AN2393 datasheet(5/35 Pages) STMICROELECTRONICS

5 / 35 page

AN2393

Main characteristics and circuit description

5/35

changing the frequency according to the feedback signal in order to regulate the output

voltages against load and input voltage variations.

The main features of the L6599 are a non-linear soft-start, a current protection mode used

to program the hiccup mode timing, a dedicated pin for sequencing or brown-out (LINE) and

a stand-by pin (STBY) for burst mode operation at light loads (not used in this design).

The transformer uses the magnetic integration approach, incorporating the resonant series

and shunt inductances. Thus, no additional external coils are needed for the resonance. The

transformer configuration chosen for the secondary winding is center-tap, and the output

rectifiers are Schottky type diodes, in order to limit the power dissipation. The feedback loop

is implemented by means of a classical configuration using a TL431 (U4) to adjust the

current in the optocoupler diode (U3). A weighted resistive divider (R53, R57, R58, R60 and

R61) is used to detect both output voltages in order to get a better overall voltage regulation.

The optocoupler transistor modulates the current from Pin 4, so the frequency will change

accordingly, thus achieving the output voltage regulation. Resistors R46 and R54 set the

maximum operating frequency.

In case of a short circuit, the current entering the primary winding is detected by the lossless

circuit (C34, C39, D11, D12, R43, and R45) and the resulting signal is fed into Pin 6.

In case of overload, the voltage on Pin 6 will overpass an internal threshold that triggers a

protection sequence via Pin 2, keeping the current flowing in the circuit at a safe level.

The third stage is a small flyback converter based on the VIPer12A-E, a current mode

controller with integrated Power MOSFET, capable of delivering (approximately) 7 W output

power on the output voltages (5 V and 3.3 V). The regulated output voltage is the 3.3 V

output and, also in this case, the feedback loop bases on the TL431 (U7) and optocoupler

(U6) to control the output voltage.

This converter is able to operate in the whole mains voltage range, even when the PFC

stage is not working. From the auxiliary winding on the primary side of the flyback

transformer (T2), a voltage Vs is available, intended to supply the other controllers (L6563

and L6599) in addition to the VIPer12A-E itself.

The PFC stage and the resonant converter can be switched on and off through the circuit

based mainly on components Q7, Q8, D22 and U8, which, depending on the level of the

signal ST-BY, supplies or removes the auxiliary voltage (VAUX) necessary to start-up the

controllers of the PFC and resonant stages. In this way, when the AC input voltage is applied

to the power supply, the small flyback converter switches on first; then, when the ST-BY

signal is low, the PFC pre-regulator becomes operative, and last the resonant converter can

deliver the output power to the load.

Note that if Pin 9 of Connector J3 is left floating (no signal ST-BY present), the PFC and

resonant converter will be not operating, and only +5V and +3.3V supplies are available on

the output. In order to enable the +24 V and +12 V outputs, Pin 9 of Connector J3 must be

pulled down to ground.

AN2393 Datasheet(PDF) 5 Page - STMicroelectronics

AN2393 Datasheet(HTML) 5 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Datasheet Download

Link URL