128 Synchronous Rectification in High-Frequency MagAmp Power Converters Volodymyr Yaskiv1, Anna Yaskiv1, Oleg Yurchenko2 1. Department of Applied Information Technologies and Electrical Engineering, Ternopil Ivan Puluj National Technical University, UKRAINE, Ternopil, 56 Ruska str., email: yaskiv@yahoo.com 2. The Institute of Electrodynamics of The National Academy of Sciences of Ukraine, UKRAINE, Kyiv, 56 Peremogy ave., email: yuon@ied.org.ua Abstract: The paper describes new approaches to high- with synchronous rectifier which does not require digital efficient high-frequency power supply design for controllers. specialized computer systems, which require high load Modern semiconductor component manufacturers specify current at low output voltage. It is suggested to use MOSFETs for synchronous rectifiers as a separate category, semiconductor power converters based on high-frequency and work on decreasing their channel resistance in the magnetic amplifiers. Paper shows the ways to increase conducting state. For instance, this parameter equals 0.2 mΩ converter’s efficiency due to the use of a synchronous for recent synchronous rectifier MOSFETs by International rectifier based on MOSFETs. Rectifier [8]. Keywords: high-frequency magnetic amplifier, synchronous rectifier, power converter, high level of the II. FUNDAMENTALS OF MAGAMP POWER load current. CONVERTERS DESIGN I. INTRODUCTION MagAmp is just a coil wound on a core of amorphous alloy with a relatively rectangular hysteresis loop (fig. 1) [9-11]. A Modern specialized computer systems require high-quality MagAmp, used as a switch, can block and delay the applied and high-efficient power supply for their proper functioning. voltage. However, MagAmp cannot interrupt the current once Reliability of such system is determined first of all by the started. Hence, MagAmps are used in pulse circuits where reliability of its power supply. One of the peculiarities of they are assisted by diode rectifiers, which cut off the current specialized computer systems is that they consume high as the applied voltage changes polarity. currents (dozens, often hundreds amperes) at low input When the voltage of negative polarity is applied to voltage (3.3V, 5V, etc.) This results in strict requirements to MagAmp, its core demagnetizes (corresponds to 1-2 slope in power converters that are used as power supplies for such fig. 1; t 1 …t 2 in fig. 2). The MagAmp core is unsaturated and systems. They include high level of reliability, efficiency, due to high resistance there flows no current through its specific power along with high quality of output voltage and winding. When the input voltage changes its polarity to its dynamic characteristics in the whole range of change of positive, MagAmp requires a certain volt-sec, which is the the load current. Moreover, operation of such power integral of voltage over time, to be applied to its terminals for converters should cause the lowest possible level of both the magnetic flux to build up in the core and reach the conductive and radiative electromagnetic interferences. saturation level (interval 2-3 in fig. 1; t 2 …t 3 in fig. 2). Nowadays, power supplies for specialized computer systems are realized as high-frequency power converters. Their efficiency is mostly defined with the operation modes of the high-frequency output rectifier, as the largest part of power converter losses at high level of load current is caused by the rectifier diodes. Technical characteristics of modern rectifying diodes (including Schottky diodes) for low output voltage applications allow to provide satisfactory efficiency for high- frequency semiconductor low and medium power DC power converters. With the appearance of high-frequency MOSFETs develops a new rectifier topology: synchronous rectifier. Its novelty consists in the use of a MOSFET instead of a rectifying diode, which is controlled in a function of voltage of high- frequency power transformer secondary winding (synchronously with this voltage) [1-3]. Works [4-7] describe digital solutions for synchronous rectifier control. The current paper introduces a simplified topology of a power converter Fig. 1. Hysteresis loop of MagAmp core material ACIT 2018, June 1-3, 2018, Ceske Budejovice, Czech Republic 129 When the magnetic inductance reaches the saturation level pull rectifier is in unconducting state (the voltage of negative (slope 3-4 in fig. 1), the MagAmp resistance approaches zero, polarity from the secondary winding W c1 is applied to its which allows the current to flow trough MagAmp’s winding gate). Demagnetizing diode VD1 is conducting. Thus, the (interval 4-1 in fig. 1; t 3 …t 4 in fig. 2). current flows through the control circuit 2, demagnetizing diode VD1, controlled MagAmp TS1, secondary winding W2 of high-frequency power transformer TV. This current is a function of the error signal obtained after comparison of regulator DC output voltage and reference voltage, and the change of the transformer secondary winding voltage due to the voltage change in the primary grid. The current causes demagnetization of MagAmp core from the saturation induction B s to some induction B 1 . The demagnetization depth is regulated with this stabilizing feedback. When the polarity of the input voltage changes to positive, remagnetization of the controlled MagAmp TS1 begins from the memorized level of induction B 1 .When the controlled MagAmp TS1 operates in control half-period, the controlled MagAmp TS2 operates in a different mode – working half- period. During this time interval the rectifier MOSFET VT2 conducts (the voltage of positive polarity from the secondary winding W c2 is applied to its gate). The demagnetizing diode VD2 in not conducting. The current flows through secondary winding W2 of the power transformer TV, the winding of Fig. 2. Ideal MagAmp waveforms: V g – transformer secondary MagAmp TS2, rectifier MOSFET VT2, inductor L, capacitor winding voltage, V MS – MagAmp switch voltage; i o – MagAmp C, load R L . The working half-period consists of two switch output current subintervals. During the first subinterval the core of MagAmp TS2 remagnetizes from the memorized level of inductance to In fig. 3 there is presented a functional scheme of a DC saturation inductance B s . The time required for this voltage regulator based on high-frequency magnetic remagnetization is considerably shorter than the amplifiers, which contains an unregulated high-frequency demagnetization time of the control half-period due to no transistor voltage inverter 1, power transformer, push-pull limitations of the remagnetization velocity (the load centre-tapped rectifier, controlled MagAmps, output filter 2, resistance is considerably smaller than the equivalent load, control circuit 3, demagnetizing diodes [11]. resistance of the control circuit). That’s why the MagAmp However, providing a high level of output current in such core saturates within the half-period of the working voltage regulator is followed by the increase of losses on the frequency. During this time subinterval the current flows diodes of high-frequency rectifier. This leads to a significant through inductor L, load R L , and reverse diode VD3, the decrease of efficiency. discharge current of the output LCD filter’s capacitor C flows through the load R L as well. After reaching saturation, the resistance of controlled MagAmp TS2 approaches zero, and the circuit current is defined with the load resistance R L (second subinterval). Changing the depth of demagnetization of the controlled MagAmps TS1 and TS2 from +B s to –B s during the control half-period, we get the pulse-width modulation within a half-period of commutation high frequency during working half-period. This provides output Fig. 3. Functional scheme of a DC voltage regulator based on high- voltage stabilization at change of the load current within its frequency magnetic amplifiers whole range. There also is a much lower level of losses in high-frequency rectifier due to significantly smaller III. MAGAMP POWER CONVERTER WITH resistance of channels of MOSFETs VT1, VT2 in conducting SYNCHRONOUS RECTIFIER state, compared to the losses on diodes (when they are used in the rectifier) as a result of both direct voltage drop and, It is suggested to substitute diodes of the output rectifier often, unsatisfactory frequency characteristics of diodes. with MOSFETs, with a possibility of their synchronous Decrease of the equivalent resistance of the regulator allows control from the respective high-frequency transformer obtaining higher level of load current along with high secondary windings. The functional scheme of such power efficiency, if the operation modes of all topology components converter is shown in fig. 4 [12]. are agreed. The DC voltage regulator operates in the following way. However, in such voltage regulator, the losses on the When the control half-period takes place, voltage of negative output filter reverse diode VD3 (about 1/3 of the load current polarity is applied to the winding of MagAmp TS1. During flows through it) do not allow achieving maximum possible this time interval, MOSFET VT1 of the high-frequency push- efficiency. ACIT 2018, June 1-3, 2018, Ceske Budejovice, Czech Republic 130 Fig. 4. The functional scheme of MagAmp power converter with synchronous rectifier Fig. 5. The functional scheme of MagAmp power converter with synchronous rectifier with MOSFET instead of output LCD filter diode It is suggested to substitute the output filter reverse diode For instance, according to the experimental research, with a MOSFET, that would be controlled in a function of efficiency of the power converter based on high-frequency voltages of additionally placed respective windings of MagAmps with output parameters of 24V, 10A, where diodes controlled MagAmps. have been used in the output rectifier and output filter, The functional scheme of DC voltage regulator is shown in constituted 92% [14]. Its input active power was equal to fig. 5 [13]. The waveforms that illustrate the principle of its 260.87 W. Which means the losses were equal to 20,87W, operation are presented in fig. 6. and about a half of those were the power losses in the output The advantage of using a synchronous rectifier in a power rectifier and filter. The use of MOSFETs with the open converter based on high-frequency MagAmps is that the load channel resistance of 0.2 mΩ in synchronous rectifier and current starts flowing through it when its MOSFETs are output filter allows to significantly decrease these losses. The already in conducting state. This is due to MagAmp operation efficiency of such power converter is expected to be ≥95%. principle. As a result, the converter’s dynamic losses are The efficiency tends to grow when designing power decreased. converters with higher output power. ACIT 2018, June 1-3, 2018, Ceske Budejovice, Czech Republic 131 [2] S. Mappu, “Control driven synchronous rectifiers in phase shifted full bridge converters,” Texas Instruments. Application Note. Power Supply Control Products, SLUA287, March 2003, 10 p. [3] B. Yang, J. Zhang, “Effect and utilization of common source inductance in synchronous rectification.,” in Proceedings of Twentieth Annual IEEE Applied Power Electronics Conference and Exposition (APEC 2005), Austin, TX, USA, 6-10 March 2005, 5 p. [4] C. Fei, F. C. Lee, Q. Li, “Digital implementation of adaptive synchronous rectifier (SR) driving scheme for LLC resonant converters,” in Proceedings of Applied Power Electronics Conference and Exposition (APEC), IEEE 2016, pp. 322-328. [5] M. S. Amouzandeh, B. Mahdavikhah, A. Prodic, B. McDonald, “Digital synchronous rectification controller for LLC resonant converters,” in Proceedings of Applied Power Electronics Conference and Exposition (APEC), IEEE 2016, pp. 329-333. [6] Y. Gu, Z. Lu, Z. Qian, G. Huang, “A novel driving scheme for synchronous rectifier suitable for modules in parallel,” IEEE Transactions on Power Electronics, Vol. 20, No. 6, November 2005, pp. 1287-1293. [7] Linear Technology, “Synchronous rectifier driver for forward converters LTC3900,” available online at: http://www.analog.com/media/en/technical- documentation/data-sheets/3900fb.pdf . [8] International Rectifier, “Automotive DirectFET power MOSFET AUIRF8736M2TR,” January 14, 2014, Fig. 6. Theoretical waveforms of MagAmp power converter with available online at: synchronous rectifier with MOSFET instead of output LCD filter https://www.infineon.com/dgdl/auirf8736m2.pdf?fileId=5 diode 546d462533600a4015355b0dade1414 . [9] MagAmp Cores and Materials, Technical Bulletin, IV. CONCLUSION BULLETIN SR-4, Magnetics Inc., available online at: Thus, the high level of load current along with high http://www.mag-inc.com/design/technical-documents . efficiency of the suggested DC voltage regulator are obtained [10] B. 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