L6566A_08 Datasheet, PDF (21/51 Pages) STMicroelectronics – Multi-mode controller for SMPS with PFC front-end

L6566A
Application information
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 T_BLANK = 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 -0.4 V. 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 (R_Z1, R_Z2)
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 (R_T) connected
from pin OSC (13) to ground. With good approximation the oscillation frequency f_osc will be:
Equation 2
f_osc
≈
2 ⋅ 10 ³
R_T
(with f_osc in kHz and R_T 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 T_osc=1/f_osc 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 T_osc has elapsed, it will be ignored and only the first negative-going edge
after T_osc 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 f_osc.
Figure 9. Drain ringing cycle skipping as the load is gradually reduced
VDS
VDS
VDS
TON
TFW
TV
Tosc
Pin = Pin'
(limit condition)
t
Tosc
Pin = Pin'' < Pin'
t
t
Tosc
Pin = Pin''' < Pin''
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