The invention relates to DC-DC converters, and in particular those that convert a higher input voltage into a lower output voltage.
Such DC-DC converters are disclosed for example in R. Köstner, A. Möschwitzer, “Elektronische Schaltungen” [“Electronic circuits”] Hansa-Verlag 1993, pages 281 to 286, and comprise a series circuit formed by an inductor and a capacitor, an output voltage for a load being tapped off across the capacitor and the load bringing about a load current, and also a changeover switch for connecting an input voltage to the series circuit or for short-circuiting the series circuit. The changeover switch is controlled by a control circuit in such a way that the changeover switch alternately for example short-circuits the series circuit for a first time duration or connects it to the input voltage for a second time duration. The ratio of the time durations (pulse width modulation) is regulated in accordance with the desired output voltage.
One problem in the case of such DC-DC converters is the dynamic behavior in the case of small output voltages and, in particular, the stability of the output voltage in the case of a changing load current. A small output voltage is to be understood as, for example, voltages of 5 V or less. In this case, primarily load current changes from large load currents to small load currents are problematic in particular when synchronous rectifiers are used rather than diodes as freewheeling components, said synchronous rectifiers being realized by correspondingly driven field-effect transistors.
Such a DC-DC converter is illustrated by way of example in FIG. 1. In this case, a push-pull output stage serving as a changeover switch has two transistors Q1 and Q2, which respectively have a correspondingly biased diode D1 and a diode D2 (body diodes) between their respective source and drain terminals. An input voltage UE is applied to the push-pull output stage in such a way that the input voltage UE is applied for example to the drain terminal of the transistor Q1 used as a synchronous rectifier, while the source terminal of the transistor Q2 is connected to ground M. The drain terminal of the transistor Q2 and also the source terminal of the transistor Q1 are connected up to one another and form the output of the push-pull output stage. The gate terminals of the transistors Q1 and Q2 are driven by a driver circuit DR by means of control voltages VG1 and VG2, the output terminal of the push-pull output stage serving as reference point for the control voltage VG1 and ground M serving for the control voltage VG2. The driver circuit DR is driven by a control circuit CTR, which feeds a pulse-width-modulated rectangular signal into the driver circuit DR.
A series circuit comprising a coil L and a capacitor C is connected between the output of the push-pull output stage and ground M, the inductor L having a parasitic resistance RS and the capacitor C having a parasitic resistance RP, which, in terms of their effect, are in series with the inductor L and with the capacitor C, respectively. Within the series circuit, the inductor L is connected with respect to the output of the push-pull output stage and the capacitor C is connected with respect to ground M. At the tap between inductor L and capacitor C, it is possible to tap off an output voltage UA with respect to ground M. The output voltage UA feeds a load resistor RL, which brings about an output current IA.
A current IL flows into the inductor L, which current, depending on the switching state of the push-pull output stage, is essentially formed either by a current IQ1 flowing through the transistor Q1 or by a current IQ2 flowing through the transistor Q2.
As illustrated in FIG. 2, the current rise diL/dt in the inductor L with transistor Q1 switched on is very much greater than the current fall diL/dt in the inductor L with transistor Q2 switched on (transistor Q1 switched off). The reason for this is that the driving voltage across the inductor L, for example in the case of an input voltage of 12 V and an output voltage of 1.5 V is very much greater if the transistor Q1 is switched on and the transistor Q2 is switched off.
This behavior is not problematic in the steady-state condition, that is to say, with a constant load current. The situation is different, however, in the case of rapid load current changes from high load to low load, that is to say for example, from full load to no load. The current IL in the inductor and the output current IA (load current) are large before the load current change, to be precise both are approximately of the same magnitude. If the load current returns abruptly to a very small value, then the impressed inductor current has to flow into the capacitor C. The current IL becomes smaller and smaller until it finally returns to the value of the output current IA (load current). In this case, it charges the capacitor C further, so that the output voltage UA increases. To a first approximation, the energy stored in the inductor L is in this case transferred to the capacitor C.
In the case of an opposite load change, by contrast, that is to say in the case of a change from small load to large load, firstly a very small or even no current IL flows in the inductor L, and virtually no output current IA. If the load current and thus the output current IA suddenly become larger, then the increased current requirement must initially be covered from the capacitor C, while the current IL through the inductor L rises. In this case, the voltage across the capacitor C (approximately the output voltage UA) decreases somewhat, to be precise, until the current IL in the inductor L has reached the magnitude of the output current IA.
The difference between output current IA and current IL through the inductor L has to be supplied by the capacitor C or has to flow into the latter. In this case, its voltage and thus the output voltage UA decrease or increase. Since the rate of change of the current IL is very much lower in the case of a current fall than in the case of a current rise (see FIG. 2), this load change case, large load current to small load current, is very much more critical than the opposite case, i.e. the change in the output voltage (in this case increase) on account of the delayed current fall in the inductor is greater than in the case of a load rise. This case has to be taken thoroughly into consideration, particularly for DC-DC converters with small output voltages, a very narrow tolerance of the output voltage and/or a high load current (=much energy in the inductor L).
Conventional control circuits react to a load change solely with altered pulse width modulation, i.e. adaptation of the ratio of the switching times of the transistors Q1 and Q2. In particular, the switch-on duration of the transistor Q1 is reduced in this case, the clocking being regularly continued. In the case where the load current suddenly decreases, the switch-on duration of the transistor Q1 is shortened further in this case until finally it is actually no longer switched on. By contrast, the switch-on duration of the transistor Q2 is correspondingly lengthened. Since, in the case of DC-DC converters with high output currents and small output voltages, the transistor Q2 (low-side switch) has an on resistance of just a few milliohms, the power loss arising in the transistor Q2 (high-side switch) is relatively small. The energy stored in the inductor L is therefore largely transferred to the capacitor C. This means that the output voltage UA may rise impermissibly.