L6566ATR datasheet(21/51 Pages) STMICROELECTRONICS

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L6566A

Application information

21/51

5.2

Zero current detection and triggering block; oscillator block

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

a negative-going edge falling below 50 mV is applied to the input (pin 11, ZCD). To do so the

triggering block must be previously armed by a positive-going edge exceeding 100 mV.

This feature is typically used to detect transformer demagnetization for QR operation, where

the signal for the ZCD input is obtained from the transformer’s auxiliary winding used also to

supply the L6566A. The triggering block is blanked for TBLANK = 2.5 µs after MOSFET’s

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

demagnetization from triggering the ZCD circuit erroneously.

The voltage at the pin is both top and bottom limited by a double clamp, as illustrated in the

internal diagram of the ZCD block of Figure 8 on page 20. The upper clamp is typically

located at 5.7 V, while the lower clamp is located at -0.4V. The interface between the pin and

the auxiliary winding will be a resistor divider. Its resistance ratio will be properly chosen

(see “Section 5.11: OVP block on page 35”) and the individual resistance values (RZ1, RZ2)

will be such that the current sourced and sunk by the pin be within the rated capability of the

internal clamps (

±3 mA).

At converter power-up, when no signal is coming from the ZCD pin, the oscillator starts up

the system. The oscillator is programmed externally by means of a resistor (RT) connected

from pin OSC (13) to ground. With good approximation the oscillation frequency fosc will be:

Equation 2

(with fosc in kHz and RT in kW). As the device is turned on, the oscillator starts immediately;

at the end of the first oscillator cycle, being zero the voltage on the ZCD pin, the MOSFET

will be turned on, thus starting the first switching cycle right at the beginning of the second

oscillator cycle. At any switching cycle, the MOSFET is turned off as the voltage on the

current sense pin (CS, 7) hits an internal reference set by the Line Feedforward block, and

the transformer starts demagnetization. If this completes (hence a negative-going edge

appears on the ZCD pin) after a time exceeding one oscillation period Tosc=1/fosc from the

previous turn-on, the MOSFET will be turned on again - with some delay to ensure minimum

voltage at turn-on – and the oscillator ramp will be reset. If, instead, the negative-going edge

appears before Tosc has elapsed, it will be ignored and only the first negative-going edge

after Tosc will turn-on the MOSFET and synchronize the oscillator. In this way one or more

drain ringing cycles will be skipped (“valley-skipping mode”, Figure 9) and the switching

frequency will be prevented from exceeding fosc.

osc

⋅

≈

Figure 9.

Drain ringing cycle skipping as the load is gradually reduced

Pin = Pin'

(limit condition)

Pin = Pin'' < Pin'

Pin = Pin''' < Pin''

VDS

TFW

Tosc

TON

VDS

Tosc

VDS

Tosc

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