SRK2000DTR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'SRK2000DTR' PDF : 17 Pages View PDF
Application information
SRK2000
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
12/19
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
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
dissipation into account. On the other hand, when current starts flowing into the body diode
of one MOSFET (or in the diode in parallel with the MOSFET), the drain-to-source voltage is
negative ( -0.7 V); when the voltage on pins DVS1,2 reaches the threshold VDVS1,2_TH
DocID17811 Rev 4
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