AN3407

Main characteristics and circuit description

Doc ID 018857 Rev 1

5/25

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Main characteristics and circuit description

The main features of the SMPS are listed here below:

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Output voltage: 48 V at 2.08 A

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Long-life, electrolytic capacitors are not used

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Mains harmonics: according to EN61000-3-2 Class-D

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Efficiency at full load: better than 90%

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Dimensions: 75 x 135 mm

2.1

VCC section

The L6585DE is supplied by applying voltage between the VCC pin and GND pin. An

undervoltage lockout (UVLO) prevents the IC from operating with supply voltages too low to

guarantee the correct behavior of the internal structures.

An internal voltage clamp limits the voltage to around 17 V and a delivery up to 20 mA. For

this reason it cannot be used directly as a clamp for the charge pump (current peaks usually

reach several hundreds of mA), but can be easily used during startup in order to charge the

VCC capacitor or during save mode in order to keep the IC alive, for example, connecting

VCC to input voltage through a resistor.

The L6585DE is supplied by the startup MOSFET Q4 and R40 charging the capacitor C25.

A charge pump connected to the auxiliary winding of the HB inductor T2 supplies the

controller via a small linear regulator represented by Q7. Once both stages have been

activated, the controllers are supplied also by the auxiliary winding of the resonant

transformer, assuring correct supply voltage during all load conditions. As the voltage on the

VCC pin reaches the turn-on threshold, the chip is enabled, and the half-bridge and the PFC

sections start at the same time.

2.2

Power factor corrector section

The PFC output voltage is controlled by means of a voltage-mode error amplifier and a

precise internal voltage reference. The PFC section achieves current mode control

operating in transition mode, offering a highly linear multiplier including a THD optimizer that

allows for an extremely low THD, even over a large range of input voltages and loading

conditions.

The controller is the L6585DE (U1), working in transition mode and integrating all functions

that are needed to perform the PFC. It delivers a stable 450 Vdc. It is a conventional boost

converter connected to the output of the rectifier bridge. It includes the coil T1, the PFC

transformer by YuJing, the diode D2 (STTH3L06U) and the PFC output capacitors C2, C3

and C4 by film type of 5 µF/800 V. The T1 provides also the information about the PFC coil

core demagnetization to pin#11 (ZCD) of the L6585DE. The T1 auxiliary winding is

connected to pin#11 (ZCD) of the L6585DE through the resistor R10. Its purpose is to

provide the information that T1 has demagnetized which is needed by the internal logic for

triggering a new switching cycle. The boost switch is represented by the power MOSFET

Q2. The T1 secondary winding (pins#8-6) and related circuitry are dedicated to power the

L6585DE during normal operation.