Drivers What is a driver MOSFET capacitances Diver operation - PDF document

Mor M. Peretz, Switch-Mode Power Supplies [7-1] Drivers • What is a driver • MOSFET capacitances • Diver operation • Gate capacitance • Gate drive calculation • Driver types • High-side driver • Isolation • Optocoupler • Boot-strap supply [7-2] Mor M. Peretz, Switch-Mode Power Supplies Driver 1

Mor M. Peretz, Switch-Mode Power Supplies [7-3] MOSFET capacitances V gs Miller capacitance 15V Q gs +Q gd V t Q Q gs +Q gd Q 1 Q 2 Q 3 [7-4] Mor M. Peretz, Switch-Mode Power Supplies Miller effect V o C gd R g G I g V s C gs            V Cgs (0) = 0 V Cgs ( ∞ ) =V GSmax 1 1 C C  k  C k m V Cgd (0) = -V O V Cdg ( ∞ ) = V GSmax    ( ) Q V C V V C total GS gs GS O gd Q V     (1 ) total o C C C eq gs gd V V max max GS GS 2

Mor M. Peretz, Switch-Mode Power Supplies [7-5] Miller effect V o C gd R g G I g V s C gs t [7-6] Mor M. Peretz, Switch-Mode Power Supplies Required drive calculation Equivalent capacitance Assuming constant drive current V S V    (1 ) o C C C t eq gs gd V max GS I g     ? Q I t C V max total g on eq gs t C V  max eq gs I g t ? on 3

Mor M. Peretz, Switch-Mode Power Supplies [7-7] Required drive calculation Gate charge Q  3 I For t1 g t r For t2     Q I t 2  V   I R g [7-8] Mor M. Peretz, Switch-Mode Power Supplies Driver types V c npn p channel s R g d d s pnp n channel V c n channel d s d s n channel 4

Mor M. Peretz, Switch-Mode Power Supplies [7-9] High-side driver V b Gate driver V GS [7-10] Mor M. Peretz, Switch-Mode Power Supplies High-side driver Transformer R s Problem of large Don Possible solution: DC restorer 5

Mor M. Peretz, Switch-Mode Power Supplies [7-11] High-side driver Optocoupler V C V C - floating supply Driver [7-12] Mor M. Peretz, Switch-Mode Power Supplies High-side driver Potential offset R e floating supply +V C Driver Logic signal is referred to GND FET is used to sustain the high voltage 6

Mor M. Peretz, Switch-Mode Power Supplies [7-13] High-side driver Boot-strap supply – floating capacitor C R e R g Driver Internal in circuit +15V [7-14] Mor M. Peretz, Switch-Mode Power Supplies High-side driver Boot-strap supply – commercial product 7

Mor M. Peretz, Switch-Mode Power Supplies [7-15] High-side driver Boot-strap supply – commercial product [7-16] Mor M. Peretz, Switch-Mode Power Supplies Current sensing • Resistive sensing • Current transformer • Measuring DC • Reset • Design • Hall effect 8

Mor M. Peretz, Switch-Mode Power Supplies [7-17] Resistive sensing I R V R I R t Parasitic inductance I V R R L R t V R [7-18] Mor M. Peretz, Switch-Mode Power Supplies Current transformer I L I L L T t I I out out t DC component via primary lost DC component may saturate transformer 9

Mor M. Peretz, Switch-Mode Power Supplies [7-19] Current transformer Pulse current - Problem due to reset   L I m S I V I R R L m R V I R m   I S  0 I S  1 : n 1 : n 0 V R t [7-20] Mor M. Peretz, Switch-Mode Power Supplies Pulse current transformer V R I I   V R t t R n ? R V R 1 : n 1 : n Reset with resistor V 2 V R I V V S m 2 R t 1 S Re sonant 2 R R effect r S V pk may damage R r > R s diode S  S V pk 1 2 10

Mor M. Peretz, Switch-Mode Power Supplies [7-21] Pulse current transformer Reset with clamp I R m V R t reset Re sonant effect Z  ( V V ) D [7-22] Mor M. Peretz, Switch-Mode Power Supplies Measuring inductor current Combining two sensors V O R S I L I I I I FET FET D D t 11

Mor M. Peretz, Switch-Mode Power Supplies [7-23] Design of current transformer V D V V  I V 2 D R n V av L m2 V R t V 2 on T S Transformer saturation is a function of the voltage  v I RD dt  max av I B   B V R max max R nA n nA f e e s V D    av on Vdt V t av on f s [7-24] Mor M. Peretz, Switch-Mode Power Supplies Hall Effect sensor Active Device Hall effect sensor Zeroing the flux by feedback I I    I n I V R f o S n n I f V o R S 12

Mor M. Peretz, Switch-Mode Power Supplies [7-25] [7-26] Mor M. Peretz, Switch-Mode Power Supplies 13

Mor M. Peretz, Switch-Mode Power Supplies [5-27] 14