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AN1842 APPLICATION NOTE

The converter topology of this SMPS is the standard fly back, working in discontinuous and continuous

current mode. The operating frequency of the circuit (~50 kHz) has been chosen in order to obtain a com-

promise between the transformer size and the input filter complexity. Hence, the input EMI filter is a simple

Pi-filter, 1-cell only, for differential and common mode noise, using a 4-sectors coil filter. A NTC limits the

inrush current at plug-in. The transformer is a slot type, manufactured by PULSE-ELDOR designed ac-

cording to the EN60950. Ferrite size is ETD34, the reflected voltage is ~95V providing enough room for

the leakage inductance voltage spike with still margin for reliability.

The reflected voltage, the switching frequency and the primary inductance have been chosen to allow the

continuous current operation of the transformer at full load, all over the input voltage range. This helps to

decrease the output capacitor size thanks to the better ratio between the rms and peak current. The net-

work D10, C14, R5 clamps the peak of the leakage inductance voltage spike ensuring reliable operation

of the PowerMOS, while C21, D11 and R19 limit the dv/dt of the drain voltage.

220FP (TO-220 insulated) has been used, mounted on a heat sink and fixed by a spring. Core of this de-

sign is the current mode primary controller, the L5991 integrating all the required blocks to manage the

control and protection of an SMPS. It is available in either DIP-16 (L5991) or in SO-16 (L5991D) packages.

The switching frequency is programmable by means of a an RC network (R11, R12, C15): during normal

operation R11 and R12 are connected in parallel by an internal switch (pin 16); when a light load is de-

tected by the controller this internal switch is opened and the resulting frequency becomes lower, pro-

grammed only from C15 and R11.

If the load is further decreased the network D4, D5, R23 provides an additional frequency reduction, pro-

portional to the load, allowing very low power consumption from the mains. Pins 15 and 3 are set in order

to allow the full duty-cycle operation and so the use of most of the energy stored in the bulk capacitor dur-

ing hold-up operations. Because of the current mode control and the possibility for the duty cycle to exceed

50%, a slope compensation circuitry has been added.

A latched, over voltage protection has been implemented by using the pin 14 and a simple resistor net-

work: in case of loop failure the circuit senses the Vcc and, when the voltage at that pin exceeds the in-

ternal threshold, the controller stops the operation until its Vcc drops below the UVLO voltage. The start-

up is done using a non-dissipative charge pump circuit to save power during standby.

The output rectifiers have been chosen in accordance with the maximum reverse voltage and their power

dissipation. The 5V rectifier is Schottky barrier type STPS5L40, a 5A-40V axial rectifier that thanks to the

low-forward voltage drop is housed in a DO-201 package.

The 12V rectifier is an STPS8H100, an high voltage Schottky rectifier offering a good trade-off between

the forward voltage drop and the maximum operating junction temperature. It is available from STM in 5

different package versions. For this design, the ISOWATT220AC (similar to a standard insulated TO-220)

has been used, mounted on a heat sink and fixed by a spring.

On both outputs, an LC filter has been added in order to filter out the high frequency ripple without increas-

ing the output capacitors size or quality. The output voltage regulation is performed by the secondary feed-

back monitoring the 5.1V output. The feedback network is the typical one that uses a TL431 driving an

optocoupler, in this case an SFH617A-2, to ensure the required insulation between primary and second-

ary. The opto-transistor drives directly the COMP pin of the L5991. Here following some waveforms during

the normal operation at full load: