This article describes a feedback control method, called ripple detection control, that is used for a low voltage and high current DC-DC power supply.
In the ripple detection control, a current ripple component is detected and AC-coupled into a feedback voltage. The feedback voltage is directly compared with the reference voltage so that it achieves a fast load-transient response.
This method provides a low converter output impedance over a wide frequency range without the need for bulky electrolytic capacitors. Consequently it saves a lot of mounting space on the printed circuit board assembly.
This control method is immune to input voltage fluctuations, making it ideal for applications utilizing an intermediate bus architecture.
Data communication equipment and intelligent household electric appliances use numerous data processing ICs. These ICs require low operating voltages (ranging from 1.0 to 1.5V) and increased load currents coupled with a reduction in power consumption and an improvement in efficiencies. Depending on processing activity the IC's load current will experience sudden increases and decreases. As a result the output voltage of a DC/DC converter can fluctuate causing system failure. A feeding configuration at the POL (Point of Load) in which a DC/DC converter is placed in very close proximity to the load is used to address such a problem, introducing placement limitations for designers. Moreover, various external capacitors are connected in parallel to achieve the low-impedance voltage source across a broad frequency range. This cost in dollars and board-space of the external capacitance elements can be significant. To cope with such conditions the ripple detection control is offered.
The ripple detection control method
The ripple detection control method improves upon a previously established technique known as a ripple converter. Figure 1 shows the principle drawing of a ripple converter. In the ripple converter, self-oscillation is achieved based on the system's delay time. The original ripple converter had some performance issues to resolve though its response characteristics were excellent.
First, the operating frequency remarkably decreases when a ceramic capacitor is used for the output capacitance. Figure 3(a) shows voltage waveforms in a ripple converter with ceramic output capacitors.
When the feedback voltage decreases below the reference voltage, Vref, the high side FET switch will be turned ON after the delay time of TdON. Because the ESR of the output ceramic capacitor is low enough, the smoothing LC circuit can be regarded as an ideal quadratic LC filter, and the output voltage of the DC-DC converter resembles the continuation of a parabola. As a result, the output voltage decreases for a while even after the FET switch turns ON. A similar delay occurs when the FET switch turns OFF. Therefore, with given system delay times (TdON, TdOFF), the operating frequency becomes comparatively low and the size of the converter becomes larger. And, if additional external capacitors are added, the operating frequency will change significantly.
Second, it is difficult to set the operating frequency to a desirable point because it uses the delay time of its own system.
To solve these performance issues, we propose a new feedback control method named 'ripple detection control' (Figure 2). An AC-ripple generator connected in parallel with the choke inductor, and variable delay circuit are added. The resistor (Rc) and the capacitor (Cc) generate a triangle-wave ripple voltage (Vtr) proportional to the inductor current. This ripple voltage is injected into the feedback voltage through a capacitor. Here the time constant of the AC ripple generator is Tc=Rc*Cc. If the AC-ripple voltage (Vtr) is designed to be larger than the output ripple voltage (Vrpl) as shown in fig3(b), the operating frequency can be adequately higher than that of the conventional ripple converter. Additionally the operating frequency continues to be stable, even when external output capacitors are added, because Vtr remains larger than Vrl.
A delay circuit is adopted to generate a stable variable delay, which is set by an external resistor. By using this delay, the operating frequency can be set to its optimal point.
Figure 1 Conventional Ripple Converter
Figure 2 Ripple Detection Control Method
Figure 3 Waveforms of each control methods