Board the electromagnetism that carries DC-DC converter generation is disturbed (EMI) the common problem that is product of content couplet network. These small circuit are normally between 1MHz and 3MHz with inferior rate of margin of accept second class is rapid switch, as a result generation exceeds the broadband EMI of 2GHz. EMI can affect the sensitivity of sensitive receiver circuit, especially beehive and system of global navigation satellite (GNSS) .

 

A kind of when measure performance of DC-DC converter EMI efficient way is small-sized magnetic field is used in time domain (H-field) probe measures rise time and ringing. Through going to coupling of magnetic field probe converter outputs inductor, can implementation blame invades a gender to measure (if pursue,1 is shown) .

 

Graph 1: probe coupling to output inductor explores typical content couplet the weaveform that halftone board carries converter of DC-DC power source to arise. Inductor is used enclose relative to bigger circle, identify very easily so. If plan institute is shown, probe should square, in order to realize the biggest coupling.

 

Detect the ringing on switch weaveform is very important, because ringing frequency is OK,change is the wide peak value in emissive character. Magnetic field probe is fast and safe, because it does not need to receive circuit repeatedly directly, need coupling to output inductor to DC-DC converter only can.

 

Headgear of explore of near field of Rohde ＆ Schwarz HZ-15 include probe of a few magnetic field (or annulus) . Because want coupling, is the electric current in taking line and element, because this used this type. That the biggest probe is too sensitive, resolution is too low, can’t keep apart emission source. Another diameter makes an appointment with 1 centimeter lesser probe (model RS H 50-1) , suit to identify in board class and explore EMI. Join probe simply 50Ω oscillograph inputs end, undertake adjustment, can obtain show good weaveform.

 

[Complier notes: The company such as Beehive, Com-Power, ETS-Lindgren, Keysight Technologies, Langer EMV, TekBox, Tektronix all offers EMI explore headgear. The company such as Beehive, Com-Power, ETS-Lindgren, Keysight Technologies, Langer EMV, TekBox, Tektronix all offers EMI explore headgear..

 

We come with mathematical method this kind of feature changes test and verify measure (if pursue,2 are shown) . Put possibly between inductor and magnetic field probe in factor of a certain unknown each other Ou (namely below the M in equality) . Because we do not know how much factor of Ou of this each other is after all, the value that cannot measure actually to amplitude and oscillograph probe so undertakes comparative. Because our target is EMI, basically pay close attention to weaveform of rise time, general switch and ringing frequency here so (if have) .

 

Graph 2: The switch of inductor of DC-DC converter output is undee (SW) passes mutual inductance (M) coupling arrives magnetic field probe.

 

DC-DC converter has signal of accurate square wave normally (VL) , nod from converter switch part (SW) and output inductor (L) input sheds ground loop, this is the signal that we want to undertake metrical with oscillograph probe. The concern of the electricity that carries inductance and voltage is as follows:

 

 

Probe of hypothesis magnetic field stands by inductor, get Ou of a few each other, m (sealed) , the output of probe is:

 

 

Amalgamative in front two formula, reach:

 

 

Put forward invariable M/L next, reach VOUT∝V.

 

Because VOUT and VL become direct ratio, because this OK and relaxed apace measures the mainest EMI character, and won’t produce join short circuit with oscillograph bougie. Stand by magnetic field probe inductor of every DC-DC converter, can measure rise time (the upper limit that shows overtone frequency) , pulse width and cycle (also consider overtone frequency) , and ringing frequency (can bring about in broadband spectrum appear wide syntonic peak value) .

 

Graph 3 with the graph the probe of 4 RT-ZS20 1.5 oscillograph that compared bandwidth to be GHz (lead short probe) the switch weaveform with probe of magnetic field of RS H 50-1 is characteristic. Outside dividing amplitude, measure knot fruit to be like.
 

Graph 3: Probe of use coupling magnetic field (upside trace) with the straight end that connect sheet probe (bottom trace) the DC-DC converter that measures equipment of network of typical content couplet outputs inductance, showed similar weaveform. But probe of use magnetic field can measure rise time, cycle and ringing quickly, and the risk that does not have circuit short circuit.
 

Graph 4: The ringing of DC-DC converter is measured, the possibility arises EMI wide peak value when 8MHz (add Gao Cixie wave) .

 

Will same magnetic field probe receives analyzer of spectrum of Siglent SSA 3032X repeatedly, its are initiative and stop frequency to be respectively 1 with 500MHz, and the resolution bandwidth that has 120kHz, the result gets 8MHz syntonic peak value inside broadband spectrum (if pursue,5 are shown) .

 

Graph 5: The broadband spectrum that DC-DC converter produces shows 8MHz syntonic peak value in Marker1 place.

 

In a lot of case that had seen in me, ringing frequency is very incidental control in 100MHz, cause the wide peak value of emissive spectrum, below this kind of circumstance, if coupling arrives aerial shape structure (it is cable normally) , cause EMI trouble possibly.