An Intelligent Inrush Current Eliminator

AN INTELLIGENT INRUSH CURRENT ELIMINATOR
Isaac
current. It is applicable to those high
the power factor correction inductors.
1. INTRODUCTION
The inrush current, related to the operation of a
power supply in an electronic equipment, and the
limitations imposed on it, have a long history.
It may be sufficient to say that it had started
with the advent of capacitors, it benefited from
the developments in some DC and some AC electric
motor applications. Therefore, it would not be
surprising to see that the fundamental principles
of the ideas we employ today, had already been
defined in the pre-WW2 literature. These even
include some impressive implementations with
electron tube controlled rectifiers.
What is new and novel is rather the style and eco-
nomics of incorporating the ideas to meet the re-
quirements of fundamental or derived topologies
with more sophistication, higher packaging den-
sity and higher "applicability
Substituting
A typical Off-Line SMPS Input Stage
filter from the figure. It does not add up to
anything important in limiting the inrush current
effectively, anyway. It would be helpful, if we
acquire an appreciation of the magnitudes of
inrush current, when it is not limited.
Let's assume that, due to a random chance,
we turn
load
resistance representing SMP
Processor,
r
R+r
line voltage. Fig.3 shows the curve of
.25 Ohms, R = 7.36 Ohms, C
=
ning and negligibly low subsequently. This would
+
J
t
0 rent limiting. Let's now examine the available
alternatives according to the historical sequence
of introduction of the fundamental idea.
(a)LIMITERS USING A LINEAR RESISTOR:
In this method; a linear power resistor, R, is
introduced into the inrush current path, Fig.4.
When applied to the Off-Line SMPS', it can be
placed either before or after the bridge
and
h) -
(t)
and
using
-+ 1
V,*&j
C R
155
Control
Fig.4- Inrush current limiter using a resistor special circuit, powered by the line or by the HF isolation transformer in the SMPS o r a sequenced combination of the both. S 2 can be controlled by processing either the voltage across the storage capacitor and/or the time, tailored to the speci - fic application. The advantages are their rela - tive simplicity and economic implementation. The disadvantages are the bulk of the
LIMITERS USING CONTROLLED RECTIFIERS:
Historically, this idea dates back to the advent of Thyrotron tubes (1). With the coming of SCR's it acquired a greater importance and with triacs a greater popularity in high power, Off -Line SMPS'.
Figs. 5a, 5b and
to limit inrush current
two of the bridge rectifiers are replaced by SCR' requiring somewhat elaborate
circuits to
control
the conduction
angles.
The alternative shown
in 156
and i t may employ a s i m i l a r scheme of conduction
angle c o n t r o l.Fig.6 shows t h e v o l t a g e a c r o s s t h e
s t o r a g e c a p a c i t o r and t h e envelope of c u r r e n t
p u l s e s. A momentary l e v e l-o f f may occur, as t h a t
c i r c l e
d by t h
e d o t t e d l i n e,i
f t h e S M P S s t a r t s up
before t h e v o l t a g e a c r o s s t h e s t o r a g e c a p a c i t o r
reaches t h e s t e a d y s t a t e
t o cope with a l l of t h e c r e d i b l e
l i n e t r a n s i e n t s with regard t o r e s e t of inrush
c u r r e n t l i m i t e r f u n c t i o n,
- pulse power from t r i g g e r i n g
c i r c u i t s when use
d with high c u r r
e n t and high
voltage c o n t r o l l e d r e c t i f i e r s and
- commutation problems which a r e,sometimes d i f f i-
c u l t t o s o l v e,depending on t h e mode of operation
and power l e v e l,when used with power f a c t o r
c o r r e c t i o n i n
d u c t o r s (8,9,10).
(c)LIMITERS USING THERMISTORS:
A Negative Temperature C o e f f i c i e n t(NTC) Thermis-
t o r can be s u b s t i t u t e d f o r R i n Fig.4 (11,12).
The use of a s i n g l e NTC Thermistor i s r a t h e r
s u i t a b l e f o r low power
SMPS',
C O N S T A N T CURRENT INRUSH LIMITERS:
The requirements
and t h e t h e o r e t i c a l
o p e r a t i o n
of
constant
c u r r e n t inrush
l i m i t e r s
have b een d e l i-
neated
i n t h e l i t e r a t u r e
and t h e l a c k
of design
attempts has
been r e l a t e d t o t h e u n a v a i l a b i l i t y
of high c u r r e n t and high voltage
t r a n s i s t o r s
(5).
Fig.7-Use of t h e r m i s t o r
t o l i m i t i n r u s h
Although,
t h e high c u r r e n t and t h e high v o l t a g e
t r a n s i s t o r s
a r e a v a i l a
b l e
now, but w e s t i l l
await
f o r a constant
c u r r e n t device with
a n e g l i g i
b l y
small
power d i s s i p a t i o n
during
t h e steady
s t a t e
it
i
3.
A l l of t h e
S e c t i o n2,
approach, i t may m e r i t t o be c a l l e d
Current Eliminator".
INRUSH CURRENT ELIMINATOR
inrush c u r r e n t l i m i t e r s,examined i n
were based on t h e t a c i t"acceptance"
of a h i g h l y o v e r-c r i t i c a l approach t o t h e steady
s t a t e magnitude. It i s p o s s i b l e and f e a s i b l e t o
arrange a c r i t i c a l approach t o t h e steady s t a t e
thus e l i m i n a t i n g t h e e x i s t e n c e of inrush c u r r e n t.
Fig.8 shows t h e e n t i r e c i r c u i t which achieves t h i s
c r i t i c a l approach. Fig.9 shows t h e v o l t a g e a c r o s s
t h e s t o r a g e c a p a c i t o r(upper t r a c e)and t h e envel-
ope of b r i d g e r e c t i f i e r output c u r r e n t(lower
t r a c e).L e t's now examine t h e o p e r a t i o n of t h e
s t a r t from t h e house keeping power
s u p p l i e s.
(a) House Keeping Power Supplies: There a r e two
i s o l a t e d house keeping power s u p p l i e s i n Fig.8.
14 p i n I C socket compatible m i n i a t u r e r e l a y s
I
w
1
..
Fig.9- Critical approach to steady state
the rail voltage to the control circuit. This
time delay allows the synchronization system to
reach the steady,state, i f the system in Fig.8
feeds more than one Power processor, and the fans
of these processors to pick up speed. If only one
processor is fed from the system, this time delay
circuit may be taken out. The ways of feeding
these house keeping power supplies from the HF
power transformer
of the SMPS are not shown.
observed at nodes (l)(lower trace), (2)(middle
trace) and (3)(upper
trace).
Conduction Angle Control Circuit: U1-B and U1-C
together with
5.0V. reference
thus generating the waveform of
Fig.12 (upper trace). This output
waveform
of
show enabling sig-
nal progression and relates it to the emitter cur-
rent of 48.
Q5, Ql-C
T3 and CR8 form the gated drive circuit for the
power transistor
48.
is the power factor correcting induc -
Constant Current Control Circuit: U18-A and
-B with U5 form the constant current control
For those power outage transients, lasting 4-5
msec., occurred during the first half of
E nabling signal generation
a line scallop, the emitter current of 48 is limited to 10 A. max. For any other outage transients, a reset of the system function
occurs.
Shutdown Circuit: If the SMPS experiences a
fault condition and shuts off, this can be communi - V. step function at
& 2
terminals, U3-A and U7-B
I
Power control
REFERENCES:
shuts the system down to prevent an unnecessary continuation of the operation.
4. NOTES
1. W. Overbeck, "Critical Inductance and Controlled
Rectifiers ", Proc. of IRE, Oct. 1939, pp.655-659 We may furnish the following notes.
2. 0. H. Schade, "Analyses of Rectifier Operation "
(a ) In the above design, Fig.8; due to the use of quad IC op -amps, quad transistor IC's and center Proc. of IRE, July 1943.
tapped power diodes, a May 79.
can b e
achieved.
enabled us to go up to 1800 W. output power
"Designing a Low Cost In -
telligent Inrush Limiter for Off -Line Conver
-
100 @ 30 A. and its remarkably even
May 1978.
6. N. 0. Sokal,
Converter
o r
140.
5. ACKNOWLEDGEMENTS
I would like to thank to Messrs.
N. Quan, M. Tung
and
C.
62
7. R. D. Middlebrook, "Input Filter Considerations
in Design and Application of Switching Regu
-
f o r a Single -phase Rectifier Power Supply "
IEEE Trans. on Industry Applications,
9. B. M. Bird and K.G. King, "An Introduction to
Power Electronics", John Wiley Sons, 1983
Chapter 2.
10. R. A.
PTC Thermistors"
Commercial Catalog, CL-10 to。