IRU3011

9

Rev. 1.6

08/20/02

www.irf.com

T

≡ Switching Period

D

≡ Duty Cycle

Vsw

≡ High-side MOSFET ON Voltage

≡ MOSFET On-Resistance

Vsync

≡ Synchronous MOSFET ON Voltage

DIr

≡ Inductor Ripple Current

DVo

≡ Output Ripple Voltage

T = 1/Fsw

Vsw = Vsync = Io

D

3T

DVo = DIr3ESR

L = 0.006

390003(4.75 - 2.8) / (2314.2) = 3.7mH

L = ESR

T = 1 / 200000 = 5

ms

Vsw = Vsync = 14.2

30.019 = 0.27V

D

≅ (2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61

35 = 3.1ms

ms

DIr = (2.8 + 0.27)31.9 / 3 = 1.94A

DVo = 1.9430.006 = 0.011V = 11mV

Ts = 125 - 3.82

3(1.8 + 0.05) = 1188C

Temperature Rise Above Ambient

Output Inductor Selection

The output inductance must be selected such that un-

der low line and the maximum output voltage condition,

the inductor current slope times the output capacitor

ESR is ramping up faster than the capacitor voltage is

drooping during a load current step. However, if the in-

ductor is too small, the output ripple current and ripple

voltage become too large. One solution to bring the ripple

current down is to increase the switching frequency,

however, that will be at the cost of reduced efficiency

and higher system cost. The following set of formulas

are derived to achieve the optimum performance without

many design iterations.

The maximum output inductance is calculated using the

following equation:

Where:

For Vo=2.8V and

DI=14.2A

Assuming that the programmed switching frequency is

set at 200KHz, an inductor is designed using the

Micrometals’ powder iron core material. The summary

of the design is outlined below:

The selected core material is Powder Iron, the selected

core is T50-52D from Micro Metal wounded with 8 turns

of #16 AWG wire, resulting in 3

mH inductance with

≈

3m

V of DC resistance.

Assuming L=3

mH and Fsw=200KHz(switching fre-

quency), the inductor ripple current and the output ripple

voltage is calculated using the following set of equations:

In our example for Vo=2.8V and 14.2A load, assuming

IRL3103 MOSFET for both switches with maximum on

resistance 0f 19m

V, we have:

Power Component Selection

Assuming IRL3103 MOSFETs as power components,

we will calculate the maximum power dissipation as fol-

lows:

For high-side switch the maximum power dissipation

happens at maximum Vo and maximum duty cycle.

8C

For synch MOSFET, maximum power dissipation hap-

pens at minimum Vo and minimum duty cycle.

Heat Sink Selection

Selection of the heat sink is based on the maximum

allowable junction temperature of the MOSFETS. Since

8C,

then we must keep the junction below this temperature.

Selecting TO-220 package gives

venders’ data sheet) and assuming that the selected

heat sink is black anodized, the heat-sink-to-case ther-

mal resistance is

ucs=0.058C/W, the maximum heat sink

temperature is then calculated as:

With the maximum heat sink temperature calculated in

the previous step, the heat-sink-to-air thermal resistance

(

8C:

≅ (2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43

≅ (2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65