STRF6624 datasheet(8/16 Pages) ALLEGRO

8 / 16 page

Series STR-F6600

OFF-LINE

QUASI-RESONANT FLYBACK

SWITCHING REGULATORS

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

™

Functional Description and Operation (cont’d)

Figure 7 - Voltage Regulation Waveforms

Normal Operation (Quasi-Resonant) Mode

Refer to the Functional Block Diagram, Typical Appli-

cation diagram (figure 6), and Quasi-Resonance Wave-

forms (figure 8).

Regulation is achieved as in fixed off-time mode but

instead of having a fixed off-time, the demagnetization of

the transformer is sensed by a second comparator. This

comparator threshold, V

th(2) is nominally 1.45 V. Quasi-

resonance sensing makes use of the natural magnetizing

and leakage inductances and self-capacitances of the

power circuit.

Figure 8 shows the drain voltage waveform, (V

DS), on

pin 3 of the STR-F66xx, as well as V

P, the voltage on the

primary of the transformer.

Once the current in the output diode stops flowing, the

primary stored energy ‘rings’ as shown by V

P and VDS.

The resonant frequency (fr) is determined by the magne-

tizing inductance of the transformer and the capacitor C4.

The addition of this capacitor sets the ringing frequency

and reduces the harmonic content in the V

DS waveform,

lowering EMI. Also since V

DS falls to a minimum during

the first half-cycle of the ring this point can be sensed and

used to turn on the MOSFET with minimum voltage

across it. Thus the MOSFET is low voltage and zero

current switched (LVS/ZCS).

Figure 8 – Quasi-Resonance Waveforms

Dwg. GK-021

VDS

t =

π√LpC4

f R = 1/2

π√LpC4

VDS (min)

VIN

VOCP

≈ 2.8 V

Vth(2)

≈ 1.45 V

Vth(1)

≈ 0.73 V

VFDBK

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