FAN6206 datasheet(10/15 Pages) FAIRCHILD

Part #	FAN6206

Description	Highly Integrated Dual-Channel Synchronous Rectification Controller

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

Direct Link	http://www.fairchildsemi.com

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FAN6206 • Rev. 1.0.2

10

Function Description

Figure 29 and Figure 30 show the simplified circuit

diagram of a dual-forward converter and its key

waveforms. Switches Q1 and Q2 are turned on and off

together. Once Q1 and Q2 are turned on, input voltage is

applied across the transformer primary side and power

is delivered to the secondary side through the

transformer, powering D1. During this time, the

magnetizing current linearly increases. When Q1 and Q2

are turned off, the magnetizing current of the

transformer forces the reset diodes (DR1 and DR2) and

negative input voltage is applied across the transformer

primary side. During this time, magnetizing current

linearly decreases to zero and the secondary-side

inductor current freewheels through diode D2. When

synchronous rectifier SR1 and SR2 are used instead of

diodes D1 and D2, it is important to have proper timing

between drive signals for SR1 and SR2.

Figure 29.Simplified Circuit Diagram of

Dual-Forward Converter

Figure 30.Key Waveforms of Dual-Forward

Converter

Figure 31 shows a typical application circuit. When a

dual-forward

converter

operates

in

continuous

conduction mode, the SR gate signals (GATE1 and

GATE2) are mainly controlled by SP and SN signals. SP

and SN signals are transferred through a pulse

transformer from XP and XN signals, which are

generated by FAN6210 (Primary-Side Synchronous

Rectifier Signal Trigger for Dual Forward Converter).

Figure 31.Typical Application Circuit

Figure 32 shows the timing diagram for continuous

conduction mode (CCM). Figure 33 shows the timing

diagram for discontinuous conduction mode (DCM).

The switching operation of SR MOSFETs Q3 and Q4 is

determined by the SN and SP signals. FAN6206 turns

on SR MOSFETs at the rising edge of the SP signal,

while it turns off the SR MOSFETs at the rising edge of

the SN signal. Within one switching cycle, SP and SN

are obtained two times.

With a voltage divider R1 and R2 connected from LPC1

to secondary winding, R3 and R4 connected from LPC2

to secondary winding, the PWM timing sequences and

frequency can be tracked precisely. The SR MOSFET is

turned on by SP signal only when the voltage level on

LPC1 or LPC2 pin is pulled LOW to GND.

During PWM-on period, the rectifying SR Q3 is turned on

by the rising edge of the SP signal after a propagation

delay (tPD-HIGH-SP1) and Q3 is turned off by the rising edge

of the SN signal after a propagation delay (tPD-LOW-SN1).

During PWM-off period, the freewheeling SR Q4 is

turned on by the rising edge of the SP signal after a

propagation delay (tPD-HIGH-SP2) and Q4 is turned off by

the rising edge of the SN signal after a propagation

delay (tPD-LOW-SN2) in CCM operation.

In DCM operation, the proprietary Linear-Predict Timing

Control (LPC) technique can provide synchronous

rectification control mechanism for freewheeling SR

MOSFET. Since SN signal is sent following with PWM

signal, the freewheeling SR MOSFET cannot be turned

off in time by SN signal before ILo linearly decreases to

zero. Therefore, the LPC mechanism is applied to turn

off Q3 in DCM mode.

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