HK1033391B - Buck converter - Google Patents

Description

Step-down converter

The invention relates to a direct current converter, namely a buck converter, comprising a controllable switch, a device for controlling the controllable switch and a device for detecting the output current of the controllable switch.

A buck converter is a dc voltage buck converter known from DE 19612365. The buck converter oscillates at a switching frequency preset by the oscillator. In order to keep the output voltage independent of the load over a wide range, the output voltage is applied to a control device which controls the electronic switch with a correspondingly adjusted pulse duty factor. In order to prevent the current flowing through the coil from exceeding a predetermined value, the control device also detects the current flowing through the coil during the conduction of the electronic switch through a sensitive resistor, and interrupts the current by the electronic switch when necessary. This also prevents the output voltage from rising undesirably.

A dc buck converter is known from DE3310678, which differs from the buck converter known from DE19612365 mentioned above only in that it does not have an oscillator of a predetermined switching frequency, but is of a self-oscillating type: the electronic switch remains conductive until the current through the coil exceeds a limit value and remains non-conductive until the output voltage drops to a certain value. From WO98/24170 a self-oscillating buck converter is known, the oscillation characteristics of which are determined solely by its output voltage and which comprises a further means for limiting the coil current with a sensitive resistor.

All of the above buck converters are therefore controlled by their output voltage. The current flowing through the coil can only be detected when the electronic switch is switched on, the detection concerned only having an effect on limiting the current.

DE3921955 discloses a switching controller which can control its output power. For this purpose, its output voltage and output current are detected. The output current is however only detected when the electronic switch is open.

A buck converter for charging a battery is known from EP 0752748. The charging circuit comprises an electronic switch and a current sensitive resistor for detecting the current flowing through the coil and switching the electronic switch on and off when the voltage of the battery is below a specific value, thereby obtaining a constant charging current averaged over time. When the battery is charged to a level where its voltage exceeds a certain value, the electronic switch is controlled independently of the output voltage of the buck converter, so that the output voltage is kept stable.

A self-oscillating buck converter is known from WO99/13559, the output current of which is sensed and held at a constant value by a current-sensitive resistor. However, as the input voltage rises, the output current also rises accordingly.

It is an object of the present invention to provide a buck converter of simple construction, dedicated to the charging of a battery, that is to say which provides a current adapted to the particular state of charge and to the prevailing environmental conditions (temperature) of the battery, irrespective of the prevailing input voltage at a particular moment. It should be noted that a nearly fully charged battery can only draw a small amount of current, while a nearly discharged battery can accommodate a larger amount of current.

To this end, the buck converter of the present invention includes means for regulating the magnitude of the output current as a function of the respective output and input voltages. It is contemplated that when the buck converter of the present invention is used to charge a battery, the output voltage of the buck converter is determined by the connected battery, reflecting the actual state of charge of the battery, so that the buck converter always provides a current that is tailored to the particular state of charge of the battery.

The step-down converter of the present invention has: a controllable switch, a control means for controlling the controllable switch, a means for detecting the output voltage of said buck converter, a means for detecting the output current of said buck converter and a means for detecting the input voltage of said buck converter, said control means comprising a comparator with hysteresis and a reference voltage source for supplying a reference voltage to said comparator, and controlling said controllable switch in such a way that the magnitude of said output current is adjusted in accordance with the output voltage, the input voltage and the output current, so that the time average of said output current decreases with the increase of said output voltage.

For example, the buck converter may be connected to a power supply via its input. The buck converter operates in a manner known in the art to reduce the input voltage at the input terminal to a lower output voltage. The accumulator is preferably connected at its output. Since the battery voltage changes only very slowly during the charging of the battery, the operation of the buck converter according to the invention with a constant output voltage, which is determined by the battery voltage, is first described here.

The controllable switch, preferably an electronic switch, in particular a transistor, is controlled by the control means in a manner known in the art, i.e. switched on and off in successive cycles. When the switch is switched on, i.e. during the time when the transistor is conducting, current flows from the input via the switch and the coil to the accumulator, so that electromagnetic energy is accumulated in the coil. When the switch is open, that is, during the time that the transistor is off, the electromagnetic energy is converted to electrical energy, causing current to flow from the coil to the battery, in which case the circuit is completed by a diode in a manner well known in the art. The means for sensing the output current of the buck converter of the present invention is preferably a current sensitive resistor that senses the output current of the buck converter during both the on and off periods of the electronic switch. The control device of the buck converter of the invention switches the switch on when the output current drops to a specific first value and switches the switch off when the output current rises to a specific second value. Since the buck converter of the present invention is controlled by its output current, this output current is independent of the magnitude of the input voltage. However, the buck converter of the present invention is designed such that the first and second specific values will change as the output voltage of the buck converter rises, so that the switch remains open for a longer period than before, causing the time-averaged output current of the buck converter to fall.

In an embodiment of the buck converter according to the invention, the first and second specific values are further configured in such a way that the first and second specific values also change when the buck converter has a higher input voltage, so that the switch remains open for a longer period than before. In this way, the so-called electronic switch storage delay time is compensated, which delays the opening of the switch when the voltage to be switched is large, so that the current is larger than when an ideal buck converter is used. Operating the buck converter of the present invention at different input voltages therefore does not pose a problem because it does not affect the magnitude of its output current.

The buck converter of the invention is preferably of the self-oscillating type, that is to say without an oscillator for controlling the opening and closing of the controllable switch. However, the buck converter of the invention has a control input by means of which the buck converter can be switched off. This can be achieved, for example, by a charge control device which emits a corresponding signal when the battery connected to the step-down converter has reached its fully charged state.

In a buck converter which is of particularly simple and therefore economical design, the invention provides that the control device comprises a first comparator with hysteresis and a first reference voltage source. The hysteresis of the first comparator results in that the first reference voltage source generates two different reference voltages depending on whether the output of the first comparator is "high" or "low".

An advantage of the buck converter also includes the structure to turn off the buck converter when the input voltage is too low to ensure proper operation of the buck converter, i.e., the buck converter automatically turns itself off when the voltage at the input drops below a minimum input voltage.

According to the invention, a step-down converter is formed which is particularly advantageous for charging a battery, so that the output impedance value is virtually infinite at low input voltages. This may be achieved, for example, by opening at least one controllable switch, disconnecting the output from the buck converter circuit or preventing current flow between the output terminals. In this way, it is ensured that the battery connected to the output of the buck converter is not discharged via the buck converter if the input of the buck converter is (accidentally) short-circuited or if the voltage is below the cell voltage value at these terminals.

The invention will be described with reference to a few embodiments shown in the drawings. Other embodiments are discussed in the description. In the drawings, there is shown in the drawings,

FIG. 1 is a block diagram of a buck converter of the present invention;

fig. 2 is a block diagram of the buck converter of the present invention shown in fig. 1, showing a preferred embodiment thereof.

Fig. 3 is a circuit diagram of a buck converter of the present invention.

The inventive buck converter shown in fig. 1 comprises a controllable switch LS and a coil L connected in series between an input terminal UE and a first output terminal UA + in a manner known in the art. The node between the controllable switch LS and the coil is connected to the cathode of the freewheeling diode D. The anode of the freewheeling diode D is grounded and connected to one end of the current sensing resistor RM. The other end of the current sensitive resistor is connected to the second output terminal Ua-. A first resistor R1 is inserted between the input UE and a point B, which is connected via a second resistor R2 to the first output Ua + and via a third resistor R3 to the second output terminal Ua-. Point B is further connected to the inverting input of a first comparator K1, the output of which first comparator K1 is coupled to the control input of the controllable switch LS via a driver TR. The non-inverting input of the first comparator K1 is connected to a first reference voltage source U1 connected between the input terminal UE and ground.

The first comparator K1 and the first reference voltage source U1 are connected in such a way that the reference voltage Uref1 (point a) supplied by the reference voltage source U1 takes two different values depending on whether the output voltage of the first comparator K1 is "low" or "high", that is to say the first comparator K1 has a hysteresis because its breakover voltage (point a) has a value different from its reset voltage (point a). The magnitude of the potential prevailing at point B is determined by the voltage drop across the resistors R1, R2, R3 and RM, and thus by the input voltage, output voltage and output current. The input voltage is divided by the first, third and current sensitive resistors, and the output voltage is divided by the second and third resistors. The first comparator K1 compares the voltage at point B with the voltage at point a. As long as the voltage at point B is lower than the voltage at point a, the output of the first comparator is "high", and vice versa: the output of the first comparator is "low" as long as the voltage at point B exceeds the voltage at point a.

Fig. 2 shows a preferred embodiment of the buck converter of the present invention. This embodiment differs from the buck converter of the present invention shown in fig. 1 in a configuration for shutting down the buck converter when a low input voltage occurs and in a configuration for preventing the battery connected to the buck converter from discharging through the buck converter when a low input voltage occurs.

The configuration of the turn-off buck converter at low input voltages comprises a second reference voltage source U2 connected between the input terminal Ue and ground, and a second comparator K2 whose non-inverting input is connected to the output of the first comparator K1, whose inverting input is connected to the second reference voltage source U2 and whose output is connected to the driver TR. The second comparator K2 compares the output voltage of the first comparator K1 with a second reference voltage supplied by a second reference voltage source Uref 2. When the input voltage is low, i.e. when the input voltage is below the second reference voltage Uref2, the output of the second comparator K2 remains "low", the controllable switch LS is kept open via the driver TR and the buck converter is prevented from oscillating. However, when the input voltage is higher than the second reference voltage Uref2, the output voltage of the second comparator K2 will follow the output voltage of the first comparator K1, that is, the controllable switch LS will be controlled by the first comparator K1 via the second comparator K2 and the driver TR.

The arrangement for preventing the battery connected to the buck converter from discharging through the buck converter in the presence of a low input voltage comprises a second controllable switch S arranged between the second resistor R2 and the first output Ua +, the control input of which is connected to the input Ue via a tenth resistor R. The second controllable switch S is preferably constituted by an electronic switch, such as a transistor having its base connected to a tenth resistor R. When the input voltage is low, the second controllable switch S will be opened, making the output resistance of the buck converter virtually infinite, preventing the battery from discharging through the second and third resistors R2, R3.

Referring to fig. 3, the first reference voltage source U1 includes a zener diode ZD1 having an anode connected to ground and one end of a sixth resistor R4, R5, R6 connected in series between the input terminal UE and ground, and a cathode connected to the fourth and fifth resistors R4, R5. The other end of the sixth resistor R6 is connected to the noninverting input terminal of the first comparator K1 (point a) and one end of the eighth resistor R8. The other end of the eighth resistor R8 is connected to the input Ue via a seventh resistor R7 and to the output of the first comparator K1 via a ninth resistor R9. The second reference voltage source U2 includes a second zener diode ZD2 having its anode grounded and its cathode connected to the inverting input terminal of the second comparator K2 and connected to the input terminal Ue via an eleventh resistor R11.

The reference voltage Uref1 (point a) generated by the first reference voltage source is a zener voltage generated from the input voltage through the fourth resistor R4 and the first zener diode ZD1 and divided by the fifth and sixth resistors R5 and R6. As previously described, the first comparator has two different values depending on whether its output is "low" or "high". When the output of the first comparator K1 is "high", the voltage at point a (breakover voltage) will be determined by the divided zener diode and the input voltage divided by the sixth, seventh and eighth resistors R6, R7, R8. Conversely, when the output of the first comparator K1 is "low", the ninth resistor R9 will be connected in parallel with the series circuit of the sixth and eighth resistors R6, R8. The voltage at point a (reset voltage) will be determined by the divided zener diode and the divided input voltage via the sixth, seventh and eighth resistors R6, R7, R8 on the one hand and the seventh and ninth resistors R7, R9 on the other hand. The difference between the breakover voltage and the reset voltage is the hysteresis voltage. The seventh and ninth resistors R7, R9 simultaneously also function as pull-up resistors at the output of the first comparator K1.

As long as the output of the first comparator K1 is "low", that is, the output voltage of the first comparator K1 does not exceed the reference voltage Uref2 of the second reference voltage source U2, the output of the second comparator K2 will likewise remain "low". Since the driver TR is not driven, the controllable switch LS will remain open. Conversely, when the output of the first comparator K1 is "high", that is, when the output voltage of the first comparator K1 exceeds the reference voltage Uref2 of the second reference voltage source U2, the output of the second comparator K2 will likewise be "high". This will cause the driver TR to be driven unless a shutdown signal is issued to the output of the second comparator K2 via the shutdown control input.

Claims (7)

1. A buck converter having: a controllable switch, a control means for controlling said controllable switch, a means for detecting an output voltage of said buck converter, a means for detecting an output current of said buck converter and a means for detecting an input voltage of said buck converter, characterized in that said control means comprises a comparator with hysteresis and a reference voltage source for supplying a reference voltage to said comparator, and controls said controllable switch in such a way that the magnitude of said output current is adjusted in accordance with said output voltage, said input voltage and said output current, so that the time average of said output current decreases with the increase of said output voltage.

2. The buck converter of claim 1, further comprising structure to turn off the buck converter when a low input voltage is present.

3. The buck converter of claim 2, wherein an output resistance value of the buck converter is infinity at the time the low input voltage is present.

4. The buck converter of claim 1, wherein the reference voltage provided by the reference voltage source takes two different values depending on whether the output of the comparator is "low" or "high".

5. A buck converter as claimed in claim 1 which is of the self-oscillating type.

6. The buck converter of claim 5, further comprising a control input operable to stop operation thereof.

7. A buck converter according to claim 1, wherein the output current of the buck converter is detectable during both on and off periods of the controllable switch.