EMC VI-J00, VI-200 specifications Measuring Output Noise, Noise coupling model

9. EMC Considerations

Design Guide & Applications Manual

The dv/dt of the switch (FET) is a noise generator. This FET is mounted on a two layer insulating and shielding assembly which is attached to the baseplate. Since ceramic is a dielectric, there is capacitance from the FET to the baseplate. (Figure 9–7) The output rectifiers are also tied to the baseplate with ceramic insulators, adding additional capacitance. The dv/dt of the FET is differentiated by these two series capacitors, resulting in a spike of noise current at 50 – 70 MHz that flows from primary to secondary. (Figure 9–8) This noise current is common-mode as opposed to differential, and therefore should not affect the operation of the system. It should be noted, however, that oscilloscopes have a finite ability to reject common-mode signals, and these signals can be abnormally emphasized by the use of long ground leads on the scope probe.

	Primary	Baseplate		Secondary
		C FET	CRectifier
Vp	Vp
ICM			ICM
		CFET	CRectifier
		I DM
	Vp
		CExternal	CExternal
		Ycaps	Ycaps

Figure 9–8— Noise coupling model

MEASURING OUTPUT NOISE

Long ground leads adversely impact the common-mode rejection capability of oscilloscopes because the ground lead has inductance not present on the signal lead. These differing impedances take common-mode signals and convert them to differential signals that show up on the trace. To check for common-mode noise, place the oscilloscope probe on the ground lead connection of the probe while the ground lead is tied to output return. (Figure 9–9) If the noise is common-mode, there will still be “noise” observed at the same test point.

NOTE: The output return must be at the same relative potential as the earth ground of the oscilloscope or damaging current may flow through the oscilloscope ground lead.

Capacitors are required from the +/–IN to the baseplate thereby shunting common-mode current, thus reducing noise current on the input power lines. The capacitor must

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

have very short leads since the frequency is high. It must also be a good capacitor (i.e., ceramic or other material that has a low ESR / ESL). This type of capacitor is most important on high input voltage units since the “dv”

is larger, but is required for all units. For off-line applications this capacitor must have the appropriate safety agency approvals.

Insert probe into female receptacle	To Scope
(Vicor P/N 06207) for proper output
differential noise measurement technique

Ground Ring on Probe

or

To Scope

Figure 9–9— Output ripple measurement technique

A capacitor from +/–Vout to the baseplate, is required since the output rectifier has a changing voltage on it, and, like the FET, can generate common-mode noise.

This capacitor is similarly recommended for high output voltage units (48 V).

Common-mode noise is not differential with respect to the output. It does, however, flow in both input and output leads of the power supply and is a noise parameter that is measured by the FCC or VDE. It can cause power systems to fail radiated emission tests, so it must be dealt with. Bypass capacitors to the baseplate with a common- mode filter on the input of the module or the main input of the power supply is required.

The common-mode filter is typically placed on the input as opposed to the output. Theoretically, since this current flows from primary to secondary, the choke could be placed in either the input or the output, but is preferably placed in the input leads for the following reasons:

1)input currents are smaller since the input voltage is usually higher;

2)line regulation of the module can correct for voltage drops across the choke; and

3)if the choke is on the output and the senses are connected to the other side of it, the stability of the loop may be impacted.

Differential output noise is the AC component of the output voltage that is not common to both outputs. The noise is comprised of both low frequency, line-related noise (typically 120 Hz) and high frequency switching noise.

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

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