Inductively controlled series resonant ac power transfer   

Abstract: An inductive power transfer pickup circuit has a pickup coil (L2) and tuning capacitor (C2) connected in series to provide a series resonant circuit. A bi-directional switch (S1) is used to vary the phase angle between the open circuit pickup coil voltage (Voc) and the pickup coil inductor current (iL) to provide a controlled AC supply to an output of the pickup.

FIELD OF THE INVENTION

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.

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.

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.

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.

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.

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.

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

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.

In one embodiment the AC supply at the output is rectified to provide a DC supply at a further output.

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.

In one embodiment the selected time period is varied to vary the phase angle.

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.

In one embodiment the current is substantially prevented from flowing by operating a switch. In one embodiment the switch comprises a bi-directional switch.

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.

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.

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.

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.

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.

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.

In one embodiment the switch comprises a bi-directional switch. A controller may be provided to control operation of the switch.

In yet another aspect the disclosed subject matter provides an IPT system including an IPT pickup according to any one of the preceding statements.

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.

An embodiment of the invention will be described by way of example with reference to FIGS. 1-16 in which:

FIG. 1 is a block diagram of a known IPT system.

FIG. 2 is a diagram showing a Series Tuned AC Processing Pickup.

FIG. 3 shows operating waveforms of the series AC processing pickup of FIG. 2.

FIG. 4 is a diagram of the pickup circuit waveform showing two operating states.

FIG. 5 is a flow chart of a computation algorithm.

FIG. 6 is a diagram showing normalized RMS output current vs. phase delay φ.

FIG. 7 is a diagram showing normalized RMS output voltage vs. controlled phase delay φ.

FIG. 8 shows pickup output voltage current characteristics.

FIG. 9 shows harmonic components of inductor current as a percentage of the maximum fundamental value at Q2=5.

FIG. 10 shows reactive load vs. real load.

FIG. 11 shows calculated waveforms when phase delay φ is (a) 0°, (b) 58° and (c) 85°.

FIG. 12 shows examples of bi-directional switches.

FIG. 13 is a block diagram for a controller.

FIG. 14 shows measured waveforms for (a) 100% power, (b) 50% power and (c) 20% power with 6Ω resistive load.

FIG. 15 shows measured waveforms for (a) 8Ω, (b) 12Ω and (c) 16Ω load with constant 80V DC voltage.

FIG. 16 shows efficiency vs. output power.

A new type of AC processing pickup illustrated herein exhibits excellent features such as simple circuitry, lower production cost and very high efficiency operation.

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.

According to one embodiment of the invention a series AC processing pickup is shown in FIG. 2 with an AC output voltage (VR2). Capacitor C2 is tuned to inductor L2 at the frequency of the primary track current i1 to form a series resonant tank. The open circuit voltage source (Voc) represents the induced voltage of the pickup. For simplicity, switch S1 is drawn as an ideal AC switch and it is the basis for controlling the output voltage.

To illustrate the circuit\'s operation, FIG. 3 shows the one period operation of the series AC processing pickup at each particular switching interval. Vg1 is the PWM control signal which turns S1 on and off. Consider the situation where Vg1 is controlled with a phase delay φ relative to the phase of Voc as shown in FIG. 3. In Mode 1 (M1, 0<t≦t1), S1 is operated by being turned on and the capacitor C2 resonates with pickup inductance L2 like a series resonant tank and the inductor current reaches a peak value and returns back to zero. When the inductor current reaches zero, S1 is operated by being turned off and the circuit enters Mode 2 (M2, t1<t≦t2). 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 (tc/ω) at the point where IR (the current through the resistor R2) changes from a positive to a negative voltage. In the beginning of Mode 3 (M3, t2<t≦t3), S1 is turned back on. Similar to M1, the circuit operates like a series resonant tank and current flows into the load resistor. In Mode 4 (M4, t3<t≦T), similar to M2, the resonant cycle is terminated and the inductor current is discontinuous. After this mode, the circuit returns back to M1, 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.

The series AC processing pickup also achieves near ideal soft switching conditions. From FIG. 3, at t1, the voltage across S1 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 S1 is turned on at t2, the pickup inductor in series with S1 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.

From the previous section, it can be seen that the phase shift between Voc and IL can be controlled by adjusting the phase delay φ. In this section, the phase delay φ 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.

The analysis procedure is greatly simplified based on the three following assumptions: The Equivalent Series Resistance (ESR) of both capacitor C2 and Inductor L2 are very small compared to the load resistor and can be neglected. The switching action of the transistors and diodes are instantaneous and lossless. Capacitor C2 and inductor L2 are perfectly tuned forming a series resonant tank with the load.

Assuming the resonant tank is perfectly tuned,

C2=1/(ω2L2)  (1)

With reference to FIG. 4, the waveform can be separated into two operating states known as the resonant state and the discontinuous state.

During the resonant state, the inductor current may be described as:

 2  i Lr  t 2 + R L 2   i Lr  t + i Lr L 2  C 2 = V oc  ω L 2  cos  ( ω   t + φ ) ( 2 )

Considering the initial condition iLr(t)|t=0=0 and

 i Lr  ( t )  t   t = 0 = V oc 

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