COVER STORY
Unique Silent Switcher designs offer high effi ciency at high switching frequencies >2MHz with ultra-low EMI
An important aspect of power supply design is ensuring it can achieve a high effi ciency, typically 90%+, whilst at the same time meeting all regulatory standards that apply. In order to achieve high effi ciency and low heat dissipation, switching regulators are replacing traditional linear regulators. The switching regulator is typically the fi rst active component on the input power line, and therefore has a signifi cant impact on the Electromagnetic Interference (EMI) performance of the complete circuit.
Getting the balance between high effi ciency and low EMI right whilst also meeting standards around radiated EMI emissions is a tough challenge for designers. A non-compliant design can be very costly not just because of the retest fees incurred but also from a time-to-market perspective. In fact, the most common reason power supply designs go over budget and miss their target completion schedule is the PCB failing during EMI compliance testing. Fortunately, there is help at hand, Matthew Johnson, Divisional Engineering Manager at Anglia introduces several high-performance switching regulators from Analog Devices that bring new levels of performance to synchronous converters. Combining high frequency switching, high effi ciency, and ultra-low EMI, these switching regulators offer very compact power solutions.
Matthew Johnson, Divisional Engineering Manager Silent Switcher®
Typical techniques for reducing radiated emissions can include spread spectrum frequency modulation, slew rate control, or use of modules with internal fi ltering and shielding. With Silent Switcher technology, Analog Devices have looked at the fundamental design of a switching regulator and developed parts from the ground up which meet Radiated EMI standards such as CISPR25 out of the box.
In a buck regulator for example, the Silent Switcher design uses two high side FETs to create opposing magnetic fi elds in the two high current loops which cancel each other out (Fig.1), the resulting effect is almost like enclosing the circuit in a metal box.
Figure. 1 – Magnetic Field Cancellation in High Current Loops
lead-frame, these can effectively act as min- iature antennas radiating EMI. Again, Silent Switcher has taken a ground up approach and employed a fl ip chip design which removes these bond wires completely reducing parasitic inductance and this source of EMI An extension to the technology, Silent Switcher 2, takes all the previous design principles and adds internal bypass capacitors and an integrated substrate to further improve EMI which is not sensitive to PCB layout. This helps to signifi cantly simplify designs and reduces performance risks even when using two-layer PC boards.
To validate the superior EMI performance, Analog Devices used a scope with a sniffi ng probe to compare the performance of a Silent Switcher device to a competitor part under the same conditions, this example demonstrated an impressive 40dB reduction in radiated emissions (Fig. 2).
More than 50 devices and modules featuring Silent Switcher® technology are already available from Analog Devices and increasing month by month, below are a small selection of these devices to suit a broad range of applications.
external clock. In addition, the device has output soft-start and output overvoltage lockout protection. The LT8337-1 variant of the device also allows external compensation for fast transient response.
LT8337
The LT8337 is a synchronous step-up (boost) switching regulator which boasts very low quiescent current. Featuring Silent Switcher architecture and optional spread spectrum frequency modulation (SSFM) to minimize EMI emissions this device delivers high effi ciency (>90%) at high switching frequencies and low output currents. The device features low radiated EMI emissions to meet the requirements of CISPR25 Class 5 Peak Limits (Fig.3).
Another common source of radiation can be the relatively long bond wires inside the device package that connect the die structure to the
8 September 2021
Figure. 2 – Comparison of Radiated Emissions vs. Competitor
Components in Electronics
The LT8337 series integrates 28V, 5A power switches, operating at a fi xed switching frequency programmable between 300kHz and 3MHz and synchronizable to an
Figure 3. Radiated EMI Performance (CISPR25 Class 5 Peak)
The LT8337 synchronous switching regulator has a wide input voltage range from 2.7V to 28V and a programmable output voltage up to 26V. The devices low VIN quiescent current in Burst Mode operation, and 100% duty-cycle capability for the integrated synchronous MOSFET in Pass Thru operation (VIN > VOUT) makes the LT8337 series ideal for battery-powered systems or any other general purpose step-up applications.
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