UCC28050, UCC28051

UCC38050, UCC38051

SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009

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BLOCK DESCRIPTION (continued)

Zero Power Block

the COMP pin to about 2.4 V. This ensures that the zero power comparator is not activated during transient

behavior (when the slew rate enhancement circuitry is enhanced).

Multiplier Block

which is obtained by a resistive divider from the rectified line. The multiplier output is approximately

0.67

improves the zero-crossing distortion and hence the THD performance of the controller in the application. The

dynamic range of the inputs can be found in the electrical characteristics table.

Overvoltage Protection (OVP) Block

The OVP feature in the part is not activated under most operating conditions because of the presence of the

slew rate enhancement circuitry present in the error amplifier. As soon as the output voltage reaches to about

5% to 7% above the nominal value, the slew rate enhancement circuit is activated and the error amplifier output

voltage is pulled below the dynamic range of the multiplier block. This prevents further rise in output voltage.

If the COMP pin is not pulled low fast enough, and the voltage rises further, the OVP circuit acts as a second

line of protection. When the voltage at the VO_SNS pin is more than 7.5% of the nominal value

of the nominal value designed for, and protects the switch and other components of the system like the boost

capacitor.

Transition Mode Control

The boost converter, the most common topology used for power factor correction, can operate in two modes

– continuous conduction code (CCM) and discontinuous conduction mode (DCM). Transition mode control, also

referred to as critical conduction mode (CRM) or boundary conduction mode, maintains the converter at the

boundary between CCM and DCM by adjusting the switching frequency.

The CRM converter typically uses a variation of hysteretic control with the lower boundary equal to zero current.

It is a variable frequency control technique that has inherently stable input current control while eliminating

reverse recovery rectifier losses. As shown in Figure 1, the switch current is compared to the reference signal

(output of the multiplier) directly. This control method has the advantage of simple implementation and still can

provide very good power factor correction.