AN1134 Datasheet, PDF (1/9 Pages) STMicroelectronics – 45W AC-DC ADAPTER WITH STANDBY FUNCTION

AN1134
®
APPLICATION NOTE
45W AC-DC ADAPTER WITH STANDBY FUNCTION
by Claudio Adragna
Purpose of this note is to provide a brief summary of the specifications and the functionality of the
evaluation board implementing a 45W, wide-range mains AC-DC adapter, based on the L5991 cur-
rent mode PWM controller.
Evaluation results are also presented so as to underline the benefits offered by the L5991 in such a
new generation of equipment that requires a superior efficiency in standby conditions, aiming at com-
pliance with energy saving standards.
Design Specifications
Table 1 summarises the electrical specification of the application. The complete electrical schematic is
shown in fig. 1 and the bill of material is listed in Table 2.
Table 1. Design Specification
Input Voltage Range (Vin)
Mains Frequency (fL)
Maximum Output Power (P_out)
Output
Normal Operation Switching Frequency (f_osc)
Light Load Switching Frequency (fSB)
Target Efficiency (@ P_out = 45W, V_in = 88 ÷ 264 Vac) (η)
Maximum Input Power (@ P_out = 0.5W, V_in = 88 ÷ 264 V_ac)
Maximum Input Power (Open load, Vin = 88 ÷ 264 V_ac)
88 to 264 Vac
50/60 Hz
45W
V_out = 18V
I_out = 2.5A
Full load ripple = 2%
70kHz
18kHz
>80%
≤2W
≤1W
The selected topology is flyback. The operation mode (@ Pout = 45W ) is CCM (Continuous Conduc-
tion Mode) at low mains voltage, DCM (Discontinuous Conduction Mode) at high mains voltage. This
design choice relieves the stress on the power components at low mains voltage, compared with a full
DCM solution. The maximum duty cycle will be limited below 50%, thus no slope compensation is
needed.
The application will benefit from the features of the L5991 PWM controller in order to minimise the power
drawn from the mains under light load conditions: low start-up and quiescent currents, and Standby
function.
Evaluation Board Functionality
The outstanding feature of this application board is the so-called Standby Function, directly available
from the L5991. When the load is such that the power demanded of the mains is greater then about
13W the switching frequency of the converter is set at fosc = 70 kHz (by means of the capacitor C5 and
the parallel of R12 and R13). When the input power falls below about 8.5W the L5991 automatically
changes the oscillator frequency to fSB =18 kHz (by disconnecting R13 internally and charging C5
through R12 only).
These thresholds are "static" values, that is are related to slow load variations. In case of step-load
changes the output of the error amplifier will experience undershoots and overshoots, thus the "dy-
namic" thresholds will be different. Namely, the dynamic threshold for the transition fosc → fSB will be
December 1999
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