SRK2000A データシートの表示（PDF） - STMicroelectronics

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SRK2000A

Synchronous rectifier smart driver for LLC resonant converters

STMicroelectronics 

Application information

SRK2000A

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

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

6.2

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

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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 VDVS1_Off and VDVS2_Off thresholds are matched in order to have the same current turn-

off level in both SR MOSFETs.

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

slightly higher than twice the output voltage; if this exceeds the voltage rating of the internal

clamp (VCCZ = 36 V typ.), a series resistor RD must limit the injected current below an

appropriate value, lower than the maximum rating (25 mA) and taking the related power

DocID025407 Rev 3

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