Make a noise for DC-DC converters

24 July 2008

Efficient high-frequency operation has long been recognised as the key to improving performance in switch-mode converters

Power conversion is usually accomplished by high-density DC/DC converter components based on highfrequency switching technologies, but highfrequency operation translates into smaller magnetics and capacitors, faster response times, smaller filters, and lower noise levels.

Notwithstanding noise performance improvements, all DC/DC converters generate EMI (electromagnetic interference) or noise. This noise, common mode, differential mode, and radiated noise, can vary among DC/DC converters from supplier to supplier and topology to topology. Although the many designs or topologies of DC/DC converter components number in the hundreds, two are dominant; fixedfrequency
PWM (pulse-width modulation) – and variable-frequency quasi-resonant zerocurrent/ zero-voltage switching (ZCS/ZVS). Design engineers working with DC/DC converters must understand the noise performance differences of these two classes.

Fixed-frequency PWM converters inherently trade off efficiency against operating frequency because of switching losses. Power, and noise, is dissipated in the switching element each time it discontinuously makes and breaks inductive current flow during its brief turn-on and turn-off transitions. Power dissipation due to switching losses increases directly with operating frequency in PWM converters until it becomes a dominant loss factor. At that point, efficiency declines rapidly, and the thermal and electrical stresses on the switch element become unmanageable. The losses result in a frequency barrier that limits achievable
power density in conventional converters.

Variable-frequency quasi-resonant ZCS/ZVS converters overcome the frequency barrier by implementing a forward converter switching at zero current and zero voltage. Each switch cycle delivers energy to the converter output, with switch turn-on and turn-off occurring at zero current and voltage, resulting in an essentially lossless switch. ZCS converters can operate at frequencies in excess of 1MHz. By eliminating the fast current discontinuities characteristic of conventional topologies, ZCS/ZVS results in a virtually lossless transfer of energy from input to output with reduced levels of conducted and radiated noise.

ZCS/ZVS converters have sinusoidal waveforms rather than the square waveforms of PWM converters. The lack of sharp edges and lower harmonic content of ZCS/ZVS results in much less excitation of parasitic
capacitance and inductance, resulting in less noise. With PWM, the input voltage is switched at a constant frequency (usually several hundred kHz) to create a pulse train. The width of the pulses is adjusted to provide the power to the load at the correct voltage. At full load, the current waveform looks square.

Both fixed and variable topologies have frequency elements that are more or less fixed, and frequency elements that vary as a function of operating conditions.

Figure 1 compares the waveforms of the current flowing through the main switch. In a module using a quasi-resonant topology, the pulse width or on-time, T1, is fixed, while the repetition rate or period, T2, is variable.
Conversely, in a module using PWM, the opposite is true; the repetition rate is fixed and the pulse width is variable. The rise/fall time, T3, is a fixed frequency in both topologies. However, in the variable-frequency
design, there are no high-frequency components associated with the leading and falling edges of the sine wave. The spectral content of the variable frequency waveform is lower in amplitude and contained in a narrower band. In the fixed-frequency waveform, the spectral content is higher in amplitude and spread over a broader range of harmonics.

Parasitic excitation results in highfrequency noise in the range of 10MHz to 30MHz. This noise can be difficult to suppress because it is often coupled to the secondary of the converter (through the transformer) as common-mode noise. With ZCS/ZVS converters, much less parasitic noise is generated due to the soft edges of the current waveform.

For applications that require low noise, an effective first step to minimise noise generated by the DC/DC converter is to select a topology, such as zero-current switching, that is inherently lower in common mode noise. Also, some products should be avoided in noise-sensitive applications. For example, control devices mounted on copper plates create parasitic capacitance from primary referenced control devices to secondary referenced control devices through the copper base, resulting in high common mode noise. In applications where no EMI requirements need to be met, bypass capacitors are typically used at the input and output pins of DC/DC converters.

Although component power modules usually incorporate some internal input and output filtering, additional external filtering is often needed to meet system requirements or agency specifications. For example, FCC
and European agencies specify the allowable levels of power supply noise that may be conducted back into the AC line. Some DC/DC converter suppliers also offer AC front ends and EMI filters as modular accessories. These filters save time and are also a means of risk prevention. The EMI filter works with the supplier’s converter modules, and, assuming proper layout, the combination is certified to meet the specified EMC directives.

In the US and Europe, conducted noise emissions are governed by the Class A and Class B limits of FCC and VDE standards. In the US, the FCC requires compliance with Class A for equipment operating in factory
settings and Class B for equipment destined for home use. In Europe, equipment for home and factory use must meet the VDE Class B standard.

Most switching power supplies today operate between 100kHz and 1MHz. Usually, the dominant peaks in the conducted noise spectrum reflected back to the power line correspond to the fundamental switching
frequency and its harmonic components. Conducted emissions standards such as EN55011 and EN55022 set quasi-peak and average limits on conducted noise reflected from the input of converters or power supply systems back to the source over the frequency range of 150kHz to 30MHz. In order to comply, all of the conducted noise, the peaks in the spectrum, must fall below the specified limits.

EMI filters are most often constructed in a single package with configurations similar to those shown in Figure 4. The EMI filter is a through-hole filter with a common mode choke and Y-capacitors (line-ground) plus two additional inductors and an X-capacitor (lineline). Transient protection is provided by Z1. This filter configuration provides sufficient attenuation to comply with the Level-B conducted emissions limit.

ROBERT MARCHETTI is senior manager of product marketing, Vicor


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