Sensing optimization by waveform check

AN3303

14/27

Doc ID 18164 Rev 2

3

Sensing optimization by waveform check

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

current can be approximately calculated as follows:

Equation 3

Actually, this calculation neglects many other factors like the IC driver propagation delay and

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

()

--------------------------------

=

_