Datasheet
SRK2000 main characteristics AN3303
6/27 Doc ID 18164 Rev 2
where I
DVS1,2.on
is the current sourced out of the DVS1,2 pins (50 µA typ.) and
V
DVS1,2_TH
is the lower clamp voltage of the DVS1,2 pins (-0.2 V typ.).
This may enable the ON threshold to be set according to the SR power MOSFET
body diode V
F
chosen for the application or the external diode connected in
parallel to the power MOSFET drain-source (e.g. Schottky rectifier).
The current sourcing out of the DVS1,2 pin is enabled after the drain-source
voltage experiences a voltage below the pre-triggering level V
DVS1,2_PT
(negative
going edge) and is disabled once the rectifier is switched on. A de-bouncing delay
(T
PD_ON
) is introduced after the current generator is activated in order to avoid
false triggering of the gate driver.
In some applications, R
D1,2
is also needed to limit the current that can be injected
into the DVS pins when the corresponding SR power MOSFET is off. In fact, when
one power MOSFET is off (and the other 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.), RD1,2 has to limit the injected current below
the maximum rating (25 mA). In addition, the SRK2000 clamping circuit dissipation
must be taken into account to avoid device overheating. In this case, Equation 1 is
used to check that the resulting V
TH.ON
is compatible with the forward drop of the
SR power MOSFET body diode (or the parallel external diode).
b) Once the power MOSFET is turned on, its drain-source voltage drops to:
Equation 2
which is negative because current flows from source to drain. When this voltage
reaches (exceeds) the turn-off threshold V
DVS1,2_Off
, the power MOSFET is
switched off. The user can set the turn-off threshold selecting between two
different values (see Reference 1) by properly biasing the EN pin during the IC
startup phase.
c) After the power MOSFET is switched off, the current still flows through its body
diode (causing the drain-source voltage to jump negative) until it becomes zero;
then the transformer winding voltage reverses and the drain-source voltage starts
increasing. As it exceeds the arming voltage VDVS1,2_A (positive going edge),
the gate drive of the second power MOSFET is armed and the operation,
described above, now applies to this rectifier.
V
DS1 2,
R
DS on()
I
SR1 2,
⋅=