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UCC38051 Datasheet(PDF) 9 Page - Texas Instruments |
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UCC38051 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 32 page UCC28050, UCC28051 UCC38050, UCC38051 SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009 9 www.ti.com BLOCK DESCRIPTION (continued) Zero Power Block When the output of the gM amplifier goes below 2.3 V, the zero power comparator latches the gate drive signal low. The slew rate enhancement circuitry of the gM amplifier that is activated during overvoltage conditions slews 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 The multiplier block has two inputs. One is the error amplifier output voltage (VCOMP), while the other is VMULTIN which is obtained by a resistive divider from the rectified line. The multiplier output is approximately 0.67 × VMULTIN × (VCOMP−2.5 V). There is a positive offset of about 75 mV to the VMULTIN signal because this 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 ( >(VREF+0.190)), the OVP feature is activated. It stops the gate drive from switching as long as the voltage at the VO_SNS pin is above the nominal value (VREF). This prevents the output dc voltage from going above 7.5% 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. |
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