2018.07.12 Si Power Device
From this article onward, we explain the differences and selective use of diodes and transistors in actual application circuits, according to the characteristics and performance of these devices. We begin with examples relating to PFC (power factor correction). For some electronic devices, PFC may be necessary; in this article we briefly explain PFC.
What is PFC?
PFC (power factor correction) refers to improvement of the power factor to bring it closer to 1. This entails bringing the power factor angle (phase angle) closer to 0° to reduce the phase difference between voltage and current, so that the apparent power is made to approach the effective power. At the same time, harmonic currents are suppressed. Suppression of harmonics is prescribed by class in the international standard IEC 61000-3-2, which stipulates the maximum allowable harmonic currents, and PFC is essentially required for relevant electronic devices.
Single PFC and Interleaved PFC
In basic PFC operation, a sawtooth-wave current is passed through an inductor, and by executing control such that the current average value is sinusoidal, the shift in the voltage and current phases is corrected. The following are examples of single and interleaved type basic PFC circuits.
As the names imply, a single-type circuit is configured from one set of a switch (transistor), diode, and inductor, whereas an interleaved circuit uses two such sets, with the switches driven 180° out of phase. Hence the inductor current in a single PFC circuit is a single sawtooth wave resulting from on/off control, whereas in an interleaved PFC circuit the sawtooth waves overlap. As a result, ripple currents are smaller and the effective frequency is doubled. The figure on the right shows the current waveforms of each inductor and the interleaved current waveform.
In the interleaved circuit, two switches are used, so switching losses are distributed and the load on a single switch is reduced, facilitating thermal design. Moreover, ripple currents are smaller and the effective frequency is higher, so that the filter size can be made compact. This is the same principle that is used in two-phase driving of DC/DC converters.
Boundary Current Mode (BCM) and Continuous Current Mode (CCM)
In general PFC control, a boundary current mode (BCM) in which switching is performed when the current reaches zero, and a continuous current mode (CCM) in which a current is flowing continuously in the inductor, are used.
In BCM, the switch turns on when the diode current becomes zero, and so a reverse recovery current does not flow in the diode. However, the current undergoes a large change from zero to the maximum value, and so the peak currents flowing in the inductor and the diode are large. On the other hand, in CCM the switches are turned on while current is flowing in the diodes, and the diodes are forcibly turned off, so that large reverse recovery currents flow and switching noise occurs. However, the continuously flowing inductor currents are substantially DC currents, and minimal ripple currents are a feature of this method.
Difference in Output Power Due to the Different Methods
The differences in the above-described single and interleaved circuits and in BCM control and CCM control appear as differences in output power and peak current characteristics. In general, interleaved circuits and CCM control are often used for circuits with high output power. The following graph is an example of comparison of these methods.
・In PFC (power factor correction), the power factor is improved, bringing it closer to 1.
・PFC can use single or interleaved circuits; when using an interleaved circuit, losses can be distributed, so that thermal design is easier.
・In PFC, either a boundary current mode (BCM) or a continuous current mode (CCM) can be used; in general, CCM is used for large power levels.