Application information

HVLED007

22/33

DS12866 Rev 1

Equation 15

Whatever the design choice is, when designing the flyback transformer make sure that its

Equation 16

4.8

Zero current detection and triggering block (pin ZCD); starter

The Zero Current Detection (ZCD) and Triggering blocks switch on the external MOSFET if

a negative-going edge falling below 200 mV is applied to the ZCD pin. To do so, the

triggering block must be previously armed by a positive-going edge exceeding 300 mV. The

voltage at the pin is bottom limited by a clamp that holds the voltage on the pin close to zero

during the ON-time of the external power switch.

This arm/trigger operation is typically used to detect transformer demagnetization for TM

operation, i.e. close to the boundary between the discontinuous and the continuous

to make the turn-on instant as close as possible to the point where the drain ringing is at a

minimum (valley switching).

The signal for the ZCD input is obtained from the transformer's auxiliary winding, typically

used also to power the HVLED007. The triggering block is blanked for at least 1 μs after

MOSFET's turn-off to prevent any negative-going edge that follows leakage inductance

demagnetization from triggering the ZCD circuit erroneously (see "Section 4.9: Overload

and short-circuit protection (OCP function)").

The interface between the pin and the auxiliary winding is a resistor divider. Its resistance

ratio is properly chosen (see "Section 4.10: Overvoltage protection (OVP function)") and the

of the power switch is within the rated capability of the internal clamp (2.5 mA):

Equation 17

At converter power-up, as the IC exits from UVLO, no signal is coming from the ZCD pin,

thus an internal oscillator ("Start Timer"), which is active as long as the gate driver (pin GD)

is low, sets GD high as its timeout elapses and enables the converter to start up. Switching

cycles is initiated by the internal starter until the output voltage and, then, the voltage

Rs

-------------------------- 1

2

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

2

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

–







1.14

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





Rs

---------------------------- = 1.4

Rs

---------



