transition when the MOSFETs
experience both high current and voltage.
Since the bottom MOSFET switches current
from/to a paralleled diode (either its own body
diode or a Schottky diode), the voltage across
the MOSFET is no more than 1V during
switching transition. As a result, its switching
losses are negligible. The switching losses are
difficult to quantify due to all the variables
affecting turn on/ off time. However, the
following equation provides an approximation
on the switching losses associated with the top
MOSFET driven by SP6134H.
P_{SH (max)} = 12C V _{rss IN (max)}I _{OUT (max)}F_S
where
Crss = reverse transfer capacitance of the top
MOSFET
Switching losses need to be taken into account
for high switching frequency, since they are
directly proportional to switching frequency.
The conduction losses associated with top and
bottom MOSFETs are determined by:
P_{CH (max)}
=
R_DS
(ON
)
I
OUT
(max
2
)
D
( ) P_CL(max)
=
R_DS
(ON
)
I
OUT
(max
2
)
1−
D
where
PCH(max) = conduction losses of the high side
MOSFET
PCL(max) = conduction losses of the low side
MOSFET
RDS(ON) = drain to source on resistance.
The total power losses of the top MOSFET are
the sum of switching and conduction losses.
For synchronous buck converters of efficiency
over 90%, allow no more than 4% power
losses for high or low side MOSFETs. For input
voltages of 3.3V and 5V, conduction losses
often dominate switching losses. Therefore,
lowering the RDS(ON) of the MOSFETs always
improves efficiency even though it gives rise
to higher switching losses due to increased
Crss.
Top and bottom MOSFETs experience unequal
conduction losses if their on time is unequal.
For applications running at large or small duty
cycle, it makes sense to use different top and
bottom MOSFETs. Alternatively, parallel
multiple MOSFETs to conduct large duty
factor.
RDS(ON) varies greatly with the gate driver
voltage. The MOSFET vendors often specify
RDS(ON) on multiple gate to source voltages
(VGS), as well as provide typical curve of
RDS(ON) versus VGS. For 5V input, use the
RDS(ON) specified at 4.5V VGS. At the time of
this publication, vendors, such as Fairchild,
Siliconix and International Rectifier, have
started to specify RDS(ON) at VGS less than
3V. This has provided necessary data for
designs in which these MOSFETs are driven
with 3.3V and made it possible to use
SP6134H in 3.3V only applications.
Thermal calculation must be conducted to
ensure the MOSFET can handle the maximum
load current. The junction temperature of the
MOSFET, determined as follows, must stay
below the maximum rating.
T_{J (max)}
= T_A(max)
+
P_{MOSFET (max )}
R_θJA
where
TA(max) = maximum ambient temperature
PMOSFET(max) = maximum power dissipation
of the MOSFET
RΘJA = junction to ambient thermal
resistance.
RΘJA of the device depends greatly on the
board layout, as well as device package.
Significant thermal improvement can be
achieved in the maximum power dissipation
through the proper design of copper mounting
pads on the circuit board. For example, in a
SO-8 package, placing two 0.04 square inches
© 2008 Exar Corporation
10/15
Rev. 2.0.0