U.S. patent application number 14/948702 was filed with the patent office on 2016-06-09 for inductively controlled series resonant ac power transfer.
This patent application is currently assigned to Auckland Uniservices Limited. The applicant listed for this patent is John Talbot Boys, Grant Anthony Covic, Hunter Hanzhuo Wu. Invention is credited to John Talbot Boys, Grant Anthony Covic, Hunter Hanzhuo Wu.
Application Number | 20160164304 14/948702 |
Document ID | / |
Family ID | 43876329 |
Filed Date | 2016-06-09 |
United States Patent
Application |
20160164304 |
Kind Code |
A1 |
Boys; John Talbot ; et
al. |
June 9, 2016 |
Inductively Controlled Series Resonant AC Power Transfer
Abstract
An inductive power transfer pickup circuit has a pickup coil
(L.sub.2) and tuning capacitor (C.sub.2) connected in series to
provide a series resonant circuit. A bi-directional switch
(S.sub.1) is used to vary the phase angle between the open circuit
pickup coil voltage (V.sub.OC) and the pickup coil inductor current
(i.sub.L) to provide a controlled AC supply to an output of the
pickup.
Inventors: |
Boys; John Talbot;
(Auckland, NZ) ; Covic; Grant Anthony; (Auckland,
NZ) ; Wu; Hunter Hanzhuo; (Auckland, NZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Boys; John Talbot
Covic; Grant Anthony
Wu; Hunter Hanzhuo |
Auckland
Auckland
Auckland |
|
NZ
NZ
NZ |
|
|
Assignee: |
Auckland Uniservices
Limited
Auckland
NZ
|
Family ID: |
43876329 |
Appl. No.: |
14/948702 |
Filed: |
November 23, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13261259 |
Jul 30, 2012 |
|
|
|
PCT/NZ2010/000203 |
Oct 12, 2010 |
|
|
|
14948702 |
|
|
|
|
Current U.S.
Class: |
307/104 |
Current CPC
Class: |
H02J 50/12 20160201;
H02J 5/005 20130101 |
International
Class: |
H02J 5/00 20060101
H02J005/00; H02J 50/12 20060101 H02J050/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2009 |
NZ |
580388 |
Claims
1. A method of providing an AC supply from IPT pickup having a
pickup coil and tuning capacitor connected in series to provide a
series resonant circuit, the method comprising varying a phase
angle between an open circuit pickup coil voltage and a pickup coil
inductor current to provide a controlled AC supply to an output of
the pickup.
2. A method as claimed in claim 1 wherein the phase angle between
the open circuit pickup coil voltage and the pickup coil inductor
current is varied by substantially preventing current flow in the
resonant circuit for a selected time period.
3. A method as claimed in claim 2 further comprising varying the
selected time period to vary the phase angle.
4. A method as claimed in claim 2 wherein substantially preventing
current flow comprises detecting when the current in the resonant
circuit is substantially zero and maintaining the current at
substantially zero for the selected time period.
5. A method as claimed in claim 2 further comprising operating a
switch in order to substantially prevent current flow in the
resonant circuit.
6. A method as claimed in claim 5 further comprising using a
bi-directional switch.
7. A method as claimed in claim 5 wherein operating the switch
comprises allowing current flow through the switch to become
substantially zero by a diode turn off characteristic.
8. A method as claimed in claim 2, further comprising comparing the
output of the pickup with a reference, and increasing or decreasing
the selected time period to change the output of the pickup toward
the reference.
9. An IPT pickup controller for a pickup having a pickup coil and a
tuning capacitor connected in series with the pickup coil, the
controller being adapted to control one or more switches to control
a pickup coil inductor current to thereby vary a phase angle
between a pickup coil open circuit voltage and the pickup coil
inductor current.
10. An IPT pickup controller as claimed in claim 9 wherein the
phase angle between the pickup coil open circuit voltage and the
pickup coil inductor current is varied by operating the one or more
switches at a selected time to substantially prevent current flow
in a resonant circuit for a selected time period.
11. An IPT pickup controller as claimed in claim 10 wherein the
controller compares an output of the pickup with a reference, and
increases or decreases the selected time to change the output of
the pickup toward the reference.
12. An IPT pickup comprising a pickup coil and a tuning capacitor
connected in series to provide a series resonant circuit, and a
controller to vary a phase angle between a pickup coil open circuit
voltage and a pickup coil inductor current to thereby provide a
controlled AC supply to an output of the pickup.
13. An IPT pickup as claimed in claim 12 wherein the phase angle
between the pickup coil open circuit voltage and the pickup coil
inductor current is varied by the controller substantially
preventing current flow in the resonant circuit for a selected time
period.
14. An IPT pickup as claimed in claim 13 wherein the controller
varies the selected time period to vary the phase angle.
15. An IPT pickup as claimed in claim 13 wherein the controller
substantially prevents current flow by detecting when the current
in the resonant circuit is substantially zero and maintaining the
current at substantially zero for the selected time period.
16. An IPT pickup as claimed in claim 13 wherein the controller
operates a switch in order to substantially prevent current flow in
the resonant circuit.
17. An IPT pickup as claimed in claim 16 wherein the switch
comprises a bi-directional switch.
18. An IPT pickup as claimed in claim 16 wherein the controller
operates the switch by allowing current flow through the switch to
become substantially zero by a diode turn off characteristic.
19. An IPT pickup as claimed in claim 13 wherein the controller
compares the output of the pickup with a reference, and increases
or decreases the selected time period to change the output of the
pickup toward the reference.
20. An IPT pickup as claimed in claim 12 further comprising a
switch connected in series with the series resonant circuit wherein
the controller operates the switch to provide the controlled AC
supply to the output of the pickup.
21. An IPT pickup as claimed in claim 20 wherein the switch
comprises a bi-directional switch.
22. An IPT system comprising an IPT pickup as claimed in claim
12.
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
Description
FIELD OF THE INVENTION
[0001] This invention relates to Inductive Power Transfer (IPT) and
has particular, but not sole, application to the provision of an AC
power source. The invention may also be used to provide a DC power
source.
BACKGROUND
[0002] IPT systems are now widely used in industry and elsewhere to
couple power from one reference frame to another without physical
contact. An example of such a system is described in U.S. Pat. No.
5,293,308, the contents of which are incorporated herein by
reference.
[0003] IPT technology allows large amounts of electrical energy to
be transferred between two loosely coupled inductors over
relatively large air gaps. An IPT system can be divided into two
sections--a primary supply and one or multiple secondary pickups.
The, or each, pickup receives power inductively from the primary.
For an IPT system used in material handling applications, multiple
secondary pickups are coupled on one long track as shown in FIG. 1,
and the coupling coefficient between the primary and secondary
inductors is typically around 0.01-0.1. In order to transfer large
amounts of power (>1 kW) to each secondary, the primary supply
generates a current in the range of 10-80 A and a frequency in the
order of 10-40 kHz to overcome the low coupling conditions.
Currently. IPT applications have been used in a wide variety of
industrial and commercial applications.
[0004] In order to improve power transfer capacity in the IPT
system, some compensation or tuning capacitor is required in the
secondary pickup. The two most common compensation topologies used
in the pickup are parallel and series tuned systems as shown in
FIG. 1. Parallel tuning gives a constant current source property
and series tuning gives a constant voltage source property. For the
series tuned pickup, the voltage source property is ideal for
driving most common types of loads. However, it is difficult to
exactly match the induced voltage of the pickup to the desired
output voltage as the tolerance in the inductor windings can easily
create a 10% deviation in the output voltage. This 10% error may
not be acceptable for many commercial or industrial loads. As such,
a switch mode controller is usually required after the pickup to
regulate the output voltage to its desired value with a minimal
amount of error.
[0005] One technique is to use primary side control to achieve
voltage regulation on the secondary pickup. This method sends
feedback signals such as output voltage of the secondary pickup
back to the primary converter via a wireless communication channel.
Generally, primary side control has two possible methods of
realization--frequency control or primary current control.
[0006] For applications such as material handling systems with
multiple secondary pickups, control on the primary side cannot be
used since regulating voltage on one pickup will affect the
operation of other pickups which may be operating at different
power levels. One conventional method to regulate the output
voltage on the secondary side is to use a linear voltage regulator
after the pickup. However, due to the tolerance of the output
voltage of the pickup and the poor efficiency of the linear
regulator, this topology is limited to low power applications.
Another method cascades a buck converter after the series tuned
pickup to regulate the output voltage with more electrical
efficiency. However, this is not ideal because of the large number
of components required which increase cost. In addition, the two
stage (AC-DC and DC-DC) conversion process has losses in each stage
which reduce efficiency. Other secondary side control techniques
directly regulate power on the AC side to deliberately tune or
detune the resonant tank circuit by adding extra reactance. One
technique to realize a variable reactance component is to use a
magnetic amplifier to produce a variable inductor. Although this
may vary the AC power directly, the use of a variable inductor in
the non-linear region of the B-H curve can limit the efficiency of
the overall system. In addition, the variable inductor is expensive
to manufacture because it has to manage the high resonant current
without fully saturating.
OBJECT
[0007] It is an object of the invention to provide an IPT system
that provides an AC power source, or to at least provide the public
with a useful choice.
SUMMARY OF THE INVENTION
[0008] In one aspect the disclosed subject matter provides a method
of providing a power supply from IPT pickup having a pickup coil
and tuning capacitor connected in series to provide a series
resonant circuit, the method including the step of varying the
phase angle between the open circuit pickup coil voltage and the
pickup coil inductor current to provide a controlled AC supply to
an output of the pickup.
[0009] In one embodiment the AC supply at the output is rectified
to provide a DC supply at a further output.
[0010] In one embodiment the phase between the pickup coil open
circuit voltage and the pickup coil inductor current is varied by
substantially preventing current flow in the resonant circuit for a
selected time period.
[0011] In one embodiment the selected time period is varied to vary
the phase angle.
[0012] In one embodiment the step of substantially preventing
current flow includes detecting when the current in the resonant
circuit is substantially zero and maintaining the current at
substantially zero for the selected time period.
[0013] In one embodiment the current is substantially prevented
from flowing by operating a switch. In one embodiment the switch
comprises a bi-directional switch.
[0014] In one embodiment the method includes the step of comparing
the output of the pickup with a reference, and increasing or
decreasing the selected time period to change the output of the
pickup toward the reference.
[0015] In another aspect the disclosed subject matter provides a
controller for an IPT pickup having a pickup coil and a tuning
capacitor connected in series, the controller including one or more
switches to control the pickup coil inductor current to thereby
vary a phase angle between the pickup coil open circuit voltage and
the pickup coil inductor current.
[0016] In one embodiment the phase between the pickup coil open
circuit voltage and the pickup coil inductor current is varied by
operating the one or more switches at a selected time to
substantially prevent current flow in the resonant circuit for a
selected time period.
[0017] In another aspect the disclosed subject matter provides an
IPT pickup comprising a pickup coil and a tuning capacitor
connected in series to provide a series resonant circuit, and a
controller to vary a phase angle between the pickup coil open
circuit voltage and the pickup coil inductor current to thereby
provide a controlled AC supply to an output of the pickup.
[0018] In one embodiment the phase between the pickup coil open
circuit voltage and the pickup coil inductor current is varied by
the controller substantially preventing current flow in the
resonant circuit for a selected time period.
[0019] In another aspect the disclosed subject matter provides an
IPT pickup comprising a pickup coil and a tuning capacitor
connected in series to provide a series resonant circuit, and
switch connected in series with the resonant circuit, the switch
being operable to vary a phase angle between the pickup coil open
circuit voltage and the pickup coil inductor current to thereby
provide a controlled AC supply to an output of the pickup.
[0020] In one embodiment the switch comprises a bi-directional
switch. A controller may be provided to control operation of the
switch.
[0021] In yet another aspect the disclosed subject matter provides
an IPT system including an IPT pickup according to any one of the
preceding statements.
[0022] The invention may also be said broadly to consist in the
parts, elements and features referred to or indicated in the
specification of the application, individually or collectively, in
any or all combinations of two or more of said parts, elements or
features, and where specific integers are mentioned herein which
have known equivalents in the art to which the invention relates,
such known equivalents are deemed to be incorporated herein as if
individually set forth.
BRIEF DRAWING DESCRIPTION
[0023] An embodiment of the invention will be described by way of
example with reference to FIGS. 1-16 in which:
[0024] FIG. 1 is a block diagram of a known IPT system.
[0025] FIG. 2 is a diagram showing a Series Tuned AC Processing
Pickup.
[0026] FIG. 3 shows operating waveforms of the series AC processing
pickup of FIG. 2.
[0027] FIG. 4 is a diagram of the pickup circuit waveform showing
two operating states.
[0028] FIG. 5 is a flow chart of a computation algorithm.
[0029] FIG. 6 is a diagram showing normalized RMS output current
vs. phase delay .phi..
[0030] FIG. 7 is a diagram showing normalized RMS output voltage
vs. controlled phase delay .phi..
[0031] FIG. 8 shows pickup output voltage current
characteristics.
[0032] FIG. 9 shows harmonic components of inductor current as a
percentage of the maximum fundamental value at Q.sub.2=5.
[0033] FIG. 10 shows reactive load vs. real load.
[0034] FIG. 11 shows calculated waveforms when phase delay .phi. is
(a) 0.degree., (b) 58.degree. and (c) 85.degree..
[0035] FIG. 12 shows examples of bi-directional switches.
[0036] FIG. 13 is a block diagram for a controller.
[0037] FIG. 14 shows measured waveforms for (a) 100% power, (b) 50%
power and (c) 20% power with 6.OMEGA. resistive load.
[0038] FIG. 15 shows measured waveforms for (a) 8.OMEGA., (b)
12.OMEGA. and (c) 16.OMEGA. load with constant 80V DC voltage.
[0039] FIG. 16 shows efficiency vs. output power.
DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
[0040] A new type of AC processing pickup illustrated herein
exhibits excellent features such as simple circuitry, lower
production cost and very high efficiency operation.
[0041] This specification discloses a new series AC processing
pickup that uses an AC switch operating near ideal soft switching
operating conditions to regulate the output voltage of the pickup
directly. The output can be either controlled AC or DC depending on
whether a rectifier is added to the AC output at the end of the
resonant network.
[0042] According to one embodiment of the invention a series AC
processing pickup is shown in FIG. 2 with an AC output voltage
(V.sub.R2). Capacitor C.sub.2 is tuned to inductor L.sub.2 at the
frequency of the primary track current i.sub.1 to form a series
resonant tank. The open circuit voltage source (V.sub.oc)
represents the induced voltage of the pickup. For simplicity,
switch S.sub.1 is drawn as an ideal AC switch and it is the basis
for controlling the output voltage.
[0043] To illustrate the circuit's operation. FIG. 3 shows the one
period operation of the series AC processing pickup at each
particular switching interval. V.sub.g1 is the PWM control signal
which turns S.sub.1 on and off. Consider the situation where
V.sub.g1 is controlled with a phase delay .phi. relative to the
phase of V.sub.oc as shown in FIG. 3. In Mode 1 (M.sub.1,
0<.ltoreq.t.sub.1), S.sub.1 is operated by being turned on and
the capacitor C.sub.2 resonates with pickup inductance L.sub.2 like
a series resonant tank and the inductor current reaches a peak
value and returns back to zero. When the inductor current reaches
zero, S.sub.1 is operated by being turned off and the circuit
enters Mode 2 (M.sub.2, t.sub.1<t.ltoreq.t.sub.2). In this mode,
no current flows through any device and the inductor current is
discontinuous, i.e. substantially zero for a selected time period,
for a phase known as the discontinuous phase (t.sub.o/.omega.) at
the point where I.sub.R (the current through the resistor R.sub.2)
changes from a positive to a negative voltage. In the beginning of
Mode 3 (M.sub.3, t.sub.2<t.ltoreq.t.sub.3), S.sub.1 is turned
back on. Similar to M.sub.1, the circuit operates like a series
resonant tank and current flows into the load resistor. In Mode 4
(M.sub.4, t.sub.3<t.ltoreq.T), similar to M.sub.2, the resonant
cycle is terminated and the inductor current is discontinuous.
After this mode, the circuit returns back to M.sub.1, repeating the
switching process. In summary, the switching action from the
equivalent AC switch generates a phase shift between the open
circuit voltage and the inductor current waveform.
[0044] The series AC processing pickup also achieves near ideal
soft switching conditions. From FIG. 3, at t.sub.1, the voltage
across S.sub.1 decreases from zero to a negative voltage while the
current through it is at zero. Because there is no current flow,
Zero-Current-Switching (ZCS) is achieved at turn off. When S.sub.1
is turned on at t.sub.2, the pickup inductor in series with S.sub.1
forces the current through it to increase slowly in the negative
direction while the voltage across it decreases to zero. For most
practical switches, the turn on is much faster than the rate of
increase of the inductor current, so the di/dt through the switch
is relatively small and a near zero current switch on condition is
obtained. In summary, if the timing of the gate drive signal for
the AC switch is accurate, the AC processing pickup achieves near
perfect soft switching conditions. A more practical method of
driving the AC switch which does not heavily rely on accurate
timing is described further below. The soft switching condition
gives the pickup desirable characteristics such as low switching
losses, low switching stress and reduced electromagnetic
interference (EMI) levels.
[0045] Analysis
[0046] From the previous section, it can be seen that the phase
shift between V.sub.oc and I.sub.L can be controlled by adjusting
the phase delay .phi.. In this section, the phase delay .phi. is
used in an exact analysis in the time domain to determine the
characteristics of the circuit under steady state operation. The
basis of the analysis method is that the conditions existing in the
circuit at the end of a particular switching period must be the
initial conditions for the start of the next switching period, and
these conditions must be identical, allowing for steady state
resonant operation.
[0047] The analysis procedure is greatly simplified based on the
three following assumptions: [0048] The Equivalent Series
Resistance (ESR) of both capacitor C.sub.2 and Inductor L.sub.2 are
very small compared to the load resistor and can be neglected.
[0049] The switching action of the transistors and diodes are
instantaneous and lossless. [0050] Capacitor C.sub.2 and inductor
L.sub.2 are perfectly tuned forming a series resonant tank with the
load.
[0051] Assuming the resonant tank is perfectly tuned,
C.sub.2=1/(.omega..sup.2L.sub.2) (1)
[0052] With reference to FIG. 4, the waveform can be separated into
two operating states known as the resonant state and the
discontinuous state.
[0053] A. Resonant State
[0054] During the resonant state, the inductor current may be
described as:
d 2 i Lr dt 2 + R L 2 di Lr dt + i Lr L 2 C 2 = V oc .omega. L 2
cos ( .omega. t + .phi. ) ( 2 ) ##EQU00001##
[0055] Considering the initial condition i.sub.Lr(t)|.sub.t=0=0
and
di Lr ( t ) dt i = 0 = V oc sin ( .phi. ) - V c ( 0 ) L 2 ,
##EQU00002##
the complete solution of the above equation is:
i Lr ( t ) = - Q 2 V oc sin ( .phi. ) .omega. L 2 sin ( .theta. v )
- t / T sin ( .omega. f t - .theta. i ) + Q 2 V oc .omega. L 2 sin
( .omega. t + .phi. ) where ( 3 ) Q 2 = ( .omega. L 2 ) / R 2 ( 4 )
T = 2 L 2 / R 2 ( 5 ) .omega. f = .omega. 1 - 1 / ( 4 Q 2 2 ) ( 6 )
.theta. i = tan - 1 ( - .omega. f V oc sin ( .phi. ) / .omega. V c
( 0 ) - V oc sin ( .phi. ) + Q 2 V oc cos ( .phi. ) + Q 2 V oc sin
( .phi. ) / ( .omega. L 2 ) ) ( 7 ) ##EQU00003##
[0056] In a similar way, considering the initial condition
V.sub.cr(t)|.sub.t=0=V.sub.c(0) and
dV cr dt i = 0 = 0 , ##EQU00004##
the complete solution to the capacitor voltage is:
V cr ( t ) = - V c ( 0 ) + Q 2 V oc cos ( .phi. ) sin ( .theta. v )
- t / T sin ( .omega. f t - .theta. v ) - Q 2 V oc cos ( .omega. t
+ .phi. ) where ( 8 ) .theta. v = tan - 1 ( .omega. f T ( V c ( 0 )
+ Q 2 V oc cos ( .phi. ) ) - V c ( 0 ) - Q 2 V oc cos ( .phi. ) +
.omega. TQ 2 V oc sin ( .phi. ) ) ( 9 ) ##EQU00005##
[0057] To investigate how long the circuit stays in the resonant
state, i.sub.L(t)=0 can be substituted in (3), resulting in the
following expression:
i.sub.Lr(t.sub.z)=0 (10)
[0058] where t.sub.z is the time the circuit operates in the
resonant state.
[0059] B. Discontinuous State
[0060] During the discontinuous state, the series resonant circuit
becomes an open circuit and the capacitor voltage remains constant
while the inductor current is zero.
V.sub.cd(t)|.sub.t=0=V.sub.c(t.sub.z) (11)
i.sub.Ld(t)=0 (12)
[0061] Because the resonant state and the discontinuous state are
repeated each half cycle (with only a polarity change), the
relationship V.sub.c(0)=-V.sub.c(T/2) must hold. Hence, the
capacitor voltage and inductor current are given by,
V c ( t ) = { V cr ( t ) 0 .ltoreq. t < t 1 V cd ( t ) t 1
.ltoreq. t < t 2 - V cr ( t ) t 2 .ltoreq. t < t 3 - V cd ( t
) t 3 .ltoreq. t < t 4 ( 13 ) i L ( t ) = { i Lr ( t ) 0
.ltoreq. t < t 1 i Ld ( t ) t 1 .ltoreq. t < t 2 - i Lr ( t )
t 2 .ltoreq. t < t 3 - i Ld ( t ) t 3 .ltoreq. t < t 4 ( 14 )
##EQU00006##
[0062] Fourier analysis can be performed on the inductor current
waveform to compute the harmonics. The in-phase and quadrature
components of both the fundamental and harmonics are given by:
I Lpn = 2 .omega. .pi. .intg. 0 .pi. / .omega. i L ( t ) cos ( n
.omega. t ) t ( 15 ) I Lqn = 2 .omega. .pi. .intg. 0 .pi. / .omega.
i L ( t ) sin ( n .omega. t ) t ( 16 ) ##EQU00007##
[0063] It is important to determine the amount of power sourced
from the primary IPT power supply for the pickup to operate. If
harmonics are ignored, the real and reactive power sourced from the
primary supply are given by:
P = Re ( Z r ) I 1 2 ( 17 ) VAR = Im ( Z r ) I 1 2 ( 18 ) Z c = -
j.omega. M 2 L 2 I Lp 1 + j I Lq 1 I sc ( 19 ) ##EQU00008##
[0064] C. Computation Routine
[0065] The above analytical analysis is very difficult as the
solution of t.sub.z and V.sub.c(0) are governed by (13) and (14)
with .theta..sub.v and .theta..sub.i as interim variables which are
associated with the auxiliary equations (7) and (9). This is in the
form of transcendental equations that can only be solved using
numeric solvers such as MATLAB or EXCEL. A computer program based
on an iterative computation, shown in FIG. 5, has been developed in
MATLAB to undertake the analysis. The program starts by
initializing the circuit parameters such as L.sub.2, C.sub.2 and
R.sub.2. The initial capacitor voltage is set as an initial
condition. With V.sub.c(0) known, t.sub.z can be calculated by
solving (10). With t.sub.z known, the capacitor voltage at half a
period can be calculated using (13). The next step is to check
whether V.sub.c(0) and -V.sub.c(T/2) have converged to a given
error index (.epsilon.<0.01). If the answer is YES, the program
terminates when the correct solution is found. Otherwise the
iteration repeats itself in the computation loop until a solution
is found. The algorithm proves to be very fast and robust.
[0066] D. Rectifier Load Modelling
[0067] The series AC processing pickup can output a controlled DC
voltage by adding a rectifier with a large DC filter capacitor. The
output voltage is maintained at a DC level with the high frequency
AC component removed. As a result, the AC voltage at the input of
the rectifier becomes a rectangular waveform with an amplitude of
the DC output voltage and two diode forward voltage drops from the
rectifier. If only the fundamental component is modelled and the
harmonic components are ignored, the RMS value of the rectangular
voltage is given by:
V ac = 2 2 ( V dc + 2 V f ) .pi. ( 20 ) ##EQU00009##
[0068] Assuming that Q2 of the circuit is relatively high during
normal operation, the input current to the rectifier can be
approximated by half sinusoids with discontinuous sections in
between. Then the RMS AC current is related to the rectified DC
current by:
I ac = .pi. I dc 2 2 ( 1 - 2 t c / T ) ( 21 ) ##EQU00010##
[0069] Therefore the equivalent AC load is:
R ac = 8 ( 1 - 2 t c / T ) ( R dc + V f / I dc ) .pi. 2 ( 22 )
##EQU00011##
[0070] The equivalent resistor can be used in (4) in the AC
analysis described in the section above to compute the operating
waveforms of the series AC processing pickup with a rectifier
load.
[0071] Pickup Characteristics
[0072] The output current (or inductor current) characteristics of
the pickup are shown in FIG. 6 for different values of Q.sub.2 or
load conditions. The normalized output current is defined by the
ratio of the output current over the short circuit current. It can
be seen that the output current asymptotically decreases as the
controlled phase delay .phi. increases from zero. The normalized
output current can be controlled from the maximum value to zero as
.phi. changes for all load (Q.sub.2) conditions.
[0073] The normalized output voltage is shown in FIG. 7 for a range
of Q.sub.2 values. It can be seen that the output voltage of the
pickup can be controlled by .phi. as a controllable voltage source.
FIG. 7 shows that the output voltage stays approximately constant
(or has very little variation) as the load resistance changes for
pickups with high Q.sub.2(5-10). Hence, this pickup demonstrates a
controllable voltage source behaviour. For low Q.sub.2 values, the
voltage source is no longer considered perfectly sinusoidal as the
harmonics contribute to the power transfer.
[0074] The output voltage-current characteristic is shown in FIG.
8. The voltage source behaviour is again demonstrated as the output
voltage stays approximately constant for a given phase delay
irrespective of output current as long as the output current is
reasonably high. It should be noted that this feature of the pickup
will give rise to significant advantages to the overall system, as
the output voltage can be controlled to any value below the open
circuit voltage without the need of an extra buck converter after
the pickup. Thus an output parameter such as voltage may be
compared with a reference value and the switch may be operated to
alter the period for which the inductor current is zero in a cycle
and thus control the phase angle to move the output parameter
toward the reference (i.e. to control the output of the
pickup).
[0075] FIG. 9 shows the first four harmonics of a series resonant
circuit AC processing pickup operating at a Q.sub.2=5 obtained from
Fourier analysis. In these figures, the amplitude of the harmonics
is expressed as a function of the fundamental component under full
load conditions. It can be seen that the amplitudes of the harmonic
components are relatively low compared to the fundamental. The
highest harmonic component for the inductor current does not exceed
6.3% of the maximum fundamental component.
[0076] The normalized reflected impedance characteristic is shown
in FIG. 10 at different values of Q.sub.2. Both the resistive and
reactive component is normalized against the factor
.omega.M.sup.2/L.sub.2. When the phase delay .phi. is at zero, only
a resistive load is reflected back on the track and the real power
is supplied by the power supply to drive the pickup. As the .phi.
increases to decrease the output power, both a real load and a
capacitive load is reflected, and the power supply has to source
both the real power and the capacitive VAR's. When .phi. increases
towards 180.degree., both the reactive and resistive load decrease
to zero. Even if the pickup reflects relatively large VAR's at
lower power, the primary power supply is not necessarily
overstressed as the overall VA has decreased significantly compared
with rated operation.
[0077] Design
[0078] In this section, the design of a 1.2 kW series AC processing
pickup according to the circuit of FIG. 2 is described. The maximum
desired output voltage is 90V and the AC load resistance is
6.OMEGA.. An asymmetrical S-shaped magnetic inductor was chosen in
this example because of its higher output power with the same
ferrite volume/length compared to traditional magnetic pickup
structures. This pickup has a V.sub.oc of 90V and an inductance
value of 115 .mu.H. The primary IPT converter uses an LCL topology
operating at a fixed frequency of 20 kHz with 125 A in the primary
track. From (1), the nominal tuning capacitance is 551 nF. In this
example, the tuning capacitance is chosen to closely match the
ideal nominal tuning capacitance. Using (4), Q.sub.2 of the circuit
is 2.4 at maximum power. Equations (13) and (14) are then used to
solve for steady state operation. FIG. 11 shows the calculated
waveforms for the circuit with a phase delay .phi. of 0.degree.,
58.degree. and 85.degree. which correspond to 100%, 50% and 20%
power, respectively.
[0079] A. Implementation of the AC Switch
[0080] The AC switch, shown in the embodiment of FIG. 2, has to
conduct current and block voltage in both directions, i.e. be a
bi-directional switch. FIG. 12 shows three possible ways of forming
the AC switch using available IGBT's or MOSFET's. Type 1 of the
bi-directional switches is not ideal for the series AC pickup as
the pickup is primarily used in low voltage high current
applications and the forward voltage drop of two diodes compared to
one diode drop in other configurations is less efficient. In the
type 2 configuration, the current path flows in either direction
taking the top or bottom path. One of the switches in this set of
devices has to have an isolated gate drive. The type 3
configuration has advantages of common ground between the
transistors, so it is easier to drive the gate. In addition, the
body diode of the switching device can be used without the need for
external diodes, provided those diodes are sufficiently low
loss.
[0081] One key requirement of practically realizing the AC switch
is to allow the switches to operate on simple gate drive waveforms.
In the description of circuit operation provided with respect to
FIGS. 2 and 3, a very precisely timed gate drive waveform at 40 kHz
is required to drive the gate. However, in practice it is too
difficult to generate this waveform with sufficient accuracy using
presently available microcontrollers. Any error in timing forces
the circuit into a discontinuous state when the inductor current is
non zero and the resulting overshoot in voltage across the switches
may cause device failure. In consequence, it is more desirable to
use the diodes turn-off behaviour to block the current in the
reverse direction and allow the circuit to naturally enter the
discontinuous state due to diode commutation. Hence, the PWM gate
signals are only required to be operating at 50% duty cycle with a
phase delay .phi. relative to the phase of V.sub.oc to allow the
circuit to enter the resonant state, while the natural diode turn
off characteristic allows the circuit to enter the discontinuous
state. Thus the switch can be operated such that the natural turn
off characteristic of a diode associated with the switch allows the
current to fall to substantially zero. If the diode was to ideally
turn off at zero current, a reverse recovery charge of zero is
required. However, a diode with a reverse recovery charge of zero
does not exist in practice so that a suitable switch that has a
diode with a low reverse recovery charge is chosen. Here the type 3
AC switch configuration from FIG. 12 was chosen, with IGBT's
(IRGP20B60PDPbF) rather than MOSFET's as the switch because this
IGBT has a body diode with a low reverse recovery charge of 80
nC.
[0082] B. Snubber Design
[0083] A simple RC snubber was designed to damp the high frequency
resonant oscillations caused by the pickup inductance and parasitic
output capacitance of the IGBT. This is required because although
an IGBT with low reverse recovery charge was chosen to reduce the
transient generated from the natural turn off of the body diode,
when the current tries to flow in the opposite direction, the
remaining voltage oscillation is still significant. This
oscillation arises because the reverse recovery current of 3 A
becomes the initial inductor current of an LC resonant circuit
comprising the pickup inductance of 115 uH and the switch output
capacitance of 130 pF. A large over shoot of more than 80% of the
steady state value appears across the switch due to the resulting
resonant oscillations. A snubber capacitance of 2.2 nF was
connected across each switch in the usual manner.
[0084] C. Component Stress
[0085] To calculate the maximum rating conditions for the
components, the phase delay .phi. has to be set slightly above zero
degrees in order to observe the peak voltage across the switches
and maximum RMS rating for the capacitor and inductor. The
calculated peak and RMS value of the voltage and current for the
capacitor, inductor and switch are listed in Table I. It can be
concluded from Table I that the switches have to be rated for both
310V and 22 A at normal operation. However, an overshoot of 10% is
still possible from the snubber design, so the switch rating should
be greater than 350V. In practice a 500V device may be used.
TABLE-US-00001 TABLE I Maximum Ratings of Components Parameter Peak
RMS V.sub.c 307 V 217 V V.sub.L 309 V 218 V V.sub.s 309 V --
I.sub.L 21.2 A 15 A I.sub.s 21.2 A 15 A
[0086] D. Controller
[0087] A practical system setup with controller for the AC
processing pickup is shown as a block diagram in FIG. 13. The phase
of V.sub.oc is measured using a separate phase sense coil L.sub.3
placed on the primary track to detect the phase of the track
current which is exactly 90.degree. out of phase with the open
circuit voltage (V.sub.oc.varies.I.sub.1.angle.90.degree.). The
phase delay .phi. is set by a computer interface while a
microcontroller adjusts the switch gate drive waveforms
accordingly. The two gate control waveforms are driven at 50% duty
cycle at the same frequency but having a phase delay of .phi.
relative to the phase of the IPT track current. One of the gate
drive signals is the inverted version of the other.
[0088] Experimental Results
[0089] The AC processing pickup as described above was coupled to a
small section of track (FIG. 13) and used to drive 1.2 kW into a 60
AC resistor. FIG. 14 shows the circuit waveforms for the series AC
processing pickup at 100%, 50% and 20% power when 0 is set to
0.degree., 58.degree., and 85.degree., respectively. From the top
trace to the bottom trace, in descending order, the traces are the
capacitor voltage, inductor current, capacitor current and switch
current. The inductor current and capacitor voltage are both
sinusoidal having low distortion at 100% power. The measurements as
shown have excellent correlation with the calculated waveforms in
FIG. 11. The amplitudes of the measured waveforms are within 10% of
the values calculated by equations (13) and (14).
[0090] A rectifier and a 2 mF filter capacitor are added at the
output of the pickup to output a controlled DC voltage. The pickup
is operated with a closed-loop controller where the output voltage
is set to a desired value in the microcontroller. The
microcontroller is configured to maintain the desired load voltage
by adjusting .phi. in accordance to the feedback of the output
voltage. FIG. 15 shows the circuit waveforms when the output
voltage is regulated to a DC voltage of 80V for 8.OMEGA., 12.OMEGA.
and 16.OMEGA. load, respectively. From top trace to bottom, in
descending order, the traces are the gate drive waveform V.sub.g1
(FIG. 13), the inductor current, the input voltage to the rectifier
and the DC output voltage. It can be see that the output voltage is
nearly constant with an error of less than 1% around the desired
value. The maximum voltage that appears across the switch is 340V
operating with the rectifier. This is in conformance with the
results deduced in the previous section.
[0091] An efficiency vs. output power plot is shown in FIG. 16 for
the overall IPT system and the series AC processing pickup by
itself outputting controlled AC to a 6.OMEGA. load. In addition,
the efficiency of the overall system and the AC processing pickup
with rectifier when outputting controlled DC to an 8.OMEGA. load is
also plotted. Referring to FIG. 13, the overall IPT system
efficiency is determined using measures of the DC input power to
the LCL primary converter and the AC output power from the
secondary pickup. Similarly, the pickup efficiency is calculated
using the AC input power delivered to a short length of primary
track that the secondary pickup is coupled upon and the AC output
power from the secondary pickup. The pickup efficiency measurement
neglects the supply (LCL converter) power losses and gives a more
meaningful measure of the conversion efficiency of the pickup
itself. The DC efficiency is measured with the losses in the
rectifier taken into account. It can be seen that the efficiency of
the pickup remains above 90% when the output power is more than 300
W to the load for either a DC or AC load. With a 1.2 kW load, the
efficiency of the AC processing pickup and the overall IPT system
can reach as high as 96% and 89%, respectively when outputting
controlled AC.
[0092] This document discloses a new IPT pickup having a series
resonant circuit in which power is processed on a cycle by cycle
basis in AC (i.e. without rectification being necessary) and which
can produce a variable AC output voltage into a load or a rectifier
filter and load combination for controlled DC. This eliminates the
bulky DC inductor required in the traditional series tuned
controllers that use a buck converter to produce a DC controlled
output. In addition, the pickup operates under near ideal soft
switching conditions which give the pickup a very high efficiency.
Although this pickup reflects back VAR's back onto the primary
track, the overall stress imposed on the primary power supply is
relatively small. The circuit operation has been theoretically
analyzed and experimental results have verified the proposed design
procedure. The AC processing pickup can be controlled over a wide
load range for a 1.2 kW system and a maximum efficiency of 96% was
obtained.
[0093] Although certain examples and embodiments have been
disclosed herein it will be understood that various modifications
and additions that are within the scope and spirit of the invention
will occur to those skilled in the art to which the invention
relates. All such modifications and additions are intended to be
included in the scope of the invention as if described specifically
herein.
* * * * *