1. Field of the Invention
This invention relates to a step up type, step down type, and flyback type switching regulators and more particularly to a switching regulator where an operating mode is detected to control the loop gain in accordance with the operating mode.
2. Prior Art
FIG. 10 shows a conventional chopper type switching regulator of a step up configuration where the output voltage is higher than the input voltage. FIG. 11 is a waveform diagram showing the behavior of the significant waveforms at different points of the circuit shown in FIG. 10. FIGS. 12A and 12B show the waveforms at different points of the circuit shown FIG. 10 in an discontinuous mode and in an continuous mode, respectively.
In FIG. 10, an error amplifier 8 amplifies the difference between the output voltage Vo and the reference voltage Vr of a reference voltage source 7 and outputs an error signal Ver as shown in FIG. 11. A reference oscillator 9 outputs a saw tooth signal Vosc as shown in FIG. 11. A PWM comparator 10 compares the error signal Ver with the signal Vosc to output a pulse-width modulated signal Vp which is fed to the base of a switching transistor 2. The comparator 10 outputs a logic "1" to turn on the transistor 2 when Vosc&gt;Ver, and the comparator outputs a logic "0" to turn off the transistor 2 when Vosc&lt;Ver. For example, if the output voltage Vo decreases due to an increase in load current, the error signal Ver decreases as depicted by a dotted line W1 in FIG. 11, so that the duty cycle of logic 1 of the signal Vp increases as depicted by W2 in FIG. 11. Thus, the transistor 2 remains on for a longer period. When the transistor 2 goes off, the electric energy stored both in a choke coil 3 and a capacitor 5 is superimposed to the energy supplied through an input terminal 1 and is directed to an output terminal 6, thereby increasing the output voltage Vo. This is the basic operation of the step up type regulator.
During switching operation, the circuit in FIG. 10 operates either in an discontinuous mode where the choke current Ich is intermittent, or in a continuous mode where the choke current exists at all times. The regulator operates in the discontinuous mode when the load is small, and in the continuous mode when the load is large.
In the discontinuous mode, for example, if the load current doubles, then the ON duty cycle of transistor 2 becomes twice to double the output power as depicted by dotted lines in FIG. 12A. The choke current Ich and collector current Itr of transistor 2 continues to increase for a longer period to increase so as to increase output power.
In the continuous mode, for example, if the load current doubles, the ON duty cycle increases only by a small amount as depicted by dotted lines in FIG. 12B. The choke current Ich and collector current Itr of transistor 2 increase to increase output power.
FIG. 13 shows a conventional chopper type switching regulator of step down configuration where the output voltage is lower than the input voltage. FIG. 14 shows the waveforms at different points in the circuit of FIG. 13. If Vosc&gt;Ver, the pulse-width modulated signal Vp goes high. The high level of Vp turns on the transistor 12 which in turn causes the transistor 2A to turn on. If Vosc&lt;Ver, the signal Vp goes low. The low level of Vp turns off the transistor 12 which in turn causes the transistor 2A to turn off. For example, if the output voltage Vo decreases due to an increase in load current, the error signal Ver decreases as depicted by a dotted line W1 in FIG. 11, so that the duty cycle of logic 1 of the signal Vp increases as depicted by W2 in FIG. 11. Thus, the transistor 12 remains on for a longer period. A longer ON period of the transistor 12 causes more electric energy to be stored across the choke coil 3 and the capacitor 5, thereby increasing the output voltage Vo. This is the basic operation of the step down type regulator.
With the aforementioned conventional step up type and step down type regulators where a regulated power supply has a capacitive load connected to the output of a negative feedback amplifier, the ON duty cycle of switching transistor must vary more greatly in the discontinuous mode than in the continuous mode in order to increase a given amount of output power. In addition, the loop gain becomes much higher when the regulator operates in the continuous mode than in the discontinuous mode. Experiment showed that the loop gain usually differs by a factor of more than ten.
In order to maintain as constant a output voltage as possible against changes in load and changes in input voltage, the loop gain should be as high as possible. Thus, the error amplifier usually has a high loop gain in a low frequency range. However, a higher loop gain causes phase delay both in the error amplifier and in the output capacitor within a feedback loop, which makes it difficult to simultaneously achieve good stability both in the continuous mode and in the discontinuous mode. This is disadvantageous to ensure good loop stability if loop gain varies greatly due to changes in mode. Conversely, improving feedback stability usually results in a lower loop gain in a high frequency range which in turn results in an increase in output resistance of the regulator. Then, the higher output resistance causes a large change in output voltage if the load changes quickly.
In order to prevent deterioration in stability and performance of feedback loop due to switching between modes, the regulator may be operated only in one mode. In this case, both stability and performance may be satisfied if changes in load and in input voltage are small. However, if changes in load and in input voltage are large, an extremely large capacitance of the output capacitor is required to reduce output resistance in the discontinuous mode. On the other hand, operating only in the continuous mode results in less peak currents and less ripple currents through the choke coil and output capacitor, being advantageous in reducing the size of choke coil. In this case, however, even when the load current is negligibly small, an unnecessary idling current must be drawn in order to prevent transition from continuous mode to discontinuous mode.