DCM rev. IL Vx Ton Toff T Ts Vx Vin VL Vin-Vo Vo -Vo - PDF document

Mor M. Peretz, Switch-Mode Power Supplies [2-1] DCM – rev. IL Vx Ton Toff T’ Ts Vx Vin VL Vin-Vo Vo -Vo [2-2] Mor M. Peretz, Switch-Mode Power Supplies Synchronous rectifier No DCM ! IL Ton Toff 1

Mor M. Peretz, Switch-Mode Power Supplies [2-3] Capacitors • Capacitor types • Output voltage ripple • Real capacitor •Parallel connection •Load step [2-4] Mor M. Peretz, Switch-Mode Power Supplies Capacitor types Polarized (electrolytic) Non Polarized C < 10 µ F Materials: Paper Tantalum Plastic Aluminum Teflon Polypropylene Minerals Mica 2

Mor M. Peretz, Switch-Mode Power Supplies [2-5] Output voltage ripple Buck IC � Q � � Vc C � IL Ts 1 � � Q 2 2 2 Vc � IL � Vc � 8C fs VESR � � V � IL R ESR ESR [2-6] Mor M. Peretz, Switch-Mode Power Supplies Output voltage ripple Boost IC Io � � � � Vc Ton on time C � � IoTon � � � � Q � � 2 Vc Vo � Vc � DTs 2RC VESR � � V � ID R ESR ESR 3

Mor M. Peretz, Switch-Mode Power Supplies [2-7] Real capacitor 20lg(|Z|) idea 1 1 � � Z l � � C 2 fC -20db/dec f [2-8] Mor M. Peretz, Switch-Mode Power Supplies Parallel connection + 100 µ 0.1 µ 20lg(|Z|) C 2 � � 1 � � dB � � � 2 f C C 1 � � 1 1 1 � � dB � � 2 f C � 2 1 � � 1 � � 1 � � dB � � dB � � � 2 f C � � 2 f C � 1 2 2 2 f f 1 f m f 2 4

Mor M. Peretz, Switch-Mode Power Supplies [2-9] Load step S C 1 C 2 R 1 R 2 Ideal response Practical response V o V o � V ( ESR ) t S t t t S [2-10] Mor M. Peretz, Switch-Mode Power Supplies Losses of SMPS • Conduction losses •RMS current •Resistor •MOSFET •ESR •Average current •Diode •IGBT • Switching losses •CV 2 • Heat conduction 5

Mor M. Peretz, Switch-Mode Power Supplies [2-11] Conduction losses V I P T 1 � � � � � P V I dt T 0 [2-12] Mor M. Peretz, Switch-Mode Power Supplies Conduction losses Resistor I Power losses are function of RMS current Why??? T 1 � � � � � P V I dt T V 0 Fourier series: 6

Mor M. Peretz, Switch-Mode Power Supplies [2-13] Conduction losses RMS calculation of typical waveform Irms � Ipk D Ipk Irms � 3 2 � � � I 2 � � � � Irms Iav � � � � 2 3 2 � � � I 2 � � � � Irms D Iav � � � � 2 3 [2-14] Mor M. Peretz, Switch-Mode Power Supplies Conduction losses MOSFET I � � 2 � � � � I � 2 � � � � P D Iav R � � � � RDS_ON DS_ON � � � � 2 3 � � 7

Mor M. Peretz, Switch-Mode Power Supplies [2-15] Conduction losses ESR I How to calculate ESR losses? Topology dependent � � 2 � � � � I � 2 � � � � P Iav ESR � � � � ESR � � � � 2 3 � � [2-16] Mor M. Peretz, Switch-Mode Power Supplies Conduction losses Diode I Conducting diode is a voltage source Power dissipation is related to the average current � P Iav V D D 8

Mor M. Peretz, Switch-Mode Power Supplies [2-17] Conduction losses Example • Buck converter • Vin = 10v ; Vout = 5V ; Iout = 5A ; fs=100kHz • L = 100uF • R DSon =10mOhm ; VD=1V ; ESR=50mOhm • Calculate efficiency [2-18] Mor M. Peretz, Switch-Mode Power Supplies Buck converter Example - solution 9

Mor M. Peretz, Switch-Mode Power Supplies [2-19] Switching losses 2 C V DS max � E C sw dss 2 � � 2 C V � � DS max � P fs sw � � 2 � � [2-20] Mor M. Peretz, Switch-Mode Power Supplies Switching losses VDS Isw tr � � � � Isw V IswV � � � DS � DS � � � � � E E t V t dt tr on off DS � � � � tr tr 6 0 � E E IswV on off DS � � P fs tr fs SW 2 3 10

Mor M. Peretz, Switch-Mode Power Supplies [2-21] Thermal considerations • Power coded to source • Temperature coded to voltage • Thermal coefficient - resistance [2-22] Mor M. Peretz, Switch-Mode Power Supplies Thermal considerations Example • Power to dissipate: 20W • � j-c = 1 o C/W ; � c-HS = 1 o C/W • Tjnct = 100 o C ; Tabm = 40 o C • Calculate � HS-amb (determine the heatsink) 11

Mor M. Peretz, Switch-Mode Power Supplies [2-23] Improving efficiency Example – MC34063 (onsemi) step up/down converter [2-24] Mor M. Peretz, Switch-Mode Power Supplies Improving efficiency Example – MC34063 (onsemi) Step-up (Boost) configuration 12

Mor M. Peretz, Switch-Mode Power Supplies [2-25] Improving efficiency Example – MC34063 (onsemi) External BJT switch [2-26] Mor M. Peretz, Switch-Mode Power Supplies 13

Mor M. Peretz, Switch-Mode Power Supplies [2-27] [2-28] Mor M. Peretz, Switch-Mode Power Supplies 14

Mor M. Peretz, Switch-Mode Power Supplies [2-29] [2-30] Mor M. Peretz, Switch-Mode Power Supplies 15

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