What’s the Problem with AC-DC Front Ends?

October 3, 2012
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The first in a series of posts looking at how the performance of AC-DC front ends can be improved looks at the limitations and issues being faced.

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.

Read the next post in the series, looking at how the next generation of ac front-end supplies are able to surmount these challenges, raising the power density bar while effectively delivering significant improvements in thermal performance.

We are running a series of webinars in November, one of which is Practical considerations for creating high performance AC-DC power systems.  Click on one of the links below to register. 

Session 1:  Tuesday, 13 November (17:00 Munich, 16:00 London, 11:00 Boston, 08:00 San Francisco)

Session 2:  Wednesday, 14 November (14:00 Munich, presented in German)

Session 3:  Thursday, 15 November (09:00 London, 10:00 Munich, 14:30 Mumbai, 17:00 Hong Kong)

Alternatively, if you are visiting electronica (13-16 November) you can attend the seminar itself. Visit Hall A6, Stand 560.

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