Transistor module

A transistor module formed on a substrate comprises a first transistor, a first recovery diode, a second transistor and a second recovery diode. A first transistor chip associated with the first transistor and a second diode chip associated with the second recovery diode are disposed adjacently on a positive potential area. The first transistor chip and the second diode chip are disposed on an output potential area. A second transistor chip associated with the second transistor and a first diode chip associated with the first recovery diode are adjacently disposed on the output potential area. At least two sides of the output potential area are adjacent to a negative potential area. Both the second transistor chip and the first diode chip are connected to the negative potential area, and one of the first and second transistor chips is disposed opposite one of the first and second diode chips located on a different potential area.

BACKGROUND

The invention relates, in general, to transistor modules, and in particular to a transistor module having a first transistor and a first recovery diode and having a second transistor and a second recovery diode.

From German Patent Disclosure DE 31 12 280 A1, a transistor module for coil excitation is known, which is intended in particular for medical equipment systems which require power supplies that generate pulse-like fields of constant intensity. An exciter current of a coil should be run up to speed within a very short time and then be kept at a constant level until the end of the pulse.

From German Patent DE 34 15 041 C2, a power supply circuit is provided for gradient coils of a magnetic resonance imaging system for making medical diagnoses. To increase the energy conversion rate within the gradient coil, energy storing devices, such as capacitors, are disposed in series with the power supply circuit and with the current source.

From German Patent DE 195 11 833 C2, a gradient current supply for a magnetic resonance imaging (MRI) system is known which includes a plurality of transistors and recovery diodes in a bridge circuit connected to a gradient amplifier. Gradient pulses to be employed are predetermined by a sequence controller to which the gradient amplifier is connected. The layout of the circuit is not the subject of DE 195 11 833 C2.

SUMMARY

The present invention is defined by the appended claims. This description summarizes some aspects of the present embodiments and should not be used to limit the claims.

A transistor module has two transistors as well as two recovery diodes associated with the transistors. The transistor module has a relatively simple construction or layout suitable, because of a low stray inductance, for a gradient amplifier with steep gradient ramps, intended for example for medical equipment applications.

This transistor module is provided with two transistor chips, each with at least one transistor, and two diode chips, each with at least one recovery diode associated with one of the transistors. A circuit substrate, which is typically rectangular and on which the chips are individually disposed, has three potential areas, namely one positive potential area, one negative potential area, and one output potential area.

Both the first transistor chip and the second diode chip, having the corresponding second recovery diode associated with the second transistor chip, are disposed on the positive potential area.

Both the second transistor chip and the first diode chip having the first recovery diode, i.e. the recovery diode associated with the first transistor chip, are disposed on the output potential area, which is connected electrically, in general with tiny bond wires, to the chips disposed on the positive potential area.

As such, each transistor chip is disposed both adjacent to the diode chip disposed on the same potential area and opposite the diode chip disposed on a different potential area. Within this component layout, the “adjacent” disposition of two components, in this case a diode chip and a transistor chip, is used to mean an arrangement on the same potential area, while the “opposite” disposition of two components is used to mean an arrangement on different potential areas.

With generally rectangular chips, the two diode chips are disposed adjacent to and opposite, respectively, two lateral lines of one transistor chip that are perpendicular to one another. Thus, the transistor chips and diode chips are disposed in a cross-shaped pattern on the circuit substrate.

The negative potential area of the transistor module is embodied either as a cohesive surface or in the form of a plurality of partial areas. In each case, the negative potential area, or portions of this surface, is adjacent to at least two sides, preferably opposite sides, of the output potential area, which generally has a rectangular basic shape. As such, one may electrically connect the chips, disposed on the output potential area, to the negative potential area using especially short lines, and these short lines may preferably extend at least approximately perpendicular to the lines which connect the output potential area to the chips disposed on the positive potential area.

While the positive potential area is located in a plane defined by the circuit substrate, the negative potential area may have a more-complex form. On the one hand, the negative potential area is defined by its partial areas bordering on the output potential area and on the other hand by the lines leading from these partial areas to the chips on the output potential area. Preferably, a plurality of lines is provided such that the plurality of lines is used to connect the chips on the output potential area to the negative potential area. Because of the plurality of lines, preferably embodied as tiny bond wires disposed at least approximately parallel to one another, a continuous surface, which preferably at least approximately spans the output potential area, is approximated.

The area of negative potential, like the positive potential area, preferably has a substantially un-branched shape, in particular a rectangular shape, and both potential areas form large surfaces located close together. As a result, magnetic fields cancel each other out over a relatively wide range.

As such, a relatively low-inductance construction of the circuit arrangement is achieved, which is A gradient amplifier with steep gradient ramps, for instance with rise and fall times on the order of magnitude of 100 μs or below, may use the circuit.

The individual chips on the circuit substrate are preferably closely spaced apart from one another such that both a spacing of one transistor chip from the diode chip having the associated recovery diode and the spacing of the transistor chip from the diode chip having the recovery diode of the other transistor chip is less than the height or width of the transistor chip. A spacing between the positive potential area and the output potential area is preferably less than the height of a diode chip.

In a preferred embodiment, the negative potential area—aside from the tiny bond wires—has a U shape, and the middle beam of the U is disposed as a connecting cross-member between the positive potential area and output potential area. A connection of two outer partial faces of the negative potential area, which border the output potential area, via the connecting cross-member is relatively advantageous for electrically connecting a plurality of transistor modules that are disposed side by side in a circuit arrangement.

In another preferred embodiment, the output potential area is substantially adjacent the positive potential area. The negative potential area may be embodied in multiple parts, including merely individual connecting pieces. This multiple parts embodiment is suitable for applications in which a single transistor module is connected separately to the negative potential.

For manufacturing, the positive potential area and the output potential area are preferably disposed in the same plane on the circuit substrate. Collectors of the transistor chips and cathodes of the diode chips are soldered to the faces of the positive and output potential areas.

Illustrative and exemplary embodiments of the invention are described in further detail below with reference to and in conjunction with the figures.

DETAILED DESCRIPTION

While the present invention may be embodied in various forms, there is shown in the drawings and will hereinafter be described some exemplary and non-limiting embodiments, with the understanding that the present disclosure is to be considered exemplifications of the invention and is not intended to limit the invention to the specific embodiments illustrated.

In this application, the use of the disjunctive is intended to include the conjunctive. The use of definite or indefinite articles is not intended to indicate cardinality. In particular, a reference to “the” object or “a” object is intended to denote also one of a possible plurality of such objects.

Similar elements are identified by the same reference numerals throughout the drawings.

FIGS. 1a,1band2each show, in simplified form, an embodiment of a transistor module1of a gradient amplifier for medical equipment, in particular for an MRI system. The transistor module1is embodied on a circuit which is also known as a half bridge and has both two transistor chips2,4and two diode chips3,5. In this embodiment, one individual transistor forms a transistor chip2,4and one individual diode forms a diode chip3,5. A capacitor, not shown, is connected on the one hand, near the bottom in the drawings, to a positive potential area8, and on the other, near the top in the drawings, to a negative potential area7. Both potential areas7,8are formed by a copper layer on a rectangular circuit substrate6and are located in a plane E1(FIG. 1b, side view).

A collector of the first transistor chip2is soldered to the positive potential area8. The emitter of the first transistor chip2is connected by a plurality of tiny bond wires in the form of lines9to an output potential area12. A gradient coil, for instance, not shown, of the medical equipment is connected to the output potential area.

The first diode chip3, whose cathode is soldered to the output potential area12, is provided for the sake of current recovery by the gradient coil. An anode of the first diode chip3is connected to the negative potential area7by a plurality of lines10, likewise in the form of tiny bond wires. The first diode chip3will be called the first recovery diode chip belonging to the first transistor chip2, or the first recovery diode chip.

In addition to the first diode chip3, the second transistor chip4is soldered with a collector to the output potential area12. An emitter of the second transistor chip4, like the anode of the first diode chip3, is connected to the negative potential area7by lines11. Further lines13connect the emitter of the second transistor chip4to the anode of the first diode chip3. The output potential area12is also connected by lines15to an anode of the second diode chip5, whose cathode is soldered to the positive potential area8and which forms the recovery diode associated with the second transistor chip4, or the second recovery diode chip.

Thus each transistor chip2,4and the associated recovery diode chip3,5is located on different potential areas12,8. Conversely, each antiparallel diode chip5,3of a transistor chip2,4is disposed jointly with that transistor chip2,4on the same potential area8,12.

The transistor chips2,4, like the diode chips3,5that are smaller by comparison to them, are approximately square. The width and height of the transistor chips2,4are indicated by BTand HT, respectively, and the width and height of the diode chips3,5are indicated by BDand HD, respectively. The surface of the circuit substrate6is for the most part filled by the potential areas7,8,12shown schematically inFIG. 2. The height of the potential areas7,8,12is indicated by the symbols HP7, HP8, and HP12, respectively.

While the positive potential area8already has a rectangular basic shape because of a corresponding copper layer on the circuit substrate6, a copper layer forming the negative potential area7has a U shape; with a middle bar of the U, oriented toward the positive potential area8, being a known as a connecting cross-member14. Between the three members of the U is the output potential area12, whose spacing Dpfrom the positive potential area8is less than the height HTof the transistor chips2,4and less than the height HDof the diode chips3,5. Between the outer members of the U, the lines10,11,13extend essentially parallel to the connecting cross-member14. These lines10,11,13approach a face, spaced apart parallel from the output potential area12, in a plane E2that effectively enlarges the negative potential area7, so that the negative potential area overall has a shape and size comparable to the positive potential area8.

An effective negative potential area7is thus formed both by the DCB (direct copper bonded) layout and by the tiny bond wires or lines10,11,13. The large potential areas7,8,12located close together or one above the other, along with the relatively short embodiment of the lines9,10,11,13,15contribute to a low stray inductance of the transistor module1, and magnetic fields may largely cancel one another out.

The spacing DBbetween the first transistor chip2and the antiparallel diode chip5, in the embodiment shown, is less than the spacing DHbetween the first transistor chip2and the associated recovery diode chip3; this latter spacing DHis approximately equivalent to the height HDof the diode chip5. The same is analogously true for the second transistor chip4and the recovery diode chip5associated with the second transistor chip4and for the chip3anti-parallel to it.

The transistor chips2,4on the one hand and the diode chips3,5on the other hand are thus disposed in a cross-shaped pattern on the circuit substrate6; each transistor chip2,4disposed on one potential area12,8is located opposite the associated recovery diode chip3,5disposed on the other potential area8,12and is adjacent to the antiparallel diode chip5,3.

InFIG. 3, another embodiment of a transistor module1is shown, which is likewise distinguished by a relatively low-inductance construction. In this other embodiment, the negative potential area7, to the extent that it is not simulated by the lines10,11,13, is present solely in the form of partial areas, which are partly degraded into connection pieces16that are not necessarily disposed in the plane E1of the circuit substrate6.

The chips3,4on the negative potential area7are thus connectable directly to the connection pieces16. A partial area, adjoining the output potential area12on the right in the drawing, of the negative potential area7, adjacent to the second transistor chip4, represents a simplification in terms of production, since corresponding tiny bond wires may simply be cut to length.

Despite the substantially reduced size, compared to the illustrating embodiment ofFIGS. 1a,1band2, of the part of the negative potential area7formed by the DCB layout, the negative potential area is also, in the exemplary embodiment ofFIG. 3as well, formed primarily by the tiny bond wires or lines10,11,13disposed above the output potential area12, approximately in the plane E2, so that the overall stray inductance is relatively quite low. A construction of the transistor module1that both has a substantially lower inductance and is substantially more compact is attained in particular also by the elimination of a connecting cross-member of the negative potential area7, which cross-member would be located between the positive potential area8and the output potential area12.

Other relative sizes, positions and areas may be used for any of the embodiments discussed above.