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MC33368 Datasheet(PDF) 7 Page - ON Semiconductor |
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MC33368 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 16 page MC33368 http://onsemi.com 7 FUNCTIONAL DESCRIPTION INTRODUCTION With the goal of exceeding the requirements of legislation on line current harmonic content, there is an ever increasing demand for an economical method of obtaining a unity power factor. This data sheet describes a monolithic control IC that was specifically designed for power factor control with minimal external components. It offers the designer a simple cost effective solution to obtain the benefits of active power factor correction. Most electronic ballasts and switching power supplies use a bridge rectifier and a bulk storage capacitor to derive raw dc voltage from the utility ac line, Figure 14. Figure 14. Uncorrected Power Factor Circuit Rectifiers Converter Bulk Storage Capacitor Load AC Line This simple rectifying circuit draws power from the line when the instantaneous ac voltage exceeds the capacitor voltage. This occurs near the line voltage peak and results in a high charge current spike, Figure 15. Since power is only taken near the line voltage peaks, the resulting spikes of current are extremely nonsinusoidal with a high content of harmonics. This results in a poor power factor condition where the apparent input power is much higher than the real power. Power factor ratios of 0.5 to 0.7 are common. Figure 15. Uncorrected Power Factor Input Waveforms Rectified DC 0 Vpk Line Sag AC Line Voltage AC Line Current 0 Power factor correction can be achieved with the use of either a passive or active input circuit. Passive circuits usually contain a combination of large capacitors, inductors, and rectifiers that operate at the ac line frequency. Active circuits incorporate some form of a high frequency switching converter for the power processing with the boost converter being the most popular topology. Since active input circuits operate at a frequency much higher than that of the ac line, they are smaller, lighter in weight, and more efficient than a passive circuit that yields similar results. With proper control of the preconverter, almost any complex load can be made to appear resistive to the ac line, thus significantly reducing the harmonic current content. Operating Description The MC33368 contains many of the building blocks and protection features that are employed in modern high performance current mode power supply controllers. Referring to the block diagram in Figure 16, note that a multiplier has been added to the current sense loop and that this device does not contain an oscillator. A description of each of the functional blocks is given below. Error Amplifier An Error Amplifier with access to the inverting input and output is provided. The amplifier is a transconductance type, meaning that it has high output impedance with controlled voltage−to−current gain (gm 50 mmhos). The noninverting input is internally biased at 5.0 V ±2.0%. The output voltage of the power factor converter is typically divided down and monitored by the inverting input. The maximum input bias current is −1.0 mA which can cause an output voltage error that is equal to the product of the input bias current and the value of the upper divider resistor R2. The Error Amplifier output is internally connected to the Multiplier and is pinned out (Pin 4) for external loop compensation. Typically, the bandwidth is set below 20 Hz so that the amplifier’s output voltage is relatively constant over a given ac line cycle. In effect, the error amplifier monitors the average output voltage of the converter over several line cycles resulting in a fixed Drive Output on−time. The amplifier output stage can sink and source 11.5 mA of current and is capable of swinging from 1.7 to 5.0 V, assuring that the Multiplier can be driven over its entire dynamic range. Note that by using a transconductance type amplifier, the input is allowed to move independently with respect to the output, since the compensation capacitor is connected to ground. This allows dual usage of the Voltage Feedback pin by the Error Amplifier and Overvoltage Comparator. Overvoltage Comparator An Overvoltage Comparator is incorporated to eliminate the possibility of runaway output voltage. This condition can occur during initial startup, sudden load removal, or during output arcing and is the result of the low bandwidth that must be used in the Error Amplifier control loop. The Overvoltage Comparator monitors the peak output voltage of the converter, and when exceeded, immediately terminates MOSFET switching. The comparator threshold is internally set to 1.08 Vref. In order to prevent false tripping during normal operation, the value of the output filter capacitor C3 must be large enough to keep the peak−to−peak ripple less than 16% of the average dc output. |
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