UCC38500

The UCC18500 family provides all of the functions necessary for an active power factor corrected preregulator and a second stage DC-to-DC converter. The controller achieves near-unity power factor by shaping the AC input line current waveform to correspond to the AC input line voltage using average current mode control. The DC-to-DC converter .

uses peak current mode control to perform the step down power conversion.
The PFC stage is leading edge modulated while the second stage is trailing edge synchronized to allow for minimum overlap between the boost and PWM switches. This reduces ripple current in the bulk output capacitor.

In order to operate with a three to one range of input line voltages, a line feedforward (V_FF) in used to keep input power constant with varying input voltage. Generation of V_FF is done using IAC in conjunction with an external single pole filter. This not only reduces external parts count, but avoids the use of high voltage components offering a lower cost solution. The multiplier then divides the line current by the square of V_FF.

The UCC18500 PFC section incorporates a low offset voltage amplifier with 7.5V reference, a highly linear multiplier capable of a wide current range, a high bandwidth, low offset current amplifier, with a novel noise attenuation configuration, PWM comparator and latch and a high current output driver. Additional PFC features include over-voltage protection, zero power detection to turn-off the output when VAOUT is below 0.25V and peak current and power limiting.

The DC-to-DC section relies on an error signal generated on secondary-side and processes it by performing peak current mode control. The DC-to-DC section also features current limiting, a controlled soft-start, preset operating range with selectable options, and 50% maximum duty cycle.

The UCC38500 and UCC38502 have a wide PFC-UVLO threshold (16.5V/10V) for bootstrap bias supply operation. The UCC38501 and UCC38503 are designed with a narrow UVLO range (10.5V/10V) more suitable for fixed bias operation. The UCC38500 and UCC38501 have a narrow UVLO threshold for PWM stage (to allow operation down to 75% of nominal bulk voltage), while the UCC38502 and UCC38503 are configured for a much wider operation range for the PWM stage (down to 50% of bulk nominal voltage).