At present, most electrical appliances on the market need AC-DC power conversion. If the AC-DC power conversion efficiency can be improved, it will help reduce household power consumption and business operation costs, and also help to improve the operating efficiency of applications such as energy storage system and battery charging. This article will show you the characteristics of power factor correction technology, as well as the product characteristics and advantages of NCP1681 PFC controller introduced by onsemi.
Improvement of AC-DC power conversion efficiency through power factor correction technology
Most electric energy in the world is supplied to almost all mains-powered equipment and AC-DC power supply in devices, which means their efficiency has a great impact on operating costs, and lower conversion efficiency will significantly increase carbon emissions. In power supply applications, almost all AC-DC power supply input bridge rectifiers produce losses, which pose a challenge to achieve the highest possible efficiency. However, removing the traditional diode bridge, PFC FET and boost diode to support the bridgeless totem pole PFC arrangement will improve the efficiency by using active switches. However, the control of new topology will be more complex, which usually requires a microcontroller and associated code, which presents an obstacle to adoption by the design team with limited time.
Although efficiency is a relatively simple concept, which represents the ratio of useful output power to input power, the input power will be significantly affected by the input power factor, which will increase with the out-of-phase movement of line current and line voltage. The product of the input power and power factor in the product is the apparent power. Using it to calculate the true efficiency will lead to a significant decrease in the overall efficiency figures. The disturbances and inefficiency generated will be propagated back through the utility network.
However, the use of power factor correction (PFC) technology can solve this situation. This concept is important and has now become a regulatory requirement. Technically, in order to improve the power factor, EMC standards such as IEC 61000−3−2 limit the power in line harmonics due to line current distortion.
In addition, in the past, peak efficiency was mainly considered, which masked the poor performance, especially when the system was running at low power level. In order to solve this problem, new schemes such as 80+ certification program require the efficiency to reach 80% at 20%, 50% and 100% of full load. The most stringent version of the 80+ standard is the "80+ Titanium standard", which stipulates that the efficiency is at least 90% at 10% load and at least 94% at full load.
Meet the requirements of "80+ Titanium standard"
The most common method is that PFC uses a boost converter to obtain a DC level higher than the mains peak voltage of the power supply from the rectified power supply. This DC level is usually 395 V, which is used to design the power supply for 90-264 VAC input, and then it is regulated by an isolated DC-DC conversion stage to generate the DC output voltage required by the power supply. A valuable by-product is that the line current will give (theoretically) a consistent power factor following the line voltage waveform.
The above method is effective. PFC can be designed to operate in continuous, discontinuous or critical conduction mode (CCM, DCM, CrM), which largely depends on whether the energy in the boost inductor is completely exhausted in each cycle. Generally, the DC-DC stage has a loss of 2%, and the line rectifier and PFC stage has a loss of 1%, although this is closer to 2% during low line operation. Because the loss of low-voltage lines is close to 4%, it is a great challenge to meet the most stringent 80+ Titanium standard, which requires an efficiency of 96% at 230VAC input and 50% load, which is a common specification in servers.
In order to solve this problem, a more efficient technology called "totem pole PFC" (TPPFC) has attracted people's interest. In this method, the active switches replaces the AC line bridge rectifier diode and absorbs the role of the boost transistor and the boost diode in the conventional boost PFC.
With the help of lossless switch, perfect inductor and no diode voltage drops, the efficiency of TPPFC circuit becomes very close to 100%. However, in the real world, MOSFET has conduction and switching losses. Although ultra-low on-resistance devices can be used (even in parallel) to minimize the conduction loss, the result usually increases the high-frequency switching loss, which, like almost all designs, needs to be weighed.
Totem pole PFC achieves higher system efficiency and power density
PCB layout also plays an important role in power conversion. Magnetic components and capacitors are usually switched at high frequency, which leads to voltage and current discontinuity and sharp edges, usually called high dv/dt and di/dt edges. Because these sharp edges are common in power converters, any parasitic inductance and capacitance in PCB layout will cause noise to be injected into each node. These problems are particularly serious in topologies using wide bandgap (WBG) devices, such as GaN and SiC. WBG devices usually have lower capacitance and switch at a faster speed, thus further increasing dv/dt and di/dt at various edges.
The topology of TPFC circuit is composed of two half-bridge configurations, in which one half-bridge, usually called “Fast Leg” switch, switches at PWM frequency and the other half-bridge, usually called “Slow Leg” switch, switches at AC line frequency. Fast leg switches play the role of switches and diodes in classical boost PFC, that is, these switches are used to adjust the output voltage and shape the input current to provide high power factor and low harmonic distortion. The slow leg switches play the role of diode bridges in classical boost PFC, and active switches with low on-resistance are used to replace diodes, thus improving efficiency.
In addition, TPFC only uses one slow leg and one fast leg device in the conduction path, while conventional boost PFC uses two bridge diodes and one active switch or boost diode in the conduction path, and fewer devices and active switches in the conduction path will replace the bridge diodes, so that TPFC topology can achieve higher system efficiency and power density than classical boost PFC.
Power factor controller supporting continuous conduction and multi-mode
This article introduces the NCP1681 PFC controller from onsemi, as it provides a code-less solution for totem pole PFC. onsemi's NCP1681 PFC controller, which is a power factor correction controller supporting totem pole continuous conduction mode (CCM) and multi-mode (CrM-CCM), can be used to drive bridgeless totem pole PFC topology. Bridgeless totem pole PFC is a power factor correction architecture, which consists of a fast switching leg driven by PWM switching frequency and a second leg running at AC line frequency. This topology eliminates the diode bridge at the input of the conventional PFC circuit, thus significantly improving the efficiency of the power stage. The controller can be configured to run under CCM or CrM-CCM.
This NCP1681 adopts totem pole PFC topology to eliminate the input diode bridge, supports CCM operation at high power level, has optional multi-mode operation, supports CCM operation at high power level and CrM operation at medium power level, with the minimum frequency foldback in DCM of 25 kHz, supports skip mode under very light load condition, adopts a novel current sense scheme, supports digital voltage loop compensation, AC line monitoring circuit and AC phase detection, has almost near unity power factor in all operating modes, and is equipped with PFC OK indicator.
NCP1681 is suitable for high-power converters from 300 W to 2.5+ kW. NCP1681 needs the information of the upslope and the downslope of the inductor to realize the average current mode control, so the current transformer is adopted in the high-frequency leg. The NCP1681 board can be divided into three sections, including EMI filter, power train and low-voltage section, and each section has specific placement and routing requirements.
NCP1681 can be widely used in power factor correction and off-line power supply. Common end products include server power supply, telecom 5G power supply, network power supply, game console power supply, ultra-high-definition TV power supply and industrial power supply.
Conclusion
TPPFC method is the first choice for efficient PFC to reach hundreds of watts, while ensuring compliance with 80+ Titanium efficiency standards and other environmental requirements of standby power consumption and no-load loss. Higher efficiency is a requirement of every application, so improvements that can be obtained from CrM-based active PFC bring benefits to users (including reducing operating costs). The WBG semiconductor, advanced PFC controller and design support tools provided by onsemi will simplify power supply design to improve efficiency and ensure customers' products can be brought to market faster.