AN3233

Main characteristics and circuit description

Doc ID 17595 Rev 1

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Main characteristics and circuit description

The main features of the SMPS are:

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Input mains range: 90

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Output voltage: 12 V at 12.5 A continuous operation

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Mains harmonics: Acc. to EN61000-3-2 Class-D or JEITA-MITI Class-D

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Standby mains consumption: <0.2 W at 230 Vac

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Efficiency at nominal load: > 91 % at 115 Vac

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EMI: according to EN55022-Class-B

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Safety: according to EN60950

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Dimensions: 65x154 mm, 28 mm component maximum height

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PCB: double side, 70 µm, FR-4, mixed PTH/SMT.

The circuit is composed of two stages: a front-end PFC using the L6563H and an LLC

resonant converter based on the L6599A and the SRK2000, controlling the SR MOSFETs

on the secondary side. The SR driver and the rectifier MOSFETs are mounted on a

daughterboard.

The L6563H is a current mode PFC controller operating in transition mode and implements

a high voltage start-up source to power on the converter.

The L6599A integrates all the functions necessary to properly control the resonant

converter with a 50 % fixed duty cycle and working with variable frequency.

The output rectification is managed by the SRK2000, an SR driver dedicated to LLC

resonant topology.

The PFC stage works as the pre-regulator and powers the resonant stage with a constant

voltage of 400 V. The downstream converter operates only if the PFC is on and regulating.

In this way, the resonant stage can be optimized for a narrow input voltage range.

The L6599A's LINE pin (pin 7) is dedicated to this function. It is used to prevent the resonant

converter from working with an input voltage that is too low which can cause incorrect

capacitive mode operation. If the bulk voltage (PFC output) is below 380 V, the resonant

start-up is not allowed. The L6599A LINE pin internal comparator has a hysteresis allowing

to set the turn-on and turn-off voltage independently. The turn-off threshold has been set to

300 V in order to avoid capacitive mode operation but allow the resonant stage to operate

even in the case of mains sag and consequent PFC output dip.

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

inductance. Therefore, no external, additional coil is needed for the resonance. The

transformer configuration chosen for the secondary winding is center-tap.

On the secondary side, the SRK2000 core function is to switch on each synchronous

rectifier MOSFET whenever the corresponding transformer half-winding starts conducting

(i.e. when the MOSFET body diode starts conducting) and then switching it off when the

flowing current approaches zero. For this purpose, the IC is provided with two pins (DVS1

and DVS2) sensing the MOSFET drain voltage level.

One of the SRK2000’s main characteristics is the ability to automatically detect light load

operation and enter sleep mode, disabling MOSFET driving and decreasing its

consumption. This function allows great power saving at light load with respect to

benchmark SR solutions.