Abstract:
A power converter that can be used in a vehicle equipped with an electrical motor provides power to both the electrical motor and other, auxiliary loads, such as a headlamp, that require a different voltage. The power converter includes a first DC power supply that generates a first voltage equivalent to the voltage required by the auxiliary loads and a second DC power supply, connected in series with the first DC power supply, that generates a differential voltage, where the sum of the differential voltage and the first voltage is the voltage required to drive the electrical motor. A DC-DC converter is connected to the second power supply and converts the differential voltage to the voltage required to drive the auxiliary loads.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method and apparatus for converting DC power to DC power of a different voltage from that of the original one. 
     FIG. 7  shows a first prior art voltage-drop type DC-DC converter  70 . The DC-DC converter  70  converts an input voltage V I  of a DC power supply  71  to an output voltage V O  that is lower than the input voltage V I . When a transistor TR is ON, a voltage V I −V O  is applied to a coil CL. The amount of change in current, ΔIL, when the transistor TR is turned on is expressed by ΔIL={(V I −V O )/L}T on  where L is the inductance of the coil CL and T on  is the ON duration of the transistor TR. When the transistor TR is turned off, a commutation diode D keeps the current flowing across the coil CL. When the transistor TR is turned off, the amount of current change ΔIL is expressed by ΔIL=(V O /L)T off  where T off  is the OFF duration of the transistor TR. When the current continuously flows across the coil CL, both current changes are equal to each other in a steady state. Therefore, the output voltage V O  is {T on /(T on +T off )}V I , which is smaller than the input voltage V I . 
   Other known types of DC-DC converters than the voltage-drop type DC-DC converter  70  include a booster type DC-DC converter and a booster/voltage-drop type DC-DC converter. 
   Recently, hybrid motor vehicles have been put to use in order to improve fuel efficiency and reduce the exhaust gas of motor vehicles. Hybrid vehicles use a running motor when they are started or when they run at a low speed, and use an engine when they run at a middle speed. The operational voltage for various kinds of units, such as a headlight, which a hybrid motor vehicle is equipped, is lower than the operational voltage for the running motor. The conventional hybrid motor vehicles therefore need two power supplies, a high-voltage power supply for the running motor and a low-voltage power supply for the various kinds of units. 
     FIG. 8  shows a prior art high-voltage and low-voltage generating apparatus  80 . The apparatus  80  has an engine  51 , an alternator  52 , a high-voltage battery  53 , a low-voltage battery  56  and a DC-DC converter  54 . The alternator  52  has a three-phase AC generator  52   a  and a three-phase full-wave rectifier  52   b  which are driven by the engine  51 . A high-voltage unit (motor)  55  is connected to the high-voltage battery  53 . The alternator  52  generates high-voltage DC power to charge the high-voltage battery  53 . The DC-DC converter  54  lowers the voltage of the high-voltage battery  53  to charge the low-voltage battery  56  and supplies the lowered voltage to a low-voltage unit  57 . 
     FIG. 9  shows another prior art high-voltage and low-voltage supplying apparatus  90 . The apparatus  90  has two alternators  52  connected to the engine  51 . The two alternators  52  respectively charge the high-voltage battery  53  and the low-voltage battery  56 . 
   The apparatus  80  in  FIG. 8  needs the large-capacity DC-DC converter  54  and two batteries  53  and  56  and thus inevitably is large and heavy. The apparatus  90  in  FIG. 9  is heavy and bulky because of the two alternators  52 . The apparatuses  80  and  90  in  FIGS. 8 and 9  are therefore unfit for hybrid motor vehicles. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a compact power converting apparatus, which stably generates at least two DC voltages, a power generating method, and a vehicle having the power converting apparatus. 
   To achieve the above object, the first aspect of the present invention provides a method of supplying power using a main DC power supply for generating a predetermined voltage to supply a first output voltage substantially equal to the predetermined voltage and a second output voltage lower than the predetermined voltage. The method includes connecting a first DC power supply for generating the same voltage as the second output voltage in series to a second DC power supply for generating a differential voltage between the first output voltage and the voltage from the first DC power supply, thereby forming the main DC power supply, connecting a DC-DC converter to the second DC power supply, and stepping down the voltage output from the second DC power supply to produce the second output voltage by using the DC-DC converter. 
   The second aspect of the present invention provides a power converting apparatus for generating a first output voltage and a second output voltage lower than the first output voltage. The apparatus includes a first DC power supply for generating the same voltage as the second output voltage, a second DC power supply, which is connected in series to the first DC power supply for generating a voltage corresponding to a difference between the first output voltage and the voltage from the first DC power supply, and a DC-DC converter, which is connected to the second DC power supply for converting the voltage from the second DC power supply to the second output voltage. 
   The third aspect of the present invention provides a method of generating a boosted voltage higher than a voltage of a main DC power supply. The method includes producing a differential voltage between a target boosted voltage and the voltage of the main DC power supply using a DC-DC converter, and producing the boosted voltage by adding the differential voltage to the voltage of the main DC power supply. 
   The fourth aspect of the present invention provides a power converting apparatus for generating a predetermined boosted voltage. The power converting apparatus includes a DC power supply, and a DC-DC converter, which is connected to the DC power supply, for producing a differential voltage between the predetermined boosted voltage and a voltage of the DC power supply. The predetermined boosted voltage is provided as a sum of the voltage of the DC power supply and the differential voltage. 
   The fifth aspect of the present invention provides a power converting method of supplying a first output voltage substantially equal to a voltage of a main battery and a second output voltage lower than the voltage of the main battery. The method includes forming the main battery by connecting a first battery for generating the same voltage as the second output voltage in series to a second battery for generating a voltage corresponding to a difference between the first output voltage and the voltage of the first battery, producing the first output voltage by adding the voltages of the first and second batteries, connecting a charge power supply for generating a voltage lower than the voltage of the main battery to an output of a DC-DC converter, producing a differential voltage between the voltage of the main battery and the voltage of the charge power supply using the DC-DC converter, and charging the main battery with a sum of the differential voltage and the voltage of the charge power supply. 
   The sixth aspect of the present invention provides a power converting apparatus for generating a first DC voltage and a second DC voltage lower than the first DC voltage. The apparatus includes a first battery for generating the same voltage as the second DC voltage, a second battery, which is connected in series to the first battery for generating a differential voltage between the first DC voltage and the voltage of the first battery, and a polarity-inverting type DC-DC converter having an input connected to the second battery and an output connected to the first battery. The DC-DC converter includes a first switching element and a first diode connected in parallel to each other, a second switching element connected between the output of the DC-DC converter and the first battery, and a second diode connected in parallel to the second switching element. 
   The seventh aspect of the present invention provides a vehicle having a running motor operable with a predetermined first operational voltage, and a subload operable with a second operational voltage lower than the first operational voltage. The running motor is connected to a main battery assembly for generating the first operational voltage. The battery assembly includes a first battery cell for generating the second operational voltage and a second battery cell, connected in series to the first battery cell, for generating a differential voltage between the first operational voltage and the second operational voltage. The vehicle includes a power converting apparatus, which is connected between the second battery cell and the subload, for converting the voltage of the second battery cell to the second operational voltage and supplying the second operational voltage to the subload. 
   Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
       FIG. 1  is a schematic block diagram of a power converting apparatus according to a first embodiment of the present invention; 
       FIG. 2  is a schematic block diagram of the power converting apparatus in  FIG. 1 , showing the flows of currents when a transistor is off; 
       FIG. 3  is an waveform diagram showing a pulse signal for driving the transistor; 
       FIG. 4  is a schematic block diagram of a power converting apparatus according to a second embodiment of the present invention; 
       FIG. 5  is a schematic block diagram of the power converting apparatus in  FIG. 4 , which is being charged; 
       FIG. 6  is a schematic block diagram of a power converting apparatus according to a third embodiment of the present invention; 
       FIG. 7  is a first prior art circuit diagram of a voltage-drop type DC-DC converter; 
       FIG. 8  is a schematic block diagram of a prior art double power-supply system for a vehicle; and 
       FIG. 9  is a schematic block diagram of another prior art double power-supply system for a vehicle. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   A power converting apparatus  100  according to a first embodiment of the present invention will now be described referring to  FIGS. 1  to  3 . 
   As shown in  FIG. 1 , the power converting apparatus  100  includes a battery assembly  1  and a DC-DC converter  5 . 
   The battery assembly  1 , which is a DC power supply for a vehicular driving motor, has a first battery cell  1   a  and a second battery cell  1   b  connected in series. The first battery cell  1   a  generates a voltage that is the same as a desired low-voltage DC output. The second battery cell  1   b  generates a differential voltage between the high-voltage DC output and the output voltage of the first battery cell  1   a . The voltage of the desired low-voltage DC output is substantially equal to the operational voltage (12V) for low-voltage units (e.g., a headlight  6 ) of a vehicle. The voltage of the high-voltage DC output is substantially equal to the operational voltage (36V) for high-voltage units (e.g., a running motor  4 ). The battery assembly  1  has a 36-V output terminal  1   d  and a 12-V intermediate terminal or tap  1   c . The charge voltage of the battery assembly  1  is 36V and the charge voltage at the intermediate tap  1   c  is 12V. 
   An alternator  3  includes a generator  3   a  and a full-wave rectifier  3   b  that are driven by an engine  2 . While the engine  2  is running, the alternator  3  charges the battery assembly  1  with a voltage of 36V. The running motor  4  is connected to the battery assembly  1 . 
   The DC-DC converter  5 , which is a polarity-inverting type or buck boost type, is connected to the battery assembly  1 . The headlight  6  is connected to the battery assembly  1  via the DC-DC converter  5 . The running motor  4  is connected between the battery assembly  1  and the DC-DC converter  5 . 
   The DC-DC converter  5  has a switching element or transistor TR 1 , an inductor L 1 , a fly-wheel diode D 1 , a current sensor CS 1 , a control circuit  7  and capacitors C 1  and C 2 . The transistor TR 1  is preferably MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The transistor TR 1  is connected in series to the inductor L 1 . That is, the transistor TR 1  has a drain connected to a positive terminal  1   d  (36-V terminal) of the battery assembly  1  and a source connected to the inductor L 1 . The fly-wheel diode D 1  is located between a ground terminal or 0-V terminal and a node between the transistor TR 1  and the inductor L 1 . The capacitor C 1  is located between the 36-V terminal  1   d  and the 12-V terminal  1   c  of the battery assembly  1 . The capacitor C 2  is located between the 12-V terminal  1   c  of the battery assembly  1  and the 0-V terminal. 
   The transistor TR 1  and the current sensor CS 1  are connected to the control circuit  7 . Also connected to the control circuit  7  are a first voltage sensor  8  for detecting an output voltage of the running motor  4  and a second voltage sensor  9  for detecting an output voltage V O  of the headlight  6 . The control circuit  7  controls the ON/OFF switching of the transistor TR 1  with a high frequency in such a way that the ratio of the detected voltage of the first voltage sensor  8  to the detected voltage of the second voltage sensor  9  becomes 3:1. This makes it possible to keep the ratio of the output voltage to the running motor  4  to the output voltage to the headlight  6  at 3:1. 
   The control circuit  7  incorporates a comparator  7   a  and a triangular wave oscillator  7   b . The comparator  7   a  compares the difference between the detected voltages of the first and second voltage sensors  8  and  9  and the output of the triangular wave oscillator  7   b  and generates a transistor drive pulse signal (see  FIG. 3 ) according to the comparison result. The control circuit  7  supplies the transistor drive pulse signal to the transistor TR 1 . The transistor drive pulse signal controls an ON time T on  and OFF time T off  of the transistor TR 1 . When the ratio of the ON time T on  of the transistor TR 1  to the OFF time T off  is 1:2, for example, the ratio of the detected voltage of the first voltage sensor  8  to the detected voltage of the second voltage sensor  9  is 3:1. When the ratio of the detected voltages of the first and second voltage sensors  8  and  9  is shifted from 3:1, the control circuit  7  changes the ON time T on  and the OFF time T off  to correct for the deviation. 
   The operation of the power converting apparatus  100  will now be discussed. 
   The DC-DC converter  5  has input terminals that are the 36-V terminal  1   d  and the 12-V terminal  1   c  of the battery assembly  1 , and 12V of the battery assembly  1  is a ground voltage in the DC-DC converter  5 . Therefore, the input voltage V I  of the DC-DC converter  5  is 24V. The output voltage V O  of the DC-DC converter  5  is −12V because the 12-V intermediate tap  1   c  is taken as a reference. 
   When the transistor TR 1  is turned on, the current flows as indicated by an arrow A in FIG.  1 . This causes the inductor L 1  to store the power that is supplied from the second battery cell  1   b . Irrespective of the switching of the transistor TR 1 , the capacitor C 2  is charged with the current from the first battery cell  1   a  and the current is supplied to the headlight  6  from the capacitor C 2 . 
   When the transistor TR 1  is turned off while the current is flowing in the inductor L 1 , the diode D 1  keeps the current flowing through the inductor L 1 , the fly-wheel diode D 1  is set on to keep this current. Then, the power stored in the inductor L 1  is supplied to the headlight  6  (an arrow C in  FIG. 2 ) as a low-voltage DC output. 
   Therefore, the power that drives the headlight  6  is supplied from both the DC-DC converter  5  and the first battery cell  1   a . When the current supplied to the headlight  6  is 100 A, for example, a current of 67 A from the DC-DC converter  5  and a current of 33 A from the battery cell  1   a  are both supplied to the headlight  6 . 
   When the power converting apparatus  100  is in a steady state, the output voltage V O  and an output current I O  are expressed by the following equations.
 
 V   O =( T   on   /T   off ) V   I 
 
 I   O =( V   I   T   on ) 2 /{2 L ( T   on   +T   off ) V   O }
 
   The control circuit  7  controls the ON time T on , and OFF time T off  in such a way as to set V O =−V I /2, so that ⅔ of the total power supplied to the headlight  6  is supplied from the DC-DC converter  5  and the remaining ⅓ is supplied from the battery assembly  1 . Specifically, the control circuit  7  monitors the output current I O  detected by the current sensor CS 1  and controls the ON time T on  and the OFF time T off  in such a manner that the ratio of the voltage supplied to the running motor  4  to the supply voltage V O  to the headlight  6  becomes 3:1. 
   When a current of 33 A or larger is supplied from the battery cell  1   a  (overloaded state), for example, the ratio of the detected voltages of the voltage sensors  8  and  9  is shifted from 3:1. In this case, after the overloaded state is released, the control circuit  7  controls the ON time T on  and the OFF time T off  in such a manner that the ratio of the detected voltages of the voltage sensors  8  and  9  becomes 3:1. 
   If there is no variation in the load of the headlight  6 , the ratio of the ON time T on  to the OFF time T off  does not change. Because the load of the headlight  6  frequently varies, however, the control circuit  7  controls the ON time T on  and the OFF time T off  based on detection signals from the first and second voltage sensors  8  and  9  in such a way that the ratio of the detected voltages of the voltage sensors  8  and  9  becomes 3:1. 
   As shown in  FIG. 3 , the voltage of the triangular wave signal of the triangular wave oscillator  7   b  periodically changes. The comparator  7   c  generates a pulse signal whose level goes to high when a comparison voltage Vc based on the difference between the detection signals of the first and second voltage sensors  8  and  9  is smaller than the voltage of the triangular wave signal and goes to low when the comparison voltage Vc is greater than the voltage of the triangular wave signal, and sends the pulse signal to the transistor TR 1 . The transistor TR 1  is turned on when the pulse signal has a high level, and is turned off when the pulse signal has a low level. When the comparison voltage Vc is equal to a predetermined value Vs, the ratio of the detected voltages of both voltage sensors  8  and  9  is 3:1. At this time, the control circuit  7  outputs a pulse signal whose ratio of the ON time T on  to the OFF time T off  is 1:2. When the ratio of the detected voltages of both voltage sensors  8  and  9  is larger than 3:1, the detected voltage of the first voltage sensor  8  is relatively large and the comparison voltage Vc is larger than the value Vs. At this time, the ON time T on  is controlled to be shorter. When the ratio of the detected voltages of both voltage sensors  8  and  9  is smaller than 3:1, the comparison voltage Vc is smaller than the value Vs. At this time, the ON time T on  is controlled to be longer. By adjusting the ON time T on  of the switching element using the triangular wave signal, the control circuit  7  controls the switching of the transistor TR 1  in such a way that the ratio of one output voltage to the other coincides with a target value. 
   The capacitors C 1  and C 2  smooth the current from the battery assembly  1 . When the capacitance of the transistor TR 1  is relatively large, the capacitor C 1  can be eliminated. 
   The power converting apparatus  100  of the first embodiment has the following advantages. 
   The DC-DC converter  5  does not directly step down the high voltage of 36V of the battery assembly  1  to the predetermined voltage of 12V, but steps down the output voltage of 24V of the second battery cell  1   b  to the predetermined voltage of 12V. Since the DC-DC converter  5  needs a small capacity, the DC-DC converter  5  can be made compact, which makes the power converting apparatus  100  compact. 
   The battery assembly  1  has the first battery cell  1   a  that outputs a voltage of 12V equal to the voltage of the low-voltage DC output, the second battery cell  1   b  that outputs a voltage of 24V or the difference between the voltage of the high-voltage DC output and the output voltage of the first battery cell  1   a , and the intermediate tap  1   c . It is therefore possible to easily secure the layout space for the first and second battery cells  1   a  and  1   b.    
   As the input terminal of the DC-DC converter  5  is connected to the second battery cell  1   b  and the output terminal to the first battery cell  1   a , the power converting apparatus  100  has a simple structure. 
   Being compact and simple in structure, the power converting apparatus  100  is suitable for use in a vehicle. 
   Because a part of the output of the DC-DC converter  5  is used to charge the first battery cell  1   a  when the discharge capacity of the first battery cell  1   a  drops down to or below a predetermined value, discharging the first battery cell  1   a  alone is suppressed. 
   Second Embodiment 
   A power converting apparatus  110  according to a second embodiment of this invention will now be described referring to  FIGS. 4 and 5 . The power converting apparatus  110  steps down or boosts the supply voltage. The power converting apparatus  110  has a second transistor (MOSFET) TR 2  in place of the fly-wheel diode D 1  used in the first embodiment. 
   Each of the first and second transistors TR 1  and TR 2  has a parasitic diode between its source and drain as indicated by dotted lines in FIG.  4 . Therefore, the use of MOSFETs for the first transistor TR 1  (switching element) and the second transistor TR 2  is equivalent to the use of a parallel circuit of a switching element and a diode. When the second transistor TR 2  is kept off, the DC-DC converter  5  functions the same as the first embodiment. 
   In the power converting apparatus  110 , therefore, the second transistor TR 2  is normally kept off and the first transistor TR 1  is switched on or off. When the output voltage of the battery assembly  1  falls below a predetermined voltage, an additional DC power supply  10  is connected to the output terminal of the DC-DC converter  5  as shown in  FIG. 5  to charge the battery assembly  1 . The additional DC power supply  10  can have the same output voltage as the output voltage of the first battery cell  1   a.    
   At the time of charging the battery assembly  1 , the first transistor TR 1  is kept off and the second transistor TR 2  is switched on and off. In this case, the DC-DC converter  5  serves as a booster type DC-DC converter. 
   The output voltage of the DC power supply  10  or the input voltage to the DC-DC converter  5  in a boost mode is expressed by V I 2 and the output voltage of the DC-DC converter  5  is expressed by V O 2. The voltage that is applied to the inductor L 1  when the second transistor TR 2  is on is V I 2 while the voltage that is applied to the inductor L 1  when the second transistor TR 2  is off is (V O 2−V I 2). When the current continuously flows across the inductor L 1 , therefore, the amount of a change in the current flowing across the inductor L 1  during the ON time T on  is substantially equal to the amount of a change in the current flowing across the inductor L 1  during the OFF time T off . This is shown in the following equation.
 
( V   I 2 /L ) T   on ={( V   O 2 −V   I 2)/ L}T   off 
 
Thus,
 
 V   O 2={( T   on   +T   off )/ T   off   }V   I 2.
 
   The control circuit  7  controls the ON/OFF switching of the second transistor TR 2  in such a way that the ratio of V O 2 to V I 2 becomes 2:1. In other words, the control circuit  7  controls the second transistor TR 2  in such a way that the ratio of the difference (24V) between the charge voltage of 36V of the battery assembly  1  and the output voltage of 12V of the DC power supply  10  to the output voltage of 12V of the DC power supply  10  is maintained at 2:1. As a result, the battery assembly  1  is charged with a voltage of 36V, which is the output voltage of 12V of the DC power supply  10  plus the boosted output voltage of 24V of the DC-DC converter  5 . 
   The second embodiment therefore has the advantages of the first embodiment and the following additional advantages. 
   The DC-DC converter  5  outputs a voltage that is the output voltage (12V) of the DC power supply  10  subtracted from the charge voltage (36V) of the battery assembly  1 , and this output voltage (24V) is added to the output voltage (12V) of the DC power supply  10 . The battery assembly  1  can be charged with the resultant voltage (36V). This means that the battery assembly  1  can be charged using the battery installed in another vehicle which has only the conventional battery for low-voltage units. 
   The power converting apparatus  110  can be constructed by providing the polarity-inverting type DC-DC converter  5  with the transistor TR 1 , which has a switching element and a diode connected in parallel, and the transistor TR 2 , which also has a switching element and a diode connected in parallel and which is used in place of the fly-wheel diode D 1 . 
   The transistors (MOSFETs) TR 1  and TR 2  serve as diodes when they are turned off in a step-down mode or a boost mode. Therefore, the structure of the power converting apparatus  110  is simpler than that of the power converting apparatus that has a parallel circuit of a switch element and a diode. 
   Third Embodiment 
   A power converting apparatus  120  according to a third embodiment of this invention will now be described referring to FIG.  6 . The power converting apparatus  120  has an insulated DC-DC converter (fly-back converter)  13  that has a transformer capability. 
   As shown in  FIG. 6 , the fly-back converter  13  is connected to the second battery cell  1   b , so that the output voltage (24V) of the second battery cell  1   b  is applied to the fly-back converter  13 . The headlight  6  is connected to the output terminal of the fly-back converter  13  and the first battery cell  1   a.    
   When a transistor TR is on, electric energy is stored in a transformer T. When the transistor TR is off, on the other hand, the electric energy stored in the transformer T is discharged. Given that the number of turns of the primary winding of the transformer T is denoted by n 1  and the number of turns of the secondary winding is denoted by n 2 , the following equation is satisfied when the secondary current continuously flows.
 
 V   O =( n   2 / n   1 ) ( T   on   /T   off ) V   I 
 
   The control circuit  7  controls the ON/OFF action of the transistor TR in such a way that the output voltage V O  of the fly-back converter  13  coincides with the operational voltage of 12V for the headlight  6 . 
   The third embodiment has the following additional advantages. 
   The use of the insulated DC-DC converter  5  permits the power converting apparatus  120  to be used as a switching power supply that must provide electric insulation between the input side device and the output side device. 
   As the power converting apparatus  120  has the insulated fly-back converter  13 , it has a simpler structure than one that has a forward converter. 
   The first to third embodiments may be modified as follows. 
   Instead of using the battery assembly  1  that has the first battery cell  1   a  and the second battery cell  1   b , the first battery cell  1   a  and the second battery cell  1   b  may be arranged separately. 
   In the first to third embodiments, the control circuit  7  that performs analog control of the ratio of the ON time of the transistor TR 1  to the OFF time thereof may be replaced with a control unit that has a CPU. In this case, the CPU computes the ratio of the ON time of the transistor TR 1  to the OFF time thereof based on the detection signals of the first and second voltage sensors  8  and  9 . Preferably, the ON/OFF switching of the transistor TR 1  is controlled using PWM based on this computed ratio. Available as this CPU is a CPU that is used in an apparatus other than the power converting apparatus  100 ,  110  or  120 . 
   The first embodiment may use current sensors which respectively detect the amount of the current flowing across the inductor L 1  and the amount of the current coming back to the battery cell  1   a  from the headlight  6 . In this case, the control circuit  7  controls the transistor TR 1  in such a manner that the ratio of the value of the two currents detected by the current sensors becomes a predetermined value (e.g., 2:1). In the second and third embodiments, the currents may be detected instead of the voltages. In this case too, the control circuit  7  controls the transistor based on the detected current values. 
   In the second embodiment, a parallel circuit of a bipolar transistor and a diode may be provided in place of the transistors (MOSFETs) TR 1  and TR 2 . 
   Instead of the MOSFET and the bipolar transistor, for example, other switching elements, such as an SIT (Static Induction Transistor) and a thyristor, may be used. 
   Instead of the running motor  4 , another unit may be connected to the power converting apparatus  100 ,  110  or  120 . The power converting apparatus  100 ,  110  or  120  may be adapted for use in a vehicle that is not equipped with the running motor  4  or may be adapted for use in a battery-powered vehicle which does not have an engine. 
   The power converting apparatus  100 ,  110  or  120  may be adapted for use in other apparatuses and equipment than a vehicle. 
   The fly-back converter  13  may be replaced with another type of DC-DC converter which has the transformer T. 
   A booster type insulated converter whose transformer T has a different turn ratio of the primary winding to the secondary winding from that of the transformer T of the fly-back converter  13  may be used. 
   It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.