Application Information: continued
Voltage applied to the FET gates depends on the applica-
PSWH(ON) = upper MOSFET switch-on losses;
PSWH(OFF) = upper MOSFET switch-off losses.
Once the total power dissipation in the switching FET is
known, the maximum FET switch junction temperature
can be calculated:
tion circuit used. Both upper and lower gate driver outputs
are specified to drive to within 1.5V of ground when in the
low state and to within 2V of their respective bias supplies
when in the high state. In practice, the FET gates will be
driven rail-to-rail due to overshoot caused by the capaci-
tive load they present to the controller IC.
TJ = TA + [PHFET(TOTAL) × RθJA],
Step 7a - Selection of the switching (upper) FET
where
TJ = FET junction temperature;
TA = ambient temperature;
PHFET(TOTAL) = total switching (upper) FET losses;
The designer must ensure that the total power dissipation
in the FET switch does not cause the power component’s
junction temperature to exceed 150°C.
RθJA = upper FET junction-to-ambient thermal resistance
The maximum RMS current through the switch can be
determined by the following formula:
Step 7b: Selection of the synchronous (lower) FET
The switch conduction losses for the lower FET can be cal-
culated as follows:
IRMS(H)
=
(IL(PEAK)2 + (IL(PEAK) × IL(VALLEY)) + IL(VALLEY)2 × D
,
PRMSL = IRMS2 × RDS(ON) = [IOUT
× ,
(1 − D)]2 × RDS(ON)
3
where
where
PRMSL = lower MOSFET conduction losses;
IOUT = load current;
IRMS(H) = maximum switching MOSFET RMS current;
IL(PEAK) = inductor peak current;
IL(VALLEY) = inductor valley current;
D = Duty Cycle.
D = Duty Cycle;
RDS(ON) = lower FET drain-to-source on-resistance.
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:
Once the RMS current through the switch is known, the
switching MOSFET conduction losses can be calculated:
PRMS(H) = IRMS(H)2 × RDS(ON)
where
PRMS(H) = switching MOSFET conduction losses;
IRMS(H) = maximum switching MOSFET RMS current;
RDS(ON) = FET drain-to-source on-resistance
The upper MOSFET switching losses are caused during
MOSFET switch-on and switch-off and can be determined
by using the following formula:
PSWL = VSD × ILOAD × non-overlap time × FSW
,
where
PSWL = lower FET switching losses;
VSD = lower FET source-to-drain voltage;
ILOAD = load current
Non-overlap time = GATE(L)-to-GATE(H) or GATE(H)-
to-GATE(L) delay (from CS51312 data sheet Electrical
Characteristics section);
PSWH = PSWH(ON) + PSWH(OFF)
VIN × IOUT × (tRISE + tFALL)
FSW = switching frequency.
=
,
The total power dissipation in the synchronous (lower)
MOSFET can then be calculated as:
6T
where
PLFET(TOTAL) = PRMSL + PSWL
,
PSWH(ON) = upper MOSFET switch-on losses;
PSWH(OFF) = upper MOSFET switch-off losses;
VIN = input voltage;
where
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:
IOUT = load current;
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],
where
PHFET(TOTAL) = PRMSH + PSWH(ON) + PSWH(OFF)
,
TJ = MOSFET junction temperature;
TA = ambient temperature;
where
PLFET(TOTAL) = total synchronous (lower) FET losses;
PHFET(TOTAL) = total switching (upper) MOSFET losses;
PRMSH = upper MOSFET switch conduction Losses;
R
θJA = lower FET junction-to-ambient thermal resistance.
14
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
