current is limited by the sense resistor (R15) and cannot definitely exceed the full load value.

R2 and R3 provide an additional DC offset on the current sense which depends on the supply input volt-

age. This is used for compensating L5991’s delay to output and also minimises the dependence of

PinSB and PinNW on the mains voltage (see table 7).

Additionally, the board includes some protection functions tipically required in AC-DC adapters, such as

overvoltage (OVP) and overcurrent protection (OCP).

OCP is inherent in the functionality of the L5991: the controller provides both pulse-by-pulse and "hic-

cup" mode current limitation (see

Application Information in the datasheet), which fully protect the con-

verter in case of overload or short circuit.

The OVP, in this specific case, is realised by sensing the supply voltage of the L5991 (generated by the

auxiliary winding) through the divider R5-R6 and feeding this partition into pin 14 (DIS). The divider ratio

is such that the OVP is tripped when the supply voltage exceeds 20V. This protection is particularly ef-

fective in case of feedback disconnection (e.g. optocoupler’s failure).

At maximum load and minimum mains voltage the converter operates at about 48% duty cycle (this is

why slope compensation is not required), however the maximum duty cycle of the L5991 is limited at

about 55% to make allowance for load transients. This implies that during transients resulting from a

large enough step-load change at minimum mains voltage, subharmonic oscillations are likely to arise. It

is, however, acceptable, this being a condition lasting few milliseconds.

To set the maximum duty cycle at 55%, L5991’s pin3 (DC) is biased through R8 and R9 at about 2.26V.

Please refer to

Application Information in L5991’s datasheet for the calculation of the voltage divider.

The evaluation board is supplied with a start-up circuit simply made up of a dropping resistor (R1+R2), in

series with a diode (D3), that draws current from upstream the bridge rectifier.

This circuit, really inexpensive, dissipates about 300 mW @ 264 Vac. The typical wake-up time is 2.8 s

at 88Vac and 0.8 s at 264 Vac. Should the wake-up time or the consumption become an issue, a more

expensive solution would be adopted. The PCB is also able to accommodate a high-voltage start-up IC

(IC2), the LR745N3 available from SUPERTEX and housed in a small TO92 package. In that case R1,

R2 and D3 would be removed and the consumption of the start-up circuit would be of few mW. The

wake-up time would be about 0.2 s independently of the mains voltage.

To enhance light load efficiency, the EVAL5991-45 board is supplied with the clamping network (for the

leakage inductance spike) made up of a Transil diode (D1) instead of the usual RCD type. The PCB is

able to accommodate the RCD clamp anyway (R23 and C16). The use of the Transil, although slightly

worsens efficiency at full load, allows to save over 100 mW that would have been dissipated on R23 at

light load.

Application board evaluation: getting started

The AC voltage, from an AC source ranging from 88 VRMS to 264 VRMS, will be applied to connector

J1 (close to the bottom left-hand corner). The 18VDC output (connector J2) is located few centimeters

on the right of J1.

Like in any offline circuit, extreme caution must be used when working with the application board

because it contains dangerous and lethal potentials. The application must be tested with an iso-

lation transformer connected between the AC mains and the input of the board to avoid any risk

of electrical shock.

There is a number of test points where significant signals can be probed:

TP1: Q1 drain voltage;

TP2: pin 6 of the L5991: output of the error amplifier;

TP3: pin16 of the L5991: standby indicator;

TP4: pin 2 of the L5991: local oscillator;

TP5: pin 1 of the TPS5904: anode of the LED of the optocoupler.

AN1134 APPLICATION NOTE

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