Overview
This article explains how differences in diode and transistor characteristics affect practical power circuit behavior and how the devices should be chosen. It uses power factor correction (PFC) as an example. Some electronic equipment requires PFC to meet standards for harmonic emission and power factor performance, so PFC is introduced first.
What is PFC
PFC refers to improving the power factor and making it approach 1. This is achieved by minimizing the phase difference between voltage and current so that the apparent power approaches the real power, and by suppressing harmonic currents. Harmonic limits are defined in international standards such as IEC 61000-3-2, so most equipment includes PFC to meet those limits.
Single-stage and Interleaved PFC
The basic PFC operation shapes the inductor current into triangular waveforms and controls the current so its average follows the input voltage sine wave, correcting the phase between voltage and current. A single-stage PFC uses one switching device (transistor), diodes, and one inductor. An interleaved PFC uses two such sets driven 180° out of phase. In single-stage PFC the inductor current is a single triangular waveform synchronized to the switch on/off periods. In interleaved PFC the triangles overlap, reducing ripple current and effectively doubling the ripple frequency. The result is lower input ripple and a higher effective frequency for filtering.
Using two switching channels spreads switching losses across devices so each device carries less load, simplifying thermal design. Lower ripple current and higher effective frequency also help reduce filter size. This principle is similar to dual-phase DC/DC converters.
Boundary Conduction Mode (BCM) and Continuous Conduction Mode (CCM)
PFC controllers typically operate in one of two modes. Boundary conduction mode (BCM), also called critical conduction mode, switches at the instant the inductor or diode current falls to zero. Continuous conduction mode (CCM) keeps inductor current flowing continuously through the switching cycle.
In BCM the diode current falls to zero before the next switching event, so there is no reverse-recovery current in the diode. However, the inductor current rises from zero to a peak each switching cycle, increasing peak currents in the inductor and diode. In CCM the diode conducts during switching events and is forced off by the switch, which can generate significant reverse-recovery current and switching noise. CCM yields a nearly DC inductor current with small ripple.
Mode Differences and Output Power
The choice between single-stage and interleaved topologies and between BCM and CCM affects output power capability and peak current characteristics. Higher-power designs commonly use interleaved PFC and CCM control.
Key points
- PFC improves power factor toward 1.
- Topologies include single-stage and interleaved; interleaved spreads losses and eases thermal design.
- Control modes include BCM and CCM; CCM is typically used in higher-power circuits.
BCM PFC: Using Diode Selection to Improve Efficiency
In practical power circuits, diodes and transistors have different characteristics and performance, so device selection matters. In power supply applications the main goal is often to improve efficiency. The following example shows how choosing different diodes affects BCM PFC efficiency.
BCM PFC: LCD TV PFC module example
The example is a single-stage PFC unit operating in boundary conduction mode typical of an LCD TV PFC. Diode D1 in the PFC uses a fast recovery diode (FRD).
Simulations compared two diodes: one with low forward voltage VF and nominal reverse-recovery time trr, and another with higher VF but fast trr. Other specifications were essentially equivalent. The main specifications used are shown below.
| Parameter | RFNL10TJ6S (low VF) | RFV8TJ6S (fast trr) | Unit |
|---|---|---|---|
| VRRM | 600 | 600 | V |
| IF | 10 | 8 | A |
| VF | 1.25 @IF=8A | 3 @IF=8A | V (typical) |
| IR | 10 | 10 | μA |
| trr | 65 | 20 | ns (max) |
The middle waveform plots diode power loss. The top waveform is the inductor current equal to the diode IF. The bottom waveform is the output voltage applied across the diode. The low-VF RFNL10TJ6S shows much lower power loss. Average losses measured: RFNL10TJ6S (low VF, nominal trr) = 0.23 W; RFV08TJ6S (fast trr, higher VF) = 0.41 W. The VF difference (1.25 V vs 3 V at IF=8 A) explains the loss difference.
These results indicate that in BCM PFC the diode VF strongly affects conduction loss, while trr has less impact. In BCM the current rises from zero to peak each cycle, so a lower VF reduces conduction loss.
For BCM PFC, selecting diodes with low VF improves circuit efficiency.
Key points
- In BCM PFC, diode VF has a large impact on losses; trr has a smaller impact.
- Choose diodes with low VF in BCM PFC to improve efficiency.
CCM PFC: Using Diode Selection to Improve Efficiency
Next, consider how diode characteristics affect efficiency in continuous conduction mode PFC.
Example: Diode impact on CCM PFC efficiency
The simplified PFC circuit used earlier is examined to see how the diode in the output stage and the MOSFET combine to affect overall efficiency. FRDs were used and three FRD variants with different characteristics were compared.
The plot below shows circuit efficiency versus FRD reverse-recovery time trr. Lower trr yields higher efficiency. The table below summarizes the FRD characteristics and measured efficiencies.
| FRD | IF (A) | VF (V) Typ. @IF max | trr (ns) Typ. @IF max, VR=400V | Efficiency (%) |
|---|---|---|---|---|
| RFNL10TJ6S | 10 | 1.1 | 100 (dIF/dt = -100 A) | 89.10 |
| RFV8TG6S | 8 | 2.3 | 25 (dIF/dt = -200 A) | 93.59 |
| RFVS8TG6S | 8 | 2.5 | 20 (dIF/dt = -200 A) | 93.87 |
Test conditions: CCM, Po = 300 W, fsw = 200 kHz, Vin = 115 Vrms, Vo = 390 V.
RFNL10TJ6S and RFV08TJ6S were the FRDs used in the previous BCM simulation. RFNL10TJ6S had the lowest VF and gave best efficiency in the BCM example. However, in CCM the influence of VF on efficiency is minimal; efficiency is dominated by the diode trr.
In the FRD waveform, the FRD conducts about 5 A forward current IF during conduction and then about 18 A reverse current IR during turn-off. This IR flows during trr and, in CCM PFC, affects the MOSFET switching. As shown, MOSFETs experience high current spikes when switching, which increases loss and reduces overall efficiency.
Conclusion: In CCM PFC, diode trr has a major impact on loss; VF has little effect. Select diodes with small trr to improve CCM PFC efficiency.
Key points
- In CCM PFC, diode trr strongly affects losses; VF has minor impact.
- Choose diodes with low trr for CCM PFC to improve efficiency.
