Datasheet

AN3303 SRK2000 main characteristics
Doc ID 18164 Rev 2 5/27
1 SRK2000 main characteristics
The main features of the SRK2000 are described below. The values of the following
parameters are reported in the SRK2000 datasheet (see Reference1).
The SRK2000 implements a control scheme specific for secondary-side synchronous
rectification in an LLC resonant converter that uses a transformer with center-tap secondary
winding for full-wave rectification. It provides two high-current gate-drive outputs, each
capable of driving one or more N-channel power MOSFETs in parallel. Each gate driver is
controlled separately and an interlocking logic circuit prevents the two synchronous rectifier
MOSFETs from conducting simultaneously.
Figure 2. Block diagram of an LLC converter with synchronous rectification
1.1 Drain MOSFET sensing and driving logic
The core function of the IC is to switch on each synchronous rectifier MOSFET whenever
the corresponding transformer half-winding starts conducting (i.e. when the MOSFET body
diode starts conducting) and then to switch it off when the flowing current approaches zero.
For this purpose, the IC is provided with two pins (DVS1 and DVS2) able to sense the power
MOSFET drain voltage level. Because each power MOSFET is turned on when its body
diode is conducting, zero voltage turn-on is achieved.
Device operations described below refer to Figure 3.
a) When the current ISR1 starts flowing through the body diode, the voltage across
the power MOSFET drain-source becomes negative; as it reaches the negative
threshold V
TH.ON
, the power MOSFET is switched on.
The threshold at which the power MOSFET turns on can be set by the following
formula:
Equation 1
V
THON
R
D
I
DVS1 2ON,
V
DVS1 2TH,
+⋅=
_