Fairchild SG6902 service manual

www.fairchildsemi.com

AN-6902
Applying SG6902 to Control a CCM PFC and
Flyback/PWM Power Supply

Summary

This application note shows a step-by-step design to a
120W/24V power adapter. The equations also can be
applied to different output voltages and wattages.

Features

 Interleaved PFC/PWM Switching
 Green-Mode PFC/PWM Switching
 No PFC Switching at Light Loads for Power Saving
 Innovative Switching Charge Multiplier-divider
 Low Startup and Operating Current
 Innovative Switching Charge Multiplier-divider
 Multi-vector Control for Improved PFC Output

Transient Response

 Average-Current-Mode Control for PFC
 Programmable Two-Level PFC Output Voltage to

Achieve the Best Efficiency

 PFC Over-voltage and Under-voltage Protections
 PFC and PWM Feedback Open-loop Protection
 Cycle-by-cycle Current Limiting for PFC/PWM
 Slope Compensation for PWM
 Maximum Power Limit for PWM
 Brownout Protection
 Over Temperature Protection
 Power-on Sequence Control and Soft-start
 20-Pin SOP and SSOP Packages

Description

SG6902 is designed for power supplies that consist of boost
PFC and flyback PWM. It requires few external
components to achieve green-mode operation and versatile
protections and compensations.

The proprietary interleave switching synchronizes the PFC
and PWM stages and reduces switching noise. At light
loads, PFC stage is turned off to save power and the PWM
switching frequency is decreased in response to the load.

For PFC stage, the proprietary multi-vector control scheme
provides a fast transient response in a low-bandwidth PFC
loop. The overshoot and undershoot of the PFC voltage are
clamped. If the feedback loop is broken, SG6902 shuts off
the switching to protect the power supply and its load.

For the flyback PWM stage, the synchronized slope
compensation ensures the stability of the current loop.
“Hiccup” operation limits a maximum output power during
the overload situations.

The difference between members of this family are shown
in the table below.

Parameter SG6902 SG6901A

Start Threshold Voltage

Minimum Operating Voltage

The Interval of OPFC Lags
Behind OPWM at Startup

PFC On/Off

OTP

Soft-Start

16V 12V

10V 10V

11.5ms 11.5ms

O X

O O

O O

© 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.2.1 • 5/1/08

AN-6902 APPLICATION NOTE

Pin Configuration

Figure 1. Pin Configuration

Typical Application

Figure 2. Typical Application

© 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.2.1 • 5/1/08 2

AN-6902 APPLICATION NOTE

Block Diagram

Figure 3. Block Diagram

© 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.2.1 • 5/1/08 3

AN-6902 APPLICATION NOTE

PFC Section

Power-On Sequence

Because the capacitor includes ±20% variation, the
capacitor 100µF is chosen.

SG6902 is active when the line voltage is higher than the
brownout threshold. The PWM stage is switching first,
then, following an 11.5ms delay time after FBPWM
voltage is higher than a PFC turn-on threshold voltage,
the PFC stage is enabled.

PFC Inductor

The switching frequency fS, output power P

efficiency n, maximum ripple current ΔI, and minimum

input voltage V

should be defined before determining

IN.min

the inductance of PFC inductor. The following equations
are utilized to determine the inductance of the PFC
inductor. Normally the maximum ripple current is 20% ~
30% of maximum input current.

()

OUT

I×=Δ

V

3.0/P2

η

)MIN(IN

1D×−=

V

O

2V

min.IN

di

LV =

dt

max

fs/D

IN.min

L

××=2V

I

Δ

For a 120W adapter power, η= 0.85, V

= 65KHz, VO = 250V, ΔI = 0.66A, D = 0.49, L =

f

S

IN(MIN)

= 90V

0.4mH.

PFC Capacitor

An advantage of using interleaving switching of PFC and
PWM stage is to reduce the switching noise. The ESR
requirement of boost capacitor is relaxed. The boost
capacitor value is chosen to remain a hold-up time of
output voltage in the event line voltage is removed.

××

=

C

O

()

η

where V

is the minimum output voltage in accordance

O.min

with the requirement of the specification.

For a 120W power supply, the capacitor is determined as:

C

© 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.2.1 • 5/1/08 4

()

>

O

()

2

t)/(P2

up-holdPWMOUT

2

ripple)normal(O

ms1585.0/W1202

××

2

6020250

−−

2

VVV −−

min.O

F86

μ

=

(1)

(2)

(3)

(4)

AC,

(5)

(6)

OUT

,

Figure 4. Interleaving Switching

Boost Rectifier and Switch

The fast reverse-recovery time of the boost diode is
required to reduce the power losses and the EMI. A 500V
voltage rating is chosen to withstand 400V boosts
potential. The average current and peak currents flow
through the boost diode and the switch, respectively, and
are given by:

η

/P22

××

I

AVG

I

AVG

PEAK

PEAK

=

π

×

=

2I

2I

OUT

V

××

π

75

×

×=

V

75

OUT

8.0/120

)Brownout(RMS

8.0/12022
A8.1

=

η

/P

)Brownout(RMS

A82.2

=×=

(7)

Oscillation and Green Mode

The resistor RI connected from the RI to GND pin
programs the switching frequency of SG6902.

1560

=

f

S

()

KR

Ω

I

For example, a 24kΩ resistor R

()

KHz

(8)

results in a 65kHz

I

switching frequency. The recommended range for the
switching frequency is 33kHz ~ 100kHz.

SG6902 provides an off-time modulation to reduce the
switching frequency in light-load and no-load conditions.
The feedback voltage of FBPWM pin is taken as
reference. When the feedback voltage is lower than about

2.1V, the switching frequency decreases accordingly.
Most of losses in a switching-mode power supply are
proportional to the switching frequency; therefore, the
off-time modulation reduces the power consumption of
the power supply in light-load and no-load conditions.
For a typical case of R

= 24KΩ, the switching frequency

I

is 65kHz at nominal load and decreases to 20kHz at light
load. The switching signal is disabled if the switching
frequency falls below 20KHz, which avoids acoustic noise.

For stability reasons, a capacitor connecting the RI pin to
GND is not suggested.