Can Asynchronous DC-DC Boost Converters (Contain Freewheeling Diodes) Still Achieve Low Emissions?

[Introduction]This article attempts to show how asynchronous converters with discrete freewheeling diodes can also achieve low emissions. It will describe the different types of converters, layouts, and packaging, and why controlled switching is so effective. It will also detail the pass test results of low-EMI evaluation circuits in CISPR 25 Class 5 radiated testing.

Introduction

Synchronous Silent Switcher® converters have set the gold standard for powerful, compact and quiet DC-DC conversion. Over the past 5+ years, we have learned about a large number of these low EMI synchronous buck and boost converters. These DC-DC converters simplify system-level EMC design in high-power, noise-sensitive environments, such as cold-crank pre-boosting, driving high-current LED strings, and high-voltage power amplifier sound systems. Monolithic (integrated power switch) boost regulators offer a more compact and efficient solution than controller-based designs, typically for 5 V, 12 V, and 24 V supply voltages.

The integrated synchronous switch and its unique layout in silicon1 are part of the “secret” of the Silent Switcher converter. On-board (integrated) switches create very tiny thermal loops, helping to minimize radiation. However, this can lead to increased costs and not all applications require synchronized switching. The cost of switching converters can be reduced if only a single power switch is integrated into the silicon chip, and an external low-cost discrete freewheeling diode can be relied on as a second switch. This practice is common in lower cost converters, but if low emissions are important, can it still be the case?

Asynchronous converters with discrete freewheeling diodes can still achieve low emissions. It is possible to use asynchronous converters for low EMI switching applications if special attention is paid to the hot loop layout and dV/dt switching edge rate in the design. It is necessary to integrate additional measures to reduce radiation in spread spectrum (SSFM). Monolithic switching regulators, such as the LT3950 60 V, 1.5 A asynchronous LED driver and LT8334 40 V, 5 A asynchronous boost converter, each integrate a single low-side power switch in the device, but rely on external freewheeling diodes, while still achieving low radiation! How does it work?

Can Asynchronous DC-DC Boost Converters (Contain Freewheeling Diodes) Still Achieve Low Emissions?

Figure 1. (A) An asynchronous monolithic boost converter has a single hot loop that includes an external freewheeling diode.

(b) Silent Switcher converter with two (opposite) thermal loops and fully integrated switches.

Freewheeling Diode vs Dead Time

Integrating one power switch instead of two in a single-chip converter can reduce chip size by 30 to 40 percent. The reduction in chip size can directly save the cost of silicon chips, and further secondary cost reductions can be achieved when silicon chips can be integrated into smaller packages. While some PCB space still needs to be dedicated to external discrete freewheeling diodes, these diodes are plentiful, reliable, and inexpensive. In boost converters, Schottky diodes with low VF have high efficiency at high output voltages and low duty cycles, arguably outperforming expensive high-voltage power FETs.

One of the reasons could be because of dead time. In a typical synchronous converter, power switch body diode conduction occurs during a preset dead time to prevent potential shoot-through problems. If the synchronous switch opens before the main switch can fully close, shoot-through occurs, causing the input or output (buck or boost) to be shorted directly to ground. At high switching frequencies and minimum and maximum duty cycle limits, dead-time control can be a limiting factor in switch design. By using low cost freewheeling diodes with low forward voltage, there is no need to provide dead time logic in the switch – very simple. In most cases, they also outperform the positive voltage drop of the body diode inherent inside the power switch (which conducts during dead time).

Simple layout and packaging

First, we can show the basic layout by starting with a simple single-chip boost converter. The LT3950 60 V, 1.5 A LED driver in Figure 2 has a simple PCB thermal loop. This hot loop (highlighted in Figure 3) consists only of small ceramic output capacitors and a similarly sized discrete freewheeling diode PMEG6010CEH. These components fit tightly into the LT3950 16-pin MSE package, as well as the switch pins and GND plane of the thermal pad. Is this enough to achieve low radiation? This is of course only part of the formula. The wire-bonded 16-pin MSE package and tight thermal loop combined with SSFM and well-controlled switching behavior (switching power transitions do not ring due to very high speeds and parasitic trace inductance) allow for low emissions.

Figure 2. The LT3950 (DC2788A) asynchronous hot loop includes a D1 freewheeling diode.

Still, the freewheeling diode and output capacitor fit tightly into the LT3950 16-pin MSE package.

The highlighted asynchronous switch node is small and compact, but not impossible. The layout of the switch node can be the key to achieving low radiation results.

Figure 3. The LT3950 LED driver is an asynchronous monolithic 1.5 A, 60 V boost converter.

The boost converter hot loop (highlighted in yellow) contains a discrete freewheeling diode that does not attenuate high frequency radiation.

Next, a single switch of an asynchronous converter can be used to form a SEPIC topology (boost and buck) to extend its usefulness beyond the intended boost use. Because it is a single switch, it is easy to break the hot loop of the boost converter and add the SEPIC coupling capacitor in it, as shown in Figure 4 and Figure 5. The top and bottom switches of most synchronous boost converters are permanently connected to a single switch node, so they cannot be converted to SEPIC. The SEPIC hot loop can be kept small if more attention is paid to the loop formed by the coupling capacitor, freewheeling diode, and output capacitor.

Figure 4. The LT8334 40 V, 5 A asynchronous monolithic boost IC is used in SEPIC applications.

The SEPIC converter hot loop (highlighted in yellow) contains a discrete freewheeling diode and a coupling capacitor that does not attenuate radiation.

Figure 5. The LT8334 monolithic 40 V, 5 A asynchronous switch integrated into a tiny 4 mm × 3 mm 12-pin thermally enhanced DFN package.

The thermal loop layout of the LT8334 SEPIC (EVAL-LT8334-AZ) includes this tiny DFN,

A ceramic coupling capacitor, a ceramic output capacitor, and a small freewheeling diode.

An integrated 5 A, 40 V switch is included in the LT8334 asynchronous boost converter. This monolithic boost converter IC is suitable for building 12 V output SEPIC converters. Figure 4 shows a standard 12 V, 2 A+ SEPIC converter with two Inductor coils for coupling capacitor C1 and coupling inductor. Since the tiny PMEG4030ER freewheeling diode D1 is not directly attached to the switch node, a 4.7 μF 0805 ceramic DC blocking coupling capacitor can easily be placed between the diode and the switch node. On the EVAL-LT8334-AZ SEPIC evaluation board, the thermal loop layout is kept small. Keeping the copper area of ​​the switch node as small as possible and as close to the switch pins as possible helps minimize radiated disturbances. Note that the entire hot loop is laid out on layer 1 and there are no vias on the switch node, or the coupled switch node on the other side of the coupling capacitor. These switch nodes should be kept as small and close as possible to achieve good results. The LT8334’s 12-pin DFN package helps keep thermal loops and radiation as small as possible.

Controlled switches are very efficient

Monolithic (including switches) switching converters can effectively help reduce emissions when used in combination with SSFM, 2 MHz fundamental switching frequency, excellent PCB layout, and well-controlled switching. If they are effective enough, it may not be necessary to take advantage of the huge advantages of Silent Switcher architecture in low radiation (Silent Switcher architecture is the ultra-low radiation gold standard, but if only to pass radiation standards, it is not necessary in all cases). In the LT3950 and LT8334, the SSFM expands up about 20% from the fundamental frequency and returns in a triangle pattern. SSFM is a feature common to low EMI switching regulators. There are several types of SSFM, but the overall goal of each type is to disperse radiated energy and reduce peak and average radiation below required limits. One goal of the 2 MHz switching frequency is to set the fundamental switching frequency above the AM radio frequency band (530 kHz to 1.8 MHz) limit so that radiation from the fundamental itself and all its harmonics does not interfere with the radio frequency. When the AM band is not a concern, lower switching frequencies can be used with confidence.

The internal switches and drivers are not affected by the switching frequency and care should be taken in the design to avoid some unwanted behavior that may degrade the EMI performance of the switching converter. Ultrafast ringing switching waveforms can produce unwanted emissions in the 100 MHz to 400 MHz range, which can be very noticeable in radiated disturbance measurements. A well-controlled switch in an IC should not behave like a radiating hammer, but rather like an effective rubber hammer with the edges of the switch suppressed. Controlled power switches can increase and decrease voltages and currents at slightly lower rates than possible. The 2 V/ns switching rate and lack of ringing in Figure 6b is a good example of such controlled switching in a monolithic converter. You can see that this internal switch turns on very softly and reaches 0 V without any subsequent harsh ringing. This contributes significantly to the LT3950’s radiation results (refer to Figures 9 to 11 below). Typically, in monolithic switching regulators, switching speed results in an increase in maximum power and a decrease in thermal performance. However, if you can carefully design, you can do more with less.

(a) (b)

Figure 6. The LT3950 controlled switch has a rising slew rate of 2 V/ns and a falling slew rate of 2 V/ns, helping to maintain high efficiency and low EMI in LED driver applications with virtually no switch node ringing .

Asynchronous Boost Controller with Gate Rate Control

In some cases, high-power DC-DC conversion requires the use of controllers and high-voltage, high-current switches external to the IC. In this case, the gate driver for the external switch remains inside the IC, but the entire switching hot loop moves outside the IC. Some creative hot loops and layouts are possible, but because of the size of the discrete MOSFETs themselves, the hot loops themselves tend to get larger.

The LT8357 high power (asynchronous) boost controller provides 24 V, 2 A (48 W) with very low emissions. It powers 3.5 mm × 3.5 mm MOSFETs at a low switching frequency for efficient conversion. In addition to a tight thermal loop (Figure 7), it enables edge rate control and radiation reduction through rising and falling gate control pins. Using a simple 5.1 Ω resistor RP (on GATEP) is sufficient to reduce the turn-on edge rate of the M1 power MOSFET and keep EMI disturbances as low as possible. Of course, some radiation filters and SSFM also help reduce radiation. The EVAL-LT8357-AZ evaluation board has additional space for radiation shielding, but for most applications it may not be necessary. This asynchronous boost controller is very similar to its monolithic version and has all the features required for high power, low EMI boost and SEPIC applications.

Figure 7. The LT8357 high-voltage boost controller has discrete gate pins to independently control the rising and falling edges of the switching edges of high-power discrete MOSFETs. The discrete gate pins are circled by yellow boxes.

Figure 8. The LT8357 boost controller in Figure 7 has excellent radiation and efficiency performance, RP = 5.1 Ω, RN = 0 Ω.

Separate gate drive pins allow controlled switch turn-on while providing fast turn-off.

In the schematic diagram, the colors represent: red RP = 0, RN = 5.1; yellow RP = 0, RN = 0;

green RP = 5.1, RN = 0; blue RP = 5.1, RN = 5.1.

Pass CISPR 25 Class 5 Radiation Standard

Low EMI evaluation circuits such as the LT3950 DC2788A have been extensively tested to evaluate their electromagnetic and conducted emissions. Figures 9 through 11 show the results of a successful radiation test with the SSFM turned on, with a 12 V input and 330 mA flowing through the 25 V LED string. Both current probe and voltage method CE results pass very strict limit criteria. In switches, FM band CE challenges are prone to occur, but the LT3950 is not affected by the FM band.

Figure 9. The DC2788A LT3950 passed (a) average and (b) peak CISPR 25 Class 5 conducted emissions tests (current probe method).

Figure 10. The DC2788A LT3950 passes (a) average and (b) peak CISPR 25 Class 5 conducted emissions tests (voltage method).

Figure 11. The DC2788A LT3950 passes (a) average and (b) peak CISPR 25 Class 5 electromagnetic emissions tests.

Setting the switching frequency to 2 MHz (adjustable range from 300 kHz to 2 MHz) allows the fundamental switching emissions to remain higher than the AM RF band (530 kHz to 1.8 MHz) without causing problems without adding Packs bulky LC AM band filters. Instead, the EMI filters used by the LT3950 can be small, high-frequency ferrite beads.

Despite the extra coupling capacitor in the hot loop and the extra port in the coupled inductor (doubling the number of switch nodes), the LT8334 SEPIC maintains low emissions. The EVAL-LT8334-AZ SEPIC 12 VOUT evaluation kit also uses 2 MHz and SSFM to provide low emissions. Similar performance can be achieved with the EVAL-LT8357-AZ boost controller. Complete radiation results, schematics, and test options for these devices can be found on analog.com and the corresponding product landing pages. Table 1 lists a new family of low-EMI asynchronous boost and SEPIC converters. Single-chip ICs and controller ICs are very practical because of their simple structure, low cost, multiple topologies, high power capabilities and low radiation. High current Silent Switcher boost converters can also be used when ultra-low emission is the primary requirement.

Table 1. New Low-EMI Monolithic Boost Converter with Switching Edge Rate Control

in conclusion

Both synchronous Silent Switchers and asynchronous monolithic switching regulators can be used for low radiation applications. Asynchronous boost converters are less expensive than ultra-high performance Silent Switcher converters. The second switch is replaced by a low-cost freewheeling diode, which has advantages at high voltages and can be flexibly reconfigured as a SEPIC. Both the small plastic package and the small carefully designed thermal switch loop area in the PCB provide low emissions when the power switching edge rate is well controlled and provides limited ringing. These features should be combined with other low EMI features such as SSFM and EMI filters. Gate drive control helps reduce and smooth switching edges for low emissions even in high power boost controllers. Pay special attention to the optimal top-level layout of the thermal loop and choose your DC-DC converter wisely for a low-e design. ADI’s family of low-EMI boost converters may be just what you need.

References

1 Steve Knoth, “High Power Density in a Small Size.” Analog Dialogue, Vol. 53, No. 4, October 2019.

About the Author

Keith Szolusha is Director of Applications at Analog Devices in Santa Clara, California, USA. Keith has been with the BBI Power Products Group since 2000, focusing on boost, buck-boost and LED driver products, while also managing the EMI room of the Power Products Division. He is a graduate of the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, with a BS in Electrical Engineering in 1997 and an MS in Electrical Engineering in 1998, specializing in technical writing. Contact: keith.szolusha@analog.com.

Kevin Thai is an applications engineer at Analog Devices in Santa Clara, California, USA. He is with the CTL Power Products Group and oversees the monolithic boost product family, as well as other boost, buck-boost and LED driver products. He received his bachelor’s degree in electrical engineering from Caltech in 2017 and his master’s degree in electrical engineering from UCLA in 2018. Contact: kevin.thai@analog.com.

By: Keith Szolusha, Director of Applications, Analog Devices Kevin Thai, Applications Engineer, Analog Devices

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