MAX17498A/MAX17498B/MAX17498C
AC-DC and DC-DC Peak Current-Mode Converters
for Flyback/Boost Applications
Thermal Considerations
It should be ensured that the junction temperature of the
devices does not exceed +125°C under the operating con-
ditions specified for the power supply. The power dissipat-
ed in the devices to operate can be calculated using the
following equation:
IN
P = V IN × I IN
where V IN is the voltage applied at the IN pin and I IN is
operating supply current.
The internal n-channel MOSFET experiences conduction
loss and transition loss when switching between on and
off states. These losses are calculated as:
=
P CONDUCTION I LXRMS 2 × R DSONLX
P TRANSITION = 0.5 × V INMAX × I PK × ( t R + t F ) × f SW
where t R and t F are the rise and fall times of the internal
nMOSFET in CCM operation. In DCM operation, since
the switch current starts from zero, only t F exists and the
transition-loss equation changes to:
P TRANSITION = 0.5 × V INMAX × I PK × t F × f SW
Additional loss occurs in the system in every switch-
ing cycle due to energy stored in the drain-source
capacitance of the internal MOSFET being lost when
the MOSFET turns on and discharges the drain-source
capacitance voltage to zero. This loss is estimated as:
P CAP = 0.5 × C DS × V DSMAX × f SW
The total power loss in the devices can be calculated
from the following equation:
P IN + P
P LOSS = CONDUCTION + P TRANSITION + P CAP
The maximum power that can be dissipated in the
devices is 1666mW at +70°C temperature. The power-
dissipation capability should be derated as the tem-
perature rises above +70°C at 21mW/°C. For a multilayer
board, the thermal-performance metrics for the package
are given below:
=
=
θ JA 48°C / W
θ JC 10°C / W
Maxim Integrated
The junction-temperature rise of the devices can be
estimated at any given maximum ambient temperature
(T AMAX ) from the following equation:
JMAX
T = T AMAX + ( θ JA × P LOSS )
If the application has a thermal-management system
that ensures that the exposed pad of the devices is
maintained at a given temperature (T EPMAX ) by using
proper heatsinks, then the junction-temperature rise of
the devices can be estimated at any given maximum
ambient temperature from the following equation:
T =
üüüüüüü T + ( θ × P )
Layout, Grounding, and Bypassing
All connections carrying pulsed currents must be very
short and as wide as possible. The inductance of these
connections must be kept to an absolute minimum
due to the high di/dt of the currents in high-frequency
switching power converters. This implies that the loop
areas for forward and return pulsed currents in various
parts of the circuit should be minimized. Additionally,
small-current loop areas reduce radiated EMI. Similarly,
the heatsink of the main MOSFET presents a dV/dt source,
and therefore, the surface area of the MOSFET heatsink
should be minimized as much as possible.
Ground planes must be kept as intact as possible. The
ground plane for the power section of the converter
should be kept separate from the analog ground plane,
except for a connection at the least noisy section of the
power ground plane, typically the return of the input filter
capacitor. The negative terminal of the filter capacitor,
ground return of the power switch, and current-sensing
resistor must be close together. PCB layout also affects
the thermal performance of the design. A number of ther-
mal vias that connect to a large ground plane should be
provided under the exposed pad of the part for efficient
heat dissipation. For a sample layout that ensures first-
pass success, refer to the MAX17498B Evaluation Kit.
For universal AC input designs, follow all applicable
safety regulations. Offline power supplies can require UL,
VDE, and other similar agency approvals.
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