1. A discharge device for charging a main capacitor via a separately-excited DC/DC converter and discharging energy from the main capacitor, the discharge device comprising a pulse width control circuit for controlling a conduction pulse width on a primary side of the separately-excited DC/DC converter,wherein the pulse width control circuit is configured to perform a PWM soft start drive of stepwise increasing the conduction pulse width on the primary side of the separately-excited DC/DC converter up to a maximum pulse width.

2. A discharge device for charging a main capacitor via a separately-excited DC/DC converter and discharging energy from the main capacitor, the discharge device comprising a pulse width control circuit for controlling a conduction pulse width on a primary side of the separately-excited DC/DC converter,wherein the pulse width control circuit is configuredto perform a PWM soft start drive of stepwise increasing the conduction pulse width on the primary side of the separately-excited DC/DC converter up to a maximum pulse width in a case where the main capacitor has a voltage less than a predetermined low voltage at the time of charging the main capacitor, andto perform a PWM drive with the maximum pulse width when the main capacitor has a voltage not less than the predetermined low voltage at the time of charging the main capacitor.

3. A discharge device for charging a main capacitor via a separately-excited DC/DC converter and discharging energy from the main capacitor, the discharge device comprising a pulse width control circuit for controlling a conduction pulse width on a primary side of the separately-excited DC/DC converter,wherein the pulse width control circuit is configuredto perform a PWM soft start drive of stepwise increasing the conduction pulse width on the primary side of the separately-excited DC/DC converter up to a maximum pulse width when the main capacitor has a voltage less than a predetermined low voltage at the time of charging the main capacitor,to perform a PWM drive with the maximum pulse width when the main capacitor has a voltage not less than the predetermined low voltage at the time of charging the main capacitor, andto perform the PWM drive with the maximum pulse width by detecting that the main capacitor reaches a predetermined voltage and terminating the PWM soft start drive in course of performing the PWM soft start drive of stepwise increasing a conduction pulse width on the primary side of the separately-excited DC/DC converter up to a maximum pulse width when the main capacitor has a voltage less than the predetermined low voltage at the time of charging the main capacitor.

4. The discharge device according to claim 2, wherein the pulse width control circuit includesa triangular-wave voltage generation circuit for generating a triangular-wave voltage at a predetermined repetition frequency;a soft-start voltage generation circuit for generating a soft-start voltage that increases with the lapse of time after starting the charging operation;a comparator for comparing the triangular-wave voltage with the soft-start voltage and outputting a signal with a duty ratio that gradually increases a conduction time at a single frequency with the time elapsed from the start of the charging operation;a soft-start voltage canceling circuit for canceling a soft start operation by detecting that a terminal voltage of the main capacitor increases to a preset voltage; anda logic circuit for charging the main capacitor on the basis of an output of the comparator until the main capacitor is fully charged and terminating the charging operation when detecting that the main capacitor is fully charged.

5. The discharge device according to claim 4, wherein the pulse width control circuit further includes a primary-coil overcurrent detection circuit for detecting that overcurrent flows on the primary side of the separately-excited DC/DC converter and turning off the logic circuit.

6. An application specific integrated circuit constituting the discharge device according to claim 4, the application specific integrated circuit comprising:a triangular-wave voltage generation circuit for generating a triangular-wave voltage at a single frequency;a soft-start voltage generation circuit for generating a soft-start voltage that increases with the lapse of time after starting the charging operation;a comparator for comparing the triangular-wave voltage with the soft-start voltage and outputting a signal with a duty ratio that gradually increases a conduction time at a single frequency with the time elapsed from the start of the charging operation;a soft-start voltage canceling circuit for canceling a soft start operation by detecting that a terminal voltage of the main capacitor increases to a preset voltage; anda logic circuit for charging the main capacitor on the basis of an output of the comparator until the main capacitor is fully charged and terminating the charging operation when detecting that the main capacitor is fully charged.

7. The application specific integrated circuit according to claim 6, further comprising a primary-coil overcurrent detection circuit for detecting that overcurrent is generated on the primary side of the separately-excited DC/DC converter and turning off the logic circuit.

8. The application specific integrated circuit according to claim 6, further comprising an external connection terminal connected to a time-constant determining element of at least one of the triangular-wave voltage generation circuit and the soft-start voltage generation circuit.

9. A discharge device for charging a main capacitor via a separately-excited DC/DC converter and discharging energy from the main capacitor, the discharge device comprising a pulse width control circuit for controlling a conduction pulse width on a primary side of the separately-excited DC/DC converter,wherein the pulse width control circuit is configuredto perform a PWM soft start drive of stepwise increasing the conduction pulse width on the primary side of the separately-excited DC/DC converter up to a maximum pulse width andto determine that the separately-excited DC/DC converter is in an overload condition and to perform the pulse width control by the use of an intermittent drive of alternately performing a charging operation and a no-charging operation on the main capacitor when a charging voltage of the main capacitor does not reach a predetermined voltage even with the continuous PWM soft start drive.