EET426 Power Electronics II Isolated Forward Converter Prepared by : Mohd Azrik Roslan EET 426 – Power Electronis II

What you should know after this lecture DC Transformer concept Isolated buck converter circuit Isolated converter advantages and disadvantages Ideal and real transformer review Leakage inductance Overlap loss EET 426 – Power Electronis II

DC Transformer Concept EET 426 – Power Electronis II

EET 426 – Power Electronis II Buck Converter Ei n Vout C R L Limitation of basic converter Single input  Single output No isolation  Can cause prob during fault Output voltage relative to the input (based on duty cycle only) Basic converter have several limitations: Single input  Single output No isolation  Can cause prob during fault Output voltage relative to the input (based on duty cycle only) EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER TRANSFORMER INSERTION POINT Ei n Vout C R L EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout R C Ei n no need for 2 primary side switches in series DC TRANSFORMER CONCEPT NOT REQUIRED: EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout R C Ei n Dfwd winding polarity (dot notation) ensures D1 forward biased when mosfet is on Dfwd reverse biased when mosfet is on resulting in forward transfer of energy EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout R C Ei n Dfwd Usually for N-Channel Mosfet, low side switching has easier drive requirement LOW-SIDE mosfet: easier drive requirements EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout R C Ei n Dfwd NO CORE DEMAGNETISATION PATHWAY EET 426 – Power Electronis II

EET 426 – Power Electronis II Problem When the transistor switch is off the transformer core must be fully reset by the end of the switching cycle. This is to avoid core saturation and the resulting increase in switch current the next time it turns on. The time available for reset reduces as the switch duty cycle increases hence reset must be possible during the minimum switch off -time. EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout C Ei n Dfwd Dreset DEMAGNETISATION PATHWAY EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER + + D1 Vout OFF R C Ei n + ON Dfwd OFF Dreset MOSFET is ON EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER MOSFET is ON EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER Vout C Ei n Dfwd Dreset ON OFF winding polarity (dot notation) ensures Dreset forward biased when mosfet is off D1 reverse biased when mosfet is off EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER OFF L + D1 Vout ON R + C Ei n Dfwd + ON OFF Dreset MOSFET is OFF EET 426 – Power Electronis II

ISOLATED BUCK CONVERTER MOSFET is OFF EET 426 – Power Electronis II

EET 426 – Power Electronis II

Isolated converter advantages Possible to have multiple outputs No common input-output connection (increase safety)  Indirect converter Output voltage polarity choice Depending upon winding polarity Output voltage can be varied dependent upon Turns ratio Duty cycle EET 426 – Power Electronis II

Isolated converter disadvantages Extra Cost Size Weight Increase losses Winding resistance Core loss Leakage inductance overlap Reduce output voltage Produce transient voltages Possible to have core saturation Need core reset EET 426 – Power Electronis II

Leakage Inductance Overlap EET 426 – Power Electronis II

EET 426 – Power Electronis II Ideal Transformer VPRIM VSEC IPRIM ISEC NP NS POWER BALANCE  =100% EET 426 – Power Electronis II

EET 426 – Power Electronis II Ideal Transformer Function Transfer energy Scale current and voltage Magnetic device Provide electrical isolation No energy storage EET 426 – Power Electronis II

EET 426 – Power Electronis II REAL TRANSFORMER Have some energy storage Magnetizing inductance (within core) Can be minimized by using gapless core High permeability core Leakage inductance (external to core) Low frequency  core saturation SMPS frequency  core loss Low frequency can cause core saturation SMPS frequency can cause core loss EET 426 – Power Electronis II

REAL TRANSFORMER NP NS RC LM IMAG practical magnetic cores have finite permeability magnetising current IMAG required to establish core flux effect represented by magnetising inductance LM Practical transformer have Finite magnetization current Finite energy associated with this magnetization current Leakage inductance in the winding CORE LOSS represented by RC EET 426 – Power Electronis II

represented by PRIMARY & SECONDARY winding resistance REAL TRANSFORMER RP RS NP NS RC LM Rp = Resistance of primary winding Rs = Resistance of secondary winding winding copper loss represented by PRIMARY & SECONDARY winding resistance EET 426 – Power Electronis II

Leakage Inductance Parasitic Element EET 426 – Power Electronis II

TRANSFORMER LEAKAGE INDUCTANCE RP LLP LLS RS NP NS RC LM TRANSFORMER LEAKAGE INDUCTANCE inductive parameter of transformers (& inductors ) due to imperfect magnetic linking between windings magnetic flux that does not link primary-secondary windings represented as primary & secondary series inductive impedances EET 426 – Power Electronis II

Leakage Inductance Effects NEED to know VALUE ISOLATED FORWARD CONVERTER REDUCES OUTPUT VOLTAGE NEGATIVE EFFECT Vout Ei n C R L SCH1 SCH2 VS,2 VS VP LL.P LL.s NP: NS NORMALLY STEP DOWN EET 426 – Power Electronis II

LEAKAGE INDUCTANCE MEASUREMENT LL,P NP NS LCR meter OPEN CIRCUIT LPRIM LCR meter primary : LL,P +LPRIM LEAKAGE INDUCTANCE is an INTEGRAL PROPERTY of the TRANSFORMER IMPOSSIBLE to measure DIRECTLY EET 426 – Power Electronis II

LEAKAGE INDUCTANCE MEASUREMENT LL,P NP NS LCR meter PERFECT SHORT CIRCUIT LPRIM LCR meter primary : LL,P EET 426 – Power Electronis II

EET 426 – Power Electronis II ISOLATED FORWARD CONVERTER Vout Ei n C R L SCH1 SCH2 VS,2 VS VP LL.P LL.s NP: NS NORMALLY STEP DOWN VS < VP IS > IP EET 426 – Power Electronis II

IMPEDANCE TRANSFORMATION NP NS VSEC VPRIM ISEC IPRIM ZSEC ZPRIM STEP DOWN CONVERTERS EET 426 – Power Electronis II

IMPEDANCE TRANSFORMATION LL,P VSEC VPRIM NS NP VSEC VPRIM LL(reflected) STEP DOWN CONVERTERS LL(reflected)< LL,P EET 426 – Power Electronis II

EET 426 – Power Electronis II ISOLATED FORWARD CONVERTER Vout Ei n C R L SCH1 SCH2 VS,2 VS VP NP: NS NORMALLY STEP DOWN LL.P LL.r VS < VP IS > IP EET 426 – Power Electronis II

ISOLATED FORWARD CONVERTER: OVERLAP Vout Ei n C R L SCH1 SCH2 VS,2 VS VP LL,r NP NS Vgs Ids ISCH1 ISCH2 Leakage inductance prevents instantaneous commutation between the output rectifiers D1 and D2 resulting in two overlap intervals during which both devices are conducting, as shown in Fig E2-3 .The overlap interval tov1 results in an effective reduction of duty cycle at Vfilter which in turn creates a reduction in output voltage. The output voltage being zero during tov2 is therefore not affected by the second overlap interval. LEAKAGE INDUCTANCE prevents instantaneous commutation between SCH1 SCH2 2 overlap intervals SCH1 & SCH2 ON SCH2 ON VS,2 = 0 EET 426 – Power Electronis II

ISOLATED FORWARD CONVERTER: OVERLAP Ei n C R L SCH1 SCH2 VS,2 VS VP LL,r NP NS Vgs Ids ISCH1 Vout ISCH2 VSEC OVERLAP INTERVAL 2 NO EFFECT on Vout The overlap interval tov1 results in an effective reduction of duty cycle at Vfilter which in turn creates a reduction in output voltage. The output voltage being zero during tov2 is therefore not affected by the second overlap interval. OVERLAP INTERVAL 1 Vs,2  DUTY CYCLE REDUCTION VL,r OUTPUT VOLTAGE LOSS DUE to OVERLAP Tsw tovlap DswTsw EET 426 – Power Electronis II

ISOLATED FORWARD CONVERTER: OVERLAP Ei n C R L SCH1 SCH2 VS,2 VS VP LL,r NP NS Vgs VSEC Vs,2 Vout VL,r Tsw tovlap DswTsw during overlap Effective Duty Cycle LOSS EET 426 – Power Electronis II

ISOLATED FORWARD CONVERTER: OVERLAP Ei n C R L SCH1 SCH2 VS,2 VS VP LL,r NP NS Vgs VSEC Vs,2 Vout VL,r Tsw tovlap DswTsw during overlap Vout LOSS NON EFFICIENCY RELATED Vout LOSS EET 426 – Power Electronis II

Isolated Forward Converter Analysis EET 426 – Power Electronis II

inductor ‘rind’ power loss input voltage Ein transformer primary turns np transformer secondary turns ns secondary current maximum Is(max) primary current max Ip(max) transistor switch duty cycle Dsw transistor on-state resistance rdson transistor power loss Pmosfet output current Iout inductor ‘resistance’ rind inductor ‘rind’ power loss Pr,ind rectifier D1 and D2 voltage drops VF DATA SHEET rectifier D1 and D2 current max Iak,max combined D1 and D2 duty cycle Drects 1 combined D1 and D2 loss Prects transformer primary loss Pprim transformer secondary loss Psec EET 426 – Power Electronis II

‘ideal’ output voltage total power loss Ploss,total input power Pin output power Pout efficiency  ‘ideal’ output voltage Vout(ideal) ‘real’ output voltage Vout real EFFICIENCY RELATED VOUT LOSS EET 426 – Power Electronis II

NON-EFFICIENCY RELATED VOUT LOSS txer leakage inductance (ref prim) Lleak(prim) (ref sec) Lleak(sec) average output voltage ‘overlap’ loss output voltage Vout NON-EFFICIENCY RELATED VOUT LOSS LEAKAGE INDUCTANCE RELATED VOUT LOSS EET 426 – Power Electronis II

NON-EFFICIENCY RELATED VOUT LOSS ‘effective’ duty cycle loss output voltage Vout NON-EFFICIENCY RELATED VOUT LOSS LEAKAGE INDUCTANCE RELATED VOUT LOSS EET 426 – Power Electronis II

EET 426 – Power Electronis II Example 1 200 V Dsw = 0.3 Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine the isolated forward converter output voltage the isolated forward converter efficiency the mosfet voltage requirement EET 426 – Power Electronis II

EET 426 – Power Electronis II Example 2 200 V Dsw = 0.4 Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine the isolated forward converter output voltage the isolated forward converter efficiency EET 426 – Power Electronis II