The LT8650S dev board for the Silent Switcher 2 buck converter. Image from ADI
Here's a quick look at the problems the Silent Switcher 2 hopes to help engineers address, including board space and thermal management.
Dealing with EMI at the Component Level
As integrated-circuit manufacturers continue to reduce switching edge-rates, electromagnetic interference becomes an increasing issue for PCB designers. During a recent interview at APEC 2019, Analog Devices' Tony Armstrong told AAC, "One of the problems with switching regulators is that they create EMI emissions and noise for downstream devices."
EMI emissions are formed when quickly changing currents combine with wire, package, and trace inductance to create electromagnetic waves that can disrupt distant parts of a circuit and prevent FCC and EU device certification.
There are several ways to deal with these emissions. Some designers add magnetic shielding to the switching portion of the IC, while others use resistors to lower the switching speed.
By sending identical current surges through the IC along mirrored paths, the emissions created along one inductive loop naturally cancel identical emissions from a second inductive loop. Image of an example circuit from ADI. Image altered by Mark Hughes.
Analog Devices decided to eliminate the emissions at the component level by using a trick known for decades: opposing current paths can self-cancel most EMI emissions inside the chip.
Bypass capacitors are included in the IC package to minimize the area of the inductive loops, further reducing emissions.
Magnetic Cancellation via an Old Trick
Armstrong explains an old military technology of magnetic cancellation in this way: "If you have opposing fields on opposite sides of the IC, and [apply] the right-hand rule, you can cancel the opposing fields out."
Image of Ampere's Right Hand Rule courtesy of Jose Fernando [CC BY-SA 4.0]
"To do the magnetic cancellation, we had to redesign the IC. We basically took the IC, broke into two parts, and then tied those two parts together again at the output," notes Armstrong. "This keeps the EMI extremely low, as much as 30 to 40 dB compared to competitive alternatives."
The result? "If you use Silent Switcher technology in systems, it is not going to cause any potential interference with noise-sensitive Bluetooth or transmission protocol circuits that might be in that system. Designers don't have to spend a lot of time and effort and/or money on shielding and filtering to mitigate noise."
Advancements with Silent Switcher 2
Silent Switcher 2 offers two key innovations over the first generation Silent Switcher:
- Filter caps are now included inside the chip.
- Lead-frame copper pillars are used for enhanced thermal performance.
"The key for us has always been this nominal 3-to-42-volt input to address the single battery automotive market," notes Armstrong. The Silent Switcher 2 has units that can handle voltage inputs at 5.5V, 8V, 18V, 20V, 42V, as well as 65V input. "The 60 V family is relatively modest right now but it is growing and this is for double battery applications as well as telecom."
At APEC 2019, Analog Devices was introducing a new 5V family, the LTC33xx. Armstrong states that the family is designed for "telecom applications and optical transport systems. The individual transports within the transmission backbone system are thermally challenged and require highly efficient ICs."
Many of these systems are sealed and are not able to dissipate heat well, so even as little as 1° C of temperature increase can lead to thermal runaway events during extended use. Any improvements in thermal efficiency are welcome and necessary to enter this space.
A thermal image of an example 72 W output, 24 V input circuit using the Silent Switcher 2. Image from ADI
Spread Spectrum Beyond Required Standards
The CISPR 25 Limit sets international electromagnetic emission limits for automobiles. The Silent Switcher 2 technology produces EMI emissions that are 30-40 dB below current regulation standards.
Example EMI/EMC curve for the LT8650S. Image from ADI
Shown above is the EMI/EMC Curve using spread-spectrum on the LT8650S from Analog Devices. The horizontal axis shows the frequency in MHz and the vertical axis shows Amplitude in dBuV/m. CISPR 25, and CLASS 5 Limits shown above in Orange, actual measurements of the LT8650 shown in Blue.
Have you worked with the Silent Switcher series before? What other methods of EMI reduction have you heard of being applied at the component level? Share your expertise in the comments below.