The UC2854B is designed as a pin compatible upgrade to the industry standard UC2854 active power factor correction circuits. The circuit enhancements allow the user to eliminate in most cases several external components currently required to successfully apply the UC2854. In addition, linearity improvements to the multiply, square and divide circuitry optimizes overall system performance. Detailed descriptions of the circuit enhancements are provided below. For in-depth design applications reference data see the application notes, UC2854 Controlled Power Factor Correction Circuit Design (SLUA144) and UC2854A and UC2854B Advanced Power Factor Correction Control ICs (SLUA177).

8.3.1 Multiply/Square and Divide

The UC2854B multiplier design maintains the same gain constant as the UC2854. The relationship between the inputs and output current is given as:

Equation 1.

This is nearly the same as the UC2854, but circuit differences have improved the performance and application.

The first difference is with the IAC input. The UC2854B regulated this pin voltage to the nominal 500 mV over the full operating temperature range, rather than the 6 V used on the UC2854. The low offset voltage eliminates the need for a line zero crossing compensating resistor to VREF from IAC that UC2854 designs require. The maximum current at high line into Iac should be limited to 250 μA for best performance.

Therefore, if V_VAC(max) = 270 V,

Equation 2.

The V_RMS pin linear operating range is improved with the UC2854B as well. The input range for VRMS extends from 0 V to 5.5 V. Since the UC2854A squaring circuit employs an analog multiplier, rather than a linear approximation, accuracy is improved, and discontinuities are eliminated. The external divider network connected to VRMS should produce 1.5 V at low line (85 VAC). This puts 4.77 V on VRMS at high line (27 VAC) which is well within its operating range.

The voltage amplifier output forms the third input to the multiplier and is internally clamped to 6 V. This eliminated an external zener clamp often used in UC2854 designs. The offset voltage at this input to the multiplier has been raised on the UC2854A/B to 1.5 V.

The multiplier output pin, which is also common to the current amplifier non-inverting input, has a −0.3 V to 5 V output range, compared to the −0.3 V to 2.5 V range of the UC2854. This improvement allows the UC2854B to be used in applications where the current sense signal amplitude is large.

8.3.2 Voltage Amplifier

The UC2854B voltage amplifier design is essentially similar to the UC2854 with two exceptions. The first is with the internal connection. The lower voltage reduces the amount of charge on the compensation capacitors, which provides improved recovery form large signal events, such as line dropouts, or power interruption. It also minimizes the dc current flowing through the feedback. The output of the voltage amplifier is also changes. In addition to a 6-V temperature compensated clamp, the output short circuit current has been lowered to 2 mA typical and an active pull down has replaced the passive pull down of the UC2854.

8.3.3 Current Amplifier

The current amplifier for an average current PFC controller needs a low offset voltage in order to minimize AC line current distortion. With this in mind, the UC2854B current amplifier has improved the input offset voltage from ±4 mV to 0 V to ±3 mV. The negative offset of the UC2854B assures that the PWM circuit will not drive the MOSFET is the current command is zero (both current amplifier inputs zero.) Previous designs required an external offset cancellation network to implement this key feature. The bandwidth of the current amplifier has been improved as well to 5 MHz typical. While this is not generally an issue at 50-Hz or 60-Hz inputs, it is essential for 400-Hz input avionics applications.

8.3.4 Miscellaneous

Several other important enhancements have been implemented in the UC2854B. A V_CC supply voltage clamp at 20 V allows the controller to be current fed if desired. The lower startup supply current (250 mA typical), substantially reduces the power requirements of an offline startup resistor. The 10.5 V/10 V UVLO option (UC2854B) enables the controller to be powered off of an auxiliary 12-V supply.

The VREF GOOD comparator assures that the MOSFET driver output remains low if the supply of the 7.5 V reference are not yet up. This improvement eliminates the need for external Schottky diodes on the PKLMT and Mult Out pins that some UC2854 designs require. The propagation delay of the disable feature has been improved to 300 ns typical. This delay was proportional to the size of the VREF capacitor on the UC2854 and is typically several orders of magnitude slower.