Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-10-08
2002-08-13
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S049000
Reexamination Certificate
active
06434025
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply unit provided with an output monitoring function. In particular the invention relates to a power supply unit with a fail-safe construction which can stably supply an output of a level within a predetermined range while monitoring the output, and which can stop the output at the time of an abnormality.
2. Description of the Related Art
Heretofore, as a direct current power supply unit, a switching power supply unit is commonly used, since this has the advantage of high efficiency, small size, light weight and so on.
This switching power supply unit will be simply described.
With this unit, an input commercial AC (alternating current) power supply is converted to DC (direct current) in a smoothing circuit comprising a rectifying circuit and a smoothing capacitor, and then supplied to a primary side of a high frequency transformer. The DC supplied to the primary side is converted to AC by ON/OFF switching of a switching element, transmitted to a secondary side of the transformer, converted to a DC output by the smoothing circuit and then supplied to a load. Moreover, this DC output is fed back to a switching control circuit for controlling the switching element. The switching control circuit incorporates an error sampling circuit, an oscillation circuit, and a pulse width modulation circuit, and detects an error between the fed back DC output and a previously set target value for the output with the error sampling circuit, carries out pulse width modulation (PWM) on a signal of the oscillation circuit with the pulse width modulation circuit in accordance with the detected error and controls the duty ratio of a PWM signal, and controls the ON/OFF period of the switching element, to thereby stabilize the target DC output and supply this to the load.
In the case of a switching power supply unit fitted with such an output monitoring function, if for example a fault occurs such that the DC output is not fed back to the switching control circuit, the switching control circuit judges the input to be zero and thus increases the output. Moreover, if a disconnection fault occurs in the primary side smoothing capacitor, the primary side voltage of the transformer periodically becomes a low level and this is reflected in the secondary side output. Furthermore, if an abnormality occurs in the secondary side smoothing capacitor, an increase of a high frequency ripple may occur.
However, with equipment demanding high safety, for example equipment used in the railway technology field, or for press machines or the like, then an erroneous operation attributed to an abnormality in the output from the power supply unit is extremely dangerous. Consequently, with power supply units it is necessary to be able to realize in a fail-safe manner, a characteristic that at the time of an output abnormality, operation is to the safe side (for example the output is stopped).
From this perspective, heretofore a technique for detecting an output abnormality to stop the power supply output of a switching power supply unit has been proposed for example in Japanese Unexamined Patent Publication Nos. 4-248317 and 10-336879.
In Japanese Unexamined Patent Publication No. 4-248317, there is shown a construction where a plurality of windings are provided in a high frequency transformer, and an output from a winding separate from a feedback control monitoring winding is monitored with an abnormality monitoring circuit, and the PWM signal is stopped based on the results of this monitoring, to thus stop the ON/OFF operation of the switching element so that the power supply output is stopped. Moreover, in Japanese Unexamined Patent Publication No. 10-336879, there is shown a construction where when the output voltage exceeds a previously set upper limit value, the output monitoring circuit outputs an abnormality signal to stop the ON/OFF operation of the switching element, and thus stop the power supply output.
However, these abnormality monitoring techniques, both only monitor for an overvoltage as output abnormality, and output abnormality on the low voltage side where the output level drops is not considered. With the conventional abnormality monitoring techniques, even if the output level drops with a circuit abnormality, this is not considered as an output abnormality, and the output is controlled to increase.
In order to detect a drop in output level as output abnormality, it is necessary to consider a time of power supply start-up. That is to say, since the output is zero at the time of power supply start-up and hence the abnormality monitoring circuit judges output abnormality and stops the switching operation of the primary side of the transformer, it is necessary to cancel the abnormality judgment in order to carry out the switching operation. However, a construction where at the time of power supply start-up, the power supply output is produced regardless of the monitoring result from the monitoring circuit, is not shown in the disclosures of Japanese Unexamined Patent Publication Nos. 4-248317 and 10-336879.
As an output error monitoring technique for a switching power supply unit, a technique which monitors not only for an overvoltage of the output but also for a drop in the output level, and stops the power supply output of the switching power supply unit at the time of output abnormality is proposed in Japanese Unexamined Patent Publication No. 7-95724.
With this technique, a pre-rectification intermediate output and a post rectification final output in the secondary side of the transformer are both monitored and when the two outputs are both abnormal overvoltages or are both abnormal low voltages, an abnormality detection signal indicating abnormality is output to an alarm monitoring circuit. The alarm monitoring circuit is constructed to, on input of the beforementioned abnormality detection signal, output a signal for stopping operation to the PWM control circuit to thereby stop the operation of the switching power supply unit.
However, in the case of Japanese Unexamined Patent Publication No. 7-95724, there is no disclosure of a specific construction for how the power supply output is started at the time of power supply start-up with the transformer secondary side output zero. With the construction of an embodiment circuit shown in
FIG. 2
of this publication, it can be supposed that an abnormality detection output is generated from the low voltage monitoring circuit at the time of power supply start-up, and hence, the PWM control circuit is not operated due to the output from the alarm monitoring circuit, and the switching power supply unit can not be started. Furthermore, even if assumed that start-up of the power supply unit is possible, with the circuit construction of
FIG. 2
, a signal indicating abnormality is considered to be generated at a high level condition. In this case, if the transmission path for the abnormality detection signal is disconnected, the output abnormality cannot be notified, so that there is a danger that the power supply output cannot be stopped.
Also with an AC power supply, as with the DC power supply, there is a requirement for realizing in a fail-safe manner a characteristic where output abnormality is detected and operation is to the safe side (for example the output is stopped). However, heretofore such a technique for an AC power supply has not been set forth.
For example, with a sensor which receives an AC signal from an AC power supply and outputs an AC signal of a level corresponding to a sensing result of an object, in order for the sensor to have fail-safe characteristics, it is necessary for a level meaning safety to be higher than a level meaning danger. This will be clear if a fault involving a disconnection in the signal transmission circuit is considered. Moreover, in the case where the output level of the sensor is dependant on the input AC signal level and/or frequency, then in spite of a danger condition due to an error in the level and/or frequency of t
Futsuhara Koichi
Sakai Masayoshi
Shirai Toshihito
Yuasa Takayuki
McDermott & Will & Emery
Riley Shawn
The Nippon Signal Co. Ltd.
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