ACT337

Rev 2, 14-Nov-12

Innovative Power

TM

- 6 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

in a constant secondary side output current profile.

The energy transferred to the output during each

peak current, and η is the conversion efficiency.

From this formula, the constant output current can

be derived:

the nominal secondary output voltage.

The constant current operation typically extends

down to lower than 40% of nominal output voltage

regulation.

Primary Inductance Compensation

The ACT337 integrates a built-in proprietary

(patent-pending) primary inductance compensation

circuit to maintain constant current regulation

despite variations in transformer manufacturing.

The compensated range is ±7%.

Primary Inductor Current Limit Compensation

The ACT337 integrates a primary inductor peak

current limit compensation circuit to achieve

constant input power over line and load ranges.

Protection

The ACT337 incorporates multiple protection

functions including over-voltage, over-current and

over-temperature.

Output Short Circuit Protection

When the secondary side output is short circuited,

the ACT337 enters hiccup mode operation. In this

threshold

and

the

auxiliary

supply

voltage

collapses. This turns off the ACT337 and causes it

to restart. This hiccup behavior continues until the

short circuit is removed.

Output Over Voltage Protection

The

ACT337

includes

output

over-voltage

protection circuitry, which shuts down the IC when

the output voltage is 40% above the normal

regulation voltage for 4 consecutive switching

cycles. The ACT337 enters hiccup mode when an

output over voltage fault is detected.

Over Temperature Shutdown

The thermal shutdown circuitry detects the ACT337

die temperature. The typical over temperature

threshold is 135°C with 20°C hysteresis. When the

die temperature rises above this threshold the

ACT337 is disabled until the die temperature falls

by 20°C, at which point the ACT337 is re-enabled.

TYPICAL APPLICATION

Design Example

The design example below gives the procedure for

a DCM flyback converter using the ACT337. Refer

to Application Circuit in Figure 6, the design for a

charger application starts with the following

specification:

The operation for the circuit shown in Figure 6 is as

follows: the rectifier bridge D1−D4 and the capacitor

C1/C2 convert the AC line voltage to DC. This

voltage supplies the primary winding of the

transformer T1 and the startup resistor R7/R8. The

primary power current path is formed by the

transformer’s primary winding, the NPN transistor,

the ACT337 internal MOSFET and the current

sense resistor R9. The network consisting of

capacitor C4 and diode D6 provides a VDD supply

voltage for ACT337 from the auxiliary winding of the

transformer. C4 is the decoupling capacitor of the

supply voltage and energy storage component for

startup. The diode D8 and the capacitor C5 rectifies

and filters the output voltage. The resistor divider

consisting of R5 and R6 programs the output

voltage.

The minimum and maximum DC input voltages can

be calculated:

FUNCTIONAL DESCRIPTION CONT’D

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ ×

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

×

=

OUTCV

SW

2

CS

P

OUTCC

V

f

R

9

.

0

V

396

.

0

L

2

1

I

η

(2)

V

90

F

μ

10

2

%

77

)

ms

4

50

2

1

(

5

2

90

2

C

)

t

f

2

1

(

P

2

V

2

V

2

IN

C

L

OUT

2

ACMIN

INDCMIN

≈

×

×

−

×

×

−

×

=

×

−

−

=

η

(3)

V

375

265

2

V

2

V

ACMAX

INDCMAX

=

×

=

×

=

(4)

Input Voltage Range

90VAC - 265VAC, 50/60Hz

10.5W

5.0V

2.5A

2.1A

Transformer Efficiency,

0.92

System Efficiency CC,

0.76

System Efficiency CV,

η

0.77