ACT337 Просмотр технического описания (PDF) - Active-Semi, Inc

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ACT337

High Performance ActivePSR™ Primary Switching Regulator

Active-Semi, Inc 

ACT337

Rev 2, 14-Nov-12

FUNCTIONAL DESCRIPTION

As shown in the Functional Block Diagram, to

regulate the output voltage in CV (constant voltage)

mode, the ACT337 compares the feedback voltage

at FB pin to the internal reference and generates an

error signal to the pre-amplifier. The error signal,

after filtering out the switching transients and

compensated with the internal compensation

network, modulates the external NPN transistor

peak current at CS pin with current mode PFWM

(Pulse Frequency and Width Modulation) control.

To regulate the output current in CC (constant

current) mode, the oscillator frequency is modulated

by the output voltage.

SW is a driver output that drives the emitter of an

external high voltage NPN transistor. This base-

emitter-drive method makes the drive circuit the

most efficient.

Fast Startup

VDD is the power supply terminal for the ACT337.

During startup, the ACT337 typically draws only

25μA supply current. The startup resistor from the

rectified high voltage DC rail supplies current to the

base of the NPN transistor. This results in an

amplified emitter current to VDD through the SW

pin via Active-Semi's proprietary fast-startup

circuitry until it exceeds the VDDON threshold 19V. At

this point, the ACT337 enters internal startup mode

with the peak current limit ramping up in 10ms.

After switching starts, the output voltage begins to

rise. The VDD bypass capacitor must supply the

ACT337 internal circuitry and the NPN base drive

until the output voltage is high enough to sustain

VDD through the auxiliary winding. The VDDOFF

threshold is 5.5V; therefore, the voltage on the VDD

capacitor must remain above 5.5V while the output

is charging up.

Constant Voltage (CV) Mode Operation

In constant voltage operation, the ACT337 captures

the auxiliary flyback signal at FB pin through a

resistor divider network R5 and R6 in Figure 6. The

signal at FB pin is pre-amplified against the internal

reference voltage, and the secondary side output

voltage is extracted based on Active-Semi's

proprietary filter architecture.

This error signal is then amplified by the internal

error amplifier. When the secondary output voltage

is above regulation, the error amplifier output

voltage decreases to reduce the switch current.

When the secondary output voltage is below

regulation, the error amplifier output voltage

increases to ramp up the switch current to bring the

secondary output back to regulation. The output

regulation voltage is determined by the following

relationship:

VOUTCV

=

2 .20 V

× ⎜⎜⎝⎛1 +

R FB 1

R FB 2

⎟⎟⎠⎞ ×

NS

NA

− VD

(1)

where RFB1 (R5) and RFB2 (R6) are top and bottom

feedback resistor, NS and NA are numbers of

transformer secondary and auxiliary turns, and VD

is the rectifier diode forward drop voltage at

approximately 0.1A bias.

Standby (No Load) Mode

In no load standby mode, the ACT337 oscillator

frequency is further reduced to a minimum

frequency while the current pulse is reduced to a

minimum level to minimize standby power. The

actual minimum switching frequency is

programmable with an output preload resistor.

Loop Compensation

The ACT337 integrates loop compensation circuitry

for simplified application design, optimized transient

response, and minimal external components.

Output Cable Resistance Compensation

The ACT337 provides programmable output cable

resistance compensation during constant voltage

regulation, monotonically adding an output voltage

correction up to predetermined percentage at full

power. There are four levels to program the output

cable compensation by connecting a resistor (R10

in Figure 6) from the SW pin to VDD pin. The

percentage at full power is programmable to be 3%,

6%, 9% or 12%, and by using a resistor value of

300k, 150k, 75k or 33k respectively. If there is no

resistor connection, there is no cord compensation.

This feature allows for better output voltage

accuracy by compensating for the output voltage

droop due to the output cable resistance.

Constant Current (CC) Mode Operation

When the secondary output current reaches a level

set by the internal current limiting circuit, the

ACT337 enters current limit condition and causes

the secondary output voltage to drop. As the output

voltage decreases, so does the flyback voltage in a

proportional manner. An internal current shaping

circuitry adjusts the switching frequency based on

the flyback voltage so that the transferred power

remains proportional to the output voltage, resulting

Innovative PowerTM

-5-

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

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