E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
These equations are suitable for initial capacitor selection. Final values should be set by testing of your pro-
totype circuit.
8.1.5 Catch Diode Selection
The catch diode DSW carries load current during the internal P-channel MOS transistors off-time. Therefore
the average diode current is depending on the duty cycle. At high input to output voltage ratios the diode
conducts most of the time. The most stressful condition for the diode is when the output is shorted. Under
this condition the diode must safely handle a peak current of:
IDSW PEAK
≥ 2 ⋅ VCLIM MAX
RS MIN
[31]
The catch diode maximum forward loss in continuous conduction mode is:
PDSWMAX ≈ IDSWAVG ⋅Vd DSW
[32]
Where VdDSW is the maximum forward drop from the diode data sheet and IDSWAVG is the average current at
the minimum duty cycle:
IDSWAVG = IOUTMAX ⋅ (1 − Duty )
[33]
The reverse (blocking) voltage VRRM of the catch diode must be at minimum VINPUTMAX .
The E910.27 high switching frequency requires a high speed rectifier DSW. The catch diode is the biggest
EMI source in the circuit. Take therefore care of diode selection. Surface mount low reverse leakage
Schottky diodes are recommended for low output voltages. Ultra high speed soft recovery rectifiers with
reverse recovery times < 30ns should be used for moderate or high output voltages, where the increased
forward drop causes less efficiency degradation.
8.1.6 Snubber Network Design
For low output current, in discontinuous conduction mode, the internal P-channel MOS transistor turns off,
and the energy stored in the inductance would cause oscillations with the parasitic capacitances without a
snubber network. Typically there are little losses in parasitic resonant circuits so many cycles of ringing nor-
mally occur. This oscillation may be an EMI issue. If required an RC snubber will easily damp the ringing.
If the snubber resistance is equal to the characteristic impedance of the resonant circuit then the resonant
circuit will be critically damped. Once the circuit has been build and is operating, the values of the snubber
components may be evaluated experimentally.
Start with a small value of capacitor CSN and place it in the circuit directly across the catch diode DSW, and
observe the voltage waveform with and without the capacitor in the circuit. Increase the value of the capaci-
tor until the frequency of the ringing to be damped has been halved. This is a near optimum value for the
capacitor CSN since it allows damping very near Q=1. Add a resistor in series with the snubber capacitor
and choose its value for optimum, so that the ringing will be aperiodic damped.
RSN ≈ 2 ⋅π ⋅ fres ⋅ LCK
[34]
Where fres = initially measured resonant frequency, and LCK = choke inductance.
ELMOS Semiconductor AG
Specification 30 / 50
03SP0357E.00 05.09.2006
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