 Thank you, and welcome to ST Microelectronics' webinar on smart in-rush current limiting using time-risters for power factor correction topologies. I'm Jeff Halvin, Product Marketing Manager for Power Discreeds in the Americas, focusing on the industrial market setting. Very happy to have you tuned in today for this very interesting topic. Let's take a look at the agenda for today's webinar. First, we'll do an overview of in-rush current limiting and why it is important in many of today's offline power supplies. We'll take a look at the traditional passive and relay-based implementations of in-rush current limiting and how we can improve them with SCRs. Following that is a quick overview of ST's SCR technology that enables multiple techniques for ICL, and then we'll dive into smart solutions for ICL that require microcontrollers to implement the smart algorithms. Finally, we'll dive into in-rush current limiting for the bridge-less totem pole PFC topology and ST's demo board, implementing the solution. Then wrap up with an overview of ST's resources to help you with your design. So first, why do we need to implement in-rush current limiting anyway? Take a look. Make sure you have a home appliance that is powered down. No energy stored in its internal power supply. What happens when you plug that device into the wall? Depending on the size of the power supply or more specifically the bulk capacitance found internally, you can get a huge spike of energy in the form of in-rush current that goes to charge up the front end electronics of the power supply. You can see how that current waveform can look when flowing through a rectified bridge based on the graphics here. The peak of this current is not only dependent on the size of the bulk capacitance of the supply, but also factors like parasitic impedances, size of the input filter components, and natural differential voltage from input voltage to output on the front end. This can cause some substantial consequences if it's not controlled. Here we are looking at a system plugged into an AC line that also has a light connected to it as well as protection provided by a circuit breaker. Uncontrolled in-rush current can have some negative consequences to this system. One, it could trip the circuit breaker causing the inconvenience of resetting it and adding to the wear of this element. Another effect can be to disturb other loads on the AC line like the light bulb shown here. Perhaps you get some flicker or deep dim at the AC line as forced sag. In fact, IEC 61000-3-3 is a standard that regulates the amount that the AC line can be disturbed and is a driving factor toward implementing in-rush current limiting. Finally, in-rush current causes excessive stress to electronics and would require over-margining the system if it's not controlled. Clearly, this isn't a newly discovered phenomena, so there must be well-established means to control in-rush current, right? Right, so let's look at the tried-and-true method that we'll eventually want to improve upon with the latest technologies. The simplest way to limit the in-rush current is by placing a resistor in series with default capacitance that needs to be charged on startup of the system. The peak of the in-rush current will be proportional to the size of the resistor used, but so are also the power losses. To help mitigate those losses, often an NTC or negative temperature coefficient resistor is used as a current limiter. As it heats due to power loss, the series resistance decreases as due to power losses. However, it'll never drop to zero, so there will be continuous unwanted power loss in the system at all times. This also can be mitigated by using bypass elements, such as relays to shunt current around the resistor and minimize those conduction losses, but that too has some drawbacks to discuss. But first, let's peek at an active way to limit in-rush current. Another method, the main topic of this webinar in fact, is control of in-rush current by controlling the differential voltage applied across the front end of the power supply itself, in particular by means of intelligently firing a silicon-controlled rectifier or SCR. A timed gate current will turn on the SCR, which displaces a traditional diode and a rectifying bridge, and the SCR will remain on until the current reaches the zero crossing of the AC cycle. At that point, the SCR automatically switches off. In this fashion, we can control how much of the input AC voltage is applied and control the amount of energy in any given line cycle that is transferred to the bulk capacitance of the power supply. We'll go much deeper on this concept later in the discussion. But there are several implementations of SCRs for in-rush current limiting that can benefit a power supply, depending on the topology, power level, and control complexity. The simplest and first method we'll investigate deeply is the bypass-parallel configuration. It's useful to power levels up to about two kilowatts and has benefits of simple self-driving control, but a drawback of still using a series resistor to limit the initial in-rush current. More complex solutions, such as the mixed bridge with SCRs or SCR-controlled totem pole PFC, can eliminate the current limiting resistor altogether, but those require a microcontroller to implement smart algorithms that drive those SCRs. Much higher power levels can be optimized at the SCRs using these topologies, and we'll touch on all three. Take a step back to the simple series resistor method of in-rush current limiting. As previously mentioned, you can use a relay to bypass this resistor and largely eliminate the conduction losses due to ICL. Many implementations actually will use a second relay in series with the current limiting resistor too to implement a standby mode in which the power supply is fully disconnected from input power and not unnecessarily drawing power when not needed. One advantage of this approach is the isolation of the relay driving coil from the high voltage AC electronics. Another can be the cost, depending on the size of the relays needed, but there are many disadvantages. Coil current consumption adds to losses. Mechanical relays are subject to wear and potentially sparks. Relays have audible clicks when enabled. They can be bulky. Finally, there's no control over the soft start time, since it's purely set by the value of the NTC. We need to address these problems with our hero, the SCR. All right, how about using those active devices? Here we present the idea of bypassing the NTC, not with a relay, but with SCRs that run parallel to the diodes in the top of the AC bridge. In this fashion, the SCRs can act in place of the diodes in the bridge and conduct to the current around the NTC and into the bulk capacitance. Keep in mind, the SCRs here do not control the inrush current. They will be inactive during startup while the diodes conduct in the series NTC limits current. Once inrush is complete and the system is in steady state, we then bypass the NTC with the SCRs to improve efficiency. The advantages of the solution are its simplicity, its reliability, smaller size than relays, and efficiency due to natural ZVS and ZCS switching. Drawback against the relay solution is the required isolated auxiliaries power supply that are used to drive the SCRs. But we can easily create a simple unregulated supply off the PFC boost inductor. We have a detailed app note called AN4606 that can be referenced for design tips on how to do this. ST actually has a patented solution for driving the SCRs in a bypass configuration in such a fashion that you avoid unnecessary losses due to the reverse conduction laws caused by DC gate current into the SCR. A small low-cost circuit with some resistors and bipolar transistors between the AUX supply and the SCR gates provide natural small gate trigger currents that are sufficient to turn on the SCRs with the correct timing. Again, the details of the implementation can be found in app note 4606 on the ST website. No digital control is required whatsoever. To evaluate this solution, ST has a very small daughter board that can be tapped into any input bridge and tested. The board is called ST eval SCR001V1 and is orderable through any standard distribution channel implemented with our TN5015H-6G SCR and D2PAC package. It's been proven to operate without a heat sink for supplies up to 800 watts, largely in banks to the patented driving circuit. It's only 26 by 26 millimeters square and is easy to tap into any power supply for evaluation. Here are the results of a one kilowatt PMC tested with the dual SCR board in bypass configuration. From 10% to full load, the SCR solution improves upon the efficiency of a relay-based bypass solution while avoiding many of the relay drawbacks. You can see the average of about three quarters of a watt savings. Certainly a very attractive alternative to the most traditional methods of in-risk current limiting. And now that I've started to sell you on the benefits of SCRs for implementing in-risk current limiting solutions, let's diverge a moment to look at the ST portfolio of SCRs that are the real stars of this show. To start, we need to understand where a thyristor, and a thyristor is a more general term for the family of semiconductors that SCRs belong to, where thyristors derive their value proposition. It's really a combination of three elements. First is the thermal performance. Since an SCR is similar to a diode and that it's created by layers of P injunctions, there's really no optimizing its voltage drop. Optimizing thermal performance has to come from the combination of die size, packaging, heat thinking, and the maximum allowed junction temperature. This is how any given SCRs current rating will be derived. Second is the part surge handling capability. How does it perform when subjective transient voltage in current conditions? Last is the application robustness. And that's demonstrated by ratings of immunity to spurious turn-on and commutation turn-off. More detail can be found in several application notes found on the ST website that are dedicated to selecting just the right thyristor for your application. ST has a very wide portfolio of SCRs, ranging from low power, sensitive gate technologies to legacy standard gate parts, and now our H series high temperature range of devices that allows industry leading 150 degrees C operating junction temperature. We have devices with voltage ratings between 400 volts and 1200 volts, current ratings up to 80 amps in a discrete package, and gate trigger currents down to as small as microamps, a combination that leads to the most innovative and comprehensive SCR portfolio on the market. Our high temperature H series SCRs are the best suited for today's high power PFC requirements for regulation and in-rush current rating. They feature high noise immunity, the mentioned 150 degrees C operating junction temperature, and up to 80 amp current ratings. There's also a wide variety of packaging options, including many surface mount. The narrow trigger current tolerances allow for easy gate drive design, and the multiple thermal advantages of the technology can simplify your mechanical design greatly. Here's a snapshot of the portfolio and variety of packaging and parameters offered. The most popular packages such as TO220s and D2PACs are represented well, but also the large D3PAC, which allows for very high powered applications with single devices. Of course, you can view the entire portfolio at our website and dig into the details in our application notes and data sheets. Parts can be sampled through your favorite distributor or orderable right on the ST online store. It's not just in-rush current limiting that benefits from our SCR portfolio. The application base is wide. From AC switching and electric vehicles to industrial battery charging to solid state relays, there are a multitude of AC applications where SCRs provide a crucial component to the optimized power design. ST offers both industrial and automotive grade versions of almost every device in the portfolio, so you're covered no matter what the application. We even offer medical grade dice. Okay, back to the in-rush current limit application focus. Let's switch over to understanding how combining the SCR with the control capabilities of a microprocessor can enable even smarter in-rush current limiting solutions for high powered and highly efficient power PFC topologies. First, a snapshot of ST's proposals for a thyristor-based in-rush current limiting solutions. For low power, you can consider using a series triac driven by a microcontroller rather than a relay or NTC. The main benefit is size and control in the startup. The IHT008V1 is a readily available evaluation board to test the solution, and further details can of course be found on st.com. We've already discussed the SCR bypass solution highlighted by the SCR001V1 evaluation board and its applicability to systems in the 500 watts to two kilowatt range. For higher powered solutions, we can consider the mixed bridge topology demonstrated by eval board ISF003V1. And the newest entry to the ST demonstration solutions is the DPST-PFC1 that uses SCRs to control in-rush current limiting in the totem pole PFC topology. These advanced solutions allow for programmable soft start, zero current switching, increased system reliability, and compact efficient designs. The principle behind the smart in-rush current limiter is fairly simple but powerful. First, we fully replace two of the four rectified bridge dials with SCRs. No longer is a bypass mechanism, but now is constant elements to both startup and steady state operation. A microcontroller can then control the on time of the bridge against the phase of the line cycle to allow only gradually increasing burst of energy through to the PFC bulk capacitance. In this fashion, that capacitance is charged smoothly due to that phase ankle control. The algorithm gradually decreases the delay from the start of the line cycle to the firing of the SCR until the SCRs are fully on for their respective halves of the full cycle. Just as a diode would operate, but with a controlled turn on. Here we see this concept implemented in a mixed bridge topology, but it's equally as applicable in a bridgeless PFC as we'll see shortly. The benefits are based on the complete elimination of the series current limiting and TC resistor. We no longer have any resistor power dissipation and as a benefit can fully control the startup time while complying to the in-rush current limiting requirements of our system and standards like IEC 61000-3-3. There can also be benefits to system efficiencies, which we'll look at now. Here's a breakdown on losses between the mixed bridge with SCR topology and a standard in-rush current limit implemented with relays and an NTC for power supply applications above 500 watts. One obvious advantage of the SCR is in standby power losses. Relay coil consumption remains constant for the series RL1 enclosed relay while in standby at about 400 milliwatts for a typical device. The SCR, even with gate current constantly applied, is only consuming approximately 25 milliwatts, more than an order of magnitude lower. How about during full power operation at steady state though? Let's look at a 500 watt application first. We'll add the contact resistance losses and diode drop losses to calculate over two and a half watts of conduction power dissipated in the bridge and the relays. The mixed bridge sums the conduction losses found in the SCRs and those of the bridge diodes totaling only 1.8 watts, about three quarters of a watt saved and with no heat thinking required on the SCRs. At high power, in this case an 1,100 watt application, the results are even more dramatic leading to a savings of over two and a half watts. The SCR solution wins on both standby and steady state losses, making it a superior solution regardless of how often the application is likely to be in standby or fully powered on. We could also take advantage of the intelligent control afforded by the microprocessor to do a smart recovery during line dropout or brownout situations. For instance, a short transient event of line drop, in this case defined as less than 30 milliseconds, does not require any special treatment and the SCRs can be kept on. The system will recover naturally without much disturbance. However, for a more lengthy dropout, we can revert to the soft start algorithm for a nicely controlled recovery. One of the major applications for the mixed bridge input rectifier is in electric vehicle charging. The broad ST portfolio of power and control electronics allow for a full power trade enabled by advanced ST technologies. Here's an example implementation of a 10 kilowatt system enabled by ST power rectifiers, silicon carbide MOSFETs, and of course the H-Series 1200 volt ADM SCRs. A fully automotive grade solution here with high reliability and ruggedness for demanding applications. Of course, ST wants to enable engineers to validate its solutions as easily as possible. So we offer a full demonstration board for the mixed bridge topology called the STEVAL-ISF003B1. This board includes the full mixed bridge power train that can be connected to a downstream continuous or discontinuous mode PFC. It's fully compliant with the latest industry standards for transient and surge operation, and it enables all the discussed benefits of mixed bridge operation from the removal of the NTC and smart operation for soft start and line drop recovery. More details can be found on ST.com for technical specs and ordering information. The same concept of the mixed bridge SCR topology can be applied to the latest trends in PFCs as well, bridge list topologies that are implemented with the latest silicon carbide devices to meet the most demanding efficiency standards. In this section, we'll take a closer look at the totem pole PFC and how to control it with SCRs. The traditional PFC is comprised from a boost stage to do pre-regulation of the rectified AC line before being fed to some DC power conditioning stage. The standard full wave rectifying bridge made of even the most efficient biomes still leads to a substantial amount of power loss. Today we can take advantage of the characteristics of silicon carbide MOSFETs to implement the bridge list totem pole topology in which the dialed losses are eliminated and efficiencies can reach well above 99% in high power systems. The design uses one leg of high-frequency switching silicon carbide bets and one leg of either dials or MOSFETs that switch at line frequency. This solution, however, still requires an inrush current limiter just as the standard bridge topology does. Traditionally done again with relays and an NTC, but of course, our friend the SCR can help improve on that solution. Here we're looking at a few different implementations of the totem pole power switches and the ST technologies applicable for each. First, we must use silicon carbide MOSFETs in the high-frequency leg due to the combination of high voltage blocking ability and robustness of the biodiode. So we propose the latest in ST second generation silicon carbide devices for the MOSFETs. But we have a few choices for the line frequency leg. The most cost effective is to use low forward drop diodes, potentially ST's STBR bridge rectifier devices. The most efficient and highest performance solution would play silicon carbide MOSFETs in this leg, but that can be costly. Also, neither of these solutions eliminates the relays and NTC. Our third choice is to drop in high voltage and current SCRs into those switch sockets. Now we can intelligently control the PFC for smart soft start and eliminate the relays in the NTC. Here we can see the conduction path through the bridge depending on line cycle and switch state of the high-frequency operation leg. When the line is positive, the inductor current will flow through SCR2, alternating between charging the inductor and charging the output capacitance depending on the switching states of the high-frequency leg. As we switch to the negative line cycle, the current path is followed through SCR1. This looks quite similar to how current would be conducted through the mixed bridge. And that's right. We can apply the same techniques of the mixed bridge to the totem pole PFC when SCRs are used in the low-frequency operation leg of the topology. The principle is exactly the same. We'll fire on the correct SCR depending on the phase of the line cycle to allow for a controlled amount of energy into the bulk capacitance. Slowly decreasing the delay applied to firing the SCR will allow for gradually more and more energy to pass through until we reach a satisfactory amount of charge on the bulk capacitance. And we can complete this off-start. The SCRs can also be kept off to disconnect the bus capacitor from the AC mains for standby operation, all without any relays. ST has a brand new evaluation kit for the totem pole topology with SCR controlled startup. It's made of three different evaluation boards. The STEval DPSTPFC1 is the main powertrain that's looking hard by MOSFETs and high voltage SCRs. It's controlled by the DSP334M1 featuring the latest in STM32 processing with embedded control algorithm. Finally, there's a handy adapter board for debugging and communication, the DP-SADP01. It's capable of up to 3.6 kilowatts of output power altogether as a kit. Let's check out the performance. First of all, the startup is smooth and controlled as is to be expected. One can see the stages of operation from off-state to the in-rush current limiting mode and then a PFC soft-start followed by steady-state operation. Transition from in-rush current limiting to PFC soft-start allows for a controlled transition from the peak line voltage up to a full 400 volt bus output. This algorithm is of course fully available in our STM32 libraries associated with the eVal board for ease of use and to speed up your design time. The performance is outstanding. Here at typical European input line voltage and frequency, we hit peak efficiencies of almost 98%, something not achievable by standard rectifying bridges and PFC combinations at these power levels. Over 97% efficiency is maintained from a thousand-watt load all the way to full load. Even more impressive is the THD performance across the whole operating range. Less than 15% THD at 20% rated power and sub 5% THD at 50% loaded above. These efficiencies allow the design to stay cool with minimal investment in mechanical heat removal. Silicon carbide MOSFETs capable of 200 degrees C operation are here staying below 85 degrees C in a typical room ambient environment. The SCRs are remaining cool at about 70 degrees C. However, to stay safe, of course, the evaluation bore is equipped with an over-temperature protection circuit connected to the microcontroller. The board will be fully released to the mass market by summertime, so if not readily available yet, you'll have to wait just a little longer. To recap the contents, it is a 3.6 kilowatt totem pole PFC with active in-rush current limiting. It uses the latest in silicon carbide MOSFETs in high-power SCRs. A viper offline auxiliary power supply provides startup power and an STM32 controls the firmware. Add in the STGAP isolated gate drivers and you have a full ST microelectronic solution that can be connected to a downstream DC to DC stage or a motor inverter stage. The benefits of the topology and control algorithm are several. Reduced standby losses and ability to disconnect from the DC bus without relays or NTCs as immunity, disurges, and transients very low common mode noise due to a smart MOSFET control scheme and of course, superb efficiency and THD performance. Of course, no solution is complete without a host of resources to help the power supply engineer with the design. So we'll wrap up by reviewing the tools at your disposal to get started. Foremost, there are evaluation boards for every option of byristor enabled in-rush current limiting. The IHT008V1 is a series triac replacement of NTC that is suitable for power levels below 500 watts. The SCR001V1 is a self-driven daughter board that can be wired in as an NTC bypass solution for power levels from about 500 watts to two kilowatts. No microcontroller needed for this one. The ISF003V1 is our mixed bridge solution for more than two kilowatt supplies using smart control of SCRs for in-rush current limiting and soft start. And finally, the brand new DPSTPFC1 totem pole solution will be available by Q3. Of course, the ST website is a fantastic place to find detailed content. There are many relevant application notes, such as AN4606, which gives tips on designing in-rush current limiting circuits with SCRs. There's APHNOT4608, that details byristor selection depending on application and APHNOT4607, that gives you the low down on byristor basics. Full selection guides and product offerings can also be found. Go to st.com slash byristor, SCR and AC switches for a full list of material. A very convenient way to find the right byristor for your application is with our Finder app. Available for both iOS and Android, it makes reviewing the ST portfolio of byristors a breeze. Within a few clicks, you can narrow down the portfolio to the parts that you need and have access to data sheets and app notes at your fingertips. I highly recommend this free download from the app and Google Play Store's. We'll give you a moment to scan the QR code now, if you'd like. In conclusion, a few points. AC input power systems face the potential for high in-rush currents and require an in-rush current limiter for standard compliance and to reduce system stresses. ST proposes the use of SCRs to create smart solutions that replace relays and NTCs. The benefits of these solutions include active soft start, high efficiency and increased reliability. A fully smart in-rush current limiting solution does require a microprocessor and associated startup auxiliary power supply. But both are mainstays of most power supply architectures already. Finally, ST makes it easy to implement these solutions with a wide array of resources for the power supply engineer, including full solution evaluation boards. And that's the end. Thank you for listening. Of course, for more information, please visit us at www.ST.com. Thank you.