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 Mor M. Peretz, Switch-Mode Power Supplies [7-2] Driver 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 1
Mor M. Peretz, Switch-Mode Power Supplies [7-4] 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 Mor M. Peretz, Switch-Mode Power Supplies [7-5] Miller effect V o C gd R g G I g C gs V s t Mor M. Peretz, Switch-Mode Power Supplies [7-6] 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 2
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 Mor M. Peretz, Switch-Mode Power Supplies [7-8] 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 Mor M. Peretz, Switch-Mode Power Supplies [7-9] High-side driver V b Gate driver V GS 3
Mor M. Peretz, Switch-Mode Power Supplies [7-10] High-side driver Transformer R s Problem of large Don Possible solution: DC restorer Mor M. Peretz, Switch-Mode Power Supplies [7-11] High-side driver Optocoupler V C V C - floating supply Driver Mor M. Peretz, Switch-Mode Power Supplies [7-12] 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 4
Mor M. Peretz, Switch-Mode Power Supplies [7-13] High-side driver Boot-strap supply – floating capacitor C R e R g Driver in Internal circuit +15V Mor M. Peretz, Switch-Mode Power Supplies [7-14] High-side driver Boot-strap supply – commercial product Mor M. Peretz, Switch-Mode Power Supplies [7-15] High-side driver Boot-strap supply – commercial product 5
Mor M. Peretz, Switch-Mode Power Supplies [7-16] Current sensing • Resistive sensing • Current transformer • Measuring DC • Reset • Design • Hall effect 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 Mor M. Peretz, Switch-Mode Power Supplies [7-18] Current transformer I L I L L T t I I out out t DC component via primary lost DC component may saturate transformer 6
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 Mor M. Peretz, Switch-Mode Power Supplies [7-20] 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 V S 1 2 pk 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
Mor M. Peretz, Switch-Mode Power Supplies [7-22] Measuring inductor current Combining two sensors V O R S I L I I I I FET D FET D t 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 V 2 t on T S Transformer saturation is a function of the voltage v I RD dt max I av 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 Mor M. Peretz, Switch-Mode Power Supplies [7-24] 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 8
Mor M. Peretz, Switch-Mode Power Supplies [7-25] Mor M. Peretz, Switch-Mode Power Supplies [7-26] Mor M. Peretz, Switch-Mode Power Supplies [5-27] 9
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