The UCC2850x family provides all of the controlfunctions necessary for an active power-factorcorrectedpreregulator and a second-stage dc-to- dcconverter. The controller achieves near-unity powerfactor by shaping the ac input line current waveform tocorrespond to the ac input-line voltage using averagecurrent-mode control. The dc-to-dc converter usespeak current-mode control to perform the step-downpower conversion.
The PFC stage is leading-edge modulated while thesecond stage is trailing-edge synchronized to allow forminimum overlap between the boost and PWMswitches. This reduces ripple current in the bulk-outputcapacitor.In order to operate with over three-to-onerange of input-line voltages, a line feedforward (VFF) isused to keep input power constant with varying inputvoltage. Generation of VFF is accomplished using IAC inconjunction with an external single-pole filter. This notonly reduces external parts count, but also avoids theuse of high-voltage components, offering a lower-costsolution. The multiplier then divides the line current bythe square of VFF.
The UCC2850x PFC section incorporates a lowoffset-voltage amplifier with 7.5-V reference, ahighly-linear multiplier capable of a wide current range,a high-bandwidth, low offset-current amplifier, with anovel noise-attenuation configuration, PWMcomparator and latch, and a high-current output driver.Additional PFC features include over-voltageprotection, zero-power detection to turn off the outputwhen VAOUT is below 0.33 V and peak current andpower limiting.
The dc-to-dc section relies on an error signal generatedon the secondary-side and processes it by performingpeak current mode control. The dc-to-dc section alsofeatures current limiting, a controlled soft-start, presetoperating range with selectable options, and 50%maximum duty cycle.
The UCC28500 and UCC28502 have a wide UVLOthreshold (16.5 V/10 V) for bootstrap bias supplyoperation. The UCC28501 and UCC28503 aredesigned with a narrow UVLO range (10.5 V/10 V) moresuitable for fixed bias operation. The UCC28500 andUCC28501 have a narrow UVLO threshold for PWMstage (to allow operation down to 75% of nominal bulkvoltage), while the UCC28502 and UCC38503 areconfigured for a much wider operation range for thePWM stage (down to 50% of bulk nominal voltage).
Available in 20-pin N and DW packages.
Combines PFC and Downstream Converter Controls
Controls Boost Preregulator to Near-Unity Power Factor
Accurate Power Limiting
Improved Feedforward Line Regulation
Peak Current-Mode Control in Second Stage
Leading-Edge/Trailing-Edge Modulation for Reduced Output Ripple