7

Rev.1.2

10 -

4. Output rectifier selection

The output rectifier is selected by the consideration of the reverse voltage rating, the current rating’ and the re-

verse recovery time and forward voltage drop for the power loss.

The reverse voltage rating should be at least 1.25 times the maximum input voltage for the consideration of

voltage arc.

The current rating should be larger than the maximum inductor current.

The diode conduction loss is due to the forward conduction and is described:

Pcond = Iout(max) * Vf * ( 1 - Vout / Vin )

Vin = input voltage ; Vout = output voltage ; Iout(max) = maximum output current ;

Vf = diode forward voltage.

The diode reverse recovery loss is due to the reverse recovery from the forward conduction to the reverse

blocking state, and is described:

Prr = Qrr * Vin / Tsw

Vin = input voltage ; Qrr = diode reverse recovery charge ; Tsw = switching period.

5. Output inductor selection

The output inductor is selected for the trade-offs between the output inductor current ripple, dc resistance for

power loss, load transient response time, and the physical size.

The output inductor current ripple determines the output voltage ripple requirement, and the inductor’s dc re-

sistance concerns the power loss. The larger the inductor value, the smaller the inductor ripple current, but the

slower the transient response time, the larger the inductor dc resistance, and hence the larger the power loss.

The inductance value is described:

L = Vout * ( 1 - Vout / Vin ) * Tsw / ΔLl

Vin = input voltage ; Vout = output voltage ;

Tsw = switching period ; ΔLl = inductor ripple current.

6. Output capacitor selection

The output capacitor is selected for the trade-offs between output ripple voltage requirement, the output volt-

age rating, the RMS current rating, the ESR and ESL for the load transient, and the physical size.

The capacitor voltage rating is recommended with 1.5 times for the maximum output voltage as conservative

guideline.

The capacitor RMS current rating is considered for stress condition’ and the trapezoid current waveform as the

simplified formula is described:

Irms = ΔIL / 2√3 ; ΔIL = inductor ripple current.

The output ripple voltage with respect to the capacitor ESR is described:

ΔV = ΔI * ( ESR + Tsw / ( 8 * C ) )

ΔI = capacitor ripple current’ which is equivalent to the inductor ripple current ;

ESR = capacitor equivalent series resistance ; Tsw = switching period.

7. PC board layout consideration

Good PC Board layout is very important in switching converter design. If designed improperly, the PC Board

could radiate excessive noise and contribute to the converter instability.