Control apparatus for DC/DC converter

A control apparatus controlling a DC/DC converter. The control apparatus switches on and off switching devices to transfer the electric power from a main storage battery to an auxiliary battery. The control apparatus includes a control circuit controlling the switching on and off, a voltage monitor detecting the over and under voltage of the main storage battery, and a gate drive circuit, including a driving transformer, driving the switching devices. The control apparatus utilizes the large current, which flows on the control circuit side of the driving transformer by short-circuiting the output side of the transformer in response to the detected over or under voltage, to transfer the isolated signal indicating the main storage battery voltage to the control circuit side. The control apparatus is more reliable than conventional control apparatuses, which employ a photo-coupler to transfer the isolated signal to the control circuit side.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the invention will be explained in detail hereinafter with reference to the accompanying drawings which illustrate the preferred embodiments of the invention. FIG. 1 is a block circuit diagram of a DC/DC converter according to a first embodiment of the invention. FIG. 2 is a block circuit diagram of the gate drive circuit used in the DC/DC converter according to the first embodiment of the invention. FIG. 3 is a circuit diagram of the voltage monitor used in the DC/DC converter according to the first embodiment of the invention. Referring now to FIG. 1 , the DC/DC converter according to the first embodiment of the invention does not include the photo-coupler used for the conventional DC/DC converters shown in FIGS. 8 and 9 . Referring now to FIG. 2 , the gate drive circuit 5 for the DC/DC converter according to the first embodiment includes a current detector Ri, a comparator 51 , a latch circuit 52 , a switching device Q 3 , and a gate driver 53 . Referring now to FIG. 3 , the voltage monitor 7 for monitoring the state of the voltage Vm of the main storage battery 2 includes an under voltage detecting section 71 and an over voltage detecting section 72 . The voltage monitor 7 outputs an output G 4 at a low level (hereinafter referred to as an “L-level”) when the voltage Vm is within a proper range to make a switching device Q 4 in FIG. 2 open. The voltage monitor 7 outputs an output G 4 at a high level (hereinafter referred to as a “H-level”) when the voltage Vm is outside the proper range, that is when the voltage Vm is an over voltage or an under voltage, to short-circuit the switching device Q 4 in FIG. 2 . When the voltage Vm is within the proper range, the current of the driving transformer on the control side in FIG. 1 converges to a small value la after the gate current feed has finished. The current detector Ri detects the voltage Va&equals;Ri×la. When the voltage Vm is an over voltage or an under voltage, the secondary winding S 1 of the driving transformer is short-circuited by the switching device Q 4 . Therefore, the current of the driving transformer takes a large value lb transiently, and the current detector Ri detects the voltage Vb&equals;Ri×lb. Thus, the state of the voltage Vm of the main storage battery 2 is detected on the control side by setting the reference value Vs of the comparator 51 such that Va<Vs<Vb. As the primary side of the driving transformer is driven, a little voltage is induced in the secondary winding S 2 even when the secondary winding S 1 is short-circuited. By employing a MOSFET, the gate voltage thereof being higher than the induced voltage, for the switching device Q 2 , it becomes unnecessary to add a specific circuit to the gate drive circuit on the side of the secondary winding S 2 to prevent switching device Q 2 from being controlled to switch on if the voltage induced in secondary winding S 2 exceeds the threshold voltage of Q 2 . When an appropriate MOSFET is not found, an appropriate additional circuit is provided to the gate drive circuit on the side of the secondary winding S 2 . The latch circuit 52 is formed, for example, of a flip-flop. When the comparator 51 detects an over voltage or an under voltage, the latch circuit 52 latches the over voltage or the under voltage. In the power supply as described above, the gate driver 53 usually feeds gate drive signals to the switching device Q 3 to make the switching device Q 3 repeat switching on and off in a short time. Even when the secondary side of the driving transformer is short-circuited, the driving current from the driving transformer quickly decreases, although the driving current increases transiently. Therefore, there remains a certain possibility that the over voltage and the under voltage of the voltage Vm is not detected at a proper time. The latch circuit 52 is disposed to detect the over voltage and the under voltage of the voltage Vm without fail. By stopping the control of the DC/DC converter as soon as the latch circuit 52 works, the control of the DC/DC converter is stopped at the drive thereof immediately after an over voltage or an under voltage occurs. For confirming on the control side whether the over voltage state or the under voltage state is continuing or not, the latched state of the latch circuit 52 is reset to return the control of the DC/DC converter to the driving state again. If the over voltage state or the under voltage state is still continuing, a voltage exceeding the reference value Vs is detected again on the control side, since the short-circuit of the switching device Q 4 is continuing, and the control of the DC/DC converter is stopped. Since any drive signal is not fed to the switching devices Q 1 and Q 2 in this occasion, the switching devices Q 1 and Q 2 keep the OFF-state thereof. Therefore, it is not necessary to consider the fault of the switching devices Q 1 and Q 2 caused by the surge voltage generated when the switching devices Q 1 and Q 2 switch from the ON-state to the OFF-state thereof. Since the latch circuit 52 does not work after the voltage Vm has returned to the proper range, the driving control of the DC/DC converter is resumed. FIG. 4 is a block circuit diagram of a gate drive circuit according to a second embodiment of the invention. The gate drive circuit according to the second embodiment is used for the DC/DC converter, which is configured as shown in FIG. 9 . Since the gate drive circuit according to the second embodiment is different from the gate drive circuit shown in FIG. 2 only in that the gate drive circuit in FIG. 4 does not include the secondary winding S 2 , the descriptions of the gate drive circuit according to the second embodiment are omitted. FIG. 5 is a block circuit diagram of a gate drive circuit according to a third embodiment of the invention. The gate drive circuit shown in FIG. 5 is used for the DC/DC converter shown in FIG. 1 . FIG. 6 is a circuit diagram of a voltage monitor used for the gate drive circuit shown in FIG. 5 . Referring now to FIG. 5 , the gate drive circuit according to the third embodiment includes a switching element Q 5 disposed on the side of the secondary winding S 2 of the driving transformer shown in FIG. 2 . The circuit including the switching element Q 5 is an additional circuit, which considers the case in which a voltage high enough to drive the switching device Q 2 is induced in the secondary winding S 2 , even when the secondary winding S 1 is short-circuited under the state of over voltage and under the state of under voltage. In the configuration as shown in FIG. 5 , the secondary winding S 2 is also short-circuited by the output G 5 from the voltage monitor shown in FIG. 6 when an over voltage or an under voltage occurs. Switching element Q 4 in FIG. 5 is controlled by the control signal G 4 output from the voltage monitor 7 of FIG. 3 , whereas switching element Q 5 in FIG. 5 is controlled by the control signal G 5 output from the voltage monitor 7 of FIG. 6 . FIG. 7 is a block circuit diagram of a gate drive circuit according to a fourth embodiment of the invention. The gate drive circuit according to the fourth embodiment considers the voltage induced in the secondary winding S 2 . Generally, the voltage induced in the secondary winding S 2 when the switching device Q 4 is open-circuited is higher than the voltage induced in the secondary winding S 2 when the switching device Q 4 is closed. The gate drive circuit according to the fourth embodiment of the invention utilizes the magnitude of the induced voltage. In detail, the gate drive circuit according to the fourth embodiment short-circuits the switching device Q 5 when the voltage induced in the secondary winding S 2 is high. The gate drive circuit according to the fourth embodiment opens the switching device Q 5 when the voltage induced in the secondary winding S 2 is low. Since the gate drive circuit according to the fourth embodiment generates the gate signal for the switching device Q 5 , not from the voltage monitor shown in FIG. 6 , but from the signal from the secondary winding S 2 , the size of the gate drive circuit according to the fourth embodiment is smaller than the size of the gate drive circuit according to the third embodiment. Switching element Q 5 is turned on automatically when the voltage induced in secondary winding S 2 is high, which voltage is changeable in accordance with the open-close situation of switch Q 1 . Thus, the gate drive circuit of FIG. 7 differs from FIG. 5 in that both a diode and the voltage monitor of FIG. 6 are not included, which makes it possible to achieve remarkable size reduction. The invention is applicable not only to detecting the state of the isolated voltage of the half-bridge-type DC/DC converter as shown in FIG. 1 or the DC/DC converter having one switching device as shown in FIG. 9 , but also to detecting the state of the isolated voltage of a full-bridge-type DC/DC converter or an inverter. The gate drive circuit for the half-bridge-type DC/DC converter includes one driving transformer. The gate drive circuit for the full-bridge-type DC/DC converter or for the inverter includes a plurality of driving transformers corresponding to the number of switching devices included in the full-bridge-type DC/DC converter or in the inverter. When the over voltage and the under voltage of the foregoing second power supply (main storage battery 2 ) is detected, the over voltage or the under voltage of the main storage battery 2 may be transmitted to the control side, which is isolated from the side of the DC/DC converter having the main storage battery 2 , by short-circuiting the secondary sides of the transformers or by short-circuiting the secondary side of one of the transformers. Since the control apparatus for controlling a DC/DC converter according to the present invention transfers the isolated data signal using a conventional driving transformer plus a few semiconductor devices added to the driving transformer, the control apparatus according to the present invention is manufactured for the same or less cost than the manufacturing cost of the conventional control apparatus which employs a photo-coupler. Since the constituent elements of the present control apparatus deteriorate with age slower than the photo-coupler used in the conventional control apparatus, the control apparatus according to the present invention exhibits improved reliability. The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.