L4990AD datasheet(14/24 Pages) STMICROELECTRONICS

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DEMONSTRATION BOARD

To evaluate the device performance, a demon-

stration board has been realized. Despite its sim-

plicity, it exploits most of the features of the

L4990.

The board embodies an application based on the

following specification of universal mains AC-DC

adapter:

Input voltage range: 85-270 Vac (50/60 Hz)

Output voltage: 15 V

Output current: 0.5 to 2A

Output voltage ripple : 300 mV (max.)

Load regulation:

± 5% (0.5 to 2 A load change)

Target efficiency @ Iout = 2 A: ; 80% over the

input voltage full range

Some preliminary decisions, concerning topology,

operating mode, switching frequency, maximum

duty cycle allowed and control technique, have

been made.

As for topology, at this power level and output

voltage, flyback is the most advantageous one,

mainly because of its simplicity, which means low

parts count, low cost and inherent high efficiency.

A peculiar design choice aiming at optimizing the

overall system concerns the operating mode: the

converter will work in continuous current mode at

low input voltages, when input current is greater,

and in discontinuous mode at higher input volt-

ages. Numerous benefits originate from that.

Compared to discontinuous current mode, con-

tinuous operation involves lower peak currents

(typ. -40%) at the same throughput power. This

implies less stress for all power components.

The transformer inductance is higher and, there-

fore, a smaller air gap is required for a given core:

this increases primary-to-secondary coupling and,

as a consequence, reduces leakage inductance

and improves energy transfer. Both efficiency and

load regulation will take advantage of that.

Another point in favor is a reduction of the mini-

mum output power that the system is able to de-

liver keeping the output well regulated.

Few components are required in addition for

slope compensation.

Actually, continuous mode flyback suffers also

from a poor dynamic behavior during load tran-

sients because of the narrow bandwidth of the

control loop due to stability problems. However,

great dynamic performance is not required to AC-

DC adapters, so this problem is of no concern.

The boundary between the two operating modes

has been set at about 150 Vac (@ Iout=2A).

The selection of the switching frequency is a mat-

ter of trade-off between achieving a small trans-

former size and high efficiency. 200 kHz seems to

be a good compromise.

In this application, the wide input voltage range

requires a large duty cycle sweep. The higher is

the maximum duty cycle, the larger is the operat-

V15=GND

V5=V13=GND

V15=VREF

V5=V13=GND

V10

DX =

tc +td

DX =

2·tc +td

D97IN498

Figure 29. Half duty cycle option.

L4990 - L4990A

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