Losses = 2 = P R I V I d Rrms Rrms Rrms S V R For a - - PDF document

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Losses = 2 = P R I V I d Rrms Rrms Rrms S V R For a - - PDF document

Jan 12, 2023 316 likes •425 views

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

slide-1
SLIDE 1

1

  • 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]

∗ Control PCM, ACM ∗ Protection

Sense Resistor Problems

∗ Inductive component

R

I

R

V

R

I

R

V L R t t

R

I

R

V

Current Sensing

slide-2
SLIDE 2

2

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 3]

Losses

Rrms Rrms 2 Rrms d

I V I R P = = N S reasonable a For least at mV 100 VR ≈ Assuming VR, VRrms same order of magnitude Pd ≈ 0.1 IRrms Irms = 10 A Pd ≅ 1 W Not practical for I>10A

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 4]

Current transformer

DC component via primary lost ! DC component may saturate transformer !

L

I

  • ut

I t t

Iout T IL L

slide-3
SLIDE 3

3

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 5]

?

IS VR t t

R D L IS VR

Current Transformer

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 6]

S

I

m

I

m

L n : 1

R

I

R

V + − IS >

R

V R + −

m L

I IS = n : 1 t

R

V

Current Transformer

slide-4
SLIDE 4

4

  • 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 : n

  • 1. DC pulses

1 : n R

R

V I t t ? VR R n I V pulse During

R

⋅ =

slide-5
SLIDE 5

5

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 9]

Resistor reset

m

I

2

V

R

V

S

R

r

R Rr > Rs Vpk might be too high for diode

1

S

2

S

2

V t

R

V sonant Re effect

2 1

S S =

pk

V

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 10]

Ct - Transformer capacitance CD - Diode capacitance RT - Transformer Resistance

m

L Ideal Sometime no resistor is used.

RT Ct Rr RS CD Im n

Reset

slide-6
SLIDE 6

6

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 11]

Reset - Clamp

m

I R

R

V

reset

t sonant Re effect ) V V (

D Z +

treset < toff

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 12]

Place to put current transformer

G D S S G D

O.K. avoid

Why ?

slide-7
SLIDE 7

7

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 13]

Measuring “DC” current with pulse current transformer

Need to measure IL

L

I t

FET

I

D

I

FET

I

D

I IL = ID + IFET D L IFET ID IL

O

V t

Long wire for illustration only

RS VO

Inductor DC Current

slide-8
SLIDE 8

8

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 15]

R 2 m

I I n I + = Im2 must be smaller than IR or 2πfsLm > R This becomes a problem at low frequencies

Problem

Im2 IR I n

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 16]

Design of current transformer

Lm2 VR I n VD V2

Transformer operation is limited by voltage NOT CURRENT R n I V nA dt B

R e max

v

≅ = ∫

s

  • n

av

  • n

av

f D V t V Vdt = =

∫

2

V

R D

V V +

av

V

  • n

t

S

T

s e max av max

f nA RD I B = If Iav large need R small. Problem at low frequencies

slide-9
SLIDE 9

9

  • Prof. S. Ben-Yaakov , DC-DC Converters

[9- 17]

DC Current transformer

I n

  • V

S

R

f

I Active Device Hall effect sensor

Null operation Φ=0 by feedback

S

  • f

R n I V I n I = = ⋅