WO2007141349A1

WO2007141349A1 - Electronic power inverter stage

Info

Publication number: WO2007141349A1
Application number: PCT/ES2006/070078
Authority: WO
Inventors: Jesús CLEMENTE BELIJAR; José GARROTE MANSILLA; Juan DÍAZ GONZÁLEZ; Juan Antonio MARTÍN RAMOS; Alberto MARTÍN PERNÍA; Pedro José VILLEGAS SÁIZ; Fernando NUÑO GARCÍA; Juan Manuel Lopera Ronda; Francisco Manuel FERNÁNDEZ LINERA; Miguel Ángel JOSÉ PRIETO; Juan Ángel MARTÍNEZ ESTEBAN
Original assignee: Suinsa Medical Systems S.A.
Priority date: 2006-06-05
Filing date: 2006-06-05
Publication date: 2007-12-13

Abstract

Electronic power inverter stage (12) for a switched power source which can be connected in cascade to the primary of a transformer (13) in which the inverter stage (12) is designed to transfer power between its input and output discontinuously in such a way that the output power from the power source is that required by the load connected to the output from the said switched power source.

Description

_STAGE. INVE1SORA ELE € thionic BE POWER _Q. BJIlO _... OF _. LAi _. NyENTION fOOOl] The present invention relates to a switching power supply comprising on the primary side of the transformer a reversing stage cascaded to a transformer die.

[0002] More specifically, the present invention will be directed to a control system that allows to reduce switching losses by reducing the cut-off current at the input (ON) and at the conduction output (OFF) of the electronic inverter switch of power. It refers, therefore, to the area of power electronics, and this invention would be applicable in equipment that requires power supplies based on this type of inverting topology, that is, in those equipment that by their characteristics require a power supply with specific levels of voltage, power (medium - high) and frequency »in which the application of the topology is clearly indicated

15 investor.

STATE OF THE TECHNIQUE

[0003] The usual scheme of a high power feeding equipment is observed in Figure 1, where a rectifying stage that includes an input filter, an inverting stage, a transformer that normally includes a resonant tank are cascaded, and then the rectification and output filtering stage.

[0004] Figure 1 shows an example of this type of high-power converters in which the inverter is implemented with four switches in full-bridge arrangement. Through the open - closed operation of these

25 switches generate a square voltage form (Fig, 2). which applies to the next stage, that is, to the input of the block that includes the transformer. The transformer used, normally introduces its parasitic components in the power topology promoting the resonant operation. For example, in high voltage generating equipment, it is common for the transformer to impose

A series inductance, L, and a parallel capacitor CP as seen in Figure 1. The resonant tank L-CP that they form can be modified by adding elements. In the example, the capacitor C _& is added to eliminate the level of continuous at the input of the transformer, being able to. additionally, be part of the resonant circuit. fOOOSj Hn in any case, the tank circuit will act as a filter for the voltage V _A B _U) applied at the input thereof and the current obtained will be sinusoidal or quasi-scnoidαl (Fig. 2),

[0006] characteristic of these resonant topologies that the same voltage and output power can be obtained with more than a combination of frequency and duty cycle (d) with the input, V _xm) , This generates the existence of different possibilities converter control: variable frequency and fixed cycle, variable cycle and fixed frequency, etc.

[0007] Whichever alternative is chosen for the control chosen, it is increasingly interesting to raise the working frequency in high-power power equipment, since it results in the decrease of the final size of the source and allows obtaining widths of older band. However, there is an important limitation or barrier in increasing the frequency and it constitutes the value of the switching losses in the power stage switches, π Currently these switches are transistors, MOSFEl ^' , IGBT, or the like. In both, the switching losses depend on the frequency and the cut-off current. For example, in an IGBT:

! <r <<- _ι_ / i * r JLLL__ ^; Í <1 1

'U t,. * <r> i t> f 'r ,, ι

where:

Pυ _\ ^τ } 'Porr are the power dissipated in each switch to the input and to the conduction output respectively.

E _0V EQH-V ( _{. C} i _cf ^e ki _ef are reference values defined by the manufacturer that depend on the selected switch.

V _C ce Ic- are the voltage and current values present at the time of switching, either ON or OFF. Y

> 5 f is the switching frequency. f0008] It is usually the case that the value of your losses absolutely determines the choice of a switch. For example, if the switch must handle a current of 120 amps maximum, it seems possible to choose a switch capable of handling 150 amps continuously. But nevertheless. 30 such choice may not be appropriate depending on the frequency of work. In fact, it may be normal to have to select a much larger switch, capable of handling 300 amps of continuous, because it will have lower losses due to better values of E _0N and BQ _FΓ (energy lost in each switching with the reference voltage and current values). Various techniques have been used to reduce switching losses, both those of driving input (ON) and those of output (OFF), 5 of which can be mentioned the use of networks of help to the switching (snubbers), BUSBAR, etc. These methods combat the increase in switching losses caused by undesirable behaviors of the switch and of! inverter wiring Another common technique to reduce losses is to switch with zero voltage or current in the switches (V <χ ^~ Q or I _C ^ O). In general, this is possible for the input or output of the switch conduction, but not for both cases simultaneously. Hn the current transistors E _U \ -> F, _{OΪ r} so it is favorable to make the input (ON) with zero current and have appreciable losses at the output (ILO). This technique is called optimal zone switching.

[§0091 Therefore, it is necessary to develop a power supply

Switching that includes a system of governing the switching elements of the inverting stage so that the switching elements are selected for the desired operation, that is, they are not oversized and, consequently, the overall performance of the power supply is high since switching losses in the inverting stage are minimized. For the

Therefore, the size and weight of the switched power supply are not penalized by the dimensions and weight of the inverting stage.

CHARACTERIZATION PE LA IN ¹ VEMCIQM

[00101] The present invention seeks to solve or reduce one or more of the drawbacks set forth above by means of an inverting stage for switching power supply 25 as claimed in claim 1. Embodiments of the invention are set forth in the dependent claims.

[00111] An object of the present invention is to provide a control system for a switched power supply that includes on the primary side of a transformer an inverting stage whose switching elements provide for at least one duty cycle, duty eyele . an output power exceeding the power demanded by the load connected to the power supply.

[0012] Yet another object of the invention is to minimize the number of components necessary to carry out the control system so that reduces the complexity weight and dimensions of the switched power supply,

[0013] Another object of the present invention is to store in a resonant circuit that includes the transformer of the switched power supply 5 the excess energy generated by the inverting stage during the work cycle / s in which the load is being fed, of so that when the output power of the inverting stage is zero, the load is fed from the electrical energy stored in the resonant circuit,

[0014] Yet another object of the present invention is to reduce the physical dimensions of the switching elements included in the inverting stage and the losses that occur in said inverting stage, since the size and weight of said stage has a great influence. the overall size and weight of the switched power supply

[0015] Another object of the present invention is to make the converter so that the resonant circuit comprises the parasitic elements of the transformer.

[0016J ^ ^lst object of the present invention to provide a control unit width modulation pulse generating corresponding control signals for each of switching elements

20 switching of the inverting stage, so that there are predetermined periods in which the inverting stage transfers power to the transformer and other predetermined periods in which there is no transfer of power from the inverting stage to the transformer. Obviously, from the switched source it was always traded! ere voltage and current to the load connected to said

25 source.

[001?! The control reduces switching losses, being compatible with control techniques such as variable frequency / fixed-variable cycle or fixed frequency / fixed-variable cycle. If used in combination with the optimal zone method, switching losses to OFF can be reduced to values of 30 5/6. Vz, or the like. In this way, switches can be used without over-voltage,

BRIEF TO THE FIGURES f0018] A more detailed explanation of the invention is given in the following description based on the attached figures in which: | 0019] Figure 1 shows in a block diagram of a high power converter according to the invention.

[0020] Figure 2 shows the voltage and current waveforms at the output of the inverter of Figure L

[002 Í J Figure 3a shows the voltage and current waveforms at the output of an inverter according to the invention, where the black switching points of the switches of a branch are indicated with black circles and where they are cut in the maximum of the current,

[0022] Figure 3b shows the voltage waveforms at the output of the power stage 10, rectified and filtered according to the invention, where the curling obtained in Figure 4b is marked with the cursors, the curling being less .

| 0023] Figure 4a shows the voltage and current waveforms at the output of the inverter according to the invention, where it is observed how the number 15 of switching is reduced by half, obtaining the same output power and voltage as in the previous case; the value of the current to be cut is also greatly reduced, in this case to zero, this example assumes the limit case of switching loss reduction, since they are reduced to practically zero, and

[0024] Figure 4b shows the waveforms of the output voltage, once rectified and filtered, where a curl is observed somewhat higher than the previous case.

DESCRIPTION OF THE INVENTION

[0025 j The operation of the switching control of the switching elements Ml to M4 of an inverting stage 12 will be explained with the aid of the 25 figures 3 a and 3 b, and 4a and 4b,

[0026] The switched converter comprises cascading a rectifier stage 1 1 that includes an input filter, the inverting stage 12, a transformer 13 which normally includes a resonant tank, and then the rectification and output filtering stage 14.

IQ [0027J Figure 3a shows the voltage. v _ΛB¡l ), and current. i] ( ₀ , output of the inverter 12. In figure 3b the output voltage can be seen, v ^. of all the power equipment on a given load. For their part in figures 4a and 4b the same have been represented previous waveforms after having added, for the same load, the control object of the present patent. output power are the same. However, the number of commutations has been reduced by half, and the cut-off currents are smaller, the switching losses have been significantly reduced

[0028J Generally, power converters should maintain the

5 constant output power even with severe variations in the input voltage to the inverter 12. Λ full load, with low input voltages a converter will typically be adjusted with working skies, d. close to maximum. d # 0.5.

However, with high input voltages, the duty cycle is reduced. becoming close to 0.25 (JPig. 3a), which should cut the maximum of

\ 0 current, if working in the optimal zone, f§029] Other control strategies give similar results: If a fixed cycle is used, dssθ, 5,} 'the frequency is varied, the current offset that will occur to maintain the constant inverter output power, will cause losses ON and OFF. whose sum is usually even higher than with control 15 in the optimal zone. The consequence is that, for the high range of input voltages to the inverter 12. it is in the worst possible loss scenario, because large current is being cut with great voltage. On the other hand, the inverter 12 works below its capacity of giving power, looking oversized.

| OO30] Kl switching control takes advantage of this extra power to reduce losses 20. Thus, for several cycles more power is provided from the input of inverter 12 by placing d '0.5 at the almost zero current offset frequency. Λ then, in some of the following cycles, the transfer of power is canceled (from the input to the output). Bl control ensures that the average power transferred to the load is what is demanded.

[G031] Hn Figure 4a shows an example in which one cycle at maximum power is followed by another with zero power. The only penalty falls on the output filter 14 which must be slightly higher for the same curling.

[0032 j Kn inverter 12. the frequency of the voltage has been reduced by half. Meanwhile the current does not vary too much in form and value, although

30 changes the moment when the switching element switches. As can be seen _, the operation can be interpreted as that the converter "loses" a period of every two. and that this loss must be compensated by increasing the duty cycle of the remaining periods, so that the input power to the resonant tank is kept constant. Expressed from another

In this way, what is done in even periods (eg) is to store energy in the Resonant tank, to recover it in the odd ones, in which the input voltage is sanded to zero (zero understanding the voltage that appears in the switching ones Ml to M4 when they are saturated).

[0033] The points at which the use of this control technique is appropriate are those at which, if not implemented. it would be necessary to cut near the maximum of the current, where the switching losses reach their highest values. Logically, these operating points are not isolated. For similar output powers and input voltages, the control to be applied is the same, varying the frequency and sky, [0034 | It should be noted that in this type of control the output voltage does not inflict e in the presence of that "dead ^" or zero voltage cycle, so it cannot be associated with an all / nothing control. The output voltage influences the frequency of the square voltage wave, that is to say in the frequency half of the resonant current.

15 | OΘ35 | The proposed control can be implemented so that the relationship between the number of periods with and without transferred power (from input to output) is not I to L but can also be implied with different relationships: 1 in 3 (1 / 3), 2 out of 3. etc.

[0036] The periods are understood to be referred to the? 0 output voltage of the inverter 12 or to the resonant current and in addition the constant periods of the output voltage of the inverter 12 can be followed, alternated or distributed in any possible pattern.

| 0037] m: n control strategies are thus obtained. in which the switching losses in average value are reduced by at least m / ii as recorded in (2):

I S

Qi

/:, ^* p) tl ff 12 »

where: neither is the number of periods in which the output voltage of the imrsor is constant: n is the total number of periods:

Pmn are losses using the proposed control; Y

P the losses obtained without using the proposed technique. In short, the proposed technique consists in governing frequency and duty cycle in a traditional way, but "losing ^" m of n periods of the inverter's output voltage, at which time it is set to zero.

J0039] The proposed control technique allows the operation of

5 power electronic inverters 12 with less losses due to the switching of the power switching elements; in some cases it allows the operation of the same under conditions of load and input voltage impossible to achieve due to the high value that would reach the switching losses, which would make the work viable - at least in regime

10 permanent- of the investors, Theoretically, it is easy to obtain this limit if we pay attention to the formula (1): the value of the total losses (the losses are considered OFF the largest) should never exceed the values defined by the manufacturer in your catalog

0040] With the use of this technique, the inverter 12 is allowed to reach 15 operating points that would otherwise be impossible to achieve, due to the excessive value of the losses,

[004 Ij In addition, even assuming that the inverter 12 can work at all required operating points, it is possible to relax the specifications of the switches Ml to M4 to be used, so that cheaper devices can be used and complying with With all operating points.

[0042] Hn with regard to the ripple of the output voltage, although at first glance it seems that it will increase, it can be said that it is not quite true, since the problems that are solved with the application of this technique appear

25 always at high frequencies; in fact that is the reason, the high switching frequency. AND! Power equipment output filter is sized for the lowest frequencies; the lowest frequency harmonics that appear in the output voltage in the case of using the technique proposed here, are of frequencies very close (in some cases even higher) to the frequencies

30 work at other operating points, so the output filter has already been designed for them. In addition, it is necessary to take into account that the low frequency components that appear in the output voltage do not constitute the fundamental harmonic, so the amplitude will be clearly lower. In summary, it can be said that the application of this technique hardly penalizes the

33 outlet filter design. J0Θ43J The control for inverters can be applied in medium-high power equipment that is based on a power converter, which incorporates a power stage inverter.

[0044] As examples of this type of equipment, the following 5 can be mentioned:

- Electron beam welding equipment, which includes a power inverter as part of the power supply: the powers in this case focus on the environment of i 5 KW. Although there are more powerful equipment, an application of this type of equipment would be in industrial radiography for

10 inspection of welds, cast iron beams, concrete, etc.

- X-ray generating equipment. In this type of equipment, the powers are higher and 65 KW can be cited as a reference value, although in this case there are also equipment of greater and lesser power. An application of this type of equipment would be in clinical radiography.

[0045] A representative and explanatory embodiment of this control technique based on an m / n ratio that is ¹ A and the constant voltage level during the stolen cycle is zero; It really is the saturation voltage of the switches Ml to M4. positive in one half cycle and negative in the other.

[0046] For all intents and purposes, these saturation voltages of switches 20 MI to M4 are considered zero, since these values are far from the working voltage values of the inverter 12.

[0047] In this preferred embodiment, the periods would be referred to the input voltage and not to the resonant current. This would be achieved if the switching elements M3 and M4 (see figure 1). or the Ml and M2 _} are left 25 on during the stolen cycle,

[0048] Figures 3a and 3b show the waveforms entering the resonant tank (3a, current and voltage) and the output voltage (3b) on a given load.

[0049] Figures 4 Ά and 4b show the same point in terms of power 30 transferred on the same load using the technique proposed here. The advantages of using this are evident, since the switching frequency is reduced by half, so that the losses due to this concept are divided by two. In the particular case shown, the switching losses are reduced to zero , since the current is in phase completely with the input voltage. 10

All this, without compromising the transmission of power to the output.

[005Oj I, the embodiments and examples set forth herein are presented as the best explanation of the present invention and its practical application and to allow specifically those skilled in the art to put in

5 practice and use the invention. However, those skilled in the art will recognize that the description and examples above have been presented for the purpose of illustration and only as an example. The description as set forth is not intended to be exhaustive or to limit the invention to the precise manner described. Many modifications and variations are possible in the light of the teaching above without departing from the spirit and scope of the claims if my mind.

Claims

J I

1, Electronic power inverter stage for switchable power supply cascadable to the primary of a transform! ' (13); characterized in that the inverter stage (12) is adapted to transfer

5 power between its input and output in discontinuous mode so that the output power of the power supply is that demanded by the load connected to the output of said switched power supply.

2, Investment stage according to claim 1; characterized in that the inverting stage (12) transfers power to the primary of the transformer (13) in a discontinuous manner,

3, Inverting stage according to claim 2; characterized in that a resonant circuit that includes the transformer (13) provides the power required by the load connected to the power supply during the work cycles in which the output power of the inverting stage (12) is absent.

4. The inverting stage according to claim 3; carjetemado because the work cycles in which the output power of the inverter stage (12) is absent can be adjacent, alternate or distributed according to a predetermined switching control algorithm.

5. Inversion stage according to claim 4; characterized in that the resonant circuit includes the parasitic elements of the transformer (13) and a series circuit of capacitor and coil,

6. Investment stage according to claim 4; characterized by the inclusion of a predetermined number m of duty cycles of the periods in which the output voltage of the inverter (12) is constant in relation to the

25 total number of periods (n) thereof.

7. Inverting stage according to claim 6: characterized in that the duty cycles of the periods may refer to the output voltage of the inverter (12) as well as the resonant current.