Figure 7. Timer anticipation for fixed frequency CCM operation (100pF mounted on TON pin)

ISR

Ip

DVS

GD

VTH_A

TSW

TSW – Tant_timer

Timer turn-off

anticipation

referred to

previous cycle

5.4.2

QR operation (with valley skipping)

In QR flyback applications, the circuit works at variable frequency and, after transformer demagnetization, a

resonance occurs due to primary inductance and total parasitic capacitance across primary switch. In this case, at

steady state, SR MOSFET turn-off is triggered by the ZCD_OFF adaptive mechanism.

During load transitions or during CC regulation operation in which output voltage may decrease, turn-off by the

adaptive ZCD_OFF comparator would be too late as the threshold needs to adapt to the new slope of the current

flowing into the SR MOSFET, while turn-off by internal timer prevents current inversion.

Timer for fixed frequency CCM is not suitable in QR operation, which inherently operates at variable frequency. In

this case, the timer operating mode consists of turning off the SR MOSFET with a fixed anticipation time Tant_timer

with respect to the first rising edge of the DVS signal, based on the duration of the previous demagnetization time

of transformer Tdem, as shown in Figure 8.

Looking at Figure 8, during the first two switching cycles (steady state operation), the turn-off is triggered by the

ZCD_OFF comparator. This is because the DVS signal reaches the adapted ZCD_OFF threshold before the timer

OFF event as the target diode Tdiode_off is larger than timer anticipation Tant_timer. In the third switching cycle, as a

consequence of the transient, the turn-off is instead triggered by timer, since current slope has decreased and the

ZCD_OFF threshold would be reached too late. In the following cycle (not shown in figure), the timer stabilizes the

anticipation to the duration of the demagnetization period of the third cycle.

Figure 8. Timer anticipation for QR operating circuit (no capacitor mounted on TON pin)

ISR

Ip

DVS

GD

VTH_A

Tdiode_off

Tdiode_off

Tdem

Tdem – Tant_timer

Timer OFF anticipation

Tzc

Tzc

Tzc =

Lstray

Rds_ON

Tant_timer

VZCD_OFF

adapted

ZCD_OFF

threshold

SRK1001

Adaptive turn-off and TIMER

DS13155 - Rev 1

page 12/27