DC/DC CONVERTER

Abstract: A DC/DC converter is provided for generating an output voltage which is higher than the input voltage, and which output voltage charges an energy store (ER). The energy store and the convertability of the converter may be tested by monitoring the pumping phase and the discharging phase of the converter, through accurate monitoring of the output voltage provided by a predetermined load (RL) as it varies within a range of values which are greater than the maximum input voltage (UBATT MAX).

Description: DC/DC Convert ez State of the Art The present invention relates to a DC/DC converter of the kind described in the precharacterising part of claim 1. Such a device is already known for use in connection with an air bag protection device in a motor vehicle. The energy store is used for providing energy to control the firing of one or more air bag gas generators for inflating the air bag or bags in the event of loss of battery power during a collision condition and it is advantageous to test the energy storage and conversion capabilities of the DC/DC converter. However, existing systems only determine a minimum energy content or do not periodically test the energy reserves, or they can only test the energy reserve by the use of additional circuitry, which thus leads to additional expense. For example, for testing purposes, the energy reserve is decoupled from the converter by a transistorized stage. Known air bag systems also use integrated switching regulators, or regulate the duty ratio and/or the frequency of the converter by means of an operational amplifier, which lead to an increase in circuitry and expense. However, this provision is inappropriate for air bag systems since regulation to a constant voltage at the output of the converter is not necessary for arbitrary load conditions and input voltages. It is therefore an object of the present invention to provide a DC/DC converter with minimal circuitry for charging and energy storage, in which the available stored energy and convertability of the converter may be periodically tested to a high degree of accuracy. Advantages of the Invention The aforegoing object is achieved by adopting the feature set forth in the characterising part of claim 1. This has the advantage that the overall function of the entire DC/DC conversion operation can be monitored and assessed as required, since, the following inequality has been established for the particular converter/energy storage capacitor combination, namely, that the minimum output voltage is greater than the maximum battery input voltage. By means of the features of the sub-claims, a DC/DC converter can be achieved in which a single comparator and constant load provide for the accurate measurement of the time period during which the converter is in its pumping phase, and also the time period in which the energy store discharges through the constant load, as determined by the values of the reference voltage at the comparator. Further, a microprocessor, which may be provided for the aforementioned measurement, may also provide a control signal for switching the converter from a discharging phase to a pumping phase, and then measuring the relevant time parameters of the following complete discharge phase. The measured time values may be used by the microprocessor to calculate the required values of the energy content of the energy store and the efficiency of the converter. Drawings The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 is a circuit diagram of one embodiment of a DC/DC converter in accordance with the present invention; and Figs. 2a to 2d show the voltage characteristic at different locations of the circuit of Fig. 1 during operation. Description of the Exemplary Embodiment The circuitry of Fig. 1 comprises a DC/DC step up converter, commonly used in connection with an air bag protection device, which operates on the known conversion principle of so-called "flyback regulation" wherein energy is stored in a coil coupled to a DC voltage supply and a higher voltage is generated by periodically shorting the coil to ground by means of a controlled power stage. As illustrated in Fig. 1, the DC voltage supply is provided by a battery voltage UJJA TT which is supplied to an inductive energy storing device ■ 10 which comprises a coil L in parallel with a "snubber" formed by R*-C-. The output of the inductive energy storing device 10 is connected at B to a power output stage 1 which comprises a field effect transistor (FET) T and a "snubber" formed by C 2 R 2 - An astable multivibrator 18 with open collector, periodically supplies a control signal to the gate of the FET T so that point B is periodically connected through the FET T, to ground, as required in accordance with "flyback regulator" operating principle outlined hereinbefore. The increased DC voltage at B charges an energy storage capacitor E R| which may be used to supply the air bag control circuitry via output A, with its desired stepped up voltage in the event of loss of battery power. The energy storage capacitor E R is also connected to the battery power supply by way of an auxiliary charging circuit 12, commonly provided in safety systems. The circuitry of Fig. 1 further comprises a regulating and monitoring switching circuit 16 included for the purpose of testing the converter and the energy store Ep, and which is realized by means of a single comparator K 2 • The converter of Fig. 1 operates as follows, with particular reference to the characteristics of Figs. 2a to 2d; The astable multivibrator 18 operates with a fixed duty ratio and a fixed frequency, and the converter is normally operated in the non-activated range of the overall system, such that, apart from a load represented by R^, no additional load is fed to the control circuitry via output A. The characteristic of the output voltage Ugp across the load R^ and apparent at output A is illustrated in Fig. 2b and varies between the limits U ER MAχ and ϋ ER MIN * The straight lines of the characteristic of Fig. 2b are intended only to show the principle of the change in output voltage Ugg in the time periods T j and T 1 (shown in Figs. 2b and 2c), and not the exact characteristic with respect to time. The converter is in pumping operation when the output voltage U ER is in the range X to Y illustrated in Fig. 2b. As such, the astable multivibrator 18 is periodically providing a control voltage to the gate of T and thus point B is periodically grounded, in accordance with the "flyback regulator" principle. Energy is therefore taken from U BATT and, through intermediate storage in the coil L, is supplied to the energy storage capacitor Ep. In this converter pumping phase, the output Up--, which is derived from the output of the comparator K 2 , exhibits a high signal as illustrated in Fig. 2c. The auxiliary charging circuit 12 assists this operation in accordance with the range of the input voltage at U RAT T > which range is illustrated in Fig. 2a. For the particular converter/energy store combination, the respective values of U ER MjN and U A - T MAχ are established such that U ER MIN > U BATT rø χ . As such, it is possible to observe and assess the overall function of the entire DC/DC conversion by means of the regulating and monitoring switching circuit 16. In this manner, a microprocessor associated with the total apparatus, detects the high signal at Up 1 , and calculates the time period T 2 . By way of example, simple evaluation criterion can then be provided as follows: T 2 should be smaller than T 2 MAX' an( ** 2 MAX is determied for, C E PR R = C 'ER MAX and U BATT = U BATT MIN . In this case, the transition X to Y, lasts the longest (T 2 MAχ ). The time period T 1 , illustrated in Figs. 2b and 2c, is characterised by the discharge operation of the converter, which operation is established as follows. The comparator K 2 constantly compares the output voltage value ϋ E p with a reference value chosen such that when ϋ ER = ϋ ER MAX tne com P arat °r is triggered and its output switched to a low signal, which is apparent at ϋ pι , and, is illustrated in Fig. 2c. This low signal now apparent at the output of the comparator K , suppresses the control of the power output stage 14 provided by the astable multivibrator 18, and so prevents point B being connected through to ground and so also prevents the build up of further voltage at point B. The energy stored at E R is discharged by way of the load R^, and the output voltage value ϋ ER falls until it reaches a value U E p MIN' at x (Fi S- 2b ) , at which point the control of the power output stage 14 provided by the astable miltivibrator 18 is re-established, and the converter reverts to its pumping phase. Since the value of the load R L , or the discharge load current, of the discharge path of the storage capacitor E R is accurately known for the non-activated control unit, namely, when the air bag system has not been activated, the energy content of E R , and also the capacitance of E R can be accurately calcuated from the low phase of the signal at UP^ , by ascertaining T j . The accuracy of measurement is limited primarily by the tolerance of the stabilized voltage of the total system, for example a tolerance of ±3% . Since Upe MTM •* > ^BATT MAX' as πlen tioned hereinbefore, the energy storage capacitor E R is not recharged, by way of the auxiliary charging circuit 12, during the discharge phase of the converter. The comparator K 2 is such that it enables determination of the limits of the two-state regulation, namely, U ER MAχ and U ER MIN , with a high degree of accuracy, and the difference between these values of the output voltage can be as small as 0.5v. The input U p to the regulating and monitoring switching circuit 16 can be used in synchronizing the monitoring microprocessor of the total system. In order to initiate a discharge phase, Y- to X' (Fig. 2b), a short low pulse, as shown in Fig. 2d, is applied to U p2 , while the converter is in its incipient discharge. The short pulse triggers the comparator K which in turn switches the converter into its pumping phase until Ug R again reaches U ER M, AX » as shown in Fig. 2b. The value of T 1 can then be ascertained as required, and used in calculations performed by the microprocessor. Thus, for a set range between a mimumin and maximum input voltage, the circuitry of the invention produces a set range between a minimum and maximum output voltage. In the normal state, i.e. in the non-activated range of the overall system, the converter acts only on the load R^, or some other defined load, and holds the voltage on the energy storage capacitor within a narrow range of voltage, namely U ER MIN U ER desired < U ER MAX for \ U BATT MIN < U BATT < U BATT MAX * When used in energy storage systems, such as air bag systems, three important converter functions can be commonly provided through the use of the single comparator K 2 and the constant load R L . Firstly, two-state regulation of the output voltage in a narrow range of variation is achieved, while the converter only supplies current to a constant load or well-defined load ( L-* * Secondly, exact periodic determination of the stored energy, with respect to time, is achieved by way of the signal obtained at Up^ and the microprocessor connected thereto. The ease with which the energy store may be tested is achieved through determining that the values of output voltage of the converter that form the range of variation thereof, are greater than the.maximum possible input voltage. By ascertaining an accurate value for the energy stored at any optional input voltage, allows for a specific extraction of energy in the case of activation of the system, i.e. the air bag system. Thirdly, monitoring of the conversion capability of the converter is achieved and the specifically chosen load R^ causes the periodic recharging of the energy store. Monitoring is likewise effected periodically by way of the signal provided at Up*-. The invention is not restricted to the details of the foregoing embodiment. For example, since the converter is operated with a fixed duty ratio and fixed frequency in pump operation, the control module 18 of the FET power switch T.- can be realised by the wiring of a single comparator as an astable multivibrator. As a further simplification, the astable multivibrator may be omitted and the control signal pulse supplied to the gate of T. | can be provided by the monitoring microprocessor of the total system. However, the provision of an independent astable miltivibrator enhances the safety and reliability of operation in the case of air bag systems.

Claims: CLAIMS 1. A DC/DC converter, for charging an energy store (E R ) and including means for testing the energy store and convertability of the converter, having an inductive energy storing device (10) which is coupled to a battery voltage and is adapted for periodic grounding by way of a power output stage (14) controlled by a control means (18), so as to generate a charging voltage higher than said battery voltage, the higher voltage charging said energy store (E R ), characterised in that said testing means comprises a load (R L ) of predetermined resistance such that the output voltage values that form the range of output voltage on the load (R- , ~ ~ -~ ~ the energy store (E R ) is controlled to periodically discharge therethrough, are higher than the maximum battery input voltage. 2. A DC/DC converter according to claim 1, wherein the testing means further comprises. a comparator (K 2 ) for comparing the voltage at the load (R L ) with a reference voltage, and for providing a controlling output to the control of the power output stage (14) whereby the control of the power output stage (14) by the control means (18) may be suppressed. 3. A DC/DC converter according to claim 2, wherein the output of the comparator (K 2 ) is adapted to be connected (U p1 ) to a microprocessor for determining the respective time periods during which the comparator (K 2 ) output exhibits its respective output signals. 4. A DC/DC converter according to claim 2 or claim 3, wherein the comparator (K 2 ) is adapted to be connected (Up 2 ) to a microprocessor for receiving a short pulse for triggering the comparator (K 2 ) so as to switch the converter from a discharging phase to a pumping phase. 5. A DC/DC converter according to any of the preceding claims, wherein said control means (18) comprises an astable multivibrator with open collector. 6. A DC/DC converter according to any of claims 1 to 4, wherein said control means (18) comprises a single comparator. 7. A DC/DC converter according to any of claims 1 to 4, wherein said control means (18) comprises a microprocessor . 8. A DC/DC converter according to any of the preceding claims, wherein said inductive energy storing device (10) comprises a snubber circuit R^,C^) in parallel with an inductor coil (L). 9. A DC/DC converter according to any of the preceding claims, wherein said power output stage (14) comprises a snubber circuit (R 2 ,C 2 ) in parallel with a field effect transistor (T). 10. A DC/DC converter according to any of claims 2 to 9, wherein said testing means comprises the regulating and monitoring switching circuits (16) of the converter .

Inventors: CRISPIN NORBERT, MATTES BERNHARD, SCHUMACHER HARTMUT, CRISPIN, NORBERT, MATTES, BERNHARD, SCHUMACHER, HARTMUT
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1988
Main classification: G01R21
IPC codes: G01R31, H02J7, H02M3