Description

STEVAL-ISA143V1

2/5

DocID025362 Rev 2

1

Description

The STEVAL-ISA143V1 system evaluation board is made up of two stages: a front-end PFC

using the L6563H, an LLC resonant converter based on the L6699 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 startup source to power on the converter.

The L6699 integrates all the functions necessary to properly control the resonant converter

with a 50% fixed duty cycle and work with variable frequency.

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

resonant topology.

The PFC stage works as 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 L6699’s LINE pin (pin 7) is dedicated to this function. It is used to prevent the resonant

converter from working with too low an input voltage, which can cause incorrect capacitive

mode operation. If the bulk voltage (PFC output) is below 380 V, the resonant startup is not

allowed. The L6699 LINE pin internal comparator has a hysteresis allowing the turn-on and

turn-off voltage to be independently set. The turn-off threshold is set to 300 V, allowing the

resonant stage to operate in case of mains sag and consequent PFC output dip.

The transformer uses the integrated magnetic approach, incorporating 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 switches on each synchronous rectifier

MOSFET whenever the corresponding transformer half-winding starts conducting (i.e. when

the MOSFET body diode starts conducting) and then switches it off when the flowing current

approaches zero. For this purpose, the IC is provided with two pins (DVS1 and DVS2)

sensing the MOSFETs’ drain voltage level.

The SRK2000 automatically detects light load operation and enters Sleep mode, disabling

MOSFET driving and decreasing its own consumption. This function allows greater power

saving at light load compared to benchmark SR solutions.

In order to decrease the output capacitor size, aluminum solid capacitors with very low ESR

were favored over standard electrolytic ones. Therefore, high frequency output voltage

ripple is limited and output LC filter is not required. This choice allows saving of output

inductor power dissipation which can be significant in high output current applications such

as this.