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Step-down converters’ switch-node voltage waveform defines the electromagnetic compatibility (EMC) habits for automotive CISPR 25 Class 5 measurements. The ringing frequency within the switch-node waveform is a crucial sign on the EMC receiver, the place the next ringing amplitude on the change node typically causes EMC points. Understanding the switch-node waveform permits predicting the converter’s EMC traits in addition to optimizing EMC filter design at an early design stage.
This text compares three automotive step-down converters to offer sensible recommendation on utilizing switch-node waveforms to foretell EMC traits for automotive CISPR 25 Class 5 measurements. That is useful to optimize EMC filter design and PCB format to satisfy CISPR 25 Class 5 requirements.
Change-node measurements
Change-node waveforms are used to check the EMC traits amongst three automotive step-down converters. Determine 1 reveals the switch-node measurement on an analysis board utilizing an lively voltage probe.
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Determine 1 Use an lively voltage probe for the switch-node measurement on the analysis board. Supply: Monolithic Energy Techniques
The switch-node voltage waveform usually has a rising time and falling time between 700 ps and a pair of ns. This requires a minimal oscilloscope bandwidth of about 1 GHz on the voltage probe tip, the place the voltage could be measured with an lively probe or a passive probe that has the required bandwidth.
For each variants, the bottom connection to the PCB should be as brief as doable to make sure that the measured ringing on the change node doesn’t embody the extra ringing from the lengthy probe floor connection.
Determine 2 reveals the right voltage probe tip place for the switch-node measurement on the analysis board. Join the GND tip as shut as doable to the IC’s PGND pin and join the probe enter tip as shut as doable to the IC’s switch-node pin. Solder the lively probe tip with a 0.7-pF enter capacitance on to the element pads by way of detachable gold-plated measuring suggestions.
Determine 2 Place the probe tip appropriately for the switch-node measurement on the analysis board. Supply: Monolithic Energy Techniques
Histogram and time pattern
Determine 3 reveals a step-down converter’s switch-node voltage (yellow hint), fSW histogram (pink hint), and time pattern (orange hint).
Determine 3 The twin frequency unfold spectrum of the MPQ4371-AEC1 consists of the switch-node voltage, fSW histogram, and time pattern. Supply: Monolithic Energy Techniques
The oscilloscope measures the switch-node voltage for every set off occasion throughout a interval of 400 µs and calculates the frequency of every switching cycle. Every calculated frequency is accrued within the histogram. The full length of this check is about 10 minutes. For the final set off occasion, the measured frequencies are represented as time pattern fSW vs. time.
The measured frequencies in Determine 3 confirm the fSW vs. time relationship from the MPQ4371-AEC1 datasheet. The time pattern waveform confirms the required twin frequency unfold spectrum modulation frequencies of 15 kHz and 120 kHz. By verifying correct IC operation, these frequencies present an outline of the anticipated fSW values for CISPR 25 Class 5 measurements.
Voltage waveform
Step down converter’s switch-node voltage waveform is measured with an lively probe. Determine 4 reveals the rising and the falling edges of MPQ4371-AEC1, by which each waveforms are overlaid on the oscilloscope by an alternating rising and falling set off. The rising edge has a rising time of 922 ps and a step response with a 273 MHz resonance frequency and a 3.2 V peak-to-peak voltage.
Determine 4 The switch-node voltage waveform for MPQ4371-AEC1 has rising and falling edges. Supply: Monolithic Energy Techniques
The MPQ4371-AEC1 step-down converter’s Quiet-FET expertise permits combining quick slewing edges with out extreme ringing. Quiet-FET expertise doesn’t considerably degrade effectivity like a snubber or bootstrap resistor (RBST), and as a substitute makes use of a minimal two-step sequential switching motion to activate the interior MOSFETs.
The resonance frequency is decided by the parasitic hot-loop inductances and capacitances. The equal hot-loop collection inductances (ESL) are outlined by the next:
ESL of the 100 nF, 0603-sized MLCC (about 800 pH)
ESL of the high-side MOSFET (HS-FET) and low-side MOSFET (LS-FET)
ESL of the bundle lead body
ESL of the PCB traces between the MLCC and IC’s VIN and PGND pins (about 700 pH/mm)
The switch-node waveform can be predicted utilizing a simulation of the PCB hot-loop community.
Frequency area
Determine 5 reveals a quick Fourier transformation (FFT) of step-down converter’s switch-node waveform. The typical fSW of 420 kHz is distributed between 384 kHz and 456 kHz (inexperienced markers) and corresponds to the measured histogram from Determine 3. The switch-node resonance frequency at 273 MHz is distributed between 250 MHz and 300 MHz (purple markers) because of twin frequency unfold spectrum modulation and corresponds to Determine 4.
Determine 5 A quick Fourier transformation is utilized to the MPQ4371-AEC1’s switch-node waveform. Supply: Monolithic Energy Techniques
Radiated emissions (RE) antenna for CISPR 25 Class 5
The vertical monopole, biconical, and log periodic antenna measurements in CISPR 25 Class 5 could be analyzed. Determine 6 reveals the radiating switching inductance at peak CISPR 25 (blue) and common CISPR 25 (yellow), the place the analyzer decision bandwidth (RBW) = 9 kHz, fSW = 420 kHz, enter voltage (VIN) = 13.5 V, output voltage (VOUT) = 3.3 V, and cargo present (ILOAD) = 2.5 A. The twin FSS modulation is useful to keep up RE under the boundaries.
Determine 6 The vertical monopole antenna measurement of MPQ4371-AEC1 passes CISPR 25 Class 5. Supply: Monolithic Energy Techniques
Determine 7 reveals the radiating objects (for instance, the harness or radiating traces on the PCB) at peak CISPR 25 (blue) and common CISPR 25 (yellow), the place RBW = 120 kHz, fSW = 420 kHz, VIN = 13.5 V, VOUT = 3.3 V, and ILOAD = 2.5 A.
Determine 7 The biconical antenna measurement of MPQ4371-AEC1 passes CISPR 25 Class 5. Supply: Monolithic Energy Techniques
Determine 8 reveals the switch-node resonance frequencies between 250 MHz and 300 MHz (akin to Determine 4 and Determine 5) at peak CISPR 25 (blue) and common CISPR 25 (yellow), the place RBW = 120 kHz, fSW = 420 kHz, VIN = 13.5 V, VOUT = 3.3 V, and ILOAD = 2.5 A. There isn’t any RE that exceeds the 250 MHz to 300 MHz resonance frequency vary.
Determine 8 The log periodic antenna measurement of the MPQ4371-AEC1 passes CISPR 25 Class 5. Supply: Monolithic Energy Techniques
Determine 9 reveals the 1.2 GHz switch-node resonance frequency inside RE at peak CISPR 25 (blue), common CISPR 25 (yellow), and the noise degree (grey), the place RBW = 120 kHz, fSW = 2.2 MHz, VIN = 13.5 V, VOUT = 3.3 V, and ILOAD = 2.5 A.
Determine 9 The log periodic antenna measurement of the MPQ4323M-AEC1 step-down converter passes CISPR 25 Class 5. Supply: Monolithic Energy Techniques
Change-node waveform for MPQ4323M-AEC1
The MPQ4323M-AEC1’s built-in, 100 nF, hot-loop MLCCs cut back the interior parasitic inductances, which shifts the resonance frequency to greater values and reduces the resonance amplitude. Determine 10 reveals an instance of a quick slewing, switching converter mixed with low inner parasitic inductances. This improves the switch-node waveform and reduces RE.
Determine 10 A quick-slewing switching converter mixed with low parasitic inductances improves the switch-node waveform of the MPQ4323M-AEC1 step-down converter. Supply: Monolithic Energy Techniques
Change-node instance on a 2-layer PCB
Determine 11 reveals two totally different step-down converters soldered on the identical 2-layer PCB. The left curve reveals the MPQ4326-AEC1 with frequency unfold spectrum modulation on a 2-layer PCB, with a switch-node resonance at 450 MHz. The fitting curve reveals a step-down converter in a suboptimal set-up with out FSS modulation and a 320 MHz resonance. The 2 converters are in contrast on the identical PCB and with the identical exterior elements.
Determine 11 Two step-down converters are in contrast in a switch-node instance on a 2-layer PCB. Supply: Monolithic Energy Techniques
The step-down converter with the suboptimal set-up signifies undesirable resonance on the rising edge (purple arrow), that means there’s a timing distinction between the HS-FET and LS-FET. This resonance is brought about through the use of a 2-layer PCB as a substitute of a 4-layer PCB. In comparison with a 4-layer PCB, a 2-layer PCB format has greater parasitic inductances inside the scorching loop, which will increase the resonance amplitude and adjustments the situation of the switch-node resonance.
The elevated amplitude is noticed with each converters. As well as, the 2-layer PCB doesn’t have the essential strong floor layer straight beneath the highest layer, leading to a bigger resonance amplitude and stronger RE.
FFT of step-down converters on a 2-layer PCB
Determine 12 reveals the FFT of the switch-node voltage waveforms for the MPQ4326-AEC1 (with FSS modulation) and step-down converter with the suboptimal set-up (with out FSS modulation) from Determine 11.
Determine 12 A quick Fourier transformation is utilized to the switch-node voltage waveforms for the MPQ4326-AEC1 (with FSS modulation) and step-down converter with a suboptimal set-up (with out FSS modulation). Supply: Monolithic Energy Techniques
MPQ4326-AEC1 makes use of frequency unfold spectrum modulation, whereas the step-down converter with the suboptimal set-up is ready to a relentless fSW. Usually, FSS modulation leads to decrease fundamentals and harmonics. Whether or not FSS modulation or a relentless frequency is extra advantageous relies on the necessities of the applying. Nevertheless, FFT reveals the variations between the 2 strategies.
MPQ4326-AEC1’s FFT reveals the switch-node resonance at 450 MHz, and the step-down converter with the suboptimal set-up reveals the switch-node resonance at 320 MHz. These switch-node resonance frequencies could be discovered within the CISPR 25 Class 5 measurements.
Perceive switch-node waveform
This text analyzed the connection between the switch-node voltage waveform and the frequency area, utilizing MPQ4323M-AEC1, MPQ4326-AEC1, and MPQ4371-AEC1 automotive step-down converters as examples. Understanding the switch-node waveform permits predicting PCB habits for CISPR 25 Class 5 measurements. The measured resonance frequency reveals up in RE measurements, enabling improved EMC filter design for suppressing the resonance frequency.
Moreover, it’s doable to evaluate anticipated frequency vary interferences at an early stage by understanding the switch-node waveform. This helps discover a appropriate step-down converter in response to the applying specs, shorten growth instances, and cut back prices by simplifying element choice for the EMC filter.
Ralf Ohmberger is a employees purposes engineer at Monolithic Energy Techniques (MPS).
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