RT7302

4

DS7302-03 May 2015

www.richtek.com

©

Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.



IN_PK

ON

L_PK

m

Vsin(θ)t

I

=

L

Figure 2. Inductor Current of CRM with Constant

On-Time Control

If the input voltage is the output voltage of the full-bridge

θ)), the

following equation :

When the converter operates in CRM with constant on-

time control, the envelope of the peak inductor current

will follow the input voltage waveform with in-phase. Thus,

high power factor can be achieved, as shown in Figure 2.

Peak Inductor Current

MOSFET Current

Input Voltage

Average Input Current

Output Diode Current

Operation

Critical-Conduction Mode (CRM) with Constant

On-Time Control

Figure 1. Typical Flyback Converter



IN

L_PK

ON

m

V

I

=

t

L

Primary-Side Constant-Current Regulation

The RT7302 needs no shunt regulator and opto-coupler

at the secondary side to achieve the output current

regulation. Figure 3 shows several key waveforms of a

conventional flyback converter in Quasi-Resonant (QR)

of the transformer.

Figure 3. Key Waveforms of a Flyback Converter

Voltage Clamping Circuit

The RT7302 provides a voltage clamping circuit at ZCD

pin since the voltage on the auxiliary winding is negative

when the main switch is turned on. The lowest voltage on

ZCD pin is clamped near zero to prevent the IC from being

damaged by the negative voltage. Meanwhile, the sourcing

signal for propagation delay compensation. The RT7302

embeds the programmable propagation delay

compensate the propagation delay effect. Thus, the output

current can be equal at high and low line voltage.

TX1

+

-

Q1

+

GD

0

0

Clamped by

controller