TY72011 查看數據表（PDF） - ON Semiconductor

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TY72011

Single Ended PWM Controller Featuring QR Operation and Soft Frequency Foldback

ON Semiconductor 

TY72011AP2

Drain Level

Valley

Switching

Possible Demag

65 mV

Auxiliary Level

0V

2

4 ms

IP = 0

Restart when Demag is too low

750.0 U

754.0 U

758.0 U

762.0 U

766.0 U

Figure 6.

Zero Crossing Detector

To detect the zero primary current, we make use of an

auxiliary winding. By coupling this winding to the primary,

we have a voltage image of the flux activity in the core.

Figure 6 details the shape of the signal in BCM.

The auxiliary winding for demagnetization needs to be

wired in FORWARD mode only. As Figure 6 depicts, when

the MOSFET closes, the auxiliary winding delivers

(Naux/Np . Vin). At the switch opening, we couple the

auxiliary winding to the main output power winding and

thus deliver: (-Naux/Ns . Vout). When DCM occurs, the

ringing also takes place on the auxiliary winding. As soon as

the level crosses-up the internal reference level (65 mV), a

signal is internally sent to re-start the MOSFET. Three

different conditions can occur:

ąă1.

In BCM, every time the 65 mV line is crossed, the

switch is immediately turned-on. By accounting for

the internal Demag pin capacitance (10-15 pF

typical), you can introduce a fixed delay, which,

combined to the propagation delay, allows to precisely

re-start in the drain-source valley (minimum voltage

to reduce capacitive losses).

ąă2.

When the IC enters VFM, the VCO delivers a pulse

which is internally latched. As soon as the

demagnetization pulse appears, the logic re-starts the

MOSFET.

ąă3.

As can be seen from Figure 6, the parasitic oscillations

on the drain are subject to a natural damping, mainly

imputed to ohmic losses. At a given point, the demag

activity on the auxiliary winding becomes too low to

be detected. To avoid any re-start problem, the

TY72011 features an internal 4.0 ms timeout delay.

This timeout runs after each demag pulse. If within

4.0 ms further to a demag pulse no activity is detected,

an internal signal is combined with the VCO to actually

re-start the MOSFET (synchronized with Ct).

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