Datasheet
Sensing optimization by waveform check AN3303
14/27 Doc ID 18164 Rev 2
3 Sensing optimization by waveform check
The board has been tested on a 150 W 12 V LLC converter (see Reference 2). The following
assertions refer to EVLSRK2000-L-40 but similar considerations can be made for all the
configurations of EVLSRK2000-x-xx.
Note that current and voltage probes can affect the IC sensing leading to malfunctioning.
Signal probing must be accomplished carefully and with minimal modification with respect to
the original circuit.
Figure 13 and 14 show key signals of the SRK2000: each SR power MOSFET is switched
on and off according to its drain-source voltage which, during conduction time, is the voltage
image of the current flowing through the power MOSFET. The measurement resolution is
not sufficient to appreciate the turn-off threshold voltage on power MOSFET drain but the
voltage step changes, corresponding to the power MOSFET turn-on and turn-off, can be
noted.
3.1 Power MOSFET turn-off compensation
Considering the SR power MOSFET R
DS(on)
and the SRK2000 turn-off threshold, turn-off
current can be approximately calculated as follows:
Equation 3
Actually, this calculation neglects many other factors like the IC driver propagation delay and
the R
DS(on)
deviation due to operative temperature. Furthermore, as previously discussed,
turn-off timing is heavily influenced by parasitic elements.
Because it is quite difficult to accurately estimate all these parameters, it is better to confront
this issue from a practical point of view. As seen in Figure 14, the power MOSFET turns off
when significant current is still flowing through it, which is diverted to the MOSFET body
diode. It can be worth delaying the turn-off to increase efficiency. This can be easily
Figure 13. Full load operation Figure 14. Full load operation - detail
CH1: SR FET drain
CH3: SR FET current
CH2: SR FET gate
CH4: primary current
CH1: SR FET drain
CH3: SR FET current
CH2: SR FET gate
CH4: primary current
I
OFF
V
DVS1 2OFF,
R
DS on()
--------------------------------=
_