Smart Inrush Current Limiter Enables Higher Efficiency In AC-DC Converters

Transcription

1 ISSUE: May 2016 Smart Inrush Current Limiter Enables Higher Efficiency In AC-DC Converters by Benoît Renard, STMicroelectronics, Tours, France Inrush current limiting is required in a wide spectrum of electronics applications, from appliances to automobiles, especially in systems where ac-dc conversion is performed to supply a dc load such as found in motor controls, battery charging and lighting. This current limiting function satisfies the limits set forth in EN , the international standard for electromagnetic compatibility. It also permits the management of overcurrent through a rectifier bridge and ultimately an increase in the reliability of the converter s bulk capacitor thanks to a smooth charge. To implement this inrush current limiting function, a specific topology built entirely in silicon uses a mixedbridge two diodes and two SCRs. This approach represents an alternative to the classic solution consisting of a standard diode bridge with thermistor and relays in series to limit the inrush current. This article begins by explaining in greater detail the reasons for implementing inrush current limiting in ac-dc applications. This is followed by a discussion of the advantages and disadvantages of the two inrush current limiting topologies the classic bridge with thermistor and relays (hereafter referred to as the thermistor and relays solution) versus the mixed bridge (also referred to as the smart solution.) Then, two considerations associated with the mixed-bridge topology are explored. One is the increased complexity of driving SCRs versus relays; another is the possibility of increased power dissipation for SCRs versus relays. With that as background, a study of power dissipation in the two topologies is presented, ultimately leading to some guidelines on where to use the mixed-bridge topology to achieve highest efficiency and where the thermistor and relays solution offers a better choice. Full-Silicon Inrush Current Limiter There is increasing use of inrush current limiting in ac-dc systems (and sometimes in dc-ac inverters), for 500- W to 7.4-kW applications, mainly to fulfill the IEC standard, which sets inrush current limits according to possible line-voltage fluctuations. For example, the standard limits the inrush current to 16 Arms for a maximum line voltage fluctuation of 3.3%. In these systems, at start-up, the bulk capacitor (C in Fig. 1) is charged until it reaches the maximum line voltage, sinking the current from the ac line. Without any limitation, the current can reach 20 times the application s steady-state current. The first reason to limit this current, and maybe the most relevant, is application safety. Proper use of current limiting will prevent products from failing when testing for compliance with the IEC safety standard for household and similar electrical appliances. Additionally, without any current limitation, the I²t of the fuse or the I FSM of the diode would have to be high, and could induce an overrating of these devices. A second reason is the reliability of the systems, especially regarding the bulk capacitor short charge time. Current limiting allows one to obtain a smooth and constant charge of this capacitor and therefore increase its life-time. Today, the most common topology for achieving this current limiting consists of a thermistor NTC or PTC, respectively for low- or high-power converter (labelled R in Fig. 1a) in series with a diode bridge and one or two mechanical relays. The first relay (S1) serves to bypass the thermistor in steady-state, while the second (S2) opens the circuit to limit standby losses How2Power. All rights reserved. Page 1 of 6

2 Fig. 1. Inrush current limiting topologies. The classic solution depicted in part a combines a standard diode bridge with a thermistor (R) and two mechanical relays (S1 and S2). An alternative approach shown in part b achieves a full-silicon solution by combining two diodes and two SCRs in what s described as a mixed bridge. Another topology, which can be implemented in full silicon, uses a mixed-bridge instead of the classic diode bridge (Fig. 1b.) A mixed bridge is composed of two diodes and two SCRs (denoted as T1 and T2 in Fig. 1b.) Smart control of the SCRs allows a limit to be imposed on the inrush current by triggering the SCRs in phaseshift mode at start-up, for the duration of a few line cycles until the capacitor is fully charged. Triggering the SCRs at the end of the sinusoidal line cycle, i.e. when the voltage across the bridge is low, reduces the peak inrush current. The main advantage of the thermistors and relays solution is the easy implementation of this circuit. Relays are controlled directly with a single low dc voltage pulse and an embedded isolated drive. However, there are drawbacks relating to the relays reliability and safety. Regarding reliability, contact resistance can increase with cycles, depending on contact materials. Furthermore, contact bounces induce high dv/dt and EMI noise. Regarding safety, the snag is the moving part of the relay that induces a poor robustness to shock and vibration. Shock and vibration robustness are required in specific applications such as automobiles and washing machines. Furthermore, in applications working in environments with flammable gas, as in industrial, medical equipment, or kitchen appliances, spark-free operation has to be ensured. Therefore, no relay can be implemented in these designs. Moreover, relay use is not a trend in high-end applications because this solution is noisy and bulky. In contrast, the mixed-bridge topology offers many advantages. As you remove all moving parts, the application becomes safer and more reliable. The electromagnetic disturbances created by the application are drastically reduced. Of course, because there is no bounce with a full-silicon implementation such as the mixed bridge, but also as SCRs have a fast response time (~100 ns), the SCR is able to turn on the bridge at ZVS (ac line zero-voltage switching) and is automatically turned off at the next zero-current crossing thanks to its latchup structure. We can note that in low-power systems (<1 kw and 230 V), the bypass is sometimes removed as the losses due to the NTC can be neglected. With the mixed bridge, in the SCRs off-state, standby losses are very low and help to obtain certifications such as Energy Star. Two Concerns With The Mixed-Bridge Solution The smart control of SCRs is a little bit more complicated to implement compared to a relay control. But this smart control enables accurate management of the inrush current (peak current, RMS current and capacitor charge duration), and of line voltage dips (IEC ) or overvoltage surges (IEC ). Furthermore, nowadays ac-dc converters typically have a smart device such as an MCU built-in to drive PFC and other protection features and sensors. Therefore, it is easy to drive the added SCRs How2Power. All rights reserved. Page 2 of 6

3 Fig. 2 illustrates the smart control when the inrush current is managed by an SCR (b) compared to a thermistor (a.) The objective is to charge the capacitor as fast as possible keeping the ac line current below 6 Arms to fulfill the requirements of the IEC standard. With the SCR control in phase-shift, the peak current at each line cycle is constant (Fig. 2b.). This smart control charges the capacitor in 120 ms instead of 400 ms with the thermistor where the current is decreasing with the device temperature (Fig. 2a.) Therefore, charging of the capacitor is four times faster with the mixed-bridge approach than with the thermistors and relay approach. Although the requirements for SCR gate driving slightly complicate the control of the inrush limiter, the major concern regarding SCRs is whether they will increase conduction losses. So we need to determine how these losses compare to those of the thermistor and relays solution. And is management of the power dissipation in SCRs significantly different than that of diode bridges? Finally, what is the impact of these conduction losses on application efficiency? Power Losses In The Mixed Bridge Fig. 2. Smart inrush current management. To evaluate the conduction losses of silicon devices versus mechanical relays, the on-state voltage versus current is measured. Due to its specific structure, the on-state voltage across the SCRs is higher than a single p-n diode. Fig. 3 shows the equivalent structure of an SCR, which is composed of a pnp and an npn transistor. A drawing of the silicon structure section (on the right) provides an illustration of the four-layer structure between the power terminals anode (A) and cathode (K) of the SCR. Thus, the voltage drop across A and K when current flows through the device is the voltage across P2-N-P1-N1. Of course, this drop is higher than a single p-n diode voltage drop How2Power. All rights reserved. Page 3 of 6

4 Fig. 3. SCR silicon structure. To highlight the difference between the conduction losses of an SCR and those of a diode, two example devices have been selected, and their voltage drops versus current density have been evaluated. These components are the same ones specified in an STMicroelectronics reference design for a 7.4-kW, 230-V application. The SCR is a 50-A, 1200-V device (part number TN5050H-12), and the diode is a 60-A, 1200-V device (part number STBR6012WY, a low V F diode.) Both devices show the same typical breakdown voltage (~1500 V) and approximately the same active die area. Devices with 1200-V ratings are often chosen for the purpose of overvoltage surges, as safety standards ask increasingly that devices withstand higher surge levels. Next, the on-state voltages of both devices versus current density are compared. Current density is figured here to compensate the slight difference in active die area between the devices. These measurements are performed at a junction temperature of 125 C, which will be approximately the junction temperature of the devices when the 7.4-kW, 230-V application is conducting in the full charge stage of operation. According to these dies sizes and currents ratings, these references are usually working with a current density between 2 A/mm² and 4 A/mm². As expected, the voltage drop is higher across the SCR: between 100 mv and 200 mv for the operating current density area. This represents approximately an 18% difference in the on-state voltage of a single SCR versus a single diode. Considering the bridge (one SCR and one diode conducting at the same time), the voltage drop across the bridge is then only 9% higher. Thus, it has only a slight impact on total power dissipation. Therefore, the power dissipation method for the mixed-bridge will be unchanged or just slightly adapted (the adaptation could be a different optimized heatsink design, the addition of thermal vias in the PCB, or a larger PCB footprint, for example). So the changes in thermal management would not necessarily add any cost to the design. What s more, the thermistor and relays topology will always be always bulkier than the mixed-bridge topology, as the SCRs only replace diodes. However, for a full comparison of the mixed-bridge inrush limiter with the thermistor and relays solution, the voltage drops of the relays (as small as they are) have to be taken into account. To that end, a 32-A, 250-V ac relay has been evaluated in terms of its on-state voltage drop versus current. Fig. 4 shows the comparison of the total circuit voltage drop during steady-state conduction of the bridge. This graph depicts the total circuit voltage drops versus current for these three cases: Diode bridge alone (two diodes, without any inrush current limiting circuit) Mixed-bridge inrush-current-limiting solution (one SCR in series with one diode) NTC + relays inrush-current-limiting solution (two relays in series with two diodes) How2Power. All rights reserved. Page 4 of 6

5 Fig. 4. Comparison of the on-state voltage drop of a single diode bridge with that of the mixedbridge and NTC+relays topologies. For a 7.4-kW, 230-V application, i.e. 32 Arms, the total voltage drop across the mixed-bridge topology is 1.87 V while the voltage drop across the topology with relays is slightly higher, 1.90 V, thus 1.6% more. So, the first key conclusion is that the mixed bridge presents lower power dissipation than the solution with relays, contrary to what one might expect. This demonstrates that the relay power consumption, due to its contact resistance, has to be considered, as it is not negligible. Furthermore, the relay used for test is, of course, new. But the contact resistance of relays increases with cycles of operation, while the dynamic resistance of silicon devices is stable in terms of reliability. In the example application, the SCRs power dissipation could likely be managed using the same method to cool the diodes in the thermistor and relays solution for example with a heatsink. However, the power dissipated by the relays can only be accommodated by using relatively bulky relays. This requirement for large relays could be an issue when space is a concern such as in a solar-panel inverter. But at lower current levels, below 10 A, the trend is inversed. Under such circumstances, there will be a lower overall voltage drop for the solution with the relays. In these cases, the power to be dissipated is lower because of the lower current (dissipated power is 60 W for 32 A and is only 4.2 W for a 3-A application), therefore there is less impact on the solution in terms of the methods of heat dissipation required. From an overall application efficiency point of view, for a 7.4-kW, 230-V application, the improvement using SCRs versus relays is just %. This gain is insignificant for most cases, but can be important in applications where very high efficiency is required (for example, with battery chargers). Conclusion An inrush current limiter circuit is implemented on the bridge side of an ac-dc converter. This is necessary to fulfill the international EMC standard IEC and for application safety and reliability. The most popular topologies to limit the inrush current are either a thermistor to limit the current with a bypass relay and a line cut-off relay or a mixed bridge composed of two SCRs and two diodes How2Power. All rights reserved. Page 5 of 6

6 For the application power designer, the major concern in choosing between these two topologies is the management of power losses. The demonstration that power losses in a mixed bridge are lower, compared to the solution with embedded relays, was performed. And this has proved, despite a higher voltage drop for the SCR than for a diode in the bridge, that the noticeable contact resistance of the relay must be taken into account, and that efficiency is nearly identical with only 0.013% improvement in efficiency for the SCR solution for a 7.4-kW, 230-V application. Therefore, a smart control of the ac-dc converter can be implemented for the same efficiency, targeting high-end applications such as automotive, solar, or home appliance. About The Author Benoit Renard obtained his degree in Analog Electronics and Microelectronics in 2009 from the Institute of Technology of the University of Tours, France. He has worked for seven years in STMicroelectronics in the Application & System Engineering department for SCR and Triac semiconductors in the ASD & IPAD division of Tours for the Automotive and Discrete Group as application engineer, specializing in ac line applications (appliances, industrial and lighting). He has forged broad experiences with many published articles, conference papers and presentations, and patents to his credit. For further reading on protection circuitry in power converters, see the How2Power Design Guide, select the Advanced Search option, go to Search by Design Guide Category and select Power Protection in the Design Area category How2Power. All rights reserved. Page 6 of 6