The adaptive turn-off consists of a ZCD_OFF comparator, where the DVS signal is compared to an adapting

threshold. This threshold is adapted in such a way that, at steady state, the measured residual conduction time of

SR MOSFET body diode after turn-off meets the target value Tdiode_off (as shown in Figure 6. ZCD_OFF

threshold adapting for target body diode conduction). The residual conduction time of the body diode is measured

between the falling edge of the driving signal and the rising edge of the DVS signal (first time exceeding the

arming voltage VTH_A).

Figure 6. ZCD_OFF threshold adapting for target body diode conduction

ISR

Ip

DVS

GD

body diode residual conduction measurement

VTH_A

target Tdiode_off

VZCD_OFF

adapted

ZCD_OFF

threshold

TD_On

The internal timer turns off the SR MOSFET with a fixed anticipation time Tant_timer with respect to the first rising

edge of the DVS signal. The IC has two different user timer operating modes optimized for fixed frequency mixed

CCM/DCM converters or for QR/DCM applications. The selection of the proper timer mode is performed through a

100 pF capacitor across the TON pin:

•

if capacitor is present, mixed CCM/DCM operation is assumed

•

if capacitor is not present, QR/DCM operation is assumed.

5.4.1

Fixed frequency mixed DCM/CCM operation

In fixed frequency operation, SR MOSFET turn-off is triggered by ZCD_OFF adaptive mechanism at load levels

where DCM operation occurs, while it is triggered by timer at higher loads where CCM operation occurs. The

latter consists of turning off the SR MOSFET with a fixed lead time Tant_timer with respect to the first rising edge of

the DVS signal, based on a switching period TSW estimate, (over a few previous cycles) as illustrated in Figure 7.

Turn-off by the timer may also intervene during low-to-high load transients, where the ZCD_OFF threshold is in

the adapting phase (because of the output voltage drop occurring as a consequence of the transient), preventing

undesired current inversions.

Most flyback controllers use operating frequency modulation to help optimize the EMI filter. For correct operation

of the SRK1000 timer, the maximum rate of change of modulated frequency must be limited, so the switching

period increase from the current cycle to the following cycle is much shorter than the timer anticipation Tant_timer

(14 ns maximum). In fact, the timer anticipation adaptation during the switching period increase is limited to

Ttimer_step every 4 cycles, and with a switching period increase longer than 14 ns from one cycle to the next, the

timer turn-off lead time steadily increases (increasing the body diode conduction).

SRK1001

Adaptive turn-off and TIMER

DS13155 - Rev 1

page 11/27