rt9238 ETC-unknow, rt9238 Datasheet - Page 20

• Manufacturer Part Number: rt9238
• Description: Vrm 8.5 Pwm And Triple Linear Power System Controller
• Manufacturer: ETC-unknow

RT9238
PWM MOSFET Selection and Considerations
In high-current PWM applications, the MOSFET
power dissipation, package selection and heatsink
are the dominant design factors. The power
dissipation includes two main loss components:
conduction losses and switching losses. These
losses are distributed between the upper and lower
MOSFET
conduction losses are the main component of power
dissipation for the lower MOSFETs. Only the upper
MOSFET has significant switching losses, since the
lower device turns on and off into near zero voltage.
The equations presented assume linear voltage-
current transitions and do not model power losses
due to the lower MOSFET’s body diode or the output
capacitances associated with either MOSFET. The
gate charge losses are dissipated by the controller IC
(RT9238) and do not contribute to the MOSFETs’
heat rise. Ensure that both MOSFETs are within their
maximum junction temperature at high ambient
temperature by calculating the
according
specifications.
necessary depending upon MOSFET power, package
type, ambient temperature and air flow.
The R
even if the same device is used for both. This is
because the gate drive applied to the upper MOSFET
is different than the lower MOSFET. Fig.15 shows the
gate drive where the upper MOSFET’s gate-to-
source voltage is approximately V
supply. For +5V main power and +12VDC for the
bias, the approximate gate-to-source voltage of Q1 is
7V. The lower gate drive voltage is about 8V. A logic-
level MOSFET is a good choice for Q1 and a logic-
level MOSFET can be used for Q2 if its absolute
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20
P
P
UPPER
LOWER
DS(ON)
=
=
I
O
I
O
2
according
2
×
is different for the two equations above
to
×
R
R
DS
A
DS
V
(
ON
(
package
IN
ON
separate
)
V
)
×
×
IN
to
V
(
OUT
V
IN
the
−
+
V
I
thermal
O
OUT
heatsink
×
duty
V
CC
)
temperature rise
IN
×
2
less the input
t
factor.
SW
resistance
may
×
F
S
The
Preliminary
be
gate-to-source voltage rating exceeds the maximum
voltage applied to V
Rectifier CR1 is a clamp that catches the negative
inductor swing during the dead time between the turn
off of the lower MOSFET and the turn on of the upper
MOSFET. For best results, the diode must be a
surface-mount Schottky type to prevent the parasitic
MOSFET
acceptable to omit the diode and let the body diode of
the lower MOSFET clamp the negative inductor
swing, but one must ensure the PHASE node
negative voltage swing does not exceed -3V to -5V
peak. The diode’s rated reverse breakdown voltage
must be equal or greater to 1.5 times the maximum
input voltage.
Linear Controllers Transistor Selection
The RT9238 linear controllers are compatible with
both NPN bipolar as well as N-channel MOSFET
transistors. The main criteria for selection of pass
transistors for the linear regulators is package
selection for efficient removal of heat. The power
dissipated in a linear regulator is
Select a package and heatsink that maintains the
junction temperature below the maximum desired
temperature with the maximum expected ambient
temperature.
When selecting bipolar NPN transistors for use with
the linear controllers, insure the current gain at the
P
LINEAR
_
Fig.15 Upper Gate Drive-direct V
=
I
O
body
+12V
×
(
V
VCC
GND
IN
−
diode
UGATE
PHASE
LGATE
PGND
CC
V
OUT
.
)
from
Q2
Q1
+5V OR LESS
DS9238-01 July 2001
conducting.
CR1
CC
NOTE:
V
NOTE:
V
GS
GS
Drive
≈ 8V
≈ V
CC
It
- 5V
is