The UCC28740 isolated-flyback power-supply controller provides Constant-Voltage (CV)
using an optical coupler to improve transient response to large-load steps. Constant-Current (CC)
regulation is accomplished through Primary-Side Regulation (PSR) techniques. This device processes
information from opto-coupled feedback and an auxiliary flyback winding for precise
high-performance control of output voltage and current.
An internal 700-V startup switch, dynamically controlled operating states, and a tailored
modulation profile support ultra-low standby power without sacrificing startup time or output
Control algorithms in the UCC28740 allow operating efficiencies to meet or exceed
applicable standards. The drive output interfaces to a MOSFET power switch. Discontinuous
conduction mode (DCM) with valley-switching reduces switching losses. Modulation of switching
frequency and primary current-peak amplitude (FM and AM) keeps the conversion efficiency high
across the entire load and line ranges.
The controller has a maximum switching frequency of 100 kHz and always maintains control
of the peak-primary current in the transformer. Protection features keep primary and secondary
component stresses in check. A minimum switching frequency of 170 Hz facilitates the achievement of
less than 10-mW no-load power.