Power Supply Specification
maximum from
Iout = 5 A to 10 A in 5 µs
Mosfet derating factor
Diode derating factor
in degres Celsius
Targeted efficiency
Extraction of the turn ratio:
Estimation of the min DC at high line and of the max DC at low line:
Calculation of the LC output filter:
Selection of the crossover frequency :
Cout >
RESR <
and
The lower condition of the RESR1 and RESR2 should be selected for the capacitor choice
Selection of the output capacitor:
Cout = 2x1000 µF (serie FM from Panasonic)
ICrms = 4.36A
Resr = 8.5 mΩ
Resr = 22 mΩ
Resr = 28.5 mΩ (57mΩ/2) @ -10°C
Verification max peak to peak output ripple current
ΔI.L <
If ICrms > ΔIL ==> Output capacitor is OK
Inductor value calculation:
Inductor selection
Verifcation of the rms current inside the output capacitor based on the inductor value selection
Secondary peak current:
Primary side peak current:
And the valley current
without magnetizing current
with magnetizing current
with magnetizing current
Computation of the rms current with 10% margin on the primary current for the magnetizing current
Rsense selection a total of 20% margin: 10% for the
magnetizing current and 10% for general margin.
Power dissipation of the Sense resistor with 20% of margin on the primary peak current
2x1.5Ω in //
where:
Mosfet Selection
Ref STP11NM50
at 125°C
Conduction losses
Turn-ON switching losses:
Turn-OFF switching losses:
I.p_pk should be increased by 10% in order to take into account the magnetizing current
Total losses without RCD clamp
Heatsink power mosfet:
Maximum Thermal resistance of the heatsink
RCD snubber
Total losses with RCD clamp
Transformer selection
Core selection
Gapping the core
Primary freewheling diode selection:
Secondary diode selection
Maximum Forward voltage at Iout max and Tj=125°C
Heatsink secondary diode:
°C/W (Maximum Thermal resistance of the heatsink)
Brown Out Calculation:
Internal parameters
Brownout resistor selection:
Switching frequency selection
If the following Rt resistor value is selected
Ramp compensation:
Down slope on secondary side:
Down slope ramp viewed on primary side transfomer over Rsense:
Natural Ramp due to the magnatizing inductance over Rsense:
Natural ramp compensation:
Internal ramp feature
Calcul Rcomp
Note : If the Natural_ramp_comp is higher than the targeted Comp_level, thus it is not needed to compensate the power supply (the magnetazing inductance create enough natural ramp compensation ): Rcomp will be equalt to zero.
Curves
RC network selection for filtering purpose on Current Sense pin
Rcomp selection:
(Selected based on the previous calulation)
RC network time constant
(Recommended Value from 150 to 300 ns)