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《ADI power products make EMI design easier》

Another way to save space is to reduce the number of components required to meet electromagnetic interference (EMI) standards and thermal requirements. Unfortunately, in many cases, simply reducing the size of the converter is not enough to meet these requirements. This article introduces advanced solutions that save space while achieving low EMI and excellent thermal performance.

Switch-mode power converters are chosen for their high efficiency, especially at high step-down ratios, but this is weighed against the EMI generated by the switching operation. In a buck converter, EMI is generated by fast current changes in the switches (high di/dt) and switch ringing caused by parasitic inductance in the hot loop.

EMI is just one of the parameters that system design engineers must consider when trying to design compact, high-performance power supplies. Many key design constraints often conflict with each other, requiring significant compromises in design limits and time to market.

To reduce EMI in a buck converter, it is necessary to minimize the radiated effects of the hot loop and minimize the source signal. There are many ways to reduce radiated EMI, but many of them will also degrade the performance of the regulator.

For example, in a typical discrete FET buck regulator, the switching edges are slowed down by external gate resistors, boost resistors, or snubbers to reduce EMI, which is the last resort to meet the stringent radiated emission standards of the automotive industry. Such quick fixes to EMI problems come at the expense of performance; such as reduced efficiency, increased component count, and larger solution size. Slow switching edges increase switching losses and duty cycle losses. The converter must operate at a lower frequency (e.g., 400 kHz) to achieve satisfactory efficiency and pass mandatory electromagnetic radiation EMI tests. Figure 1 shows a typical switch node voltage waveform with fast and slow switching edges, respectively. As shown, the switching edge speed is significantly slower, resulting in increased switching losses and a significant increase in the minimum duty cycle or step-down ratio, not to mention other negative effects on performance.

Reducing the switching frequency also increases the physical size of the converter inductor, output capacitor, and input capacitor. At the same time, a large π filter is required to pass the conducted emission test. As the switching frequency decreases, the inductor L and capacitor C in the filter need to be increased accordingly. Under low-line full load conditions, the inductor current rating should be greater than the maximum input current. Therefore, a large inductor and multiple capacitors are required in the front end to meet stringent EMI standards.

For example, at a switching frequency of 400 kHz (instead of 2 MHz), in addition to increasing the size of the inductor and capacitor, the inductor and capacitor in the EMI filter must also be relatively large to meet the conducted EMI standards required in automotive applications. One reason is that they must attenuate not only the 400 kHz switching fundamental frequency, but also all harmonics up to 1.8 MHz. A regulator operating at 2 MHz does not have this problem. Figure 2 shows the size comparison of a 2 MHz solution and a 400 kHz solution.

Shielding may be a last resort to reduce EMI, but shielding takes up space that the application may not have available and requires additional mechanical design and testing iterations.

To avoid the AM frequency bandwidth and maintain a small solution size, switching frequencies of 2 MHz or higher are preferred for automotive applications. Once the AM band is avoided, it is only a matter of ensuring that high-frequency noise (also known as harmonics) and switching ringing are minimized. Unfortunately, high-frequency switching typically causes electromagnetic emissions to increase from 30 MHz to 1 GHz.

Some switching regulators have fast and clean switching edges to reduce EMI, such as the Silent Switcher® devices in the ADI Power family ^. Let’s first look at some other useful features.

Figure 1. Slow switching edges mean that there are significant switching losses in addition to duty cycle losses.

Figure 2. Size comparison of 2 MHz solution and 400 kHz solution.

Spread spectrum frequency modulation (SSFM) is a technique that dithers the system clock within a known range, thereby distributing the EMI energy over the frequency domain. While the switching frequency selected for common switching power supplies is typically outside the AM band (530 kHz to 1.8 MHz), unmodulated switching harmonics within the AM band may still not meet stringent automotive EMI requirements. Adding SSFM functionality can significantly reduce EMI within the AM band and other areas.

Figure 3. The ultralow EMI LT8636 5 V/5 A step-down converter in spread spectrum mode with 7 A peak current and an operating voltage range of 5.7 V to 42 V.

Figure 3 shows an ultra-low EMI and high efficiency 12 V to 5 V/5 A converter using the LT8636 Silent Switcher monolithic buck regulator operating at a 2 MHz switching frequency. Figure 4 shows the conducted and radiated EMI performance of the test demonstration circuit at 14 V input and 5 V, 5 A output. On the front end, small inductors and ceramic capacitors help filter out conducted noise, while ferrite beads and ceramic capacitors help reduce radiated noise. Two small ceramic capacitors are placed on the input and ground pins to minimize the hot loop area while separating the hot loops and helping to eliminate high frequency noise.

To improve EMI performance, the circuit is set to operate in spread spectrum mode: SYNC/MODE = INTVCC. Triangular frequency modulation is used to adjust the switching frequency from the value set by RT to about 20% higher, that is, when the LT8636 is set to 2 MHz, the frequency will vary between 2 MHz and 2.4 MHz at a 3 kHz rate.

It is clear from the conducted EMI spectrum that the peak harmonic energy is spread out, reducing the peak amplitude at any particular frequency—the noise is reduced by at least 20 dBμV/m due to the spread spectrum feature. It is also clear from the radiated EMI spectrum that the spread spectrum mode also reduces radiated EMI. The circuit meets the stringent automotive CISPR 25 Class 5 radiated EMI requirements with only a simple EMI filter on the input side.

Figure 4. CISPR 25 radiated EMI with and without spread spectrum mode.

The number of electronic devices in automotive applications has only increased, and most devices require more supply current with each design iteration. With active load currents this high, heavy-load efficiency and proper thermal management become primary considerations. Reliable operation depends on thermal management, and uncontrolled heat generation can lead to costly design issues.

System designers are also concerned about light-load efficiency, which is just as important as heavy-load efficiency because battery life is determined primarily by the quiescent current at light or no load. Full-load efficiency, no-load quiescent current, and light-load efficiency must be weighed in silicon and system-level design.

_{To achieve high efficiency at full load, the R DS(ON)} of the FETs, especially the bottom FET, should be minimized , which seems simple. However, transistors with low R _DS(ON) usually have relatively high capacitance, which increases switching and gate drive losses, and also increases die size and cost. In contrast, the LT8636 monolithic regulator has very low MOSFET conduction resistance and high efficiency under full load conditions. The LT8636 has a maximum output current of 5 A continuous current and 7 A peak current in still air without any additional heat sink, which simplifies reliable design.

To improve light load efficiency, regulators operating in low ripple Burst Mode® charge the input capacitor to the desired output voltage while minimizing input quiescent current and output voltage ripple. In Burst Mode, current is delivered to the output capacitor in short pulses followed by a relatively long sleep period during which most of the control (logic) circuitry is turned off.

To improve light load efficiency, a larger value inductor can be selected because more energy can be delivered to the output during the short pulses, and the buck regulator can also stay in sleep mode longer between each pulse. By extending the time between pulses as much as possible and minimizing the switching losses of each short pulse, the quiescent current of a monolithic buck converter can reach 2.5 μA in a monolithic regulator such as the LT8636. Typical parts on the market are tens or even hundreds of μA.

Figure 5 shows a high efficiency solution for 3.8 V/5 A output from a 12 V input for an automotive application using the LT8636. The circuit runs at 400 kHz to achieve high efficiency and uses a XAL7050-103 10 μH inductor. Efficiency is maintained over 90% at loads as low as 4 mA and as high as 5 A. Peak efficiency is 96% at 1 A.

Figure 5. Efficiency of a 12 V to 3.8 V/5 A solution using the XAL7050-103 inductor (f _SW = 400 kHz).

Figure 6 shows the efficiency of the solution from 1 μA to 5 A. The internal regulator is powered from the 5 V output through the BIAS pin to minimize power dissipation. Peak efficiency is 95%; full load efficiency is 92% for a 5 V output from a 13.5 V input. Light load efficiency remains at or above 89% for loads as low as 30 mA for 5 V applications. The converter was run at 2 MHz with the XEL6060-222 inductor tested to optimize heavy and light load efficiency in a relatively compact solution. Light load efficiency can be further improved to above 90% with a larger inductor. The current in the feedback resistor divider is minimized when it appears at the output as load current. .

Figure 6. Efficiency of 13.5 V to 5 V and 3.3 V solutions using the XEL6060-222 inductor and LT8636 (f _SW = 2 MHz).

Figure 7 shows the thermal performance of the solution at 4 A constant load and 4 A pulsed load (8 A pulsed load total) and 10% duty cycle (2.5 ms)—13.5 V input in still air ambient room temperature. Even at 40 W pulsed power and 2 MHz switching frequency, the LT8636 case temperature remains below 40°C, allowing the circuit to safely operate up to 8 A for short periods of time without a fan or heat sink. This is achieved in a 3 mm × 4 mm LQFN package due to the thermally enhanced packaging technology and the high efficiency of the LT8636 at high frequencies.

Figure 7. Thermal map of the 3 mm × 4 mm LT8636 showing temperature rise under 13.5 V to 5 V/4 A constant load plus 4 A pulsed load (10% duty cycle).

Space is increasingly at a premium in automotive applications, so power supplies must be reduced in size to fit on the board. Increasing the switching frequency of the power supply allows for the use of smaller external components such as capacitors and inductors. Also, as mentioned previously, switching frequencies above 2 MHz (or below 400 kHz) keep the fundamental frequency outside the AM radio band in automotive applications. Let’s compare a commonly used 400 kHz design to a 2 MHz design. In this case, increasing the switching frequency fivefold to 2 MHz reduces the required inductance and output capacitance to one-fifth of that of the 400 kHz design. Seems like a no-brainer. However, even ICs that support high frequencies may not work in many applications, as using a high-frequency solution comes with its own trade-offs.

For example, high-frequency operation in high step-down ratio applications requires a low minimum on-time. Based on the equation V _OUT = T _ON × f _SW × V _{IN , a minimum switch on-time (T ON} ) of about 50 ns is required to produce a 3.3 V output voltage from a 24 V input voltage at a 2 MHz operating frequency . If the power IC cannot achieve this low on-time, pulses must be skipped to maintain a low regulated output—essentially defeating the purpose of a high switching frequency. In other words, the equivalent switching frequency (due to pulse skipping) could be in the AM band. With a minimum switch on-time of 30 ns, the LT8636 allows direct conversion from high V _IN to low V _{OUT at 2 MHz. In contrast, many devices are limited to a minimum of >75 ns, requiring them to operate at a low} frequency (400 kHz) to achieve a higher step-down ratio to avoid skipping pulses.

Another common problem with high switching frequencies is that switching losses tend to increase. Switching-related losses include switch conduction losses, turn-off losses, and gate drive losses—all of which are approximately linearly related to the switching frequency. Reducing the switch on and off times can improve these loss characteristics. The LT8636 switch on and off times are very short, less than 5 V/ns, which can achieve minimum dead time and minimum diode time, thereby reducing switching losses at high frequencies.

The LT8636 used in this solution features a 3 mm × 4 mm QFN package and a monolithic structure with integrated power switches while providing all the necessary circuit functions, which together form a solution with minimal PCB space occupation. The large exposed ground pad under the IC conducts heat to the PCB through a very low thermal resistance (26°C/W) path, reducing the need for additional thermal management. This package is designed to be FMEA-compliant. Silent Switcher technology reduces the PCB area of ​​the hot loop, so the radiated EMI problem at this high switching frequency can be easily solved with a simple filter, as shown in Figure 3.

With careful IC selection, compact, high-performance power supplies for automotive applications can be produced without repeated trade-offs. That is, high efficiency, high switching frequency, and low EMI can be achieved simultaneously. To illustrate the compact design that can be achieved, the solution chosen in this article uses the LT8636, a 42 V, 5 A continuous/7 A peak monolithic step-down Silent Switcher regulator in a 3 mm × 4 mm LQFN package. In this IC, the V _IN pins are separated and placed symmetrically on the IC, which separates the high-frequency hot loop and makes the magnetic fields cancel each other to suppress electromagnetic radiation EMI. In addition, the synchronous design and fast switching edges can improve heavy-load efficiency, while the low-ripple burst mode operation is beneficial to light-load efficiency.

The LT8636’s 3.4 V to 42 V input range and low dropout voltage also make it suitable for automotive applications, enabling it to operate during automotive start-up or load dump conditions. In automotive applications, system designers often face many trade-offs when trying to reduce the size of the power solution, but with the design in this article, designers can achieve all performance goals without making trade-offs.