SRK2000DTR(2012) データシートの表示（PDF） - STMicroelectronics

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SRK2000DTR
(Rev.:2012)

Synchronous rectifier smart driver for LLC resonant converters

STMicroelectronics 

SRK2000

Application information

5.1.3

Remote on/off control

Whichever configuration is used, since a voltage on the EN pin 45 mV below VEN_On

disables the gate drivers, any small-signal transistor can be used to pull down the EN pin

and force the gate drivers into an off state.

Finally, it should be noted that during power-up, power-down, and under overload or short-

circuit conditions, the gate drivers are shut down if the Vcc voltage is insufficient: < VCCOff in

case of pull-up resistor configuration, < 0.975 ÞVCC_G in case of resistor divider

configuration (the coefficient 0.975 depends on the hysteresis on the Enable pin threshold).

5.2

Drain voltage sensing

In the following explanations it is assumed that the reader is familiar with the LLC resonant

half bridge topology and its waveforms, especially those on the secondary side with a

center-tap transformer winding for full-wave rectification.

To understand the polarity and the level of the current flowing in the SR MOSFETs (or their

body diodes, or diodes in parallel to the MOSFETs) the IC is provided with two pins, DVS1-

2, able to sense the voltage level of the MOSFET drains.

Figure 7. Typical waveform seen on the drain voltage sensing pins

The logic that controls the driving of the two SR MOSFETs is based on two gate-driver state

machines working in parallel in an interlocked way to avoid both gate drivers being switched

on at the same time.

There are four significant drain voltage thresholds: the first one, VDVS1,2_A (= 1.4 V),

sensitive to positive-going edges, arms the opposite gate driver (interlock function); the

second, VDVS1,2_PT (=0.7 V), sensitive to negative-going edges, provides a pre-trigger of the

gate driver; the third is the (negative) threshold VTH-ON that triggers the gate driver as the

body diode of the SR MOSFET starts conducting; the fourth is the internal (negative)

threshold VDVS1,2_Off where the SR MOSFET is switched off (selectable between -12 mV or

-25 mV by properly biasing the EN pin).

The value of the ON threshold VTH-ON is affected by the external resistor in series to each

DVS1-2 pin needed essentially to limit the current that might be injected into the pins when

one SR MOSFET is off and the other SR MOSFET is conducting. In fact, on the one hand,

when one MOSFET is off (and the other one is conducting), its drain-to-source voltage is

Doc ID 17811 Rev 2

11/17

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