WO1997001211A1

WO1997001211A1 - Load resonant converters

Info

Publication number: WO1997001211A1
Application number: PCT/GB1996/001498
Authority: WO
Inventors: Helen Phyllis Geraldine Pollock
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 1995-06-23
Filing date: 1996-06-19
Publication date: 1997-01-09
Anticipated expiration: 1997-12-23
Also published as: GB9512806D0; AU6232996A

Abstract

A load resonant converter (1) comprises a resonant circuit (3) having a plurality of resonant frequencies when connected to a load RLOAD and different apparent resistances at at least two of its resonant frequencies, and a controller (5) serves to control the resonant circuit (3) so as to provide an output corresponding to a first impedance transformation when the resonant circuit (3) is caused to resonate at a first resonant frequency and so as to provide an output corresponding to a second impedance transformation when the resonant circuit (3) is caused to resonate at a second resonant frequency. Such a load resonant converter (1) may be controlled so as to provide two or more distinct output power levels, or alternatively so as to provide output power levels within a predetermined acceptable range when connected to a variable load by switching the operating frequency of the circuit between two or more resonant frequency values in dependence on the load voltage.

Description

"Load Resonant Converters"

This invention relates to load resonant converters, and is concerned more

particularly with power control of such converters. It is particularly useful for welding, laser and induction heating power supplies and for capacitor charging power supplies.

In an arc welding power supply for supplying a pulsed current output for welding, the workpiece forms part of the electrical circuit and must therefore be isolated from the circuit by means of an isolation transformer for safety reasons. A

smoothing inductance is also required to reduce the current ripple on the current supply

to the welding arc. The size of both these magnetic components depends on the frequency of operation of the circuit, and, at normal supply frequencies, they can be quite large. This is disadvantageous not only because of the bulk and cost of such

components, but also because it imposes a limit on the response time of the circuit.

The size of these components can be reduced if the frequency of operation of the

converter is increased. Higher frequency operation also allows a rapid response to

current in the arc, and thus allows a rapid initial rise of current to establish the arc

quickly and subsequent control of the current to ensure that burn back leading to excess

deposition of welding wire does not occur . The size and performance of laser and

induction heating power supplies can also be improved by high frequency operation,

whether or not such power supplies incoφorate isolation transformers.

Series-parallel load resonant converters have been developed for such an application as they can operate at high switching frequencies and since they include resonant components connected both in series and in parallel with the load which limit

the current and voltage when the load is either open circuited (due to extinguishing of

the arc) or short circuited (due to the electrode coming into contact with the workpiece). Such resonant converters typically includes either a full bridge or a half bridge arrangement of switching devices supplied from a d.c. supply rail, and a resonant circuit containing a series leg, a parallel leg and a load leg each of which

consists of inductive, capacitative or resistive elements or a combination of these. The

load leg inductance can arise from the leakage inductance of an isolation transformer,

the inductance of the load, an additional inductor, or any combination of these. When such a circuit is excited at a resonant frequency, the current through the series leg and hence through the switching devices naturally commutates to zero, and can be controlled so that this is the point of switching. This enables the converter to operate at high frequency without exceeding the thermal rating of the switching devices.

However it is difficult to obtain effective power control of such converters while

maintaining zero current switching.

One known method of controlling such a converter is by fixed frequency

pulse width modulation (PWM) as described in "Fixed frequency PWM series-parallel

resonant converter" , A.K.S. Bhat, IEEE Industrial Applications Society Conference,

San Diego, 1989. Such a method uses a full bridge converter and controls the

switching devices so that there is a period of time around each zero crossing of the

current during which only one of the four switching devices is in operation (zero volt loop operation). This has the effect of decreasing the time within each resonant cycle

for which the full supply voltage excites the resonant circuit and thus enables the output power level of the converter to be varied by varying the duration of the zero volt loop period. However zero current switching is not maintained as the zero volt loop period

is increased, and switching losses therefore limit the maximum frequency at which the

converter can operate.

Another known method of controlling such a converter is by a frequency control technique (also referred to as phase control) in which the power output is varied by moving the operation of the resonant circuit away from resonance, as described in

"Theoretical and experimental studies of the LCC-type parallel resonant converter",

I. Bataresh, R. Liu, C. Lee, A. Upadhay, IEEE Trans, on Power Electronics, vol. 5,

No 2, April 1990. However this control method suffers from the disadvantage that,

when the voltage is no longer in phase with the current, zero current switching is not

maintained, and this again reduces tlie maximum frequency at which the converter can

operate.

A further control method involves deadtime control of a half-bridge

converter in which, in order to vary the power output, a delay is introduced before

each switching device is turned on, as described in "Current pulse control of high

frequency series resonant d.c. link power converter", Y. Murai, S. Mochizuki, IEEE

Industrial Applications Society Conference, San Diego, 1980. This control method

maintains zero-current switching but introduces a significant current ripple which increases as the power is reduced.

It is an object of the invention to provide a novel load resonant converter in

which the output is controllable without introducing large switching losses or significant output current ripple.

According to the present invention there is provided a load resonant

converter comprising a resonant circuit having a plurality of resonant frequencies when

connected to a load and different apparent resistances at at least two of its resonant

frequencies, and control means for controlling the resonant circuit so as to provide an

output corresponding to a first impedance transformation when the resonant circuit is caused to resonate at a first resonant frequency and so as to provide an output corresponding to a second impedance transformation when the resonant circuit is

caused to resonate at a second resonant frequency.

A resonant frequency of the resonant circuit occurs when the applied voltage

across the circuit is in phase with the applied current, that is when the circuit appears

resistive, and it should be appreciated that the term "apparent resistance" is used to

denote the resistance which the circuit appears to have when in this condition.

Such a load resonant converter may be controlled to operate at two or more

different resonant frequencies so as to provide two or more distinct output power levels

due to the circuit having a different apparent resistance associated with each resonant frequency. When used in a welding power supply, for example, such a converter can

be controlled so as to provide a low power output during simmering and a high power

output during actual welding. At each power level zero-current switching, high efficiency and low output current ripple can be maintained.

Alternatively such a load resonant converter may be controlled to operate at

two or more different resonant frequencies so as to provide output power levels within

a predetermined acceptable range when connected to a variable load by switching the operating frequency of the circuit between two or more resonant frequency values in dependence on the load voltage. Thus, for example, on charging of a capacitor so that

the voltage across the capacitor increases from substantially zero to a maximum

allowable value, the output power level may be maintained within an acceptable range

by switching the operating frequency of the circuit between two or more resonant frequency values.

It will be appreciated that a criterion for the resonant circuit to have more

than one resonant frequency is that there should be more than one current path through

the circuit, each path consisting of inductive, capacilative or resistive elements or any

combination of these. The load can be placed in any leg of the circuit. If it is in a

series leg of the circuit, further resistance must be present in at least one parallel leg

of the circuit. The load may be any energy conversion device which serves to dissipate

electrical energy or to convert it to another form. For example the load may be a

welding arc which may be considered as an e.m.f. in series with a resistance and which serves to dissipate electrical energy in the form of heat.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which:

Figures 1 and 2 are circuit diagrams of full bridge and half bridge series parallel load resonant converters;

Figure 3 is a plot of the magnitude and phase of admittance against

frequency for a load resonant converter in accordance with the invention;

Figure 4 is a plot of the switch current and voltage of such a load resonant converter in accordance with the invention at two different power levels;

Figure 5(a) is a block diagram showing a control arrangement of a load

resonant converter in accordance with the invention;

Figure 5(b) is a block diagram of a another control arrangement of a load

resonant converter in accordance with the invention;

Figure 6 is a circuit diagram of a further load resonant converter in

accordance with the invention, having full power control, an isolation transformer and

rectified load current; and

Figure 7 is a plot of the range of power levels achievable with the load

resonant converter of Figure 6 relative to the r.m.s. of the exciting voltage.

The full bridge resonant converter 1 of Figure 1 comprises four power

switching devices S I , S2, S3 and S4, each of which is shown as an insulated gate bipolar transistor (IGBT), and four freewheeling diodes Dl , D2, D3 and D4 connected

in parallel with the switching devices. In addition the converter 1 , which is provided

with a d.c. supply 2, includes a resonant circuit 3 comprising a series leg consisting of an inductance Ls and a capacitance C_s, a parallel leg consisting of an inductance L_p and

a capacitance C_P, and a load leg consisting of an inductance L_, a capacitance C_L and

a load represented by a resistance R_LOΛD- Furthermore the switching devices SI , S2, S3 and S4 are switched on and off by a controller 5 in a manner which will be

described in more detail below so as to excite the resonant circuit 3 at a resonant frequency in order to supply current to the load.

Figure 2 shows a half bridge resonant converter 10 comprising only two

power switching devices SI and S2 in a half bridge arrangement, and only two

freewheeling diodes Dl and D2. Two capacitances Cl and C2 are connected in series

across the supply voltage to provide a connection point and split d.c. supply, and the

resonant circuit 3 and other components are otherwise similar to those described with

reference to the converter of Figure 1 and are accordingly denoted by the same

references.

The component values used in the resonant circuit 3 of each converter 1 and

10 can be chosen so that the circuit has two or more resonant frequencies and so that

the circuit delivers a different level of power to the load at each resonant frequency for

a given supply voltage. A resonant frequency of the circuit occurs when the applied

voltage across the circuit is in phase with the applied current, that is when the circuit appears resistive, and it can be shown by circuit analysis that this condition occurs when the imaginary part of the numerator of the impedance equation is zero, that is

when ω⁶ -I- k,ω⁴ + k,,ω² + k. = 0

where ω is the resonant frequency and k,, k,, and k. are coefficients determined by the

component values used in the circuit.

In this example, since the imaginary part of the numerator is a polynomial

of the sixth order in ω, it has three pairs of roots and consequently the circuit has three

pairs of resonant frequencies + ω₀, +ω, and ±ω₂. Furthermore the component values

can be chosen so that the circuit has a different apparent resistance at each resonant frequency, so that the power level is different at each resonant frequency.

A half bridge resonant circuit having three resonant frequencies, 50 kHz, 75

kHz and 100 kHz, and having a corresponding ratio of different power levels 1 : 0176

: 0.45, is obtained by selecting the following values for the components shown in

Figure 2: C_s = 45 nF, L_s = 135 μH, C_P = 45 nF, L_P = 14 μH, 1^ = 80 μH, R_L =

15.125 Ω, C_L= ∞ (i.e. not present). The apparent resistance of such a circuit is 15 Ω

at 100 kHz, 83 Ω at 75 kHz and 33 Ω at 50 kHz, and the variation of the magnitude

and phase of admittance of the circuit against frequency is shown schematically in

Figure 3. As may be seen the circuit has three different power levels at the three

resonant frequencies, ω₀, ω„ ω₂, so that control of the converter to change it from one

resonant frequency to another produces a change in the power level whilst maintaining zero current switching.

Figure 4 shows the switch current and voltage of the converter initially during full power operation with the circuit operating at one resonant frequency, and

subsequently during reduced power operation with the circuit operating at another

resonant frequency. At the bottom of the figure the alternate switching on and off of

the power switching devices SI and S2 is indicated diagrammatically for the two

different operational modes.

Two possible methods of control of such a circuit at three power levels will

now be described by the way of example with reference to Figures 5a and 5b which

should be considered in combination with the circuit diagram of Figure 2.

Figure 5a shows the simplest control arrangement. In this example the

controller 5 incorporates a clock generator 101 for each of the resonant frequencies of

tlie resonant circuit. A power selector 102 determines which clock generator signal is

selected for the control of the resonant converter. A delay compensation block 103

modifies the duty cycle of the output of the clock generator to account for delays in the

power circuit. This ensures that the voltage applied by the power switching devices

across the resonant circuit passes through zero as near as possible to the zero crossing

of the current. The gate drives 104 switch the power switching devices of the

converter 105 at the selected resonant frequency and the corresponding output power

level in the load is achieved. Changes in power level in the load are achieved by changing the selected power level using the power selector 102 resulting in the use of

a different clock generator 101 in order to change the resonant frequency of the circuit,

for example from 75 kHz to 100 kHz.

Figure 5b shows an altemative control arrangement which controls the circuit at various power levels with the capability of maintaining resonant operation when there is variation in the load at any power level. In this example the controller 5

incorporates a current sensor 11, such as a Hall effect device, which senses the current

in the series leg of the resonant circuit 3 and supplies an output voltage proportional to the sensed current to a comparator 12 which produces an output signal identifying

the time at which the series leg current is zero. A phase-locked loop 13 incoφorates

a divider 14 serving to produce a signal having a frequency which is 64 times the

frequency of the comparator signal. This higher frequency signal is supplied to an

advance circuit 15, the output of which is in advance of the output signal of the

comparator 12 by a predetermined number of cycles of the high frequency signal. This

results in a signal at the output of the advance circuit 15 which is supplied to a digital

controller 16 and which is advanced to take account of delays in the gate drives 17 for

driving the power switching devices of the converter 18 so that the devices are turned on as near to zero switch current as possible.

The digital controller 16 includes a clock generator at each of the resonant

frequencies from which the control signal for the power switching devices is initially

derived until the current in the resonant circuit reaches its steady state value. The switching of the converter between the power levels can be effected

manually by the operator or automatically by an appropriate programming device or

in response to sensed load conditions. Where such a converter is used in a pulsed welding power supply, for example, tlie converter may be operated at a reduced power

level to maintain the arc in a simmer condition, whereas the full power level may be used for welding.

Such a converter is therefore capable of providing power at substantially

different power levels whilst operating from a fixed d.c. supply voltage and

maintaining zero current switching throughout. Furthermore the power switching

devices are operated with negligible switching losses even though switching at

frequencies five times their normal recommended frequency. There is very rapid

switching between different power modes.

Figure 6 shows a further resonant converter 20 in accordance with the

invention which incorporates an active rectifier circuit 21 , a resonant circuit 22, an

isolation transformer 24 and a full bridge diode rectifier 25. The active rectifier circuit

21 consists of power switching devices S_Λ and S„, diodes D_Λ, D_n, D_c, D_D, D_P, D_F, a

capacitance C_Λ and inductances L_Λ, L . and is connected between an a.c. source 23 and

the d.c. supply to the resonant circuit 22. The active rectifier circuit 21 consists of an

input filter, a full bridge diode rectifier and a buck boost converter. In the buck boost

converter the output voltage is greater than or less than the input voltage, the duty cycle

of the simultaneously switching power switching devices S_Λ and S_n being adjustable to vary the ratio of the output voltage to the input voltage. This enables the d.c. supply

voltage to the resonant circuit to be varied whilst also achieving near unity power factor

operation from the a.c. source.

In the embodiment shown in Figure 6, the load is isolated from the resonant circuit by means of the isolation transformer 24. In applications, such as welding, the workpiece forms part of the electrical circuit and must be isolated from the power

converter for safety reasons. The transformer is reduced in size significantly with high

frequency operation. The leakage inductance of the transformer forms part of the

resonant circuit. The transformer can have a different number of turns on the primary

and secondary windings to allow for variation in the designed output voltage of the circuit. There may also be multiple secondary windings.

In Figure 6 the load is connected to the isolation transformer 24 by the full

bridge diode rectifier 25 so as to provide a direct current in the load at each power

level. The rectifier or any other means of rectifying the load current can be connected

without the use of the isolation transformer if isolation of the load is not required.

Figure 7 shows the range of power achievable using such a resonant

converter 20 with minor variation of the d.c. supply voltage from the active rectifier.

For example, at an r.m.s. fundamental excitation voltage of 300 V, changing the

frequency of operation of the circuit from 75 kHz to 50 kHz to 100 kHz produces a

change in the output power level of the circuit from 1 kW to 2.7 kW to 6 kW (corresponding to resistance values of the resonant circuit of 83, 33 and 15 Ω). Thus,

by changing both tlie resonant frequency at which the circuit is operated and the supply

voltage using an a.c. - d.c. converter or some other means, a complete power range can be achieved while maintaining zero current switching in the converter. Further

flexibility can be achieved by moving the frequency of operation away from the

respective resonant frequency.

Various modifications of the above described converters are possible within the scope of the invention to suit particular requirements. For example the converter

may be designed to have, and to be operated at, three or more resonant frequencies.

Furthermore the converter may be controlled so as to vary the frequency of operation within a small frequency range to allow fine control of the power level. Such

operation introduces power losses during switching, but, since it is used only to provide

fine tuning of the power output relative to discrete power levels at each resonant

frequency, the switching losses are less than would be obtained in conventional phase

controlled resonant circuits over a wider power range.

The above description is confined to load resonant converters for providing

a power output at two or more distinct output power levels. However such load

resonant converters may also be used for supplying power to a variable voltage load

such as a capacitor. For example, the resonant converter of Figure 6 may be used for

charging a capacitor which is substituted for the resistive load, although the active

rectifier circuit 21 , and the isolation transformer 24 are not essential where a capacitor is to be charged. During charging of a capacitor by a conventional power supply, the

voltage across the capacitor increases substantially from zero to a maximum allowable

value so that the power supplied to the capacitor varies in dependence on the capacitor

voltage. However, if the capacitor is charged by the resonant converter of Figure 6,

the resonant frequency of the resonant circuit can be controlled so as to allow the

power level to be maintained within an acceptable range during the complete capacitor

charging cycle, even though the capacitor voltage will vary significantly during such

charging. This is achieved by switching the circuit between two or more resonant

frequency values so as to maintain the apparent circuit resistance within a narrow range

while the load varies over a much wider range.

Thus, for example, in one embodiment of capacitor charging resonant converter in accordance with the invention, the capacitor is initially charged with the

resonant circuit resonating at a first resonant frequency and, when the power supplied

goes outside an acceptable range, the circuit is then caused to resonate at a second

resonant frequency such that the power level is maintained within the acceptable range.

Subsequently, when the power supplied goes outside the acceptable range during

continued charging of the capacitor, the circuit is caused to resonate at a third resonant

frequency, thus again maintaining the power level within the acceptable range. It will

be appreciated that such voltage dependant control of the resonant frequency can be

used to maintain the power level of the circuit within an acceptable range over the

whole capacitor charging cycle. Whether the resonant circuit is controlled so as to vary the output power

level or so as to maintain the output power level within an acceptable range when supplying a variable load, it will be appreciated that the control of the resonant circuit is such as to provide an appropriate impedance transformation between the load

impedance and the apparent circuit impedance, the value of the impedance

transformation being selectable by appropriate control of the circuit resonant frequency.

Claims

CLΔ1MS

1. A load resonant converter comprising a resonant circuit (3, 22) having a

plurality of resonant frequencies when connected to a load and different apparent

resistances at at least two of its resonant frequencies, and control means (5) for controlling the resonant circuit (3, 22) so as to provide an output corresponding to a

first impedance transformation when the resonant circuit (3, 22) is caused to resonate

at a first resonant frequency and so as to provide an output corresponding to a second

impedance transformation when the resonant circuit (3, 22) is caused to resonate at a

second resonant frequency.

2. A converter according to claim 1, wherein the control means (5) is arranged

to control the resonant circuit (3, 22) so as to provide an output at a first power level when the resonant circuit (3, 22) is caused to resonate at the first resonant frequency and so as to provide an output at a second power level when the resonant circuit (3, 22)

is caused to resonate at the second resonant frequency.

3. A converter according to claim 1 , wherein the control means (5) is arranged

to control the resonant circuit (3, 22) so as to provide outputs within a predetermined

power range to a variable load such that, at a first load voltage, an output within said

range is provided due to the resonant circuit (3, 22) being caused to resonate at the first

resonant frequency and, at a second load voltage, an output within said range is

provided due to the resonant circuit (3, 22) being caused to resonate at the second resonant frequency.

4. A converter according to claim 1 , 2 or 3, wherein the resonant circuit (3, 22)

comprises a series capacitance (Cs) and/or a series inductance (Ls) in series with the

load and a parallel capacitance (Cp) and/or a parallel inductance (Lp) in parallel with

the load.

5. A converter according to any preceding claim, wherein the resonant circuit (3, 22) incoφorates an isolation transformer (24) for connection of the load to the circuit.

6. A converter according to any preceding claim, wherein the resonant circuit

(3, 22) incorporates power switching devices (SI , S2, S3, S4) adapted to be turned on

and off alternately by the control means (5).

7. A converter according to any preceding claim, wherein the control means (5)

comprises first means (101) for producing a first frequency signal for exciting the

resonant circuit (3, 22) at the first resonant frequency in response to operation of a

selector (102) in order to select an output at the first power level and second means

(101) for producing a second frequency signal for exciting the resonant circuit (3, 22)

at the second resonant frequency in response to operation of the selector (102) in order

to select an output at the second power level.

8. A converter according to claim 7, wherein the control means (5) incoφorates

delay compensation means (103) for modifying the duty cycle of the frequency signals

to compensate for delays in the resonant circuit (3, 22).

9. A converter according to any preceding claim, wherein the control means (5) includes sensing means (11) for sensing the current in the resonant circuit (3, 22) and

for producing a feedback signal indicative of the timing of the sensed current for

controlling the resonant circuit (3, 22).

10. A converter according to claim 9, wherein the control means (5) includes advancing means (15) for providing a control signal which is advanced relative to the feedback signal in order to compensate for delays in the resonant circuit (3, 22).

11. A converter according to any preceding claim, wherein rectifying means (21)

is provided for supplying the converter with a direct current voltage from an alternating

current supply (23), the rectifying means (21) being adjustable to vary the magnitude

of the supply voltage and hence the output power level of the converter.

12. A converter according to any preceding claim, wherein the control means (5)

includes fine frequency adjustment means for effecting fine frequency adjustment of

the frequency at which the resonant circuit (3, 22) is excited in order to fine tune the

output power level of the converter.