Version 1.0 , November 2001 Application Note AN-SMPS-16822CCM-1 CoolSET Design of Flyback SMPS in continous conduction mode operation Authors: Junyang Luo Meng Kiat Jeoh Published by Infineon Technologies AG http://www.infineon.com 3RZHU0DQDJHPHQW6XSSO\ N e v e r s t o p t h i n k i n g Design of Flyback SMPS in continous conduction mode operation Contents: DESIGN OF FLYBACK SMPS IN CONTINUOUS CONDUCTION MODE OPERATION.................................................... 3 Introduction to current mode control............................................................................................................. 4 Benefit of CCM compared to DCM in flyback converter with current mode control .................................... 4 Low peak current and RMS current............................................................................................................... 4 Drawback of CCM compared to DCM in flyback converter with current mode control ............................... 5 SMPS CIRCUIT WITH TDA16822 ........................................................................................................................ 7 Introduction to CoolSET™ TDA16822.......................................................................................................... 7 SMPS CIRCUIT DESIGN ON CCM OPERATION WITH TDA16822.......................................................................... 7 Transformer design........................................................................................................................................ 9 Voltage regulation loop design .................................................................................................................... 10 EXPERIMENTAL TEST ......................................................................................................................................... 13 CONCLUSION ..................................................................................................................................................... 18 REFERENCES...................................................................................................................................................... 18 Page 2 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Design of Flyback SMPS in continuous conduction mode operation Abstract For SMPS in flyback configuration, discontinuous conduction mode (DCM) is popular because of its simple transfer function characteristics. However, in some applications, continuous conduction mode (CCM) is preferred to achieve high efficiency. In the paper, CCM operation is discussed in detail. CoolSET™ TDA16822, which includes the PWM controller and CoolMOS™, is introduced for SMPS application. It can work at both of DCM and CCM. The application circuit with TDA16822 is described briefly. Based on the same output specification requirement, two evaluation boards with TDA16822, for DCM and CCM operation respectively, are set up. The experimental test is done for these two boards and the performances for DCM and CCM are summarized in the final. Introduction to continuous conduction mode in flyback converter with current mode control the difference between discontinuous (DCM) and continuous conduction mode (CCM) DCM CCM VSW VSW VDC_IN+VR VDC_IN+VR VDC_IN VDC_IN 0 0 ton treset ton t toff t toff TS TS φ φ 0 0 iPRI t t iPRI 0 0 t t iSEC iSEC IO IO 0 0 t t Figure 1 Typical waveforms of DCM and CCM operation Page 3 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation The typical voltage, current and magnetic flux waveforms in continuous conduction mode (CCM) for flyback converter are shown in Figure 1. For comparison, the typical waveforms in DCM are demonstrated together. It can be seen that in CCM neither the primary current nor the secondary current is truly continuous. Hence, for flyback converter, continuous conduction mode refers to the incomplete demagnetization of the transformer core over a cycle of operation. Introduction to current mode control Current mode control is very popular in SMPS design. In current mode control, a control voltage VFB directly controls the inductor current that feeds the output stage and thus output voltage. It has a several advantages over the conventional direct duty cycle control: (1) The peak current of the switch can be limited by simply putting an upper limit on the control voltage and the overload protection is obtained. (2) One pole corresponding to the primary inductor is removed from the control-to-output transfer function VO(s)/VFB(s), thus simplifying the compensation in the negative-feedback system. (3) Good line regulation is obtained. The duty cycle will be adjusted directly to accommodate the changes in the input voltage, resulting in excellent rejection of input line transients. Benefit of CCM compared to DCM in flyback converter with current mode control Low peak current and RMS current The shapes of primary and secondary current waveforms in CCM operation are trapezoid instead of triangular in DCM operation. So at the same output power situation the peak current and the equivalent RMS current are lower than those in DCM operation. Then the conduction loss on the primary switch and secondary rectifier diode is less than that in DCM. It will help to increase the total efficiency of the SMPS. Page 4 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Drawback of CCM compared to DCM in flyback converter with current mode control (1) A right-half-plane zero exists and provides additional phase lag while boosting gain. It makes good compensation difficult. (2) The overload protection threshold changes with input voltage variation. For DCM operation, the output power Pout is obtained as: 1 2 Pout = ηPin = ηLP I peak f S 2 (1) where, η is efficiency of the SMPS, Pin is the input power, LP is the primary inductance of the transformer, Ipeak is the peak current passing through the power MOS and fs is the switching frequency of the power MOS. For fixed switching frequency operation, Pout is only dependent on Ipeak which is controlled by VFB. Hense the upper limit VFB will limit the maximum Pout which is not dependent on the input line voltage for DCM operation. However for CCM operation, the equation (1) is not valid any more. It should be modified as follow. The primary current waveform is shown in Figure 2. Then the input power Pin is derived as: Pin = Vin I a D = N ratioVO (1 − D) I a (2) where, Vin is the input DC voltage, Ia is the average inductor current during turn-on period, D is the switching duty cycle, Nratio is the turn ratio of the transformer and VO is the output voltage. The inductor current ripple ∆IL is ∆I L = Vin N V (1 − D ) DTS = ratio O LP LP f S (3) Page 5 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation ∆I L Ipeak Ia Figure 2 Primary current waveform in CCM operation 1 I peak = I a + ∆I L 2 (4) from Equation (4), it is obtained V N V (1 − D ) 1 I a = I peak − ∆I L = I peak − in DTS = I peak − ratio O 2 2 LP 2 LP f S (5) combine Eq(2) and Eq(5), then Pin = N ratioVO ( 1 − D )( I peak − N ratioVO ( 1 − D ) ) 2 LP f S Pout = ηPin = ηN ratioVO ( 1 − D )( I peak − N ratioVO ( 1 − D ) ) 2 LP f S (6) (7) It can be seen that the maximum output power is not only dependent on the peak current but also the duty cycle, i.e. input voltage. Page 6 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation SMPS circuit with TDA16822 Introduction to CoolSET™ TDA16822 The TDA16822 is a current mode control pulse width modulator with built in CoolMOS™ transistor and working at fix switching frequency fS=100kHz. It fulfills the requirement of minimum external control circuitry for a flyback application. The block diagram with the typical application circuit is shown in Figure 3. There are only 6 active Pins with DIP8 package. To safeguard the system, some basic protection function such as IC undervoltage lockout, maximum duty cycle limitation, overload and open loop protection, overvoltage protection during startup and latched thermal shutdown are built in. With external current sense resistor, the maximum peak current limitation is adjustable. With the propagation delay compensation, the current overshoot dependent on di/dt is minimized. Because there is no demagnetized protection for this IC, it can work in both DCM and CCM. SMPS circuit design on CCM operation with TDA16822 In this section we concentrate on the design for CCM operation. The detail circuit design steps of transformer and the regulation loop will be introduced with a design example of 8.8V/1.7A SMPS. For DCM design, please refer to the application notes of “CoolSET™ TDA16831…-34 for OFF-Line Switch Mode Power Supplies” and “CoolSET™ application note supplement”. The target specification is as below. Universal AC input: 85VAC~265VAC. Output voltage VO: 8.8V Output current IO: 1.7A Efficiency: 80% Page 7 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Figure 3 Block diagram and typical application of TDA16822 Page 8 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Transformer design (1) maximum duty cycle Dmax The transformer turn ratio of primary to secondary could be obtained as: N ratio = Vds max − Vi max Vo + Vdiode (8) where, Vdsmax is the maximum voltage across drain to source of the MOSFET, Vimax is the maximum input DC voltage and Vdiode is the on state voltage drop of secondary rectifier diode. Then the maximum duty cycle Dmax is shown as: ( Vo + Vdiode )N ratio Vi min + ( Vo + Vdiode )N ratio Dmax = (9) where, Vimin is the minimum input DC voltage. If Dmax is higher than 0.5, the slope compensation is required for the system stability. In this design, we choose Vdsmax=450V, Vimax=380V, Vimin=90V, VO=8.8V and Vdiode=0.5V, and then the turn ratio is N ratio = Vds max − Vi max 450 − 380 = = 7.53 Vo + Vdiode 8 .8 + 0 .5 (10) and D max = (Vo + V diode ) N ratio (8.8 + 0.5) ⋅ 7.53 = = 0.44 Vi min + (Vo + V diode ) N ratio 90 + (8.8 + 0.5) ⋅ 7.53 (11) Slope compensation is not necessary for this design. (2) transformer primary inductance LP For CCM operation, there is no upper limit for primary inductance LP to guarantee the demagnetization of the transformer. The larger the LP, the lower the inductor current ripple ∆IL and the lower the RMS values of the primary current through the MOSFET and secondary current through rectifier diode. Accordingly, the conduction losses of the MOSFET and rectifier diode could be lower. Hence the LP is set as large as possible within the limitation of core size. In this design, we choose LP=1.85mH with EFD20/N67 core type. Page 9 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation (3) Maximum primary peak current Ipeak_max, primary RMS current through MOSFET IPRMS_max and secondary RMS current through rectifier diode ISRMS_max from Eq. (2), (3) and (4): I peak _ max = Pout 1 1 N ratioVO ( 1 − D ) + ∆I L = + = 0.589 A Vi min D 2 LP f S ηVi min D 2 (12) i P ( t )dt = Pout ∆I D ( 3( )2 + L ) = 0.32 A ηVi min D 3 4 (13) iS ( t )dt = I ( ∆I L * N ratio )2 1− D ( 3( O )2 + ) = 2.29 A 3 1− D 4 (14) Pin I PRMS _ max = 1 TS I SRMS _ max = 1 TS TS ∫ 0 TS ∫ 0 2 2 2 (4) Primary and secondary turn numbers, NP and NS To prevent the saturation of the transformer, the NP should be: NP ≥ I peak _ max LP (15) Bmax Amin where, Bmax is the maximum flux density of the core and Amin is the minimum magnetic cross section 2 area of the core. In this case, Amin=31mm and we choose Bmax=0.33T. And then, NP ≥ I peak _ max LP Bmax Amin = 106.5 (16) we choose NP=108. And NS = NP = 14.3 N ratio (17) we choose NS=14. Voltage regulation loop design (1) Power stage transfer function For current mode control flyback converter in continuous conduction mode operation, its transfer function of output voltage vo to control voltage vFB is shown as follow. Page 10 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation GS ( S ) = vO ( S ) KN ratio R( 1 − D ) = ⋅ v FB ( S ) 1+ D SLP D ) N ratio R( 1 − D )2 SCR 1+ 1+ D ( 1 + SCRC )( 1 − 2 (18) where, K is the gain of iL(S) to vFB(S), R is the load resistance, C is output capacitance and RC is the parasitic ESR of output capacitor. According to TDA16822 datasheet, K could be calculated as follow. K= iL ( S ) 1 = v FB ( S ) Rsense AV (19) where, Rsense is the primary current sense resistance and AV is the gain of PWM operating amplifier. It can be seen a right half plane (RHP) zero exists in GS(S). The RHP zero provides additional phase lag while boosting gain. It is also a moving zero varied with D. Hence, large bandwidth ωC is not normally obtainable with CCM operation in flyback converter. In the application, Rsense=1.5Ω AV=3.65 from TDA16822 datasheet R=8.8/1.7=5.2Ω C=2200µF RC=0.06Ω 80 0 60 -20 40 -40 20 Phase Angle Gain(db) The gain and phase response characteristics of GS(S) are shown in Figure 4 and 5. GS(S) 0 -20 G(S) GS(S) -60 -80 -100 -120 -40 -140 GR(S -60 -80 100 GR(S G(S) -160 101 102 103 104 f(HZ) 105 Figure 4 Gain response 106 107 -180 100 101 102 103 104 f(HZ) 105 106 107 Figure 5 Phase response Page 11 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation (2) Feedback loop design The feedback loop circuit is shown in Figure 6. It consists of compensation network (TL431, R1, R2, R3, R4, C1 and C2) and optocoupler. Figure 6 Feedback loop circuitry The total transfer function GR ( S ) = v FB ( S ) of the feedback loop is vO ( S ) v FB ( S ) GC RFB 1 + S ( C1 + C 2 )R4 = ⋅ vO ( S ) R3 SC1 R1 ( 1 + SC 2 R4 ) (20) where, GC is the current transfer ratio of the optocoupler. In this design, GC=100% (optocoupler: SFH617-3) RFB=3.7KΩ (from TDA16822 datasheet) R1=6.2KΩ R2=2.4KΩ R3=1KΩ R4=15KΩ C1=0.22µF C2=10nF The gain and phase response characteristics of GR(S) and final G(S)=GS(S)+GR(S) are shown in Figure 4 and 5. The cross frequency is around 500Hz and the phase margin is about 82°. Page 12 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Experimental test Based on the same output specification (8.8V/1.7A), two SMPS boards, operating in CCM and DCM respectively, are set up with TDA16822 implementation. The main differences of the two boards are the transformer design and current sense resistor. For DCM design, the primary inductance is 409µH and sense resistor is 1Ω. For CCM design, the primary inductance is 1850µH and sense resistor is 1.5Ω. The testing is done for both of the two boards and the performances are demonstrated for the comparison. Load efficiency (Figure 7): The graph shows the efficiency changes with load current at 85VAC and 265VAC inputs. It can be seen that the efficiency of CCM board is generally 5% higher than that of DCM board. 85 84 83 81 75 70 65 CCM 85VAC CCM 265VAC DCM 85VAC DCM 265VAC 60 55 0.2 CCM 82 fficiency (%) Eficiency (%) 80 0.4 0.6 0.8 1 1.2 Iout (A) Figure 7 Load regulation 80 79 78 DCM 77 76 75 1.4 1.6 1.8 74 50 100 150 200 250 300 VAC (V) Figure 8 Line regulation Line efficiency (Figure 8): This graph shows the efficiency changes with AC input voltage at full load current. Again, 5% improvement on efficiency is obtained from CCM operation. Standby /No load power dissipation (Figure 9): This figure shows the power loss at standby/no-load condition. There is not much difference on the standby input power for CCM and DCM operation. Both of them are below 1W. Overload protection threshold (Figure 10): This figure shows the overload protection threshold, i.e. maximum output power set by the circuit. It can be seen, for DCM board, the maximum output power is almost a constant value and not dependent on the input voltage. However, for CCM board, the threshold value increases with the increasing of the line voltage. From 16W at 85VAC to 21.6W at 265VAC, it corresponds to 35% overshoot. This phenomenon has be predicted by the mathematical derivation in Eq(7). Page 13 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation 22 1 CCM DCM Overload Protection Threshold (W) 0.9 CCM 0.8 Pin (W) 0.7 0.6 0.5 0.4 0.3 0.2 50 100 150 200 250 300 21 20 19 18 17 DCM 16 15 50 100 150 200 250 300 VAC (V) VAC (V) Figure 9 Standby /No load power dissipation Figure 10 Overload protection threshold Figure 11 shows the test waveform at full load, 220VAC input for DCM board. Figure 12 shows the test waveform at full load, 220VAC input for CCM board. It can be seen that the primary turn on current does not start from zero and the shape of the current waveform is trapezoid in CCM, instead of triangular in DCM. And the peak current is I peak = 0.8 / 1.5 = 0.53 A it is lower than that in DCM board which is I peak = 0.9 / 1 = 0.9 A Vds VFB VRsense Figure 11 test waveforms at full load, 220VAC input for DCM board. Channel 1: Vds, 200V/div, Channel 2: VRsense, 0.5V/div, Channel 3: VFB, 1.82V/div, time: 2µs/div. Page 14 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Vds VFB VRsense Figure 12 test waveforms at full load, 220VAC input for DCM board. Channel 1: Vds, 200V/div, Channel 2: VRsense, 0.5V/div, Channel 3: VFB, 2.10V/div, time: 2µs/div. Figure 13 and Figure 14 show the dynamic performance of DCM operation. Due to the large bandwidth ωC=3KHz, SMPS response immediately and there is almost no DC ripple when the load is switched between no load and full load. Figure 15 and Figure 16 show the dynamic performance of CCM operation. Compared to DCM board, CCM operation has high DC ripple when the load is switched between no load and full load. From no load to full load, the response time is around 10ms and the transient voltage drop is 0.2V. From full load to no load, the response time is about 40ms and the transient voltage overshoot is 0.12V. This is because of the narrow bandwidth ωC=500Hz which leads to the slow response for the load regulation. Hence the output voltage suppressor is normally required in CCM operation to prevent the voltage overshoot when the load is suddenly cut off. Page 15 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation VO IO Figure 13 test waveforms for DCM board when the load is switched from no load to full load (1.7A), test condition: 220VAC input. Channel 2: VO, 0.1V/div, Channel 4: IO, 1A/div, time: 0.2ms/div. VO IO Figure 14 test waveforms for DCM board when the load is switched from full load (1.7A) to no load, test condition: 220VAC input. Channel 2: VO, 0.1V/div, Channel 4: IO, 1A/div, time: 0.2ms/div. Page 16 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation VO IO Figure 15 test waveforms for CCM board when the load is switched from no load to full load (1.7A), test condition: 220VAC input. Channel 2: VO, 0.1V/div, Channel 4: IO, 1A/div, time: 2ms/div. VO IO Figure 16 test waveforms for CCM board when the load is switched from full load (1.7A) to no load, test condition: 220VAC input. Channel 2: VO, 0.1V/div, Channel 4: IO, 1A/div, time: 10ms/div. Page 17 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Conclusion The benefit and drawback of the CCM operation for flyback converter are analyzed in detail in this paper. Experiments are done and the steady state and dynamic performances are demonstrated for both of DCM and CCM operation. DCM is much popular for SMPS design because of its simple transfer function characteristic and good line and load regulation. Most importantly the maximum output power can be set at a constant value and no power overshoot during fault. However, CCM can provide the higher efficiency which is required by more and more applications. In CCM operation, both of the peak current and RMS current are lower than that of DCM operation. Although its overload protection threshold varies with the input voltage, CCM operation is still attractive for the applications with high efficiency requirement. Authors: Junyang Luo and Meng Kiat Jeoh Infineon Technologies Asia Pacific Pte. Ltd., No. 25 New Industrial Road,Singapore 536211 Email: [email protected], [email protected] References 1. Harald Zöllinger and Rainer Kling, “CoolSET™ TDA16831 … -34 for OFF-Line Switch Mode Power Supplies,” Infineon Technologies Application Note, AN-SMPS-1683X-1 version 1.2, May 2000. 2. Harald Zöllinger and Rainer Kling, “CoolSET™ application note supplement,” Infineon Technologies Application Note, AN-SMPS-16822-1, version 1.0, July 2000. 3. Infineon Technologies, “TDA16822 Off-line Current Mode Controller with CoolMOS™ on Board”, Infineon Technologies Datasheet, April 2000. Page 18 of 20 AN-SMPS-16822CCM-1 Design of Flyback SMPS in continous conduction mode operation Revision History Application Note AN-SMPS-16822CCM-1 Actual Release: V1.0 Date:25.09.2001 Page of Page of actual prev. Rel. Previous Release: V1.0 Subjects changed since last release Rel. 20 ---------- First Issue For questions on technology, delivery and prices please contact the Infineon Technologies Offices in Germany or the Infineon Technologies Companies and Representatives worldwide: see the address list on the last page or our webpage at http://www.infineon.com CoolMOS and CoolSET are trademarks of Infineon Technologies AG. We listen to Your Comments Any information within this dokument that you feel is wrong, unclear or missing at all? Your feedback will help us to continously improve the quality of this dokument. Please send your proposal (including a reference to this dokument) to: [email protected] Edition 2001-03-01 Published by Infineon Technologies AG, St.-Martin-Strasse 53, D-81541 München © Infineon Technologies AG 2000. All Rights Reserved. Attention please! The information herein is given to describe certain components and shall not be considered as warranted characteristics. Terms of delivery and rights to technical change reserved. We hereby disclaim any and all warranties, including but not limited to warranties of non-infringement, regarding circuits, descriptions and charts stated herein. Infineon Technologies is an approved CECC manufacturer. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office in Germany or our Infineon Technologies Representatives worldwide (see address list). Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may only be used in life-support devices or systems with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. 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