1 Prof. S. Ben-Yaakov , DC-DC Converters [9- 4] Current - PDF document

Prof. S. Ben-Yaakov , DC-DC Converters [9- 1] Current Sensing � 9.1 Resistor � 9.2 Current transformer Prof. S. Ben-Yaakov , DC-DC Converters [9- 2] Current Sensing ∗ Control PCM, ACM ∗ Protection I R Sense Resistor V R Problems I R ∗ Inductive component t I V R R L R t V R Prof. S. Ben-Yaakov , DC-DC Converters [9- 3] Losses = 2 = P R I V I d Rrms Rrms Rrms S V R ≈ For a reasonable 100 mV at least N Assuming V R , V Rrms same order of magnitude P d ≈ 0.1 I Rrms I rms = 10 A P d ≅ 1 W Not practical for I>10A 1

Prof. S. Ben-Yaakov , DC-DC Converters [9- 4] Current transformer I L I L L T t I I out out t DC component via primary lost ! DC component may saturate transformer ! Prof. S. Ben-Yaakov , DC-DC Converters [9- 5] Current Transformer D L V R I S R I S t V R ? t Prof. S. Ben-Yaakov , DC-DC Converters [9- 6] Current Transformer + − L I m S I V I R R L m R V I R m − + I S > I S = 0 1 : n 1 : n 0 V R t 2

Prof. S. Ben-Yaakov , DC-DC Converters [9- 7] Conclusions � Linear Current transformers are useful only if current to be measured is AC � In Power Electronics, Pulse Current transformer is useful for measuring pulsed current Prof. S. Ben-Yaakov , DC-DC Converters [9- 8] Pulse Current transformer Design 1. DC pulses V R I t t ? 1 : n R V R 1 : n I = ⋅ During pulse V R R n Prof. S. Ben-Yaakov , DC-DC Converters [9- 9] Resistor reset I V V m 2 R R R r S V 2 R r > R s V R S t 1 V pk might be too S high for diode Re sonant 2 effect S = S V 1 2 pk 3

Prof. S. Ben-Yaakov , DC-DC Converters [9- 10] Reset C D I m n C t - Transformer capacitance C t C D - Diode capacitance R r R S R T - Transformer Resistance R T Ideal L m Sometime no resistor is used. Prof. S. Ben-Yaakov , DC-DC Converters [9- 11] Reset - Clamp I R m V R t reset t reset < t off Re sonant effect Z + ( V V ) D Prof. S. Ben-Yaakov , DC-DC Converters [9- 12] Place to put current transformer O.K. avoid D G S D G S Why ? 4

Prof. S. Ben-Yaakov , DC-DC Converters [9- 13] Measuring “DC” current with pulse current transformer Need to measure I L I L = I D + I FET L D I L I D I FET I L I I I I FET FET D D t Inductor DC Current V O V O R S t Long wire for illustration only Prof. S. Ben-Yaakov , DC-DC Converters [9- 15] Problem I n I m2 I R I = + I I m 2 R n I m2 must be smaller than I R or 2 π f s L m > R This becomes a problem at low frequencies 5

Prof. S. Ben-Yaakov , DC-DC Converters [9- 16] Design of current transformer V D V I V + V 2 D R n V av L m2 V R V 2 t on T S Transformer operation is limited by voltage NOT CURRENT = ∫ v I RD dt = I B av max ≅ B V R max nA f max R nA n e s e If I av large need R small. V D ∫ = = Vdt V t av on av on Problem at low frequencies f s Prof. S. Ben-Yaakov , DC-DC Converters [9- 17] DC Current transformer Active Device Hall effect sensor Null operation Φ =0 by feedback I I ⋅ = = I n I V R f o S n n I f V o R S 6