Ldc > Lac improves the ac rejection ratio at the operating frequency of Iac (current in ac winding). However, even Schottky rectifiers can be significantly more lossy than the synchronous type, notably at high currents and low voltages. A  K of ~0.6 is a good tradeoff. The drain to source voltage of each SR MOSFET is directly sensed to determine the level of conducted current so that the MOSFET … Two winding resonant inductor with two windings for Lac. The dc winding only requires a low dc resistance, so normal wire is used. Attach the drive leads across Ldc, the sense leads across Lac and read Lm directly. The timing is very important, as a short circuit across the input power must be avoided and can easily be caused by one transistor turning on before another has turned off. For example, the typical data sheet in Fig. This unit with its smaller dual windings or triple windings can be packaged in a lower profile package, allowing better thermal flow. (Click for enlarged view). The LTC3900 is a secondary-side synchronous rectifier driver designed to be used in isolated forward converter power supplies. In such applications, active rectification becomes necessary.[1]. However, a +5V gate-to-source does not fully enhance the channel (Fig. The control scheme in this IC allows for each synchronous rectifier to be switched on as the corresponding half-winding starts conducting and switched off as its current goes to zero. The semiconductor diodes are replaced with active switching elements: transistors that may be power MOSFETs or power bipolar transistors that are switched on and off at the required times to allow the rectification to occur. You can directly measure Lm with most LCR meters with four leads. When addressing very low-voltage converters, such as a buck converter power supply for a computer CPU (with a voltage output around 1 volt, and many amperes of output current), Schottky rectification does not provide adequate efficiency. Prior art for an EMI filter. Why did I patent this configuration when so many papers have been written on it? Severns, Rudy, Armijos, Jack., MOSPOWER Applications Handbook, Siliconix Incorporated, 1984. In low voltage converters (around 10 volts and less), the voltage drop of a diode (typically around 0.7 to 1 volt for a silicon diode at its rated current) has an adverse effect on efficiency. The controller enables high efficiency designs for flyback and quasi resonant flyback topologies. 5, L2 is a two winding device. (Click for enlarged view). The channel also sits at infinite resistance during capacitance measurements causing a second problem in using Fig. 11. In Fig. On the other hand, operating in the MOSFET III quadrant, a synchronous rectifier presents a resistive i-v characteristics, as shown in Fig. Determine this capacitor value to resonate with Lac, leakage inductance. For more information on this article, CIRCLE 334 on Reader Service Card. The TEA1999TK is a dedicated controller IC for synchronous rectification on the secondary side of flyback converters. Note both SPICE simulations contain the same results proving the theory. Synchronous rectifiers can improve power efficiency, particularly in low-voltage, medium power applications. Figure 1 is prior art for reference only. Both two- and three-winding devices operate in the same manner as explained below: Ldc and Lac, which when loosely coupled (0.5< K<0.90) each form a leakage inductance. In transformers, current goes in the primary dot and out the secondary dot. SR improves efficiency, thermal performance, power density, manufacturability, and reliability, and decreases the overall system cost of power supply systems. Despite this fact, the reason that the synchronous rectifying type achieves higher efficiency than the nonsynchronous type is that a transistor (especially a MOSFET) is used for the low side switch, substantially eliminating the loss generated by a diode and permitting optimal timing control. Any place we have an ac and dc current component in an inductor winding, we should look at this topology to reduce the filter components. This article will examine the advantages of SR © 2020 Endeavor Business Media, LLC. The RCI will scale from 25 watts to kW and it will operate from 50Hz to over 850 Hertz, filling the needs of commercial and military devices. This means the effective input capacitance, Cinput(3rd-Quad), and gate charge, QG(3rd-Quad), are independent of third quadrant drain current. In practice, the MOSFET gate has a positive bias using a constant current source while the negative current source biases the drain. The RCI patent is currently pending (US 62/170,844) and the search results are very promising. It is also used in switched-mode power supplies (SMPS).[1]. 1, synchronous rectification requires a gate drive circuit to control the MOSFET switches. The decreasing voltage drop across the rectifying element directly relates to increased converter efficiency. (Click for enlarged view). Using SPICE simulation, I realized the simplicity of this inductor configuration. The charging current and magnitude determine the charging rate of the device capacitance. The RT9624D can be supplied from 4.5V to 13.2V. The control circuitry for active rectification usually uses comparators to sense the voltage of the input AC and open the transistors at the correct times to allow current to flow in the correct direction. 7. It can be used in all power supplies needing high efficiency, like: • Chargers • Adapters • Flyback power supplies with very low and/or variable output voltage 4 Ordering information Table 1. Figure 2 shows both a two-winding and three-winding PFC circuit to help explain the RCI operation, but there can be many implications as shown in Fig. Due to the separation of ac and dc currents, a higher efficiency unit can be made in the same volume. Enhancing the channel during negative drain voltage creates an effective input capacitance Cinput(3rd-Quad) value, as shown in Fig. 2 Pred Capacitor to store previous cycle timing for SR MOSFET. 2. The NCP43080 is a synchronous rectifier controller for switch mode power supplies. This occurs because the negative drain voltage causes the MOSFET body diode to be forward biased where it ceases to be a capacitor. The IR11688 synchronous rectifier (SR) control IC drives a pair of N-channel power MOSFETs forming the rectifying output stage of a resonant half-bridge converter. Then add a high-frequency (Ferrite) core to the ac winding and terminate with the resonant capacitor. The sign displayed is the phase. A programma 3 plots input capacitance, Ciss, Miller capacitance, Crss, and output capacitance, Coss, as a function of forward drain voltage for quadrants one and two. I believe that the previous papers have made this design very difficult to understand and scared off the engineers and magnetics manufacturers. The Table lists data points extracted from Fig. 1. 3, Coss rises toward infinity. The PFC Inductors I can buy are single windings of high power dissipation due to high resistance at the switching frequency (Rac losses). [2], Active rectification has many applications. The UCC24624 high-performance synchronous rectifier (SR) controller is dedicated for LLC resonant converters to replace the lossy diode output rectifiers with SR MOSFETs and improve the overall system efficiency. MOSFET capacitances change as a function of drain voltage, VDS. The N-Channel MOSFET is optimized for low gate charge, low R DS(ON) MOSFET synchronous converter input capacitance affects performance. Hi-Rel Synchronous Rectifier MOSFETs IR offers a comprehensive portfolio of High Reliability synchronous rectifier MOSFETs for harsh environments. The APR348 MOSFET driver. Three winding PFC input circuit. TEA1993TS SR controller The TEA1993TS is a dedicated controller IC for synchronous rectification on the secondary side of flyback converters. (Click for enlarged view). High Frequency Synchronous Rectifier Driver 2020/07/01 Ver 2 Page 2 TYPICAL APPLCATION CIRCUIT PIN DESCRIPTION Pin Symbol Description 1 Timing Discontinuous current filter timing adjustment resistor connection. Dicklyon ( talk ) 00:11, 6 December 2010 (UTC) Sign up for Power Electronics eNewsletters. The SRK2000A device implements matched turn-off MOSFET thresholds. When a Power MOSFET is used in bridge topologies and as synchronous rectifier in the secondary side of a power supply, body-drain diode features together with FoM become critical. The LTC3900 has other advantages, such as protection features not found in other drive methods. adopting Synchronous Rectification (SR)—the use of MOSFETs to achieve the rectification function typically performed by diodes. The designer did not have a good understanding of skin effect nor proximity losses. The synchronous rectifier MOSFET contains an integral, parasitic body diode that can act as a clamp and catches the negative inductor voltage swing during this dead time. As shown in Fig. Resonant-coupled inductor circuits. By contrast, the on-resistance of an Nch-MOSFET, for example, is exceedingly low, measuring 50mΩ. The UCC24624 SR controller uses drain-to-source voltage sensing method to achieve on and off control of the SR MOSFET. Can be used in any topology, input or output, diverting switching current to ground. Here, we set the gate current to 1mA and the drain current to 35A. However, the ISL9N310 Power MOSFET (at ½ rated current, 17.5A) has a conduction drop of 0.175V, decreasing at a rate of 56μV/°C. Using active rectification to implement AC/DC conversion allows a design to undergo further improvements (with more complexity) to achieve an active power factor correction, which forces the current waveform of the AC source to follow the voltage waveform, eliminating reactive currents and allowing the total system to achieve greater efficiency. This information is useful for applications using the power MOSFET as a first quadrant switch. The TEA2095TE is a dedicated controller IC for synchronous rectification on the secondary side of resonant converters. The most efficient solution is to use a MOSFET driver, like the LTC3900, that is synchronized to the primary PWM controller. This is not just about PFC inductors but input and output filters. A low coupling factor (K) is obtained by placement of the two windings on opposite sides of the core and gap. (Click for enlarged view). 1, synchronous rectification requires a gate drive circuit to control the MOSFET switches. As shown in Fig. Therefore, you don't achieve optimum performance using logic level drive signals. The voltage drop across the transistor is then much lower, meaning a reduction in power loss and a gain in efficiency. ID=35A. Why were they all using transformer circuits to describe this coupled inductor? It incorporates the sensing stage and driver stage for driving the SR MOSFET which rectifies the output of the secondary transformer winding. The constant voltage drop of a standard p-n junction diode is typically between 0.7 V and 1.7 V, causing significant power loss in the diode. Then add a low-frequency inductor (Powder Iron to operate at 120Hz) to connect the dc winding to the bridge. Using an LCR meter, measure LAC, Ldc, and Lm. Under certain current level, the forward-voltage drop of a synchronous rectifier can be lower than that of a diode rectifier, and consequently reduces the rectifier conduction loss [A1]. Enslin. These three devices would simulate the RCI. A unique feature of this IC is its intelligent automatic sleep mode. I measured the total inductance of each inductor and their mutual inductance, and Figure 5 (see all figures below): Lm = Mutual inductance between LAC and LDC windings, LdL = Leakage inductance in the LDC winding, LaL = Leakage inductance in the ac winding. Figures 6 and 7 are SPICE simulations for the RCI. This graph illustrates why it's desirable to replace passive diodes with active transistors, especially for dc-dc converters with very low output voltages and high currents. 7. A typical Schottky diode will have a 25°C forward drop on the order of 0.33V, declining at a rate of 1.6 mV/°C. T. Grossen, E. Menzel, J.J.R. To reduce voltage drops across the rectifying elements and increase total converter efficiency, power MOSFETs Q1 and Q2 replace the traditional rectifier diodes. How Does Power Factor Correction Impact Your Utility Bill? Here’s a step-by-step design of a resonant-coupled inductor employed in a PFC EMI filter that doesn’t impact power factor. Then I remembered Lloyd Dixon’s seminars for Unitrode that covered coupled inductors and “current steering”. Input current of the RCI unit. 4, on page 31. The two gate driver stages have their own sensing inputs and operate independently. Historically, vibrator driven switches or motor-driven commutators have also been used for mechanical rectifiers and synchronous rectification. These two variables control the turn-on and turn-off times of the MOSFET switch. Yes, there is lots of prior art but the theory given is very confusing. 4, L2 has three windings and in Fig. The gap should be placed away from the windings or the fringing flux will increase winding resistance. My challenge was that I could not divert the switching current from the input filter and circulate it in another loop. Because the MOSFET switches are charge-coupled devices, this gate driver must supply a current capable of charging the MOSFET's input capacitance. 8. With the appearance of high -frequency MOSFETs develops a new rectifier topology: synchronous rectifier. They can be made with an on-resistance as low as 10 mΩ or even lower. Input current of the original unit. [1] Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5-1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop. If this is true, then why is it not in general use, why can’t I buy these Resonant Coupled Inductors (RCIs) off the shelf? Therefore, we address the specified drain current(s) on a typical data sheet. Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power BJTs. The switching speed of a power MOSFET charge-controlled device depends on the speed with which an associated gate driver circuit can charge its input capacitance. To evaluate the gate drive requirements during synchronous rectification, you must study first and second quadrant capacitance. LaL resonates with the resonant capacitor (Cr) to form a low-impedance series resonant circuit at the switching frequency and shunts the ac current to ground, away from the output and the IDC current is blocked by Cr. MOSFETs have a constant very low resistance when conducting, known as on-resistance (RDS(on)). However, it's of little value to synchronous rectifier applications where the MOSFET channel enhancement happens during the reversal of the drain voltage — a third quadrant operation. Fig. The RCI will divert >30db of switching current from the input filter. Core magnetics with some needed cutting are readily available. Fig. 10. The RT9624D is a high frequency, synchronous rectified, single phase MOSFET driver designed for normal MOSFET driving applications and high performance CPU VR driving capabilities. The word PFC (Power Factor Correction) is used throughout this document to denote one configuration that is widely known to the profession, but the RCI can be used in many different configurations as shown in Fig. We derived Fig. In terms of a voltage drop, it is significantly lower than the VF for diodes. Over the past 40 years many of my designs included Power Factor Correction (PFC) ranging from 50 W to over 5.0 kW and operating from 30 kHz to 150 kHz switching frequency, utilizing CCM, CrCM and DCM modes and 50 Hz to 850Hz input power. 1. 6. I would like to hand off this design to the right company. 5, above, plots third quadrant gate charge, QG(3rd-Quad), for the ISL9N310 Power MOSFET using the circuit in Fig. One classic solution replaces standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (as low as 0.3 volts). So I went back and searched the many papers on the subject, finding lots of higher math and transformer equivalent circuits. That is what I needed but did not understand the theory at that time. To meet this design problem I came up with a solution and patented it. GreenChip synchronous rectifier controller 7.2 Start-up and UnderVoltage LockOut (UVLO) The IC leaves the UVLO state and activates the synchronous rectifier circuitry when the voltage on the VCC pin is above Vstartup (8.5 V typical). 4. Top is the current in the LDC winding at about 2.2 Adc with low switching ripple. The charging current and magnitude determine the charging rate of the device capacitance. Called SRK1000A and SRK1000B, they include adaptive switching logic “such that the synchronous rectifier mosfet is switched on as soon as current starts flowing through its body diode and it is then switched off as current approaches zero,” according to the company. As shown in Fig. Optimum MOSFET Selection for Synchronous Rectification 5 Application Note AN 2012-05 V2.4 May 2012 gate-charge, Qg., so we need to make sure that the gate voltage (Vg) is set high enough for it to happen. Two winding PFC input circuit. Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power BJTs. Showing three full cycles of the switching waveforms. (Click for enlarged view). This limitation is usually dealt with either by placing several transistors in parallel, thereby reducing the current through each individual one, or by using a device with more active area (on FETs, a device-equivalent of parallel). 3. The applicable power stage VIN range is from 5V to 24V. the external synchronous rectifier MOSFET, which replaces the diode for the rectification of the voltage on the secondary winding of the transformer. A MOSFET actively controlled to act as a rectifier—actively turned on to allow current in one direction but actively turned off to block current from flowing the other direction—is sometimes called an ideal diode. John W. Worman, Fairchild Semiconductor, Mountaintop, Pa. Design a PFC Resonant Coupled Inductor That Doesn’t Distort Power Factor. Using ideal diodes rather than a standard diodes for solar electric panel bypass, reverse-battery protection, or bridge rectifier reduces the amount of power dissipated in those diodes, improving efficiency and reducing the size of the PC board and weight of the heat sink required to deal with that power dissipation.[3][4][5][6][7][8]. Manufacturing costs are similar to a PFC inductor. The placement of core gaps is also important to Rac. Prior art coupled inductors were composed of one winding containing both ac and dc currents. This design has the current being shared (split into ac and dc) by the primary and secondary. There is no magic, just solid engineering. 7, showing that a +10V gate voltage will enhance the MOSFET channel to 96% using +15V as the 100% benchmark. OB2301W is a high performance and tightly integrated secondary side synchronous rectifier for switch mode power supply system. As the K factor is reduced, Lm gets smaller and the leakage inductances get larger. (Click for enlarged view). This device will operate in CCM, CrCM and DCM of operation. 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 … However, this body diode is lossy, is slow to turn off, and can cause a 1% to 2% efficiency drop. You can see that the Csub input(3rd-Quad) value bares no resemblance to the traditional MOSFET capacitance values, shown in Fig. When the voltage drops below 8.0 V (typical), the UVLO state is reentered and the SR MOSFET gate driver outputs are Note the slope of the ac current: if Ton × Ein/dI =Lm Then: 4µH × 400V/4.8A = 333µH (same value shown in 7). Image used courtesy of Diodes Incorporated. This caused the winding to be large and difficult to package and keep cool. 9. 7, above). It is frequently used for arrays of photovoltaic panels to avoid reverse current flow that can cause overheating with partial shading while giving minimum power loss. To answer this we evaluated drain currents of 5A to 35A in 5A steps and found that the gate-voltage wave-forms for each current overlaid each other perfectly. Not a pretty package, but a good start in understanding this topology. Although 35A represent the maximum continuous drain current in the ISL9N310 data sheet, not all applications operate at maximum drain current. The resultant magnetic package is about the same volume as a standard PFC inductor but in a lower profile that aids system packaging, thermal flow and noise shielding. 3. (LdL + LaL) that reduce their original (K=1) inductance such that: LdL impedes the flow of ac current to the external circuit, but has low resistance to Idc, current in the dc winding. Maybe I can educate the engineering community and reap some small benefits. The ac winding is sized according to the needs of the converter, which is known by those in the art. 3 for evaluating synchronous rectification gate charge requirements. Fig. SECONDARY SIDE SYNCHRONOUS RECTIFICATION SWITCHER Description The APR34709 is a secondary side Combo IC, which combines an N-Channel MOSFET and a driver circuit designed for Synchronous Rectification (SR), supports DCM and Quasi-Resonant Flyback Topologies. 2a. 3. Externally adjustable minimum off−time and on−time blanking periods provides flexibility to drive various MOSFET package types and PCB layout. It has typical values of 500µH to control ripple current in the winding and switching device. It incorporates the sensing stage and driver stage for driving the SR MOSFET, which is rectifying the output of the secondary transformer winding. Use LaL to calculate Cr at the switching frequency: It is important to note that a custom device is not necessary to get good results. Presently, the synchronous rectifier article is completely wrong in its definition. Such a MOSFET-based ideal diode is not to be confused with an op-amp based super diode. This page was last edited on 18 February 2020, at 19:02. It incorporates two driver stages for driving the SR MOSFETs, which rectify the outputs of the central tap secondary transformer windings. All rights reserved. Most manufacturers' data sheets graph these functions. SPICE simulation using the calculated leakage inductance values as shown above and the coupling factor set to 1. As Lm is reduced, the IAC component increases, affecting the external circuit. Recently, I was designing a PFC unit for 800 Hz operation and had difficulty designing the input filter that eliminates EMI, but does not distort the power factor caused by the large currents in the X capacitors. (Click for enlarged view). 2 compares the voltage drop across various rectifying elements as a function of temperature for Schottky diodes and MOSFETs. The diode is incidental; the synchronously controlled switch is key, whether BJP, MOSFET, or mechanical. 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