Multiloop Control for Fast Transient DC–DC Converter
THE demand for fast load transient performance has grown significantly, affecting the power supplies of modern high-speed processors—especially processors targeting to achieve a fast transition from the low-power idle mode to the high-speed active mode. There is a massive load current change when the system switches from an idle mode to an active mode. Ideally, the regulator should maintain a voltage level that is almost constant, which means that there should be a negligible output voltage overshoot/undershoot and rapid response time. To accomplish these stringent requirements, various research works on fast transient dc–dc converters have been proposed.
Among these methods, the increase of system bandwidth has been the most general solution for the majority of the analog circuits designed according to the linear control theory. In the design of a switching mode dc–dc converter, the wide system bandwidth can be achieved by adopting the current-mode control method. On top of that, an adaptive pole–zero position circuit has been proposed to instantly move the pole and zero pair of the compensation network to higher frequencies, in order to temporarily extend the bandwidth of the system during the transient event. The pole and zero are moved back immediately after the transient event to stabilize the system. However, a careful design targeting system stability during the whole process needs to be taken into consideration once the bandwidth is tentatively changed. Another commonly used method to improve the transient response is to increase of the slew rate of the error amplifier.
Different current-boosting modules are used to increase the source/sink current at the output of the error amplifier during the transient period. The required boosting current must be large enough to realize an obvious improvement on the transient response, and at the same time, it needs to avoid the over-response oscillation caused by the excessive boosting current. Moreover, current-boosting modules introduce more power consumption, which degrades the overall efficiency.
Besides that, the use of nonlinear control is an alternative to realize fast transient response. For example, hysteresis control can offer immediate feedback during load variations. However, this method has a drawback—the high electromagnetic interference (EMI) noise due to its variable switching frequencies. Circuits can be added to lock the switching frequency and thus abate the EMI noise, but such additions bring more complexities into the circuit design.
The conventional current-mode buck dc–dc converter is highlighted with the blue dashed line. Differing from the conventional type-III compensation network which contains three poles and two zeros to boost the phase to ensure stability, the proposed method generates the same number of poles and zeros to boost the mid-band gain to significantly enhance the response strength of the compensator, thus increasing the transient response. At the same time, the proposed scheme also helps to manage the output voltage ramping speed during the converter power-up.
Theoretically, the transient performance depends mainly on two factors—the response speed and the response strength. The former can be interpreted as the delay time from the load transient event to the change on the control signal, and the latter refers to the amount of the amplitude changes on the control signal. Taking the widely used type-II compensation design as an example here, the structure of the type-II compensation designs can be realized by either the operational amplifier–resistor–capacitor (OpAmp–RC) topology [as shown in Fig. 2(a)] or the transconductance–capacitor (Gm–C) topology [as shown in Fig. 2(b)]. During the load step, the variation in the output voltage results in a change in the feedback signal . The control signal Vc can only respond gradually, due to the compensator’s integrating effect. Its response speed depends on the bandwidth of the compensator.
- Difficult to achieve fast load transient performance.
- Current boosting modules in existing system require more power, which degrades the overall efficiency.
- High electromagnetic interface (EMI) noise.
Step-down method of DC -to-DC power converter it is called Buck Converter. The operation of electromechanical device of DC-DC converter is converts direct current (DC) from certain level of input voltage to another voltage level. This paper proposes the novel design implementation of fast transient response current-mode buck converter with ac coupled feedback (ACCF). Where, ACCF is the modified design of a conventional Capacitor multiplier. The previous method of DC to DC Converters requires more power to achieve the fast transient to voltage conversion and it has high electromagnetic interface (EMI) noise. To overcome this problem this work presents a novel design of DC-DC converter with ACCF. ACCF circuit used to eliminate the bulky soft-start circuit when the ramping speeds of the output voltage during power-up. A Present Proposed system uses current mode- controller to improve response in speed and also increasing load transient voltages. The proposed scheme has been implemented in input voltage 2.6V ~ 4.2V and 45nm CMOS technology and compared in terms of Voltage, power, area and delay of the DC-DC converter will be calculated.
This paper utilizes a current-mode controller to improve the response speed aspect by increasing the system bandwidth in order to enhance the load transient response. On top of that, a novel ACCF is paralleled around the compensator to boost the response strength at the mid-band (as shown in Fig. 3), which is lacking in the conventional designs. The output impedances of both gm0 cell and ACCF are assumed to be infinity (which will be discussed in Section III). The transfer function of the conventional type-II compensation network can be expressed in (3) which includes two poles and one zero (located at p1, p2, and z1, respectively). With the additional ACCF, three poles and two zeros will be generated (located at p 1, p 2, p 3,z 1, and z 2, respectively), and the newly generated poles and zeros will boost the compensation mid-band gain as derived in (4). The respective Bode plot can be found in Fig. 4 accordingly
Active Compensation Capacitor
Active capacitor has been proposed and used in the amplifier to amplify capacitance in . In this paper, the large passive compensation on-chip capacitor Cc1 is replaced by the equivalent active capacitor in the proposed buck converter to reduce the footprint, as shown in Fig. 6. The schematic of the active capacitor will be also modified and used in realizing the ACCF, which will be discussed later. Equations (5)–(7) explain the derivation of the equivalent active capacitor from the passive capacitor mathematically. The equivalent circuit of (7) is modeled in Fig. 6(b) as an active capacitor. The current mirror is used as the current control current source to amplify the ac current in this work. Fig. 7 shows the circuit implementation of the active capacitor by using the current mirror. A (N + 1) times smaller passive capacitor is utilized in parallel with the current mirror, which has an amplification factor of N (N = 19 in this paper), to realize the same capacitance as Cc1(Cc1 = 80 pF in this paper). Fig. 8 shows an approximately seven-time-reduction in the silicon area with the use of the equivalent active capacitor. The higher the value of N, the smaller is the footprint of the active capacitor.
Multiloop Control for Transient Enhancement and Soft-Start
The main structure of the ACCF circuit is modified from the active capacitor circuit , where an ordinary on-chip capacitor C f 1 is ac coupled from Vout, amplified by the current mirror (transistor M2-to-M1 with the ratio M:1, M > 1), and connected back to the main control loop at the gm0 cell output node, as shown in Fig. 9. The circuit implementation of the gm0 cell is shown in Fig. 10. Resistor Rgm is inserted in between the source terminals of MP1 and MP2 to sense the voltage differences (V) between the input terminals (V+ and V−) of the two super-source-followers; hence, gm0 of the cell will be inversely proportional to the value of Rgm . The output current is generated by the two pairs of current mirrors formed by MP3, MP4, MN3, and MN4 at the output stage, and it will charge the compensation network at the output of the gm0 cell. The mid-band gain (Amid-band) of the gm0 cell is, therefore, proportional to Rc while inversely proportional to Rgm , as expressed in (8). Eventually, the process, voltage and temperature variations of the two resistors are effectively canceled. Nevertheless, Rc should be placed close to Rgm in the layout for a better matching purpose
- Fast load transient performance has been achieved.
- Low power consumption.
- Low electromagnetic interface (EMI) noise.
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Multiloop Control for Fast Transient DC–DC Converter
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