Advanced Power
Electronics Corp. APU3048
Output Capacitor Selection
Soft-Start Programming
Boost Supply Vc
Input Capacitor selection
Inductor Selection
6
If the high value feedback resistors are used, the
input bias current of the Fb pin could cause a slight
increase in output voltage. The output voltage set point
can be more accurate by using precision resistor.
The input filter capacitor should be based on how
much ripple the supply can tolerate on the DC input line.
The ripple current generated during the on time of
control MOSFET should be provided by input capacitor.
The RMS value of this ripple is expressed by:
Where:
D is the Duty Cycle, simply D=VOUT/VIN
IRMS is the RMS value of the input capacitor current.
IOUT is the output current for each channel.
For VIN1=12V, IOUT1=4A and D1=0.275
Results to: IRMS1=1.78A
And for VIN2=5V, IOUT2=4A and D2=0.36
Results to: IRMS2 =1.92A
For higher efficiency, a low ESR capacitor is
recommended.
For VIN1=12V, choose one Poscap from Sanyo
16TPB47M (16V, 47uF, 70mΩ, 1.4A)
For VIN2=5V, choose one 6TPC150M (6.3V, 150uF,
40mΩ, 1.9A).
The criteria to select the output capacitor is
normally based on the value of the Effective Series
Resistance (ESR). In general, the output capacitor must
have low enough ESR to meet output ripple and load
transient requirements, yet have high enough ESR to
satisfy stability requirements. The ESR of the output
capacitor is calculated by the following relationship:
The soft-start timing can be programmed by
selecting
the soft start capacitance value. The start up time of the
converter can be calculated by using:
tSTART = 75 x Css (ms) ---(2)
Where:
CSS is the soft-start capacitor (mF)
For a start-up time of 7.5ms, the soft-start
capacitor will be 0.1uF. Choose a ceramic capacitor at
0.1uF.
To drive the high-side switch it is necessary to
supply a gate voltage at least 4V greater than the bus
voltage. This is achieved by using a charge pump
configuration as shown in Figure 9. The capacitor is
charged up to approximately twice the bus voltage. A
capacitor in the range of 0.1mF to 1mF is generally
adequate for most applications.
Where:
δVO = Output Voltage Ripple
δIO = Output Current
δVO=75mV and δIO=3A, results to: ESR=25mΩ
The Sanyo TPC series, PosCap capacitor is a
good choice. The 6TPC150M 150uF, 6.3V has an ESR
40mΩ. Selecting two of these capacitors in parallel for
each output, results to an ESR of around 20mΩ which
achieves our low ESR goal.
The capacitor value must be high enough to
absorb the inductor's ripple current. The larger the value
of capacitor, the lower will be the output ripple voltage.
The resulting output ripple current is smaller then
each channel ripple current due to the 180o phase shift.
These currents cancel each other. The cancellation is
not the maximum because of the different duty cycle for
each channel.
The inductor is selected based on output power,
operating frequency and efficiency requirements. Low
inductor value causes large ripple current, resulting in
the smaller size, but poor efficiency and high output
noise. Generally, the selection of inductor value can be
reduced to desired maximum ripple current in the
inductor (δi); the optimum point is usually found
between 20% and 50% ripple of the output current.
For the buck converter, the inductor value for
desired operating ripple current can be determined
using the following relation:
Where:
VIN = Max Input Voltage
VOUT = Output Voltage
δi = Inductor Ripple Current
fS = Switching Frequency
δt = Turn On Time
D = Duty Cycle