Medical X-ray appliances use high-voltage power supplies that must be able to work with very different energy requirements. Two techniques can be distinguished in X-ray medical imaging: fluoroscopy and radioscopy. The former involves low power radiation with a long exposure time, while radioscopy requires large power during short radiographic exposure times. Since the converter has to be designed by taking into account the maximum power specification, it will exhibit a poor efficiency when operating at low power levels. Such a problem can be solved by using a new multilevel LCC topology. This topology is based on a classical series-parallel resonant topology, but includes an additional low-voltage auxiliary transformer whose function depends on the X-ray technique considered. When radioscopy operation is selected, the transformer will allow the power to be shared between two full-bridges. If fluoroscopy mode is activated, the auxiliary full bridge is disconnected and the magnetizing inductance of the auxiliary transformer is used to increase the resonant inductor in order to reduce the resonant currents, thus improving the efficiency of the converter.

Many industrial and medical applications (X-ray generation, electron beam welding, electrostatic precipitators [

Not only do the electrical specifications represent a technological problem for the power stage that has to cope with such a wide power range: the design is also complicated by the tolerances in the power grid as defined by different AC grid regulations all around the world. Taking this into account, the DC input voltage for this power stage is set to have a value ranging from _{i}_{i}

Taking into account all of the conditions indicated above, it is necessary to select a topology that can deal with the variation ranges considered for the input voltage and for the output power, without affecting its performance. LCC resonant topology is commonly used for these applications (

The fact of using resonant inductors and capacitors makes the LCC-PRC (Parallel Resonant Converter) specially suited for high-voltage DC power supplies and distributed DC/DC applications, where the transformer parasitic elements are usually incorporated into the basic operation of the whole circuit. Due to the high output voltage demanded, a high transformer ratio is required. The large number of turns in the secondary side of this transformer will determine the value of the parasitic capacitance in the topology. In a similar way, the leakage inductance will depend on the winding geometrical structure and the number of turns in the primary side. These parasitic elements will affect the converter operation, so including them in the power topology becomes mandatory; as mentioned above, the PRC-LCC converter fulfils this objective.

Compared to the series resonant converter or the parallel resonant converter, the LCC-PRC also exhibits better performance at output voltage regulation, and a better reliability with short-circuit or open-circuit condition [

The inclusion of a large dead time between each pulse, like that associated to burst-mode operation, involves a large low-frequency ripple in the output voltage, which, due to the higher harmonic content, could be audible. Additional low-pass-filter is required at the high-voltage side and also at the input of the converter to minimize EMI (electromagnetic interference) problems. During stand-by mode (period between burst pulses), a resonance appears between _{S}_{S}_{P}

Appropriate design of the LCC-PRC converter is always made for the worst operating conditions (lowest input voltage, highest output power, and voltage); this is the way to guarantee that the converter specifications are met for any other operating point. When these worst-case conditions are considered, transferring the maximum power to the load while providing a high output voltage requires that a high parallel capacitance and a low series inductance be present in the circuit.

Once the LCC resonant tank is obtained considering these worst-case operating conditions (maximum output power), it must be guaranteed that the power topology can also work under low-power conditions (some tens of watts), while keeping the output voltage at values of hundreds of kV. Since it is necessary to have a high resonant current in the topology in order to obtain the nominal output voltage and regardless of the power demanded, the converter efficiency will be penalized when operating under low-power conditions. This high resonant current is necessary to provide the voltage required across the parallel resonant capacitor in order for the topology to work properly. Note that the maximum value for this voltage does not depend on the power demanded: its value is determined by the output voltage only. Hence, Equation (1) shows how to calculate the value of the resonant current:

Thus, the main drawback of the LCC topology appears because of the wide power range associated to X-ray applications. In this case, the efficiency at low power decreases dramatically, which makes it essential to adapt the LCC topology in such a way that it is possible to control the value of the resonant current as a function of the power being transferred to the load.

The goal of this paper is trying to overcome this problem by modifying the resonant inductor during fluoroscopy (low-power) operation. The proposed multilevel converter solves these problems by including an additional full bridge that can also contribute to increasing the maximum output power (>100 kW), which is the actual tendency in X-ray generators. In reference to [

As already indicated, depending on the X-ray technique used, there are two different conditions that the multilevel must operate at. Each of these situations defines two different output specifications, whose nominal operation values are:

Fluoroscopy: 1.2 kW–120 kV

Radioscopy: 100 kW–150 kV

So, different power specifications may be covered by using a multilevel converter like that in

The utility of the auxiliary transformer is twofold. On the one hand, it allows the voltage across the resonant tank to be defined by varying the duty cycle of the two full bridges included in the topology. On the other hand, it can be used to increase the value of the resonant inductance when using the fluoroscopy technique. This increase of the resonant inductor is achieved by turning the slave bridge off, which results in the resonant inductance being the series association of the magnetizing inductance of the auxiliary transformer plus the leakage inductance of the high-voltage transformer of the main bridge. This new situation involves a specific design of the auxiliary transformer in order to avoid saturation during the fluoroscopy operation.

During the radioscopy mode operation, both full-bridge converters are synchronized in such a way that the voltage generated by each one (_{A}_{B}_{AB}_{A}_{B}

The current, handled by the resonant tank, is the same as the one that flows through the main and auxiliary full-bridge converters. _{1}, and that of the slave bridge, _{2}.

The multilevel converter proposed in this paper allows two control actions. By changing the duty cycle of the auxiliary bridge, _{2}, the first harmonic amplitude of the voltage at the input of the resonant tank, _{AB}_{1}, is kept constant regardless of any variations in the grid DC voltage, _{i}_{AB}_{1} = 750 V. The other control parameter, _{1}, should be used to keep the output voltage constant during the radiography process.

Knowing that the voltage across the resonant tank will always have the same value (_{AB}_{1} = 750 V) poses a great advantage in the design: all the components of the resonant tank need to be calculated for a single, high-value voltage. If this high-voltage value were not unique, the only way to ensure that the nominal output power demanded by the X-ray tube can be provided would be by designing all of the components for the lowest input voltage (_{AB}_{1} = 400 V), which always results in a larger parallel capacitor, _{P}_{O}_{O}_{cmin}_{2} allowing input voltage control (

As stated above, when a traditional design is considered (

Using Equations (2)–(4) above, and considering a minimum operation frequency _{cmin}_{α}_{S}_{P}_{S}

Considering these values, the current through the series inductor at _{L}_{i}

However, if a multilevel structure, like the one in _{AB}_{1} = 750 V for _{i}_{S}_{P}_{S}

_{L}

This situation was to be expected, since now the same power transferred to the load comes from two full bridges with the same input voltage (_{i}_{1} and _{2}, are almost the same to supply the maximum power.

The number of switches in the multilevel converter is doubled, but their specification of maximum current is halved; thus, the final cost does not increase significantly as far as the switches are concerned [_{C}_{1}, _{2}, _{O}_{X}_{O}_{i}_{O}_{B}_{B}_{S}^{0.5}, some plots can be obtained to observe the boost capability of this converter (

From the equations above, the first harmonic of the input voltage to the resonant tank can be obtained. This first harmonic can be used to determine the gain of the converter by using the set of equations published in reference [

However, in the fluoroscopy mode, when low output power is required, the slave bridge is turned off and the auxiliary transformer participates in the topology by leaving its magnetizing inductance in series with the resonant inductance (_{S}_{m}

Thus, in radioscopy mode, the resonant inductance is made up by the main transformer leakage inductance only (_{S}_{m-aux}

Operation in fluoroscopy mode also evinces a different performance when using a single-full-bridge topology (traditional design, _{ON}_{OFF}_{CE}_{C}_{ON} _{OFF}_{C}_{ON}_{OFF} _{CE}_{C}

One of the most critical parts in the design of the high-voltage power supply is the main transformer. This is the component that must guarantee the equipment isolation and raise the output voltage to the values specified by the load (150 kV). The inclusion of a high-voltage transformer has an important effect in the topology operation. Due to the large values of the transformer parasitic elements, they must be included as resonant topology elements. By doing so, the need of additional, bulky reactive elements can be avoided. But parasitic elements can only be calculated before construction if the transformer geometry is accurately characterized [

To facilitate the transformer parameters calculation and assembly, the secondary winding was built by using two windings of

The plastic material used to hold the PCB’s is Arnite, which exhibits low H_{2}O absorption (0.1% D-570 test), dielectric constant at 1 MHz

The electric field between two conductive layers (

Using this expression, together with the dielectric constant of the insulation material, _{i}

The total energy _{T}_{1}:

The leakage inductance can be obtained in a similar way; in this case, the leakage magnetic energy (FEA tools) in the windings must be calculated and associated to the series inductance _{S}

All of these equations allow designers to define the geometry of the main transformer that will provide the desired resonant elements _{S}_{P}_{S}

The process for designing the HV transformer starts with the specification of the turn ratio _{O}_{i}_{C}_{C}_{i}_{O}_{S}_{P}_{S}_{P}_{F}_{F}

In order to validate the effectiveness of the proposed topology, the resonant tank was built by using discrete components (_{S}_{P}_{S}_{O}_{1} =1:80 (

Nanocrystalline material Vitroperm 500 F has been used to build the auxiliary transformer in order to avoid saturation. The number of turns in this transformer is N1_{aux} = N2_{aux} = 3 (Γ = 1).

The two full bridges mounted with IGBTs FF300R12KS4 can be seen in

The waveforms in

All of the resonant elements designed for the application were built as discrete components to simulate the HV transformer, thus referring to all of the voltages and currents to the primary side. In the case of the traditional LCC converter, the resonant tank required is: _{S}_{P}_{S}_{S}_{P}_{S}

The significant difference obtained in the value of the resonant current when comparing the traditional and the multilevel LCC converter determines the great advantage of using the proposed stage.

Both _{O}_{2} to keep the amplitude of the first harmonic at the resonant tank _{AB}_{1} constant, in order to deal with AC grid variations. The duty-cycle _{1} will also help to control the output power. On the other hand, the multilevel voltage _{AB}

One of the main advantages of the multilevel converter proposed is that it succeeds in joining, in the same topology, two different types of operation (fluoroscopy and radioscopy) that have been typically solved with two converters independently designed for each case. The resonant elements in the traditional LCC converter are kept the same, but in the multilevel converter the slave bridge is disconnected to include the magnetizing inductance of the auxiliary transformer in the resonant tank. Therefore, the new resonant inductance is _{S}_{S}_{P}

The multilevel converter one more time exhibits a low amplitude resonant current, while reducing the switching frequency to 41 kHz (_{m}_{S}_{P}

Since the traditional LCC converter is designed for high power operation, its frequency should be increased (>60 kHz) when the power demanded is very low (fluoroscopy) in order to control the output power. Under these conditions, the converter has to manage high reactive power at a higher frequency, as can be noticed from the high resonant current and the low duty-cycle (

Therefore, in the multilevel converter the amplitude of the resonant current has been highly reduced, thus power losses are improved thanks to both effects: low switching frequency and low resonant current amplitude. The efficiency in fluoroscopy mode for the traditional (Trad) and multilevel (Mult) converter has been measured at different loads (_{O}_{1} = 1:80). The input voltage was fixed to 300 V. With this operation mode the output power is typically lower than 1.2 kW. The traditional LCC converter is penalized by the low output power demanded, since it must be designed for the maximum output power specified. The efficiency in radioscopy mode was also analyzed (

The design of power supplies for high-voltage applications is conditioned by the wide range of input power and voltage required. This situation is especially complex in X-ray generations, where specifications define power ranges from hundreds of watts to hundreds of kW, but it also appears in other applications such as electron beam welding or electrostatic precipitators.

Traditional LCC Full Bridge converters are typically designed for high-power operation (radioscopy), while low-power situations (fluoroscopy) are solved by increasing the switching frequency and the reactive power. The resonant current cannot be reduced because it is defined by the output voltage and the value of the resonant capacitor, _{P}

In this work, a multilevel LCC resonant converter is proposed that includes two full-bridge topologies operating in parallel with different duty cycles. This allows the voltage at the input of the resonant tank to be kept constant, _{AB}

Additionally, it is possible to significantly reduce the output power (fluoroscopy mode) by disconnecting the slave bridge. This result in the magnetizing inductance of the auxiliary transformer being added to the resonant circuit, and thus, switching frequency, resonant currents, and the reactive power being substantially reduced.

The final conclusion is that a multilevel topology has been proposed to replace traditional LCC resonant converters in X-ray power supplies. The topology proposed succeeds in reducing the resonant current for any of the operating modes used in these applications. This reduction results in an increased efficiency being obtained with simple modifications: including an additional bridge (whose IGBTs only need to conduct currents half the value of those in the main bridge) and a low-voltage 1:1 transformer that must handle the whole current of the converter.

This work has been co-funded by the Plan of Science, Technology and Innovation of the Principality of Asturias through Project FC-15-GRUPIN14-122, and by the Spanish Government with the action TEC2014-53324-R.

This paper is part of a research carried out by A. M. Pernía and Juan A. Martín-Ramos, whereas Juan Díaz, Miguel J. Prieto and Pedro J. Villegas assisted with FEA and mathematical simulations.

The authors declare no conflict of interest.

Output power and output voltage specifications for X-ray power supplies.

Series-parallel LCC resonant topology.

Multilevel topology proposed.

Voltage (_{AB}_{L}_{1}, _{2} represent the gate delay signals.

Resonant current _{L}_{i}

Resonant current I_{L} versus output voltage and switching frequency at _{i} = 750 V.

Output voltage versus frequency for _{2} = 0.2 and

Resonant circuit for radioscopy and fluoroscopy.

Secondary winding of the high-voltage transformer.

Cross section of the high-voltage transformer and external connection.

Parasitic capacitance of two conductive layers. Geometrical model of a PCB (printed circuit board).

High-voltage transformer.

LCC resonant multilevel converter.

Evolution of voltage _{AB}_{O}_{L}_{O}_{O}

Evolution of voltage _{AB}_{O}_{L}_{O}_{O}

Evolution of voltage _{A}_{L}_{O}_{O}_{i}

Evolution of voltage _{A}_{L}_{O}_{O}_{i}

Efficiency during fluoroscopy operation for different loads and output voltage (referred to the primary side) by using the traditional LCC (Trad) or the multilevel LCC (Multi).

Efficiency during radioscopy operation for different loads and output voltage (referred to the primary side) using the traditional LCC or the multilevel LCC.

Design comparison for Fluoroscopy operation. (Output voltage: 120 kV. Output power: 1.2 kW).

_{O} |
_{O} |
_{L} |
||

400 V | 120 kV | 1200 W | 309 A | 80.5 kHz |

750 V | 120 kV | 1200 W | 310 A | 81.1 kHz |

400 V | 120 kV | 1200 W | 60 A | 53.7 kHz |

750 V | 120 kV | 1200 W | 61 A | 54.5 kHz |

Design parameters for traditional and multilevel LCC converter.

_{P} |
Γ_{1} |
|||

950 nF | 630 nF | 10 µH | 80 | 50 kHz |

_{P} |
Γ_{1} |
|||

330 nF | 220 nF | 38 µH | 80 | 50 kHz |

330 nF | 220 nF | 163 µH | 80 | 50 kHz |