Abstract:
A DC/DC bridge having a power stage provided with controllable semiconductor switches controls a direct currently load, and comprises two bridge sections, one of which bridge sections conducts direct currently while the other bridge section is controlled via pulse width modulation to regulate the current magnitude. Two of three bridge arms of the DC/DC bridge are connected in parallel and semiconductor switches in the parallel-connected bridge arms are controlled via pulse width modulation to form a pulse width modulation controlled bridge section.

Description:
This application is a Continuation-In-Part of U.S. application Ser. No. 10/352,205, filed Jan. 28, 2003, now U.S. Pat. No. 6,697,273, which is the National Phase of PCT International Application No. PCT/FI02/00767 filed on Sep. 24, 2002, on which priority is claimed under 35 U.S.C. § 120, the entire contents of which are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   FIELD OF THE INVENTION 
   The present invention relates to a DC/DC bridge for controlling a direct-current load, said bridge having a power stage provided with controllable semiconductor switches and comprising two bridge sections, one of which bridge sections conducts direct current while the other one is controlled via PWM to regulate the current magnitude. 
   DISCUSSION OF THE BACKGROUND ART 
   Elevator motor drives may be either alternating-current or direct-current motor drives. The drives may be controlled e.g. via PWM (Pulse Width Modulation). Today, alternating-current motor drives are fairly common, especially because of the simple construction of alternating-current motors. However, direct-current motor drives are still in use. It is not always appropriate to replace a direct-current motor drive with an alternating-current motor drive, because e.g. gearless direct-current motor drives and the direct-current motors used in them are durable, and it is therefore unnecessary to replace them with alternating-current motor drives. Besides, replacing the motor entails relatively high costs. 
   Since the trend is towards alternating-current motor drives, it is also hardly rational to specifically develop power electronics systems for use in the control of direct-current drives. In addition, semiconductor components, such as IGBTs, used in alternating-current drives are becoming increasingly efficient and economical. Moreover, it is to be expected that new regulations concerning network harmonics may result in future use of traditional thyristor bridges used in direct-current drives becoming more difficult. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to overcome the drawbacks of prior-art technology and achieve a new type of PWM-controlled DC/DC bridge for a direct-current load, such as a direct-current motor. The solution of the invention is based on a new principle of power electronics topology, wherein it is possible to utilize the technology of alternating-current drives in the control of direct-current motors. In the solution of the invention, a PWM-controlled bridge section consists of two bridge arms, and the semiconductor switches in these bridge arms are turned on alternately in the conducting state. 
   By using the technology of the invention, it is possible to manufacture modern and economical PWM-controlled direct-current motor drives e.g. for elevator applications. By applying the invention, the power electronics technology and components of alternating-current drives manufactured in large production series can be utilized in direct-current drives. The conductor rails and other structures of the power stage circuits can be identical to those used in alternating-current drives. In addition, the use of semiconductor switches, such as IGBTs can be optimized even in direct-current applications. And, in particular, cost savings are achieved in development and production methods as the same methods can be used in both alternating-current and direct-current drives. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described in detail with reference to an example and the attached drawings, wherein: 
       FIG. 1  presents a prior-art direct-current bridge; 
       FIG. 2  presents a network bridge in an alternating-current drive; 
       FIG. 3  presents a direct-current drive according to the invention, comprising a DC/DC bridge according to the invention; 
       FIG. 4  presents a timing diagram for a DC/DC bridge according to the invention; and 
       FIG. 5  illustrates the layout and cooling of the semiconductor switches in the DC/DC bridge of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  presents a prior-art PWM-controlled H-bridge H 1  of a direct-current motor drive intended e.g. for an elevator application. It consists of two bridge arms H 11  and H 12  connected to a direct-current source UDC, each arm containing controllable semiconductor switches S 11 -S 14  for the positive and negative poles, consisting of e.g. an inverse-parallel connection of an IGBT transistor and a diode. The bridge feeds a direct-current load L 1 , such as the direct-current motor of an elevator. 
     FIG. 2  correspondingly presents a network bridge A 1  as used in a prior-art three-phase alternating-current drive to rectify the alternating voltage of the network UAC into a direct voltage UDC, and an alternating-current inductor unit P 1  connected before it. The bridge has in the upper and lower arms controllable semiconductor switches S 21 -S 26 , which are connected to each network phase and which may also consist of e.g. an inverse-parallel connection of an IGBT transistor and a diode. 
   In principle, the electric motor drive of the invention is like a prior-art three-phase alternating-current drive comprising a network converter (rectifier bridge), a motor bridge and inductors. The present invention relates expressly to the DC/DC bridge feeding the motor. The network bridge and the inductors may be e.g. as presented in FIG.  2 . According to the invention, the same type of controllable semiconductor switches can be used in both the rectifier bridge and the motor bridge. 
     FIG. 3  presents a DC/DC bridge B 1  according to the invention, which is used to feed a direct-current motor M 1 , a direct current Idc flowing in the motor. The bridge is controlled by a control unit BC 1 . Magnetization of the motor is accomplished using a separate magnetizing unit MA 1 , which is controlled in a known manner. 
   The DC/DC bridge presented in  FIG. 3  comprises two bridge sections B 11 , B 12 . The first bridge section in the bridge according to  FIG. 3  comprises two arms B 111  and B 112 , each containing controllable semiconductor switch units S 21 -S 24 . The bridge arms are connected to each other on the power stage circuit board via a connector CONN 1 . The second bridge section B 12  consists of a single arm with controllable semiconductor switch units S 25 -S 26 . Thus, the bridge as a whole has three arms, corresponding to a three-phase DC/AC bridge. For each semiconductor switch, a controller BC 21 -BC 26  is provided in the control unit BC 1 . 
   The modulation of the DC/DC bridge of the invention is done as follows: The PWM-controlled direct-current drive of the invention can use the same kind of semiconductor switches S 21 -S 26  for PWM control as are used in an alternating-current drive. The direct-current drive can be selected e.g. by a switch SW 1  in the control unit. Instead of alternating current, a zero-frequency is used (non-rotating field). In the direct-current drive of the invention, not all of the semiconductor switches have to switch a high-frequency PWM control signal, typically of a frequency of a few kHz; two of the switches, S 25  and S 26 , can be used to select the torque direction (up/down), and the switches in question conduct pure DC current. The switches S 21 -S 26  used for PWM control are turned on/off at a frequency of a few kHz to control the magnitude of the current ldc (the torque). 
   The greatest power dissipation occurs in the switches S 21 -S 26  used for PWM control. In the control method of the invention, the switches used for PWM control are turned into conduction alternately in different arms, so the switches in question are only operated for about 50% of normal conduction time or less. This action is visualized in the timing diagram in  FIG. 4 , which represents the conduction times of switches S 21 , S 23  and S 26  as well as the conduction times of diodes S 22  and S 24 . 
   From a thermal point of view, the invention provides the advantage that the power stage of an alternating-current drive can be fully utilized in a direct-current drive. Thermal dissipation in the IGBTs used in the circuit is typically double the dissipation in the switches that are not used in PWM control. Therefore, according to the invention, switches S 21  and S 23  are only kept conducting for at most 50% of the time in a cyclic manner. The conduction period varies according to the voltage (current) required. 
   As compared with the prior-art H-bridge, the circuit additionally provides the advantage that the ripple of the direct current ldc, which produces a disturbing noise, is reduced to about half of the original level. This advantage can be utilized by reducing the switching frequency of the PWM switches S 21 -S 24 , which will diminish thermal dissipation in the switches and increase the load capacity of the bridge. 
   In practice, direct-current switches may have higher dissipation because direct-current losses are slightly greater than switching-PWM losses. Therefore, switches S 25  and S 26  are placed on the air intake side (arrow) of the cooling segment COOL 1  ( FIG. 5 ) as it is cooler than the middle part of the segment or the air exit side, as indicated by the temperature curve TEMP. 
   It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the claims presented below. Besides IGBT, the switches used may also consist of other fully gate-controlled components. Besides a direct-current motor, the load may also consist of e.g. a braking magnet of an elevator. 
   The arrangement of the invention can be used in a wide range of electric drives. By applying the principle of the invention, it will be possible to cover the power range of direct-current elevator drives by using the latest IGBT technology. Alternating-current and direct-current drive modules can be assembled on the same production line, and the application can be selected mainly via software control.