Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
1999-09-16
2001-01-23
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S053000, C363S089000, C363S070000, C323S901000
Reexamination Certificate
active
06178102
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates generally to power supplies and, more particularly, to such a power supply for use with pulse width modulated amplifiers.
2. Description of the Related Art.
Large amplifiers made for magnetic resonance imaging (MRI) gradient amplifiers have often comprised a major portion of the cost of the overall MRI system. As gradient amplifier sizes continue to increase to provide increased functionality for imaging, the size and cost of the amplifier's DC power supplies has likewise risen.
Pulse Width Modulated (PWM) amplifiers offer a very attractive simplification to the DC power supply system in that they typically can share one supply between all three axes of amplifiers, X, Y and Z. Since all three axes are not driven to full capacity at any one time, the aggregate DC supply demand is not three times that of the maximum demand of any one axis. A worst case would be that the total demand is only twice the maximum demand of one axis.
If three separate supplies had been required, the result would be that the system would need to make three supplies of the worst case size to meet any possible demand. Thus it is evident that by sharing supplies the PWM amplifier has reduced the power supply to ⅔ of the size of designs requiring three separate and isolated supplies. Since the supply is both smaller and simpler, it would be typical that the power supply cost has been halved by supply sharing.
The reason that PWM amplifiers can operate with a shared supply is that they do not require topologies such as grounded bridges which have floating supplies to overcome voltage based limitations of their output power controlling semiconductor devices. When semiconductors are used in non-dissipative modes as switches, they are less limited by voltage induced failures and higher voltages are possible.
The opportunities for optimization do not end with the improvements due to commonality. A further benefit derives when it is noted that the construction of very large amplifiers no longer require the use of galvanic isolation to isolate the output DC potentials from the AC mains potentials. Traditionally, isolation is done in small products to minimize the lethal exposure of users to primary-side power when contacting the secondary side. With a large amplifier having DC outputs of hundreds of volts and hundreds of amps, any direct contact of a user with the secondary side would be fully lethal, whether there were galvanic isolation or not. Safety must be derived by other means.
Strangely, gradient amplifiers have retained the isolated designs that were appropriate for small products in other applications. Galvanic isolation provides no useful feature once AC mains line transients have been filtered and arrested. By continuing to design with galvanic isolation, the cost and size of the supply have been inflated.
Ideally, a gradient amplifier would be operated with two DC supply feeds which are electrically centered (plus and minus voltage) about ground. This allows the amplifier to operate with a no output of zero volts on all of its full-bridge output terminals. Failure to so operate can result in both a hazard to amplifier technicians exposed to net DC voltages on the load when there is no signal and electrolysis within water cooled gradient coils that allow direct impingement of the cooling water with the gradient coils.
Typically, non-galvanically isolated supplies are less expensive to implement than those requiring all power to flow through transformers having isolated primaries and secondaries. Large amounts of unregulated power can be obtained in a non-isolated manner by simply rectifying the AC mains (three-phase). Being three-phase the resultant ripple voltage on the DC output is relatively small compared to the DC component.
If the three-phase input AC power does not provide connection to the neutral feed, all non-isolated full-wave rectifier circuits will of necessity be of classic 6-pulse delta form. If the neutral is provided, the rectifier could also be of 6-pulse wye form with the neutral at DC common potential.
PWM gradient amplifiers do not require power supplies of varying voltages. Ideally, the operating DC voltages are fixed and not subject to variations in line or load. Traditionally, the design of PWM gradient amplifier DC power supplies has in cluded regulating power supplies that would be capable of regulating output voltage over a wide range of voltage. Such regulators are appropriate for laboratory use where the desired voltage can vary from use to use. The only gradient amplifier requirement for diversity of output voltage is to be able to shut-down (zero output volts) when required by fault or safety related conditions .
SUMMARY OF THE INVENTION
The present invention, in one form thereof, involves a regulated power supply for a PWM amplifier. The power supply generates a DC voltage output from an AC voltage input. A significant portion of the DC voltage output is generated by rectifying the AC voltage with the remaining portion of the DC voltage output generated by a regulated power source.
The substance of this invention is that when operating non-galvanically isolated power supplies it is also possible to construct a regulated power supply with only a portion of its power converted by a regulating member, further reducing the cost of implementation. Such practice will also minimize size and maximize power efficiency. Power that is not processed will not likely result in dissipation. The regulating power-processing portions of a power supply are generally the most expensive and lossy per KW processed.
In one example the desired DC supply voltages are just over +/−400 Vdc and the desired AC input voltage is 400 Vac or 480 Vac (delta measured) three-phase. Even with low line voltage (400 Vac−10%) over half of the DC output voltage (and power) is derivable without being regulated or processed other than simply being rectified.
At high line (480 Vac+10%) the net rectified output is still less than the desired DC output voltage. In any case only the difference voltage between the desired output voltages and the available unregulated DC voltages need be processed through regulating converters. The higher the unregulated AC input voltage the less power needs to be processed through the regulating portions of the power supply.
The present invention provides a power supply for supplying a DC voltage from an AC supply voltage provided by an AC voltage source. The power supply comprises a plurality of input lines coupled to the AC voltage source, a rectifier coupled to the input lines, first and second regulators coupled the rectifier and generating controlled positive and negative DC voltage components, and a plurality of output lines coupled the regulators. The rectifier rectifies the AC supply voltage into an unregulated positive DC voltage component and an unregulated negative DC voltage component. The first regulator augments the unregulated positive DC voltage component with the controlled positive DC voltage component to create a positive DC voltage. The second regulator augments the unregulated negative DC voltage component with the controlled negative DC voltage component to create a negative DC voltage. The positive DC voltage and the negative DC voltage are provide on the output lines.
The present invention also provides a power supply for supplying a DC voltage from an AC supply voltage provided by an AC voltage source. The power supply comprises a plurality of input lines coupled to the AC voltage source, a transformer coupled to said input lines, a primary rectifier coupled to said transformer, first and second regulators coupled to said primary rectifier and generating controlled positive and negative DC voltage components, and a pair of output lines coupled to said regulators. The transformer transforms the AC supply voltage to a first AC voltage. The rectifier rectifies the first AC voltage to a first unregulated positive DC voltage component and a first
Baker & Daniels
Crown Audio, Inc.
Vu Bao Q.
Wong Peter S.
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