Application Information: continued
where
I
where
RMS(H) = maximum switching MOSFET RMS current;
P
RMSL = lower MOSFET conduction losses;
IL(PEAK) = inductor peak current;
IOUT = load current;
D = Duty Cycle;
RDS(ON) = lower FET drain-to-source on-resistance.
IL(VALLEY) = inductor valley current;
D = Duty Cycle.
Once the RMS current through the switch is known, the
switching MOSFET conduction losses can be calculated:
The synchronous MOSFET has no switching losses, except
for losses in the internal body diode, because it turns on
into near zero voltage conditions. The MOSFET body
diode will conduct during the non-overlap time and the
resulting power dissipation (neglecting reverse recovery
losses) can be calculated as follows:
P
RMS(H) = IRMS(H)2 × RDS(ON)
where
PRMS(H) = switching MOSFET conduction losses;
IRMS(H) = maximum switching MOSFET RMS current;
PSWL = VSD × ILOAD × non-overlap time × FSW,
R
DS(ON) = FET drain-to-source on-resistance
where
SWL = lower FET switching losses;
VSD = lower FET source-to-drain voltage;
ILOAD = load current
The upper MOSFET switching losses are caused during
MOSFET switch-on and switch-off and can be determined
by using the following formula:
P
Non-overlap time = GATE(L)-to-GATE(H) or GATE(H)-
to-GATE(L) delay (from CS51313 data sheet Electrical
Characteristics section);
PSWH = PSWH(ON) + PSWH(OFF)
VIN × IOUT × (tRISE + tFALL)
F
SW = switching frequency.
=
,
6T
The total power dissipation in the synchronous (lower)
MOSFET can then be calculated as:
where
PSWH(ON) = upper MOSFET switch-on losses;
PSWH(OFF) = upper MOSFET switch-off losses;
PLFET(TOTAL) = PRMSL + PSWL
,
VIN = input voltage;
where
IOUT = load current;
PLFET(TOTAL) = Synchronous (lower) FET total losses;
PRMSL = Switch Conduction Losses;
PSWL = Switching losses.
Once the total power dissipation in the synchronous FET is
known the maximum FET switch junction temperature can
be calculated:
tRISE = MOSFET rise time (from FET manufacturer’s
switching characteristics performance curve);
tFALL = MOSFET fall time (from FET manufacturer’s
switching characteristics performance curve);
T = 1/FSW = period.
The total power dissipation in the switching MOSFET can
then be calculated as:
TJ = TA + [PLFET(TOTAL) × RθJA],
PHFET(TOTAL) = PRMSH + PSWH(ON) + PSWH(OFF)
,
where
TJ = MOSFET junction temperature;
where
T
A = ambient temperature;
PHFET(TOTAL) = total switching (upper) MOSFET losses;
PRMSH = upper MOSFET switch conduction Losses;
PSWH(ON) = upper MOSFET switch-on losses;
PSWH(OFF) = upper MOSFET switch-off losses.
PLFET(TOTAL) = total synchronous (lower) FET losses;
θJA = lower FET junction-to-ambient thermal resistance.
R
Step 8: Control IC Power Dissipation
Once the total power dissipation in the switching FET is
known, the maximum FET switch junction temperature
can be calculated:
The power dissipation of the IC varies with the MOSFETs
used, VCC, and the CS51313 operating frequency. The aver-
age MOSFET gate charge current typically dominates the
control IC power dissipation.
TJ = TA + [PHFET(TOTAL) × RθJA],
The IC power dissipation is determined by the formula:
where
TJ = FET junction temperature;
TA = ambient temperature;
PHFET(TOTAL) = total switching (upper) FET losses;
PCONTROLIC = ICCVCC + PGATE(H) + PGATE(L)
,
where
R
θJA = upper FET junction-to-ambient thermal resistance
PCONTROLIC = control IC power dissipation;
ICC = IC quiescent supply current;
VCC = IC supply voltage;
Step 7b: Selection of the synchronous (lower) FET
PGATE(H) = upper MOSFET gate driver (IC) losses;
PGATE(L) = lower MOSFET gate driver (IC) losses.
The upper (switching) MOSFET gate driver (IC) losses are:
The switch conduction losses for the lower FET can be cal-
culated as follows:
PRMSL = IRMS2 × RDS(ON) = [IOUT
× ,
(1 − D)]2 × RDS(ON)
PGATE(H) = QGATE(H) × FSW × VGATE(H)
,
15
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