While front-end ac-dc converters have changed significantly in terms of efficiency, density, and regulations, power factor issues continue to encumber the users of these supplies. It affects the ac input mains resulting from out-of-phase line current due to reactive loads, yet the harmonics injected back into the ac line because of poor power factor have been the most troubling consequence.
The problem seems to become more widespread with switch-mode ac-dc converters, which are so prevalent in electronic equipment today. The standard bridge rectifier/hold-up capacitor scenario in these converters presents a predicament where drawing current from the ac power line in a non-sinusoidal form further aggravates the harmonic distortion problem.
Since the front-end section acts as a peak detector, a current flows to charge the capacitor only when the instantaneous ac voltage exceeds the voltage on the capacitor. A single-phase off-line supply draws a current pulse during a small portion of the half-cycle duration. Between those current peaks, the load draws the energy stored in the hold-up capacitor. The harmonic content of the typical pulsed current waveform generates non-efficient added RMS currents, affecting the real power available from the mains.
Meeting Requirements
Consequently, in Europe there are stricter regulations to limit the harmonic distortion permitted on the ac mains line. One well-known electromagnetic compatibility (EMC) directive, EN61000-3-2, was introduced to limit the reflected harmonics that are sent back from electronic equipment into the mains. It applies to all electronic systems consuming more than 75 W.
Though improvements have been gradual, modern ac-dc front-end solutions continue to employ boost converter topology with full-bridge diode rectification. Although this methodology addresses the power-factor limitation of the ac-dc front end to well over 0.9, there are many tradeoffs in terms of electromagnetic interference (EMI), thermal management, and power components when the input voltage range is wide.
To leapfrog in performance, the ac front end faces many challenges in terms of power density, conversion efficiency, and flexibility. And the problem seems to get tougher as the power levels go higher. Because active power-factor correction (PFC) adds components, it tends to reduce power density, efficiency, and reliability.
Boosting power density translates into thermal challenges, which means conversion efficiency must be further improved. Thus, driving the overall performance bar to a new high level with high reliability and compliance to regulations for harmonics, EMI, and safety extra-low voltage (SELV) outputs over the universal range of 85 V ac to 264 V ac is a daunting task.
The Role Of The DC-ZVS
Surmounting these challenges in the next generation of ac front-end supplies and achieving optimal balance between density, thermal management, and scalability require a topology optimized for this application. Vicor’s offering employs a combination of its patented double-clamp zero-voltage switching converter (DC-ZVS) topology and its patented adaptive-input cell technology consisting of two DC_ZVS cells configured dynamically (and automatically switched as required) either in series or in parallel to efficiently handle a wide variation of input voltage.
The double clamp converter includes isolation with unconventional clamp and storage elements, and, as implemented operates all of the switches using ZVS switching. This enables switching frequencies in the megahertz range with efficiency above 95%. The adaptive cell feature allows the converter to operate at the same peak efficiency over the universal input range.
Coupled with the development of advanced packaging technology, this approach would help raise the power density bar while effectively delivering significant improvements in thermal performance.
The unique merger of architecture, topology, and advanced packaging can deliver unparalleled power density with a very thin profile. It provides the system architect more flexibility and scalability to add differentiating functionality for competitive advantages.
For instance, filters can be embedded inside the package to meet EMI regulations like EN55022 class B standard, as well as line surge protection for compliance with EN61000-4-5. In other words, a new ac front-end solution can raise the power density, efficiency, and flexibility bars while complying with all the necessary regulatory specs.
For more differentiation, an optimized controller IC would include interfaces that permit the ac-dc front end to communicate with the system host controller. Additionally, such controllers can be designed to incorporate features that will permit parallel operation for applications such as N+1 redundancy or three-phase operation.
Since the ac-dc front end’s output can be within the SELV output requirements, it essentially realizes a complete isolated solution from wide input ac mains to low dc output with several new bells and whistles. In essence, it can be treated as an ac building block that can be further combined with an array of modular dc-dc converter building blocks to achieve a wall-plug to point-of-load (POL) solution with an unmatched combination of power density, efficiency, and scalability.
In short, an optimal combination of architecture, topology, and advanced packaging can overcome these challenges and deliver a power-factor-corrected ac-dc front-end solution that promises to spark new trends in power density, conversion efficiency, and flexibility while meeting and exceeding the total harmonic distortion, EMI, and SELV output requirements.