MAX15004_11 Datasheet, PDF (18/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
Selecting V_CC Resistor (R_VCC)
The V_CC external supply series resistor should be sized
to provide enough average current from V_OUT to drive
the external MOSFET (I_DRIVE) and I_SUPPLY. The V_CC is
clamped internally to 10.4V and capable of sinking
30mA current. The V_CC resistor must be high enough to
limit the V_CC sink current below 30mA at the highest
output voltage. Maintain the V_CC voltage to 8V while
feeding the power from V_OUT to V_CC. For a regulated
output voltage of V_OUT, calculate the R_VCC using the
following equation:
R _VCC
=
(V_OUT − 8)
(I_SUPPLY + I_DRIVE)
See Figure 5 and the Power Dissipation section for the
values of I_SUPPLY and I_DRIVE.
Flyback Converter
The choice of the conversion topology is the first stage
in power-supply design. The topology selection criteria
include input voltage range, output voltage, peak cur-
rents in the primary and secondary circuits, efficiency,
form factor, and cost.
For an output power of less than 50W and a 1:2 input
voltage range with small form factor requirements, the
flyback topology is the best choice. It uses a minimum
of components, thereby reducing cost and form factor.
The flyback converter can be designed to operate
either in continuous or discontinuous mode of opera-
tion. In discontinuous mode of operation, the trans-
former core completes its energy transfer during the
off-cycle, while in continuous mode of operation, the
next cycle begins before the energy transfer is com-
plete. The discontinuous mode of operation is chosen
for the present example for the following reasons:
• It maximizes the energy storage in the magnetic
component, thereby reducing size.
• Simplifies the dynamic stability compensation design
(no right-half plane zero).
• Higher unity-gain bandwidth.
A major disadvantage of discontinuous mode operation
is the higher peak-to-average current ratio in the primary
and secondary circuits. Higher peak-to-average current
means higher RMS current, and therefore, higher loss
and lower efficiency. For low-power converters, the
advantages of using discontinuous mode easily surpass
the possible disadvantages. Moreover, the drive capabil-
ity of the MAX15004/MAX15005 is good enough to drive
a large switching MOSFET. With the presently available
MOSFETs, power output of up to 50W is easily achiev-
able with a discontinuous mode flyback topology using
the MAX15004/MAX15005 in automotive applications.
Transformer Design
Step-by-step transformer specification design for a dis-
continuous flyback example is explained below.
Follow the steps below for the discontinuous mode
transformer:
Step 1) Calculate the secondary winding inductance
for guaranteed core discharge within a mini-
mum off-time.
Step 2) Calculate primary winding inductance for suffi-
cient energy to support the maximum load.
Step 3) Calculate the secondary and bias winding
turns ratios.
Step 4) Calculate the RMS current in the primary and
estimate the secondary RMS current.
Step 5) Consider proper sequencing of windings and
transformer construction for low leakage.
Step 1) As discussed earlier, the core must be dis-
charged during the off-cycle for discontinuous mode
operation. The secondary inductance determines the
time required to discharge the core. Use the following
equations to calculate the secondary inductance:
( ) ( ) _{LS ≤}
V_OUT + V_D × D_OFFMIN ²
2 × I_OUT × f_OUT(MAX)
D_OFF
=
t _OFF
t_ON + t_OFF
where:
D_OFFMIN = minimum D_OFF.
V_D = secondary diode forward voltage drop.
I_OUT = maximum output rated current.
Step 2) The rising current in the primary builds the
energy stored in the core during on-time, which is then
released to deliver the output power during the off-time.
Primary inductance is then calculated to store enough
energy during the on-time to support the maximum out-
put power.
L_P
=
V_INMIN2 × D_MAX ² × η
2 × P_OUT × f_OUT(MAX)
D = ^tON
t_ON + t_OFF
D_MAX = Maximum D.
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