OB2578T is an excellent primary side regulation power switch with CC/CV operation for medium level power AC/DC charger and adapter applications. The device operates in CCM/QR mode to provide high efficiency along with several functions of built-in protections. It removes the need for secondary feedback circuitry to lower the total bill of material cost. Proprietary Constant Voltage (CV) and Constant Current (CC) control is integrated as shown in the figure below.

In CV control, the controller changes the mode of operation according to line voltage and load condition. At full loading, the controller operates in fixed frequency CCM in low line voltage and quasi-resonant (QR) mode in high line voltage. The primary side regulation power supplies up to high power without the efficiency limitation of DCM or audible noise.

In CC control, OB2578T samples the primary-side average current and the demagnetization pulse to regulation the output current. The current and output power setting can be adjusted externally by the sense resistor Rs at CS pin.

OB2578T offers comprehensive protection coverage with auto-recovery feature including Cycle-by-Cycle current limiting, VDD OVP, OLP, SCP, OTP etc.

OB2578T consumes less than 75mW input power at no-load condition with high line voltage.

OB2578T is offered in SOP8 and DIP8 package.

Figure.1. Typical CC/CV Curve


n Primary-side sensing and regulation operates in CCM/QR mode without TL431 and opto-coupler

n High precision constant voltage and current regulation at universal AC input

n Fixed frequency (65kHz) CCM mode operation with low line voltage at full load

n Integrated 4A MOSFET

n Quasi-resonant operation for high efficiency in high line voltage

n Programmable CV and CC regulation

n Good dynamic response

n Built-in line compensation for tight CC regulation

n Built-in fixed cable compensation

n Built-in primary winding inductance compensation

n Built-in control loop compensation

n Built-in leading edge blanking (LEB)

n Ultra low start-up current and low operating current

n Comprehensive protection coverage with auto-recovery

l VDD over voltage protection (VDD OVP)

l VDD under voltage lockout with hysteresis (UVLO)

l Cycle-by-cycle current limiting

l Feedback open loop protection (OLP)

l  Output short circuit protection (SCP)


Medium level Power AC/DC offline SMPS for

n Cell phone charger

n Tablet PC

n AC/DC adapter

n Set-top box power supplies