DP2525B datasheet(7/11 Pages) ETC

7 / 11 page

DP2525

Low Cost Primary Side Regulation(PSR) PWM Power Switch

7/11

Rev.2.0

PERATION DESCRIPTION

DP2525 is a high performance, multi mode, highly

integrated DCM (Discontinuous Conduction Mode)

Primary Side Regulation (PSR) controller. The built-

in high precision CV/CC control with high level

protection features makes it suitable for offline

small power converter applications.

System Start-Up Operation

Before the IC starts to work, it consumes only

startup current (typically 3uA) which allows a large

value startup resistor to be used to minimize the

power loss and the current flowing through the

startup resistor charges the VDD hold-up capacitor

from the high voltage DC bus. When VDD reaches

UVLO turn-on voltage of 18V (typical), DP2525

begins switching and the IC operation current is

increased

1mA

(typical).

The

hold-up

capacitor continues to supply VDD before the

auxiliary winding of the transformer takes the

control of VDD voltage.

Fig.1

Once DP2525 enters very low frequency FM

(Frequency

Modulation)

mode,

the

operating

current is reduced to be 0.8mA typically, which

helps to reduce the standby power loss.

PSR

Constant

Voltage

Modulation

(PSR-CVM)

In primary side control, the output voltage is sensed

on the auxiliary winding during the transfer of

transformer

energy

the

secondary.

Fig.2

illustrates the CV sampling signal timing waveform

in DP2525. As shown in Fig.2, it is clear that there

is a down slope representing a decreasing total

rectifier Vf and its voltage drop as the secondary

current decreases to zero. To achieve an accurate

representation of the secondary output voltage on

the auxiliary winding, the CV sampling signal blocks

the leakage inductance reset and ringing. When the

sampling

process

over,

the

internal

sample/hold (S&H) circuit captures the error signal

and amplifies it through the internal Error Amplifier

(EA). The output of EA is sent to the Primary Side

Constant Voltage Modulator (PSR-CVM) for CV

control. The internal reference voltage for EA is

trimmed to 2V with high accuracy.

During the CV sampling process, an internal

variable current source is flowing to FB pin for

Cable Drop Compensation (CDC). Thus, there is a

step at FB pin in the transformer demagnetization

process, as shown in Fig.2. Fig.2 also illustrates the

equation for “demagnetization plateau”, where Vo

and VF is the output voltage and diode forward

voltage; R1 and R2 is the resistor divider connected

from the auxiliary winding to FB Pin, Ns and Na are

secondary

winding

and

auxiliary

winding

respectively.

When system enters over load condition, the output

voltage falls down and the FB sampled voltage

should be lower than 2V internal reference which

makes system enter CC Mode automatically.

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