ISL6722A Datasheet, PDF (16/24 Pages) Intersil Corporation – Flexible Single Ended Current Mode PWM

ISL6722A, ISL6723A
The losses associated with MOSFET operation may be
divided into three categories: conduction, switching, and
gate drive.
The conduction losses are due to the MOSFET’s ON
resistance.
Pcond = r_DS(ON) • Iprms²
W
(EQ. 30)
where r_DS(ON) is the ON resistance of the MOSFET and
Iprms is the RMS primary current. Determining the
conduction losses is complicated by the variation of r_DS(ON)
with temperature. As junction temperature increases, so
does r_DS(ON), which increases losses and raises the
junction temperature more, and so on. It is possible for the
device to enter a thermal runaway situation without proper
heatsinking. As a general rule of thumb, doubling the +25°C
r_DS(ON) specification yields a reasonable value for
estimating the conduction losses at +125°C junction
temperature.
The switching losses have two components, capacitive
switching losses and voltage/current overlap losses. The
capacitive losses occur during turn on of the device and may
be calculated as follows:
Pswcap
=
1_--
2
•
C
f
et
•
V
i
n²
•
f_s
w
W
(EQ. 31)
where Cfet is the equivalent output capacitance of the
MOSFET. Device output capacitance is specified on
datasheets as Coss and is non-linear with applied voltage.
To find the equivalent discrete capacitance, Cfet, a charge
model is used. Using a known current source, the time
required to charge the MOSFET drain to the desired
operating voltage is determined and the equivalent
capacitance may be calculated in Equation 32.
Cfet
=
-^I--^c---^h----^g-----^•----^t
V
F
(EQ. 32)
The other component of the switching loss is due to the
overlap of voltage and current during the switching
transition. A switching transition occurs when the MOSFET
is in the process of either turning on or off. Since the load is
inductive, there is no overlap of voltage and current during
the turn on transition, so only the turn off transition is of
significance. The power dissipation may be estimated as
Equation 33:
P
s
w
≈
1_--
x
•
Ip
pk
•
V
i
n
•
To
l
•
f_sw
(EQ. 33)
where Tol is the duration of the overlap period and x ranges
from about 3 to 6 in typical applications and depends on
where the waveforms intersect. This estimate may predict
higher dissipation than is realized because a portion of the
turn off drain current is attributable to the charging of the
device output capacitance (Coss) and is not dissipative
during this portion of the switching cycle.
Ip p k
V _D-S
Tol
FIGURE 9.
The final component of MOSFET loss is caused by the
charging of the gate capacitance through the device gate
resistance. Depending on the relative value of any external
resistance in the gate drive circuit, a portion of this power will
be dissipated externally.
Pgate = Qg • Vg • f_sw
W
(EQ. 34)
Once the losses are known, the device package must be
selected and the heatsinking method designed. Since the
design requires a small surface mount part, a SOIC-8
package was selected. A Fairchild FDS2570 MOSFET was
selected based on these criteria. The overall losses are
estimated at 400mW.
Output Filter Design
In a flyback design, the primary concern for the design of the
output filter is the capacitor ripple current stress and the
ripple and noise specification of the output.
The current flowing in and out of the output capacitors is the
difference between the winding current and the output current.
The peak secondary current, Ispk, is 10.73A for the 3.3V
output and 4.29A for the 1.8V output. The current flowing into
the output filter capacitor is the difference between the
winding current and the output current. Looking at the 3.3V
output, the peak winding current is Ispk = 10.73A. The
capacitor must store this amount minus the output current of
2.5A, or 8.23A. The RMS ripple current in the 3.3V output
capacitor is about 3.5A_RMS. The RMS ripple current in the
1.8V output capacitor is about 1.4A_RMS
Voltage deviation on the output during the switching cycle
(ripple and noise) is caused by the change in charge of the
output capacitance, the equivalent series resistance (ESR),
and equivalent series inductance (ESL). Each of these
components must be assigned a portion of the total ripple
and noise specification. How much to allow for each
contributor is dependent on the capacitor technology used.
16
FN9237.1
July 11, 2007