Technical Articles/White Papers

Low Noise Silent Switcher Architecture Simplifies EMI Design

Automotive, transportation, and industrial applications are noise sensitive and demand low EMI power solutions. Traditional approaches control EMI with slowed down switching edges or lower switching frequency. Both have undesired effects, such as reducing efficiency, increasing minimum on- and off-times, and requiring a large solution. Alternative solutions such as an EMI filter or metal shielding add significant costs in required board space, components, and assembly, while complicating thermal management and testing.

Our low noise μModule® technology offers a breakthrough in switching regulator design. The LTM8003 regulator within the µModule package uses a proprietary Silent Switcher® architecture to minimise EMI emissions while delivering high efficiency at high switching frequencies. The architecture of the regulator and the internal layout of the µModule device are designed so that the input loop of the regulator is minimised. This significantly reduces the switching node ringing and the associated energy stored in the hot loop, even with very fast switching edges. This quiet switching offers excellent EMI performance while minimising the ac switching losses, allowing the regulator to operate at high switching frequencies without significant efficiency loss.

This architecture, combined with spread spectrum frequency operation, greatly simplifies the EMI filter design and layout, which is ideal for noise sensitive environments. Figure 1 shows a simple EMI filter on the input side, enabling the demo circuit to pass the CISPR 25 Class 5 standard with plenty of margin, as shown in Figure 2.

Figure 1. A 5V converter with a simple EMI filter at the input passes CISPR 25 Class 5.

Figure 2. DC2416A demonstration circuit passes radiated EMI spectrum CISPR 25 Class 5.

Continuous 3.5A with Peak Current Capability 6A

The internal regulator is capable of safely delivering up to 6A of peak output current, and no extra thermal management – airflow or heat sink – is required for the LTM8003 to continuously support a 3.5A load at 3.3V or 5V from a nominal 12V input. This meets the needs of the battery-powered applications in industrial robotics, factory automation, and automotive systems.

Wide Operation Temperature Range from –40°C to +150°C

Automotive, industrial, and military applications require power supply circuits to operate continuously and safely in ambient temperatures over 105°C or require significant headroom for a thermal rise. The LTM8003H is designed to meet specifications over a –40°C to +150°C internal operating temperature range. The internal over-temperature protection (OTP) monitors the junction temperature and stops switching when the junction temperature is too hot.

Figure 3a shows a 3.5A, 5V solution that operates from a wide-ranging 7V to 40V input. The thermal performance at a nominal 12V input is shown in Figure 3b. The typical efficiency is above 92% with a 12V input and 2A load.

Figure 3. A 5V, 3.5A solution for 7V to 40V inputs using the H-grade version. Thermal imaging shows no need for bulky heat mitigation components.

Negative Output –5V from +3.5V to +35V Input

Figure 4 shows a solution for a –5V, 4A output from a nominal 12V input, with a maximum of 35V input. The BIAS pin should be connected to GND.

Figure 4. A –5V supply from a +5V to +35V input delivers current up to 4A.

Conclusion

The LTM8003 is a wide input and output range, low noise, 3.5A step-down µModule regulator featuring the Silent Switcher architecture. Inputs from 3.4V to 40V can produce outputs from 0.97V to 18V, eliminating the need for intermediate regulation from batteries or industrial supplies. The pinout is specifically designed to be FMEA compliant, so the output stays at or below the regulation voltage during adjacent pin shorts, single-pin shorts to ground, or pins left floating. Redundant pins enhance electrical connections in the event a solder joint weakens or opens due to vibration, aging, or wide temperature variations, such as in automotive and transportation applications.

A complete solution fits a compact space not much larger than the 6.25mm × 9mm × 3.32mm, BGA footprint of the LTM8003, including the input and output capacitors. The quiescent current of typically 25µA and wide temperature operation from –40°C to +150°C (H-grade) make it ideal for circumstances where space is tight, the operational environment is harsh, and low quiescent current and high reliability are mandatory. Its features help minimise design effort and meet the stringent standards for industrial robotics, factory automation, avionics, and automotive systems.

Figure 5. A complete step-down solution is barely larger than the 6.25mm × 9mm footprint of the LTM8003 µModule regulator.

About the Author

Zhongming Ye is a senior applications engineer for power products at Analog Devices, Inc., in Milpitas, California. He has been working at Linear Technology (now part of ADI) since 2009 to provide application support on various products including buck, boost, flyback, and forward converters. His interests in power management include high performance power converters and regulators of high efficiency, high power density, and low EMI for automotive, medical, and industrial applications. Prior to this, he worked at Intersil for three years on PWM controllers for isolated power products. He obtained a Ph.D. in electrical engineering from Queen’s University, Kingston, Canada. Zhongming was a senior member of IEEE Power Electronics Society. He can be reached at zhongming.ye@analog.com.

Electric vehicles, large-scale battery storage stacks, home automation, industrial, and telecom power all require converting high voltages to ±12 V, where dual polarity rails are required for powering amplifiers, sensors, data converters, and industrial process controllers. One challenge in all of these systems is creating a compact, efficient dual polarity regulator that can operate over a temperature range of –40°C to +125°C—especially important in automotive and other high ambient temperature applications.

Linear regulators are well understood and typically top the list of candidates for bipolar supplies, but are not suitable in the high input voltage, low output voltage applications previously mentioned, mainly due to heat dissipation in the linear regulator at high step-down ratios. Furthermore, a dual polarity solution requires at least two integrated circuits (ICs): one positive output linear regulator and a negative converter. A better solution would use a single switching regulator that produces both outputs from a relatively high input, at good efficiency and regulation, while fitting tight spaces and reducing cost.

This article presents two elegant circuits that generate ±12 V outputs from a wide 30 V to 400 V input voltage range, both using a single high voltage LT8315 converter. One circuit is an isolated flyback topology; the other is based on a nonisolated buck topology. The LT8315 itself is a high voltage monolithic converter with integrated 630 V/300 mA MOSFET, control circuitry, and high voltage start-up circuit, inside a thermally enhanced 20-lead TSSOP package.

Isolated Dual Polarity Flyback Regulator with No Optocoupler

Flyback converters are widely used in multi-output applications to provide galvanic isolation, improve safety, and enhance noise immunity. Outputs can be positive or negative, depending on which side of the output is grounded. Traditionally, the output voltage regulation is achieved using optocouplers to transfer information from the secondary-side reference circuitry to the primary side. The problem is that optocouplers add significant complexity and degrade reliability due to propagation delay, aging, and gain variation, etc. Typically, the one output connected to the feedback pin of the IC dominates the regulation loop, while other outputs are loosely controlled through the transformer windings, resulting in poor regulation of those outputs.

The LT8315 requires no optocoupler and samples the reflected, isolated output voltage from a tertiary winding on the power transformer. Also, the output voltage is sensed when the secondary current is almost zero to achieve excellent load regulation. In a dual output design, this unique sensing scheme allows each output to be closely regulated—both outputs can dominate the regulation. As a result, a typical ±5% load regulation is easily achieved.

The LT8315 solution shown in Figure 1 operates under quasi-resonant boundary conduction mode. The primary MOSFET has a minimum turn-on loss because the MOSFET turns on when the switch node rings to its valley. There is no diode reverse recovery loss on the secondary side. A 3 kV reinforced insulation transformer is the only component across the isolation barrier, enhancing system reliability and meeting stringent high voltage power isolation requirements. Figure 2 shows the full load efficiency curve under different input voltages. This flyback converter achieves 85.3% peak efficiency when the input is 70 V and both load currents are 50 mA.

Figure 1 shows the complete schematic of a flyback converter with a wide input range from 30 V to 400 V. It outputs ±12 V and maintains tight regulation with load currents from 5 mA to 50 mA. This flyback converter has 85.3% peak efficiency, as shown in Figure 2.

Figure 1. A complete ±12 V/50 mA isolated flyback converter for a wide input range, 30 V to 400 V.

Figure 2. Full load efficiency vs. input voltage for the flyback converter in Figure 1.

Figure 3. Schematic of a nonisolated dual inductor buck converter using a single LT8315 IC: 30 V to 400 V input to ±12 V outputs at 30 mA each.

Nonisolated Dual Polarity Buck Regulator with Two Inductors

The LT8315’s high voltage input ability can be applied in nonisolated solutions by using off-the-shelf inductors. A buck regulator with dual inductors, requiring only a few components, is shown in Figure 3. This converter accepts an extremely wide-ranging input—30 V to 400 V—and produces ±12 V/30 mA outputs. This circuit can achieve efficiency as high as 87% at full load for both outputs with a 30 V input.

In this topology, LT8315’s GND pad is intentionally ungrounded and connected as the common switch node for driving both outputs. For PCB layout, LT8315’s GND pad’s size should be constrained within the exposed pad area to reduce electromagnetic interference to other components because the GND trace is a relatively noisy switch node in this topology. Diode D2 and two 1% resistors at the FB pin form the feedback path regulating the positive output voltage. D2 is necessary to prevent the FB pin discharging whenever the MOSFET conducts. The resistive voltage divider does not need to take into account the forward voltage drop of D2 because the forward voltage of D2 and D3 are equal and cancel; therefore, the feedback network tracks and closely regulates the positive output voltage.

The negative rail comprises a low voltage coupling capacitor CFLY, a second inductor L2, a catch diode D4, and the negative output capacitor CO2. According to the inductor volt-second balance for the circuit loop of CO1-L1-CFLY-L2, the average voltage across L1 and L2 is zero, so the coupling capacitor CFLY’s voltage is equal to the positive output voltage. CFLY charges up L2 during the on-time of the MOSFET, while D4 provides a path for the L2 discharge during the MOSFET off-time. The negative output voltage is indirectly regulated based on the voltage of CFLY remaining constant and equal to the positive output voltage. As shown in the regulation curve of Figure 4, the negative supply maintains ±5% regulation for a load range of 3 mA to 30 mA at various input voltages, when the positive load is at a full 30 mA.

Figure 4. Negative 12 V load regulation curves at various input voltages for the dual inductor buck converter in Figure 3.

Conclusion

This article presents two dual polarity converter solutions for a wide 30 V to 400 V input range: one isolated, the other nonisolated. The LT8315 is used in both, due to its high voltage integrated MOSFET, no optocoupler feedback loop, and internal high voltage startup circuit. Other features include low ripple Burst Mode® operation, soft start, programmable current limit, undervoltage lockout, temperature compensation, and low quiescent current. LT8315’s high level of integration simplifies the design of high voltage input and dual polarity output circuits for a wide variety of applications.

About the Author

Zhijun (George) Qian is a senior engineer at Analog Devices. He is responsible for power product applications of various nonisolated and isolated converters. He obtained his B.S. and M.S. from Zhejiang University, and his Ph.D. from University of Central Florida, all in power electronics. He joined Linear Technology (now part of ADI) in 2010. He can be reached at george.qian@analog.com.

About the Author

William Xiong graduated from Cal Poly, San Luis Obispo in 2017 with a bachelor’s degree in electrical engineering. He started working at Analog Devices as an applications engineer in July 2017 and works with buck, boost, and isolated topologies such as flyback and forward converters. He can be reached at william.xiong@analog.com.