Today, regulatory agencies (EN/IEC61000-3-2) have legislation requiring units that draw power over 75 watts have a specified-level of harmonic content. This legislation was enacted to allow the maximum power to be transmitted over the power lines and to limit the harmonic content on these lines. The best method of meeting this requirement is having the current drawn from a power line closely following the shape of the incoming voltage. To meet these objectives, the use of a Power Factor Correction (PFC) circuit, which actively forces the input current to follow the line’s voltage, is one of the most desirable options. These circuits are quite complex in nature.
To accomplish this function of the PFC requires that the current be measured and be presented to the controller IC. In the higher power units this means large input currents. Since the currents are quite large, any voltage across a sense resistor will result in large power losses in the sense resistor. To avoid this, a current sense transformer is often used. This is put in series with the switch that is controlled by the PFC controller and can measure the current very accurately without dissipating much power. However there is a problem with this method of measuring the current when the controller demands duty cycles that approach 100%. Since the current transformer is a magnetic device the volts/seconds integral across the transformer over several cycles must sum to zero. Because of the inherent parasitic elements in the current transformer, it may not be able to reset (sum to zero) with very short off times. When this happens the transformer tends to walk up the magnetic flux curve and approach saturation. If this occurs, the information that the transformer is presenting to the controller becomes corrupted and can result in circuit failure. For this reason a maximum duty cycle clamp may be needed
PWM (pulse width modulated) converters, of which a PFC is one type, operate by turning a switch on and off with a controlled duty cycle. During the on time, energy is stored in a magnetic device and then during the off time, the magnetic device releases that energy.
The ratio of the maximum on time to the minimum off time is often controlled by the clock which is a ramp generator in most systems. The maximum on time is usually the time it takes to charge a capacitor with a voltage ramp and the minimum off time is the time it takes to discharge the ramp voltage. In the case of the UCC3817 these currents are set by two current sources controlled by a single resistor. The discharge current is designed to be a factor of 19 times the charge current.
In a PFC controller the duty cycle required for operation without distortion often approaches 100% so a 19 to 1 duty factor though it appears high is reasonable and if the current sensing is done resistively it is. If the current sensing is done with a current transformer and the frequency is high the current transformer may not be able to reset at maximum duty cycle.
The solution to this problem is to increase the mandatory off time which can be done by increasing the fall time of the ramp voltage thus insuring that the current transformer has sufficient time to reset. This may result in slightly higher crossover distortion but it should be acceptable.