IE46524B1 - Dc/dc converter - Google Patents

Abstract

A. Switching regulator. [FR2386862A1]

Description

The invention relates to a DC/DC converter of the type comprising input terminals for a d.c. voltage energy source, output terminals for load, a filter having an inductor, a capacitor and a diode connected in parallel to the capacitor and to the inductor, a switching device which is connected to control load current between the input terminals and the output terminals and which has a power-switching transistor which can be switched over into a conductive state and into a blocking state by means of control elements, a first comparator connected to the external load and which compares the output voltage with a reference voltage and passes a signal to the control elements if they differ, and a control-switching transistor which monitors the load current supplied to an external load in a resistor and passes a signal to the control elements if a permissible value is exceeded so as to block the power switching transistor.

DC/DC converters having these features will be referred to herein as of the type defined.

In a known converter of this type, described in U.S. Patent Specification No. 3,396,326, the inductor also has a secondary winding which emits a signal for blocking the power-switching transistor when the current - 2 46524 flowing through the induction coil decreases. This secondary winding has substantial disadvantages: 1. Monitoring of a complete discharge of the energy contained in the induction coil is not ensured and there is a risk that the induction 5 coil nevertheless runs in saturation, which leads to overloading of the converter; 2. Inductors having a secondary winding are not commercially available; and 3. Only the dynamic behaviour, and not the static behaviour, of 10 the inductor can be monitored using a secondary winding.

The object which is to be achieved by the present invention, consists in providing a DC/DC converter in which large variations in the input voltage of, for example, 1 : 10 and large fluctuations in the output load are permissible, without individual elements of the converter being destroyed by overloading. In particular, running the inductor in saturation is to be avoided.

A DC/DC converter of the type defined comprises, according to the invention, a second comparator which compares the voltage at the input of the inductor with the voltage at the output of the inductor and which emits a signal 0 so as to block said power-switching transistor as soon as the voltage at the output of the inductor is greater than the voltage at the input of the inductor.

Reference is now made to the accompanying drawing, in which: Figure 1 shows a circuit diagram of a known DC/DC converter; and Figure 2 shows a circuit diagram of a DC/DC converter according to an embodiment of the invention.

The converter shown in Figure 1 has two terminals at the input d.c. voltage UE and two terminals at the output d.c. voltage U.A. The two positive terminals are interconnected via a power-switching transistor T and - 3 46524 an induction coil DR arranged in series. A comparator LD is connected to the positive output terminal and to the base of the transistor T. This comparator is also connected to an auxiliary power supply voltage UH and to a reference voltage UR, which are indicated only by an arrow. A filter which consists of the induction coil DR, a capacitor C, and a diode D is also present, the diode being arranged parallel to the capacitor C and to the induction coil DR.

The mode of action of this converter, which is in itself known, is as follows. Via the comparator LD, the output' voltage UA has the effect of switching the switching transistor T on and off, that is to say, as soon as the output voltage exceeds a certain value, the switching transistor T is blocked via the comparator which continuously compares the output voltage to a reference voltage UR. As a result, the output voltage UA falls so that the comparator LD makes the transistor T conductive again. As long as the switching transistor T is conductive, the filter, that is to say the induction coil DR and the capacitor C, is connected to the input voltage UE, as a result of which the current i-j rises.

When the switching transistor T is blocked, the energy can flow out of the coil DR via the diode D and the current i^ becomes smaller. It is clear that the output voltage UA depends on the time tj during which the switching transistor T is conductive, and on the oscillating period t? of the system, that is to say UA/UE = with t2 = 1/f if f is the frequency of the system. Optimum matching of the components of this converter to the particular requirements is essential since the efficiency, that is to say the losses, depends on the suitable selection of the elements. In particular, the induction Coil DR must be sized in such a way that it does not reach saturation over the entire input range UE and with the highest permissible load on the output of the converter.

To avoid these difficulties, the known converter has been improved according to the present invention.

According to Figure 2, the converter embodying the invention likewise possesses two terminals at the input d.c. voltage UE and two terminals - 4 46524 at the output d.c. voltage UA. The two positive terminals are again interconnected via a power-switching transistor T2. A first comparator LDl is connected to the positive output terminal and, via the control transistors T4, T5 and TI, to the base of the switching transistor T2. The comparator LDl is also connected to a source DDC of an auxiliary power supply voltage and to a reference voltage UR. A filter which consists of an induction coil or inductor DR and a capacitor C, is likewise present, A diode D is arranged parallel to the capacitor C.

The components described so far do not differ in their arrangement from the known converter according to Figure 1.

According to the invention, however, the following additional components are also provided. 1) A second comparator LD2 is connected on one side to the positive output terminal and on the other side, via the control transistors T5 and TI, to the base of the switching transistor T2. Moreover, the comparator is connected to a point A in front of the induction coil and to the source DDC of an auxiliary voltage. 2) The base of a second switching transistor T3 is connected to the collector of the switching transistor T2. Moreover, a first resistor R3 is located between the positive input terminal and the collector of the switching transistor T2, a second resistor R4 is located between the base of the second switching transistor T3 and the collector of the first switching transistor T2 and a third resistor R5 is located between the collector of the second switching transistor T3 and the base of the switching transistor T4. 3) In addition, a time function element TM1 is present, which is connected via an input terminal E and via the control transistors T5 and TI to the base of the switching transistor T2. 4) Parallel to the time function element TM1, a level switch LD3 which is dependent on the auxiliary voltage or the rate of increase thereof, is also connected to the input terminal E.

The mode of action of the converter described is as follows: 1) In normal operation, the actual value of the output voltage UA is compared in the comparator LDI with the reference voltage UR. As soon as the output voltage UA becomes larger than the reference voltage UR, the comparator LDI switches over from the value 0 to the value l11. This switching-over to the value 1 has the effect that the control transistor T4 becomes conductive. As a result, the control transistor T5 is blocked. This has the consequence that the control transistor Tl is likewise blocked and thus also the switching transistor T2. As soon as the output voltage UA becomes smaller than the reference voltage UR, the comparator LDI switches back again from the value 1 to the value 0, as a result of which the control transistor T4 is blocked and the control transistors T5 and Tl as well as the switching transistor T2 thus becomes conductive. This process takes place in steady or normal operation, that is to say as long as the load is equal to or smaller than the maximum permissible load. 2) When the converter output is overloaded or short-circuited, the duty factor of the switching transistor T2 rises to such an extent that the inductor DR reaches saturation. On saturation of the inductor DR, the current in the switching transistor T2 rises and leads to its sudden destruction. To prevent this destruction, it is necessary to monitor the rise of current.

The current is monitored with the aid of the resistors R3, R4, R5 and the switching transistor-T3. Using these elements, the power-switching transistor T2 can be protected from destruction by a sudden excessive current. This current monitoring alone, however, cannot prevent destruction of the switching transistor T2 by thermal overloading since the current monitoring operates fast, that is to say at more than 100 kHz and with a duty factor of about 50%. At a high steady input voltage UE, the power losses in the switching transistor T2 reach such a level that overheating and destruction take place. Furthermore, the inductor DR is unable to release the energy stored therein. Thus, the inductor DR operates virtually continuously in saturation. It is therefore also - 6 463 24 necessary to monitor the inductor DR. Monitoring of the inductor DR is carried out with the aid of the second comparator LD2; the latter compares the output voltage UA after the inductor DR with the voltage at the point A before the inductor DR. As long as the voltage at the point A is smaller than the output voltage UA, the inductor DR still releases energy. The switching transistor T2 must therefore be blocked. As long as the voltage at the point A is smaller than the output voltage UA, the comparator LD2 remains on the value 0, as a result of which the control transistors T5 and Tl and hence also the switching transistor T2 are blocked. When the voltage at the point A is equal to or greater than the output voltage UA, the comparator LD2 switches over to the value 1 and the transistor T5 becomes conductive, and the control transistor Tl and the switching transistor T2 thus also become conductive.

In steady or normal operation, the period of energy release from the inductor DR is shorter than the control period; in this case, the comparator LD2 has no significance.

In case of a short-circuit, monitoring of the current will start first and, subsequently, monitoring of the inductor will start. This combination of monitoring the current and monitoring the inductor results in a favourable duty factor of the switching transistor T2, so that the converter is short circuitproof.

When the converter shown in Figure 2 is switched on, the auxiliary power supply voltage in the source DDC of an auxiliary voltage must first be built up, before the converter can operate. Thus, the time function element IMI is required; this blocks the control transistors T5 and Tl and thus also the switching transistor T2, until the auxiliary voltage has been built up.

With the aid of the level switch LD3, it is possible to ensure that the converter starts to operate only at a certain auxiliary voltage or at a certain rate of increase of auxiliary voltage.

Claims (2)

1. A DC/DC converter of the type defined, comprising a second comparator which compares the voltage at the input of the inductor with the voltage at the output of the inductor and which emits a signal “0 so as to 5 block said power-switching transistor as soon as the voltage at the output of the inductor is greater than the voltage at the input of the inductor.

2. A DC/DC converter of the type defined, substantially as described herein with reference to Figure 2 of the accompanying drawings.