VIPER15HDTR datasheet(22/40 Pages) STMICROELECTRONICS

22 / 40 page

Operation description

VIPER15

22/40

Doc ID 15455 Rev 5

7.7

Frequency foldback function and valley skipping mode

The switching frequency, in Quasi Resonant mode, is not fixed and it depends on both the

load and the converter’s input voltage. The switching frequency increases when the load

decreases, or when the input voltage mains increases, and vice versa. In principle it could

reach an infinite value. To avoid that, the VIPER15 taps the maximum switching frequency of

the application by its control logic.

The frequency limit is realized with an internal oscillator switching at 136 kHz for VIPER15L

or at 225 kHz for the VIPER15H, sees the parameter FOSClim on Table 8 on page 8. This

oscillator is synchronized with power MOSFET turn-on. When the power MOSFET is off, if

the first negative-going edge voltage of the ZCD pin, resulting from transformer’s

demagnetization, appears after at least one oscillator cycle has been completed, the

MOSFET is turned ON and the oscillator re-synchronized.

Otherwise, if the first negative-going edge voltage appears before completing one oscillator

cycle, the signal is ignored. Due to the ringing of the drain voltage, the ZCD pin will

experience another positive-going edge voltage that arms the circuit and a subsequent

negative-going edge voltage. Again, if this appears before the oscillator cycle is complete, it

is ignored, otherwise the MOSFET is turned ON and the oscillator re-synchronized. In this

way, one or more drain ringing cycles will be skipped (Figure 29 on page 22 shows the so

called “valley-skipping mode”) and the switching frequency will be prevented from exceeding

the limit FOSClim.

Figure 29.

Drain ringing cycle skipping as the load is progressively reduced

When the system operates in valley skipping-mode, uneven switching cycles may be

observed under some line/load conditions, due to the fact that the OFF-time of the power

MOSFET is allowed to change with discrete steps of one ringing cycle, while the OFF-time

needed for cycle-by-cycle energy balance could fall in between. Thus one or more longer

switching cycles will be compensated by one or more shorter cycles and vice versa. This

mechanism is natural and there is no appreciable effect on the converter’s performances

and on its output voltage.

The operation described so far does not consider the blanking time TBLANK after power

MOSFET's turn OFF. Actually TBLANK does not come into play as long as the following

condition is met:

Equation 2

where D is the MOSFET duty cycle. If this condition is not met, the time during which

MOSFET's turn-ON is inhibited is extended beyond TOSClim by a fraction of TBLANK. As a

consequence, the maximum switching frequency will be a little lower than the internal limit

set by the oscillator and valley-skipping mode will take place slightly earlier than expected.

Pin=Pin'

(limit condition)

Pin=Pin''< Pin'

Pin =Pin''' <Pin''

TFW

Tosc

TON

Tosc

lim

OSC

BLANK

lim

OSC

BLANK

•

−

≤

Obsolete

Product(s)

- Obsolete

Product(s)

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