IRU3138

9

Rev. 1.0

01/29/04

www.irf.com

Choose IRLR3715Z for control MOSFET and IRFR3711Z

for synchronous MOSFET. These devices provide low

on-resistance in a D-Pak.

The MOSFETs have the following data:

The total conduction losses will be:

The switching loss is more difficult to calculate, even

though the switching transition is well understood. The

reason is the effect of the parasitic components and

switching times during the switching procedures such

as turn-on / turnoff delays and rise and fall times. The

control MOSFET contributes to the majority of the switch-

ing losses in synchronous Buck converter. The synchro-

nous MOSFET turns on under zero voltage conditions,

therefore, the turn on losses for synchronous MOSFET

can be neglected. With a linear approximation, the total

switching loss can be expressed as:

The switching time waveform is shown in Figure 7.

Figure 7 - Switching time waveforms.

From IRFR3711Z data sheet we obtain:

These values are taken under a certain condition test.

For more details please refer to the IRFR3711Z datasheet.

By using equation (12), we can calculate the total switch-

ing losses.

Feedback Compensation

The IRU3138 is a voltage mode controller; the control

loop is a single voltage feedback path including error

amplifier and error comparator. To achieve fast transient

response and accurate output regulation, a compensa-

tion circuit is necessary. The goal of the compensation

network is to provide a closed loop transfer function with

the highest 0dB crossing frequency and adequate phase

margin (greater than 45

8).

The output LC filter introduces a double pole, –40dB/

decade gain slope above its corner resonant frequency,

and a total phase lag of 180

8 (see Figure 8). The Reso-

nant frequency of the LC filter is expressed as follows:

Figure 9 shows gain and phase of the LC filter. Since we

already have 180

8 phase shift just from the output filter,

the system risks being unstable.

Figure 8 - Gain and phase of LC filter.

Where:

T = Switching Period

---(12)

3

2

T

3

IRFR3711Z

---(13)

1

2

p3

10%

90%

Gain

0dB

Phase

0

8

-180

8

Frequency

Frequency

-40dB/decade

IRFL3715Z

IRFR3711Z