Drivers Prof. S. Ben-Yaakov , DC-DC Converters [8- 2] Driving a - PDF document

Prof. S. Ben-Yaakov , DC-DC Converters [8- 1] Drivers Prof. S. Ben-Yaakov , DC-DC Converters [8- 2] Driving a MOSFET L D V in V o R L C 1

Prof. S. Ben-Yaakov , DC-DC Converters [8- 3] Driver Requirements C gd R g I g C ds V s C gs V gs Miller capacitance 15V Q gs +Q gd V t Q Q gs +Q gd Q 1 Q 2 Q 3 � Capacitors are non-linear Prof. S. Ben-Yaakov , DC-DC Converters [8- 4] Gate Current V S 15V L D V in V o t V gs R g R L I g C t V S V gs I g t 2

Prof. S. Ben-Yaakov , DC-DC Converters [8- 5] Calculating Required Drive V S t * Assumption: Constant Current I g ? t ? Method A: Equivalent capacitor V o C gd I g R g R g G I g G V s C eq C gs V s Prof. S. Ben-Yaakov , DC-DC Converters [8- 6] Gate capacitance Q = C total eq V gs max V o C gd R g G V o I g I g V Cgd C gs V s V Cgs V Cgs (0) = 0 ∆ V Cgs ( ∞ ) =V GSmax V Cgd (0) = -V O ∆ V Cdg ( ∞ ) = V GSmax = + + Q V C ( V V ) C total GS gs GS O gd Miller Effect Q V = = + + C total C C ( 1 o ) eq gs gd V V GS max GS max 3

Prof. S. Ben-Yaakov , DC-DC Converters [8- 7] Gate capacitance Calculating required I g (constant), t on : V = + + C C C ( 1 o ) eq gs gd V GS max = ⋅ = ⋅ Q I t C V total g on eq gs max C V I = eq gs max g t on Prof. S. Ben-Yaakov , DC-DC Converters [8- 8] Calculating Required Drive Method B: Gate Input Charge V gs V o dependent Q Q total = ⋅ Q I t total g on Q I = total g t on 4

Prof. S. Ben-Yaakov , DC-DC Converters [8- 9] Gate Drivers V c npn Push pull - complementary R g V c =V gs max pnp p channel s d O.K. Watch for through-shoot d current s n channel Prof. S. Ben-Yaakov , DC-DC Converters [8- 10] Drivers V c n channel d s BAD d s n channel V c R g Logic OR Gate PWM level driver controller 5

Prof. S. Ben-Yaakov , DC-DC Converters [8- 11] Commercial driver � Totem pole Prof. S. Ben-Yaakov , DC-DC Converters [8- 12] Example V P Given: required t r t r I G I G - ? Question: Why can I G be considered constant ? ∑ Q ⋅ = ∑ = t I Q I r G G t r 6

Prof. S. Ben-Yaakov , DC-DC Converters [8- 13] Example − 7 ≈ = If t 100 nS 10 S r Σ Q ≈ 50 nQ 100 nS = = I 2 A 50 nQ Significant current ! Prof. S. Ben-Yaakov , DC-DC Converters [8- 14] Exercise 10meg R4 D1 out Dbreak L1 L2 V1 {L1} C1 RL {Vin} {L1*(n*n)} 220u {Load} IC = 6 out_gnd drain S1 K K1 0 gate PARAM ET ERS: K_Linear + + COUPLING = 1 n = 0.5 - - L1 = l1 Vin = 12 V1 = 0 V2 Sbreak L1 = 300u L2 = l2 V2 = 15 Load = 10 TD = 0 0 TR = 0.01u TF = 0.01u PW = 10u PER = 20u 0 � Replace the switch with a mosfet, put a 10 Ohm resistor is series with gate and measure the input current to gate 7

Prof. S. Ben-Yaakov , DC-DC Converters [8- 15] High-Side Drive The problem Possible solution : Transformer coupling V b R s Gate driver V GS Problem 1.Wide range of D on Prof. S. Ben-Yaakov , DC-DC Converters [8- 16] Driver isolation Possible solution : Like Forward ? R g n 3 n 1 ? n 2 Problem #2 - Leakage L lkg C in R g L m 8

Prof. S. Ben-Yaakov , DC-DC Converters [8- 17] Transformer isolation � Power and signal pass via transformer – High Frequency modulation Prof. S. Ben-Yaakov , DC-DC Converters [8- 18] Transformer isolation 9

Prof. S. Ben-Yaakov , DC-DC Converters [8- 19] Transformer isolation � Signal coding and reconstruction Prof. S. Ben-Yaakov , DC-DC Converters [8- 20] Driver isolation V C Solution #2: Optical Driver V C - floating supply R e Solution #3 +V C Driver Potential offset 10

Prof. S. Ben-Yaakov , DC-DC Converters [8- 21] Potential offset + floating C supply C R e R g Driver Internal in circuit +15V Available for HB, FB Prof. S. Ben-Yaakov , DC-DC Converters [8- 22] HB Driver 11

Prof. S. Ben-Yaakov , DC-DC Converters [8- 23] HB Driver Prof. S. Ben-Yaakov , DC-DC Converters [8- 24] Low-side drive 12

Prof. S. Ben-Yaakov , DC-DC Converters [8- 25] Steering diodes � Why the diodes ? Prof. S. Ben-Yaakov , DC-DC Converters [8- 26] Turn “off” R L R G R L R G DRIVER C GS C GS V CEsat L S L S I I 13

Prof. S. Ben-Yaakov , DC-DC Converters [8- 27] Lock current close to source DRIVER R f V CC Ω 10 PG G Trace current path to make sure current is locked Prof. S. Ben-Yaakov , DC-DC Converters [8- 28] Current path High Low 14

Prof. S. Ben-Yaakov , DC-DC Converters [8- 29] Real life example Ref µ V C 0 . 1 F UC3823 PG ∆ V G V CC V CC Ref V C PG G 15