Application Report SNVA674C – August 2012 – Revised December 2014 AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Vijay Choudhary ABSTRACT In many applications, one or more low-cost, simple to use, isolated power supplies working from input voltages up to 100 V are needed. Traditional solutions use flyback converters to generate this bias supply. Flyback designs typically utilize asymmetric transformers turns ratios for primary and secondary power windings, with an optocoupler and reference, or an auxiliary winding for feedback regulation. Additionally, flyback converters need an elaborate compensation design for stability. This results in a tedious design process, bulky solution, with a higher component count and cost. An isolated buck converter (Fly-Buck) uses a synchronous buck converter with coupled inductor windings to create isolated outputs. Isolated converters utilizing Fly-Buck topology use a smaller transformer for an equivalent power transfer as the transformer primary and secondary turns ratios are better matched. There is no need for an optocoupler or auxiliary winding as the secondary output closely tracks the primary output voltage, resulting in smaller solution size and cost. This article presents the basic operating principle of an isolated buck converter. The operating current and voltage waveforms are explained and design equations are derived. The design example shows a step-bystep procedure for designing a practical two-output 3 W isolated buck converter. 1 2 3 4 5 Contents Fly-Buck Converter .......................................................................................................... 1 Maximum Output Current Equations ...................................................................................... 4 Design Example.............................................................................................................. 5 Conclusions ................................................................................................................. 10 References .................................................................................................................. 10 List of Figures 1 Complete Schematic for an LM5017–Based Isolated Converter ...................................................... 9 2 Efficiency at 750 kHz, VOUT1 = 10 V ........................................................................................ 9 3 Steady State Waveform (VIN = 48 V, IOUT1 = 100 mA, IOUT2 = 200 mA) ................................................ 9 4 Step Load Response (VIN = 48 V, IOUT1 = 0, Step Load on IOUT2 = 100 mA to 200 mA). ............................ 9 .............................................................................. ........................................... 5 List of Tables 1 1 Isolated Buck Regulator Design Equations 2 Component Calculation/Selection Steps for a Two-Output Isolated Buck 6 Fly-Buck Converter An isolated buck converter, also known as Fly-Buck converter, is created by replacing the output filter inductor (L1) in a synchronous buck converter with a coupled inductor (X1) or flyback-type transformer, and rectifying the secondary winding voltage using a diode (D1) and a capacitor (COUT2). The topology can be extended to any number of isolated secondary outputs. It also can be used to generate one or more inverting outputs. All trademarks are the property of their respective owners. SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Copyright © 2012–2014, Texas Instruments Incorporated 1 Fly-Buck Converter www.ti.com VIN SW L1 VOUT Q1 COUT Q2 1a) A Synchronous Buck Converter VOUT2 D1 N2 COUT2 X1 VIN SW Q1 VOUT1 N1 Q2 COUT1 1b) An Isolated Buck Converter (Fly-buck) Creating an Isolated Buck Converter by Modifying a Synchronous Buck Converter The primary output voltage equation is identical to a buck converter and is given by Equation 1: VOUT1 = TON V = D x VIN TON + TOFF IN (1) and the secondary output voltage is given by Equation 2: VOUT2 = N2 VOUT1 - VF N1 (2) where VF is the forward voltage drop of the secondary rectifier diode, and N1, N2 are the number of turns in the primary and secondary windings, respectively. The secondary output (VOUT2) closely tracks the primary output voltage (VOUT1) without the need for additional transformer winding or an optocoupler for feedback across the isolation boundary. Isolated Buck Converter Switching Sub-Intervals shows the operating modes in an isolated configuration during TON, when the high-side buck switch is on; and TOFF, when the low-side switch is on. Current in the two windings is also shown. During TON, the current in the secondary winding is zero as the secondary diode is reverse biased by a voltage equal to VIN x N2 N1 (3) The current in the primary winding is the same as the magnetizing current (similar to a buck converter inductor). 2 AN-2292 Designing an Isolated Buck (Fly-Buck) Converter SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Fly-Buck Converter www.ti.com Creating an Isolated Buck Converter by Modifying a Synchronous Buck Converter (continued) VOUT2 IL2 = 0 VIN COUT2 SW VOUT1 Q1 IM Q2 COUT1 IL1 = IM 2a) TON (Q1: ON, Q2: OFF) VOUT2 IL2 VIN COUT2 SW IM Q2 IL2 VOUT1 COUT1 ' IL1 = IM + I 2b) TOFF (Q1: OFF, Q2: ON) Isolated Buck Converter Switching Sub-Intervals During TOFF, the current in the secondary winding is decided by the resonant tank formed by COUT1, the leakage inductance of the coupled inductor, and COUT2. The current in the primary winding is the sum of the magnetizing current (similar to a buck converter inductor current), and the reflected current from the secondary winding. These operating waveforms are shown in Isolated Buck Operating Waveforms. SW1 SW2 GND VOUT2 IGND IL1 IL2 TON TOFF Isolated Buck Operating Waveforms SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Copyright © 2012–2014, Texas Instruments Incorporated 3 Maximum Output Current Equations www.ti.com Isolated Buck Operating Waveforms (continued) 2 Maximum Output Current Equations On a cycle-by-cycle average basis, the winding and output currents have the following relationship Isolated Buck Output Stage with Coupled Inductor. IL1 = IOUT1 (4) IL2 = IOUT2 (5) and SW2 IOUT2 VOUT2 D1 IL2 + N1:N2 L2 SW1 L1 COUT2 IOUT1 VOUT1 IL1 + COUT1 Isolated Buck Output Stage with Coupled Inductor The combined inductor current waveform (iL1+ iL2), which is equal to the magnetizing current, is identical to a buck converter. The peak inductor and switch current during on-time is given by Equation 6: isw(peak) = iL1(peak) = IL1 + IL2 + 'IL1 + 'IL2 'IL1 = IOUT1 + IOUT2 + 2 2 (6) where we make use of the fact that during on-time (TON) there is no current in secondary winding. Therefore, the maximum total load current is given by Equation 7: IOUT1 + IOUT2 = ILIM (MIN) - 'IL1 2 (7) where the total load current is defined as the sum of the load currents at the two outputs. For turn-ratios (N2/N1) not equal to unity, IOUT2 should be multiplied by the turn-ratio in Equation 7, shown here in Equation 8: IOUT1 + IOUT2 'IL1 N2 = ILIM (MIN) 2 N1 (8) The maximum peak-to-peak current ripple in the primary winding is given by Isolated Buck Regulator with Three Outputs: (VIN (MAX) ± VOUT) VOUT 'IL1 = 4 L1 x fSW VIN (MAX) AN-2292 Designing an Isolated Buck (Fly-Buck) Converter (9) SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Design Example www.ti.com Isolated Buck Output Stage with Coupled Inductor (continued) VOUT3 COUT3 N3 VOUT2 COUT2 N2 X1 VIN SW Q1 Q2 VOUT1 N1 COUT1 Isolated Buck Regulator with Three Outputs Table 1 presents equations for non-equal turn-ratios and three windings (Isolated Buck Regulator with Three Outputs). The generalization to any number of windings is straightforward. Table 1. Isolated Buck Regulator Design Equations Description Equations Output Voltages VOUT1 = TON V = D x VIN TON + TOFF IN N2 VOUT2 = VOUT1 - VF N1 N3 VOUT3 = VOUT1 - VF N1 Cycle-by-Cycle Average Quantities (11) (12) IL1 = IOUT1 (13) IL2 = IOUT2 (14) IL3 = IOUT3 (15) Peak Currents in HS FET and Primary Winding isw(peak) = iL1(peak) = IOUT1 + Primary Winding Peak-to-Peak Current Ripple 'IL1 = 3 (10) (VIN (MAX) ± VOUT) L1 x fSW 'IL1 N3 N2 IOUT3 + IOUT2 + 2 N1 N1 VOUT VIN (MAX) (16) (17) Design Example The design example illustrated in Two Output Isolated Buck Reference Schematic details the design procedure for a two-output isolated buck converter. Design Specifications Input Voltage Range (VIN) 36 V - 72 V Primary Output Voltage (VOUT1) 10 V Secondary Output Voltage (VOUT2) 10 V Primary Load Current (IOUT1) 100 mA Secondary Load Current (IOUT2) 200 mA Switching Frequency (fsw) 750kHz SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Copyright © 2012–2014, Texas Instruments Incorporated 5 Design Example www.ti.com VOUT2 D1 + NS LM5017 VIN 36V-72V BST CIN + X1 CBST SW VIN COUT2 VOUT1 Rr NP Cr + + RON RUV2 RON COUT1 Cac RUV1 RFB2 VCC UVLO RTN D2 FB + RFB1 CVCC Two Output Isolated Buck Reference Schematic In this example, we start with a standard two-output circuit using TI’s 100 V synchronous buck regulator, LM5017, and calculate the component values. We begin with buck converter component calculations and qualify some of the steps for the isolated configuration. The calculation steps are listed in Table 2. Table 2. Component Calculation/Selection Steps for a Two-Output Isolated Buck Component Name Calculation Steps Selected Value/Rating RFB1, RFB2 This parameter is selected by the user. Choose RFB1=1kΩ 1kΩ, 7.16kΩ RFB2 VOUT1 = 1.225V x (1 + ) RFB1 (18) VOUT1 - 1) x RFB1 = 7.16 k: :RFB2 = ( 1.225 (19) CVCC Select a 1µF capacitor of 16 V or higher rating as recommended in the LM5017 datasheet. 1µF, 16 V CBST Select a 0.01µF capacitor of 16 V or higher rating, as recommended in the datasheet. 0.01µF, 16 V CIN Input capacitor should be large enough to limit the input voltage ripple 0.47µF + 2.2µF, 100 V IOUT (MAX) CIN t 4 x f x 'VIN (20) Choosing a ΔVIN=0.5 V gives a minimum CIN=0.2μF A standard value of 0.47μF is selected. A larger bulk capacitor is usually needed to suppress inductive spikes in the input voltage. A 2.2μF bulk capacitor is selected in this case. Input capacitor should be rated for the maximum input voltage under all conditions. RON From datasheet, VOUT1 f SW = .x RON 130kΩ (21) Where K = 1 x 10–10 → RON=133 kΩ RUV1, RUV2 UVLO resistors RFB1 and RFB2 set the UVLO threshold and hysteresis according to the following relationship: VIN (HYS) = IHYS x RUV2 4.42kΩ, 125kΩ (22) and VIN (UVLO, rising) = 1.225V x R ( RUV2 + 1) (23) UV1 where IHYS=20μA. Setting UVLO hysteresis of 2.5 V and UVLO rising threshold of 36 V results in RUV1=4.42kΩ; and RUV2=125kΩ 6 AN-2292 Designing an Isolated Buck (Fly-Buck) Converter SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Design Example www.ti.com Table 2. Component Calculation/Selection Steps for a Two-Output Isolated Buck (continued) Component Name Calculation Steps X1 A coupled inductor or a flyback type transformer is required for this L1=33 µH, 1:1 turns ratio topology. Energy is transferred from primary to secondary when the synchronous switch of the buck is ON. Using Equation 16 for the peak inductor current equation in Table 1, the maximum inductor current ripple that can be tolerated is given by: N2 'IL1 = (0.7 - IOUT1 - IOUT2 x ) x 2 = 0.8A (24) N1 Selected Value/Rating Using Equation 17 for the peak-to-peak inductor current ripple equation, the minimum inductor value is given by: L1 = (VIN (MAX) ± VOUT) 'IL1 x fSW VOUT = 14.4 PH VIN (MAX) (25) A higher value such as 22 µH or 33 µH for primary inductance can be selected to keep the primary winding and high-side switch current below the minimum peak current limit. For our design, a 33 µH value is selected for primary inductance. For this chosen primary inductance, in the primary inductor current ripple during TON is Equation 26 'IL1 = (VIN (MAX) ± VOUT) L1 x fSW VOUT VIN (MAX) (26) A 1:1 turns ratio is selected, resulting in Equation 27 N2 VOUT2 = V - VF | 9.3V N1 OUT1 D1 The voltage across D1 when the high side buck switch is on is N2 VD1 = VIN N1 (27) 100 V, 1A DLFS1100–7 (28) For a VIN_MAX=72 V, a 100 V Schottky is selected. SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Copyright © 2012–2014, Texas Instruments Incorporated 7 Design Example www.ti.com Table 2. Component Calculation/Selection Steps for a Two-Output Isolated Buck (continued) Calculation Steps Selected Value/Rating COUT1 In a buck converter, 'IL1 'VOUT = x f x COUT1 1 µF, 25 V, X7R f Component Name (29) and therefore for an output voltage ripple of ~50 mV gives, COUT1 = 1.16 µF. Selecting a standard value of 1 µF results in ΔVOUT~ = 60 mV at VIN=72 V and ΔVOUT~ = 50 mV at VIN= 36 V. The figure below shows the primary winding current waveform (IL1). The reflected secondary winding current adds to the primary winding current. Because of this the output voltage ripple is not the same as in a non-isolated buck converter. The output capacitor value calculated in Equation 29 should be used as the starting point. Actual optimization of output capacitor over the whole line/load range must be done experimentally. A better approximation of the primary output capacitor voltage ripple is given by Equation 30: (IOUT2 x N2 ) x TON (MAX) N1 | 75 mV 'VOUT1 = COUT1 (30) TON(MAX) x IOUT2 x N2/N1 IL1 IOUT2 IL2 TON(MAX) x IOUT2 Current Waveforms for COUT1 Ripple Calculation (31) As can be seen from the primary inductor current waveform in the above figure, in case of low leakage, the primary winding current reverses immediately when the secondary winding starts conducting. Therefore, the reflected secondary winding current induced primary output ripple voltage is not phase-lagged with respect to the switch node waveform. Therefore, the reflected load current induced voltage ripple does not need to be compensated for with the ripple injection circuit. If lower output voltage ripple is required, a higher value should be selected for COUT1 and/or COUT2. COUT2 A simplified waveform for secondary output current (IOUT2) and the current in the secondary winding is shown in the figure below. 1 µF, 25 V, X7R IOUT2 IL2 TON(MAX) x IOUT2 Secondary Current Waveforms for COUT2 Ripple Calculation (32) The secondary output current (IOUT2) is sourced by COUT2 during one time TON. Ignoring the current transitions time in the secondary winding, the secondary output capacitor ripple voltage can be calculated using Equation 33: 'VOUT2 = IOUT2 x TON (MAX) COUT2 (33) For a 1:1 transformer turns ratio the primary and secondary voltage ripple equations are identical. Therefore, COUT2 is chosen to be equal to COUT1 (1 µF) to get comparable ripples on primary and secondary outputs. If lower output voltage ripple is required, a higher value should be selected for COUT1 and/or COUT2. 8 AN-2292 Designing an Isolated Buck (Fly-Buck) Converter SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Design Example www.ti.com Table 2. Component Calculation/Selection Steps for a Two-Output Isolated Buck (continued) Component Name Calculation Steps Rr, Cr, Cac Type III ripple circuit as described in the LM5017 datasheet is preferred 46kΩ, 0805, 1000 pF, 0.1 µF (25 for isolated configuration. Type I and Type II ripple circuits suffer from V) larger jitter as the reflected load current affects the feedback ripple. For a constant on time converter to be stable, the injected in-phase ripple should be larger than the capacitive ripple on COUT1. Selected Value/Rating VOUT L1 Rr C OUT Cr R FB2 Cac GND To FB R FB1 Type II Ripple Circuit (34) Using type III ripple circuit equations, the target ripple should be greater than the capacitive ripple generated at the primary output. Cr = 1000 pF Cac = 0.1 PF (VIN (MIN) - VOUT) x TON RrCr d 50 mV (35) Resulting in Rr= 180kΩ. This is the borderline case of stable ripple. Half to a fourth of this resistance should be selected for sufficient margin for variations in TON, COUT1, and other components. For this design Rr = 46.4kΩ; is selected for robust operation. D2 (optional) D2 is an optional diode connected between VOUT1 and VCC regulator 20 V, 50 mA output. When VOUT1 is > VCC the VCC supplied from VOUT1. This results in reduced losses in VCC regulator inside the IC. The final schematic for the isolated power supply is shown in Figure 1. The experimental results for this circuit are presented in Figure 2, Figure 3, and Figure 4. D1 VOUT2 9.5V R7 T1 20V-100V VIN (TP1) BST 2 4 C1 + 2.2 F GND (TP2) + C5 0.47 F R3 130 N SW VIN 33 H (1:1) 7 8 + 0.01 F C2 R8 RON LM5017 46.4 N VCC UVLO FB (TP7) R2 UVLO 8.25 N EXP TP8 TP10 RTN 1 6 R9 0 (TP5) VOUT1 10V (TP3) 0 R6 C7 C8 0.1 F R4 7.32 N D2 + C3 1 F GND (TP4) 5 + U1 IGND C9 2200 pF 2000V 3300 pF R1 127 N 3 (TP6) R10 + C4 2 N 1 F 0 SW (TP11) R5 1 N C6 1 F Figure 1. Complete Schematic for an LM5017–Based Isolated Converter SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Figure 2. Efficiency at 750 kHz, VOUT1 = 10 V AN-2292 Designing an Isolated Buck (Fly-Buck) Converter Copyright © 2012–2014, Texas Instruments Incorporated 9 Conclusions www.ti.com Figure 3. Steady State Waveform (VIN = 48 V, IOUT1 = 100 mA, IOUT2 = 200 mA) 4 Figure 4. Step Load Response (VIN = 48 V, IOUT1 = 0, Step Load on IOUT2 = 100 mA to 200 mA). Conclusions An isolated buck converter (or Fly-Buck) converter was presented that does not require any additional winding or optocoupler for regulating an isolated output. The operating principle of the topology was also presented along with operating current and voltage waveforms. The relationship between the primary and the isolated output voltages and output currents were presented. We also developed design equations for estimating the peak primary switch current for specified load currents. The equations in this design can also be used to determine the maximum load current that the converter can provide for a given peak current limit. Simplified approximations for output voltage ripples were also presented. A detailed design procedure was presented for a 3W two-output isolated buck converter with a primary output and an isolated output using a 100 V synchronous buck regulator IC. An isolated buck converter can be used to replace a flyback converter for low-power isolated regulator applications with potential savings in complexity, number of components, and cost. Learn more about flyback converters on the Power Management website at www.ti.com. 5 References LM5017: 100 V,600 mA Constant On-Time Synchronous Buck Data Sheet (SNVS783) LM5160 Wide Input 65 V, 1.5A Synchronous Step-Down DC-DC Converter (SNVSA03) 10 AN-2292 Designing an Isolated Buck (Fly-Buck) Converter SNVA674C – August 2012 – Revised December 2014 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Revision History www.ti.com Revision History Changes from B Revision (May 2013) to C Revision ...................................................................................................... Page • • • • • Changed 8 to 4 ............................................................................................................................ 6 Changed value from 1.24 to 0.2 ......................................................................................................... 6 Added two sentences ..................................................................................................................... 6 Added 0.47 +............................................................................................................................... 6 Added reference ......................................................................................................................... 10 NOTE: Page numbers for previous revisions may differ from page numbers in the current version. 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