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| Energy-Efficient Peak-Current Controlled Power Conversion IC Family Delivers 3 to 28 Watts in Universal-Input Flyback Power Supplies By Kent Wong and John Jovalusky APEC 2007 Anaheim, California Scope of Paper and Presentation * * * * * * Introduction of IC control scheme concept and its benefits IC operation and power supply interaction Defining the state machine state-change criteria and limits Descriptions of IC pin functionality & important internal functions Power supply performance results Conclusion APEC 2007 Session T8: Semiconductor Devices Slide# 2 Introduction: Basic IC Functionality * * * ON/OFF control enables/disables MOSFET switching cycles Since MOSFET & controller are integrated, IDRAIN is sensed directly Enabled cycle switch on-time ends when IDRAIN reaches ILIMIT APEC 2007 Session T8: Semiconductor Devices Slide# 3 Basic Functionality Continued: State Machine Operation and Supply Stability/Responsiveness * State machine adjusts ILIMIT based on number of consecutive enabled or disabled cycles * Pole-zero placement, slope compensation and gain/phase bode plotting are eliminated * Transient load responsiveness is fast yet stable, and equal to or better than that of a well compensated PWM controlled power supply APEC 2007 Session T8: Semiconductor Devices Slide# 4 Overview of How IC Interacts with Power Supply * * VOUT is compared to a reference (Zener Diode) When VOUT > ref set-point, Zener and opto-LED conduct, phototransistor pulls current from EN/UV pin, which disables switching * As switching cycles are skipped, VOUT drops below ref set-point, Zener and opto-LED stop conducting..., which enables switching Controller continually enables/disables MOSFET switching to keep VOUT in regulation APEC 2007 Session T8: Semiconductor Devices Slide# 5 State Machine Operational Overview & Benefits * Since cycles are skipped while VOUT > the reference set point, ON/OFF control realizes very low no-load power consumption * The state machine automatically adjusts the MOSFET ILIMIT according to the load - (ILIMIT is raised as the load increases and lowered as the load decreases) APEC 2007 Session T8: Semiconductor Devices Slide# 6 Defining State-Machine State-Change Limits * Frequency boundary between CCM and DCM operation - The boundary frequency between CCM and DCM operation of a peak current limited, ON/OFF controlled, flyback converter is determined by the steady-state minimum inductor current (IMIN) as a function of ILIMIT (1) I min = I LIMIT - (1 - D)TS Vo (V T - L( I LIMIT - I min ))VoTs TV = I LIMIT - in S = I LIMIT - s ( O || VIN ) nL L n LVinTs L L nL - where 1:nL is the transformer primary to secondary turns ratio, Ts is the switching period (1/frequency), L is the primary winding inductance value, VIN and VO are the input and output voltages and D is the duty cycle. Solving for the inductance at which IMIN = 0 gives the CCM/ DCM boundary, and yields the critical switching frequency value of (2) f crit = VO || VIN ) I LIMIT L nL ( 1 APEC 2007 Session T8: Semiconductor Devices Slide# 7 State-Machine State-Change Limits, continued * (3) Power Delivery in DCM versus CCM - Power delivery in DCM is simply PO , DCM = 12 LI LIMIT f s 2 - (4) When operating in CCM, power delivery is a little more complex (1 - D) I LIMIT + I min 1 VIN I + I min V V ( ) = VO ( )( LIMIT ) = ( O || VIN )[ I LIMIT - ( O || VIN )( )] 2 2 2 Lf s nL nLVIN + VO nL nL PO ,CCM = VO I O = VO - When a switching cycle is skipped in CCM and steady state operation ceases, ^ the inductor current perturbation, iL (0) , must be taken into account. Thus, the inductor current, I min - p (i) , at the end of the cycle following the skipped cycle is (5) I min -p ( i ) = I min + i L ( 0 ) ( - ^ Vo )i = I n L V in LIMIT - Ts VO ( || V L nL IN ) + iL (0 )( - ^ Vo )i n L V in and the power delivered in a train of^ m CCM switching cycles in time (m+1)Ts following a perturbation, i L (0) , would thus be (6) PO ,CCM m m ^ L L TV V 2 2 2 = ( I LIM -I min - p (i )) = [mI LIM - ( I LIM - s ( 0 || VIN ) + iL (0)( - 0 )i ) 2 ] L nL nLVIN 2(m + 1)Ts i =1 2(m + 1)Ts i =1 APEC 2007 Session T8: Semiconductor Devices Slide# 8 Defining State Machine State-Change Criteria Case Power & fs are minimum. Pertinent to the intermediate and the full ILIMIT states One switching cycle followed by (n - 1) skipped cycles, f s = ( 1 ) f clk , uses Equation 3: n 11 Pmin = ( ) LI n2 2 LIM - state Power delivery and fs are at a maximum. Pertinent to the intermediate ILIMIT states and the lowest ILIMIT state ( n - 1) switching cycles followed by one skipped cycle, n -1 fs = ( ) f clk , uses Equation 3: n Pmax = ( n -1 1 2 ) LI LIM - state f clk n2 DCM only f clk DCM @ min fs CCM @ max fs One switching cycle followed by (n - 1) skipped cycles, f s = ( 1 ) f clk , uses Equation 3: n Pmin 11 = ( ) LI n2 2 LIM - state ( n - 1) switching cycles followed by one skipped cycle, in CCM (involves accounting for the perturbation as a result of the skipped cycle), uses Equation 6: Pmax = n -1 ^ TV V L 2 [(n - 1) I LIM - ( I LIM - s ( 0 || VIN ) + i L (0)( - 0 ) i ) 2 ] 2(n)Tclk L nL n LVIN i =1 f clk CCM only One switching cycle followed by (n - 1) skipped cycles, but IL zero, even during ( n - 1) skipped cycles. converter is always in CCM, f s = ( 1 ) f clk , uses Equation 4: n Pmin = ( VO V n || VIN )[ I LIM - state - ( O || VIN )( )] nL 2 Lf clk nL ( n - 1) switching cycles followed by one skipped cycle, in CCM (involves accounting for the perturbation as a result of the skipped cycle), uses Equation 6: Pmax = n -1 ^ TV V L 2 [(n - 1) I LIM - ( I LIM - s ( 0 || VIN ) + i L (0)( - 0 ) i ) 2 ] 2(n)Tclk L nL n LVIN i =1 fclk is the internal IC oscillator (clocking) frequency APEC 2007 Session T8: Semiconductor Devices Slide# 9 State Machine State-Change Criteria Depicted APEC 2007 Session T8: Semiconductor Devices Slide# 10 IC Pin Function Descriptions * DRAIN (D) Pin: - Power MOSFET drain and high-voltage current source (start up circuit) connections * BYPASS / MULTI-FUNCTION (BP/M) Pin: - Bias supply bypass capacitor connection point - Internal ILIMIT level selection function - Input for latching shutdown function * ENABLE / UNDER-VOLTAGE (EN/UV) Pin: - Feedback input to switching controller - Input for under-voltage lockout function * SOURCE (S) Pin: - Power MOSFET source connections and controller ground reference point APEC 2007 Session T8: Semiconductor Devices Slide# 11 Internal IC Circuitry and Functionality Produces and regulates IC's supply voltage. Eliminates external start-up resistor string Internal ENABLE signal determines if next cycle is skipped or not Sets voltage level used by comparator to detect ILIMIT Voltage across MOSFET D-S detects ILIMIT Prevents the initial current spike from falsely triggering the ILIMIT comparator APEC 2007 Session T8: Semiconductor Devices Slide# 12 Flyback Converter Designed Around the IC APEC 2007 Session T8: Semiconductor Devices Slide# 13 Power Supply Performance Results: Efficiency APEC 2007 Session T8: Semiconductor Devices Slide# 14 Power Supply Performance Results: No-load Power Consumption without a Bias Winding APEC 2007 Session T8: Semiconductor Devices Slide# 15 Power Supply Performance Results: No-load Power Consumption with a Bias Winding APEC 2007 Session T8: Semiconductor Devices Slide# 16 Power Supply Performance Results: Transient Load Response APEC 2007 Session T8: Semiconductor Devices Slide# 17 Power Supply Performance Results: Conducted EMI APEC 2007 Session T8: Semiconductor Devices Slide# 18 Conclusion * Integrating a high-voltage MOSFET with a simple controller: - Shortens the duration and the cost of the design process - Enables the integration of functionality that is otherwise impossible ON/OFF control and the state machine give the following benefits: - Consistently high active-mode efficiency over the entire load range - Very low light load and no-load power consumption - Delivers optimized responsiveness without the loop compensation exercise - Eliminates audible sound production at no-load and light loading Careful Modeling of state machine state-change criteria and limits: - Minimized the number of ILIMIT levels required & kept change criteria simple - Enabled excellent transient load response without unnecessary state changes * Power supply performance results show that the concept is sound - Low cost, good performing power supplies can be quickly designed * * APEC 2007 Session T8: Semiconductor Devices Slide# 19 |
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