Off-line Power Supply Controller

•

Transformerless Off-line

Power Supply

•

Wide 100VDC to 400VDC

Allowable Input Range

•

Fixed 5VDC or Adjustable

Low Voltage Output

•

Output Sinks 200mA, Sources

150mA Into a MOSFET Gate

•

Uses Low Cost SMD Inductors

•

Short Circuit Protected

•

Optional Isolation Capability

The UCC3888 controller is optimized for use as an off-line, low power, low voltage,

regulated bias supply. The unique circuit topology utilized in this device can be

visualized as two cascaded flyback converters, each operating in the discontinu-

ous mode, both driven from a single external power switch. The significant benefit

of this approach is the ability to achieve voltage conversion ratios as high as 400V

to 2.7V with no transformer and low internal losses.

The control algorithm utilized by the UCC3888 sets the switch on time inversely

proportional to the input line voltage and sets the switch off time inversely propor-

tional to the output voltage. This action is automatically controlled by an internal

feedback loop and reference. The cascaded configuration allows a voltage conver-

sion from 400V to 2.7V to be achieved with a switch duty cycle of 7.6%. This topol-

ogy also offers inherent short circuit protection since as the output voltage falls to

zero, the switch off time approaches infinity.

The output voltage is set internally to 5V. It can be programmed for other output

voltages with two external resistors. An isolated version can be achieved with this

topology as described further in Unitrode Application Note U-149.

UCC1888

UCC2888

UCC3888

3/97

FEATURES

DESCRIPTION

OPERATION

With reference to the application diagram below, when input voltage is first applied,

8.4V. At this time, DRIVE goes high turning on the power switch, Q1, and redirect-

voltage. This provides a constant (line voltage)

• (switch on time) product.

Note: This device incorporates patented technology used under license from Lambda Electronics, Inc.

TYPICAL APPLICATION

UDG-96013