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1 Click to edit Master title style Fly-Back Converter CCM Vs CM (Continuous Conduction Mode Vs iscontinuous Conduction Mode) Giridharan Shanmugavel 1

2 Click to edit Contents Master title style Fly-back Converter q Fly-back converter topology q Power Conversion Steps q Energy in the Inductor q CCM Vs CM q Typical esigns comparison q Stress and Losses in components q Steady State Transfer Functions q ynamic Performance q Comprehensive MAXIM Solutions

3 Click Objectives to edit Master and Audience title style q esired Objective for this session: This presentation aims to outlay the Flyback converter, and its operation in Continuous and iscontinuous modes. Steady state design performance metrics are discussed, with an application example. ynamic performance differences in the two modes are highlighted. Comprehensive Maxim Power Management solutions are proposed. q Target Audience: Power Converter esigners and Application Engineers focused on Isolated /Non-isolated flyback converter, and Magnetic componentdesigners 3

4 Click Fly-back to edit Master Converter title style N:1 R Load V IN Q Salient Points q Suitable for low power (<50W )applications q Very high voltage transfer ratio possible ( Transformer ) q Can Provide galvanic isolation q Tolerate wide range of input voltage q Least component count among isolated power converters 4

5 Click Power to edit Conversion Master title Steps style N:1 N:1 L P L S L P L S R Load R Load V IN 1 Q V IN Q Step#1: MOSFET is in ON State. Energy is transferred from source to primary winding of inductor [Output Capacitor continues to deliver the load] Step#: MOSFET is in OFF State. Energy is delivered from the secondary winding of inductor to output capacitor [Output Capacitor continues to deliver the load] (The Transformer is actually a Two-Winding Inductor) 5

6 Click to edit Master title style Energy in the Inductor (CCM) V o I Lp V in 1 T s.t s (1-).T s V in /L p I Lp I Ls V o I Ls -V o /L S V in φ Inductor has Residual Energy at the beginning and end of T s Continuous Conduction Mode (CCM) 6

7 Click to edit Master title style Energy in the Inductor (CM) V o (1-- ).T s V in I Lp T s.t s.t s I Ls V o I Lp V in /L p -V o /L S V in I Ls 3 V o φ V in New State Inductor has Zero Energy at the beginning and end of T s iscontinuous Conduction Mode (CM) 7

8 Click to CCM edit Master Vs CMtitle style (1-- ).T s T s.t s.t s I Lp I Ls φ In CCM, extends to (1-) 8

9 Click Typical to edit Master Application title style R N:1 R LOA L P L S C OUT (85~65)V AC 50/60Hz C IN Controller Q Offline Power Supply Universal Input Voltage (85~65Vac, 50/60Hz) Typical in Smart Meters Neutral referenced (non-isolated) output voltage 1V~15V, 1A output 9

10 Click Typical to edit esign Master Example title style R 5:1 C OUT R LOA L P L S uf (85~65)V AC 50/60Hz C IN 68uF F S = 50kHz Controller Q Parameter CCM CM Unit L P uh L S uh I Peak,Primary Amp I Peak,Secondary Amp I RMS,Primary Amp I RMS,Secondary Amp For the same uty Cycle, Vo is greater in CM!! For the same requirements, Transformer is smaller in CM!! NOTES: C IN is chosen based on holding time requirement C OUT is chosen based on output ripple requirement 10

11 esign Performance Click to edit Master title style (Steady State) q Applying Volt-Second balance, the Input output relationships are derived CCM: V V O IN 1 = N ( 1 ) CM: q Resolving for (writing in terms of circuit parameters) CCM: V V O IN 1 = N ( 1 ) CM: Where, the K parameter relates to circuit parameters as: K L CCM: V o is related to: Circuit parameters (N) and Operating point (V IN, ) = LS is thesecondary Inductance S R is the LoadResistance R TS TS is the SwitchingTime Period V V O IN V V IN 1 = N O = K CM: V o is related to: Circuit parameters (N, L S, T S ) and Operating point (V IN,, R LOA ) Operating uty Cycle () is different for CCM & CM (for the same input-output conditions, and N) 11

12 esign Performance Click to edit Master title style (Steady State) q Peak current and voltage stress in Power devices Stress Variable CCM CM Peak MOSFET (Q) Voltage Same (assume same N) Peak MOSFET (Q) Current Lower Higher Peak iode () Voltage Same (assume same N) Peak iode () Current Lower Higher q RMS current in circuit and indicator of losses Higher RMS currents due to peaky nature of currents in CM (for the same input voltage and output load conditions) Higher losses in CM operation than in CCM q Larger current excursions in CM Larger flux excursions in the magnetic circuit Higher losses in magnetic coupling media in CM Typically Higher losses in CM than in CCM (for the same input-output conditions) 1

13 esign Performance Click to edit Master title style (ynamic conditions) q Control transfer functions, for Voltage Mode Control (simplified and expressed for N=1) Transfer Function CCM CM Control to Output Voltage ( ) R o s VIN ( 1 ) v = ~ o ( s) ( ) ( ) ~ s 1 L 1 L C P P d ( s) 1+ s + s R ( 1 ) ( 1 ) v ~ ~ d L 1 s P VIN = K 1 RC 1 + s Control Transfer Functions simpler in CM Inductor current is no longer a State in CM (Initial and Final Energy levels are zero) Right Half Plane Zero present in CCM (Limits the maximum achievable Closed Loop Bandwidth) v o~ ~ d Gain ( s) ( s) Phase CM CCM Log(f) 13

14 Click to edit RHP Master Zerotitle style Step Energy gained in the primary I PRIMARY Time uty Cycle lost in the secondary I SECONARY uty Cycle Loss > Energy Gain q For a uty Cycle Step: The energy gained by inductor is not available for transfer immediately ue to discontinuous nature of diode current, the energy transfer is reduced The output voltage reduces (for a given load) momentarily Eventually, the inductor energy build is sufficiently large to build output voltage Time q Quick uty Cycle changes = Momentary reduction in output voltage! Indirectly limits the rate of control Limits the closed loop bandwidth 14

15 Click MAXIM to edit Master Solutions title style MAX17499/MAX

16 Click MAXIM to edit Master Solutions title style Secondary-side Regulated, Isolated Power Supply 16

17 Click to edit Summary Master title style q Key highlights: v An overview of basic operation of the Flyback converter v Operation in Continuous and iscontinuous modes v Steady state design performance metrics (application example) Operating point for a given input-output conditions Stress in circuit elements v ynamic performance differences in the two modes v Physical insight into the Right Half Plane Zero v Comprehensive Maxim Power Management solutions 17

18 Click to edit Master title style Thank You 18