SRK2000 datasheet(13/19 Pages) STMICROELECTRONICS

13 / 19 page

DocID17811 Rev 4

13/19

SRK2000

Application information

(-0.2 V typ.), an internal current source IDVS1,2_On is activated; as this current exceeds

50 µA, the gate of the MOSFET is turned on. Therefore, the actual triggering threshold can

be determined by Equation 6.

Equation 6

For instance, with RD = 2 k, the triggering threshold is located at

- (2 k

  50 µA) - 0.2 V = -0.3 V.

To avoid false triggering of the gate driver, a debounce delay TPD_On (= 250 ns) is used after

sourcing IDS1,2_On (i.e. the current sourced by the pin must exceed 50 µA for more than 250

ns before the gate driver is turned on). This delay is not critical for the converter’s efficiency

because the initial current is close to zero or anyway much lower than the peak value.

Once the SR MOSFET has been switched on, its drain-to-source voltage drops to a value

given by the flowing current times the MOSFET RDS(on). Again, since the initial current is

low, the voltage drop across the RDS(on) may exceed the turn-off threshold VDVS1,2_Off, and

determine an improper turn-off. To prevent this, the state machine enables the turn-off

comparator referenced to VDVS1,2_Off only in the second half of the conduction cycle, based

on the information of the duration of the previous cycle. In the first half of the conduction

cycle only an additional comparator, referenced to zero, is active to prevent the current of

the SR MOSFET from reversing, which would impair the operation of the LLC converter.

Once the threshold VDVS1,2_Off is crossed (in the second half of the conduction cycle) and

the GATE is turned off, the current again flows through the body diode causing the drain-to-

source voltage to have a negative jump, going again below VTH-ON. The interlock logic,

however, prevents a false turn-on. It is worth pointing out that, due to the fact that each

MOSFET is turned on after its body diode starts conducting, the ON transition happens with

the drain-source voltage equal to the body diode forward drop; therefore there is neither

a Miller effect nor switching losses at MOSFET turn-on. Also at turn-off the switching losses

are not present, in fact, the current is always flowing from source to drain and, when the

MOSFET is switched off, it goes on flowing through the body diode (or the external diode in

parallel to the MOSFET).

Unlike at turn-on, the turn-off speed is critical to avoid current reversal on the secondary

side, especially when the converter operates above the resonance frequency, where the

current flowing through the MOSFET exhibits a very steep edge while decreasing down to

zero: the turn-off propagation TPD_Off delay has a maximum value of 60 ns.

The interlock logic, in addition to checking for consistent secondary voltage waveforms (one

MOSFET can be turned on only if the other one has a positive drain-to-source voltage

> VDVS1,2_A) to prevent simultaneous conduction, allows only one switching per cycle: after

one gate driver has been turned off, it cannot be turned on again before the other gate drive

has had its own on/off cycle.

The IC logic also prevents unbalanced current in the two SR MOSFETs: if one SR MOSFET

fails to turn on in one cycle, the other SR MOSFET is also not turned on in the next cycle.

DVS

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