EMC VI-200, VI-J00 specifications Radiated Noise, Noise Considerations

Design Guide & Applications Manual 9. EMC Considerations

For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies

RADIATED NOISE

Radiated noise may be either electric field or magnetic field. Magnetic radiation is caused by high di/dt and is generally what is measured by FCC, VDE or MIL-STD-461. Vicor converters utilize zero-current-switching, with the advantage over PWM non-zero-current-switching being that zero-current-switching topologies contain minimal discontinuities in the switched current waveforms, resulting in lower di/dt’s. Electric field radiation (caused by dv/dt) is “near-field,” i.e., it decays rapidly as a function of distance and as a result does not typically affect radiated measurements.

Radiation can be minimized by proper board layout. Keep all leads with AC current short, twisted or routed as overlapping planes to minimize loop cross-sectional area.

Also keep in mind the effects of capacitive coupling — even when not expected. Do not put an unshielded filter on the opposite side of the PCB from the module. Conducted noise can be capacitively coupled around the filter. Do not route input and output leads in the same cable bundle. Again, no special precautions, just good design practice.

NOISE CONSIDERATIONS

All switchmode power supplies generate a certain amount of “noise”, yet it remains one of the least understood parameters in power conversion.

VI-200s and VI-J00s both use the same topology, so their operation is very similar. These products are zero-current- switching converters — i.e., the current is zero when the main switch is turned on or off. While the switch is on, the current through the switch or the primary of the transformer is a half-wave rectified sine wave. Similar in operation to a resonant converter, these products are commonly referred to as quasi-resonant converters. The LC resonant frequency is fixed so the on-time of the switch is about 500 ns. When the switch turns on, energy builds up in the leakage inductance of the transformer (L) and then “transferred” into the capacitor on the secondary side of the module. (C, Figure 9–6) The energy processed in each pulse is fixed, and is ultimately the energy stored in this capacitor, 1/2 CV2. Since the energy in every pulse is fixed, the repetition rate of the pulse train is varied as a function of load to regulate the output voltage. Maximum repetition rate occurs at minimum line, full load and is approximately twice the LC time period or 1 µs. If the load drops by 50%, then the repetition rate is approximately one-half of maximum (since the energy in every pulse is fixed). Therefore the pulse repetition rate varies linearly with load, to a first order approximation.

	Vs
	L
+IN	+ OUT
	C
Vp	Ip
	–OUT
–IN

Figure 9–6— Basic zero-current-switching converter topology (VI-200 / VI-J00)

Since the energy in every pulse is related to the square of the applied voltage (CV2), the pulse repetition rate varies as approximately the square of the line voltage. For example, a 300 V input unit can vary from 200 – 400 V, or a factor of two, therefore it follows that the repetition rate must vary by approximately a factor of four to regulate the output. As previously established, the current in the primary is a half-wave rectified sine wave, but the voltage on the primary is a square wave. Since this voltage is a square wave, it contains harmonics of the fundamental switching frequency. It also includes frequencies, that extend to 70 MHz.

These frequencies can be of interest in the following circumstances. Rapidly changing voltages (high dv/dt) can generate E-fields (primarily near-field) which do not usually cause system noise problems since they significantly decrease as a function of distance. For this reason, E-fields are not measured by agencies such as the FCC or VDE. These agencies do, however, measure the magnetic radiation caused by high frequency currents in a conductor. The half-wave rectified sine wave in the transformer is an example of this, but since there are minimal discontinuities in the current waveform and the loop cross-sectional area is very small, the resultant E-field is very small. E-fields can be a problem if sensitive circuitry is located near the module. In this case, a shield can be positioned under the label side of the module as a discrete element or as a ground plane on the PCB. The other effect that occurs as a result of the 50 – 70 MHz component on the main switch is common-mode noise. (Figure 9–7)

	Parasitic
	Capacitance
FET	Rectifier
Shield	Shield
Ceramic	Ceramic

Baseplate

Figure 9–7— The shield layer serves to reduce the capacitance

vicorpower.com 800-735-6200 Applications Engineering 1-800-927-9474 Rev. 2.1

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