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A Current-Mode Buck DC-DC Converter with Frequency Characteristics Independent of Input and Output Voltages Using a Quadratic Compensation Slope
Abstract
By using a quadratic compensation slope, a CMOS current-mode buck DC-DC
converter with constant frequency characteristics over wide input and
output voltage ranges has been developed. The use of a quadratic slope
instead of a conventional linear slope makes both the damping factor in
the transfer function and the frequency bandwidth of the current
feedback loop independent of the converter's output voltage settings.
When the coefficient of the quadratic slope is chosen to be dependent on
the input voltage settings, the damping factor in the transfer function
and the frequency bandwidth of the current feedback loop both become
independent of the input voltage settings. Thus, both the input and
output voltage dependences in the current feedback loop are eliminated,
the frequency characteristics become constant, and the frequency
bandwidth is maximized. To verify the effectiveness of a quadratic
compensation slope with a coefficient that is dependent on the input
voltage in a buck DC-DC converter, we fabricated a test chip using a
0.18µm high-voltage CMOS process. The evaluation results show that
the frequency characteristics of both the total feedback loop and the
current feedback loop are constant even when the input and output
voltages are changed from 2.5V to 7V and from 0.5V to 5.6V,
respectively, using a 3MHz clock.
We propose a fast and precise simulation method for replicating the transient responses and frequency characteristics of switching power converters using a mixture of real circuits and behavioral models. The method used a behavioral simulation tool (Verilog-A), which was supported by an analog simulator such as SPECTRE®. The time-step became variable, however, the nonlinear operation of the circuit was considered and state-space equations were formulated by applying the hybrid-dynamical-system control scheme in behavioral modeling [1]. Each macro block can have either the real schematic and behavioral model. The transient simulation operates in such a way that the analog simulator advances the simulation step by step while obtaining results from behavioral blocks. Further application of the periodic steady state (PSS) and periodic stability (PSTB) analyses, allowed us to obtain the frequency characteristics in short time. For verification, we simulated the circuits of the current-mode buck DC-DC converter IC, which was previously fabricated using a 0.18-um CMOS process. When we chose blocks of a buffer, the switching and output circuits and RS-FF real transistor circuits, while the rest behavioral models, we achieved transient response and frequency characteristics that were 13 and 75 times faster, respectively, than the analog simulator alone, while keeping less than 3.9% and 2.5% of errors compared with IC's evaluation results.
Frequency characteristics of the total feedback loop of a current-mode buck DC-DC converter in the discontinuous conduction mode (DCM) are examined by formulating an equivalent small-signal transfer function using a proposed block structure that does not suffer direct influence from the current feedback. Utilizing this transfer function, it is clarified that the type of the compensation slope in a current feedback loop affects the loop frequency characteristics in DCM, and in terms of stability, high voltage gain and large 0-dB frequency bandwidth, the quadratic compensation slope is more effective than the linear compensation slope. The validity of the proposed block structure and formulations and the effectiveness of applying a quadratic compensation slope instead of a linear compensation slope in DCM are verified by comparing calculation results, SPICE simulation results and evaluation results using circuit and measurement data from an MOS current-mode buck DC-DC converter IC that had been previously fabricated. More than 10-dB higher voltage gain, three times larger 0-dB frequency bandwidth, more than 10-degree larger phase margin and faster recovery time in load current change are realizable in DCM compared with the case using a linear compensation slope.
A current-mode control power convertor model that is accurate at
frequencies from DC to half the switching frequency is described for
constant-frequency operation. Using a simple pole-zero transfer
function, the model is able to predict subharmonic oscillation without
the need for discrete-time z -transform models. The accuracy of
sampled-data modeling is incorporated into the model by a second-order
representation of the sampled-data transfer function which is valid up
to half the switching frequency. Predictions of current loop gain;
control-to-output; output impedance; and audio susceptibility transfer
functions were confirmed with measurements on a buck converter. The
audio susceptibility of the buck converter can be nulled with the
appropriate value of external ramp. The modeling concentrates on
constant-frequency pulse-width modulation (PWM) converters, but the
methods can be applied to variable-frequency control and discontinuous
conduction mode
Owing to its simplicity in topological structure, the flyback switching converter has enjoyed dominance in the low-power, less than 200 W, power supply market. For example, in personal computers, switching converters in flyback configuration certainly hold an undisputable edge over other topologies. Beside being simpler in design, flyback operation also offers one distinctive advantage, which is the inherent output short circuit protection. This natural benefit stems from the fact that, for this topology only, the energy transferring action takes place during the flyback phase when the main switch is nonconducting. Therefore, the switch is isolated from the short induced peak surge and the maximum power available is limited to the extent that a pulse-width modulator can stretch.
A CMOS Current-mode DC-DC converter using a quadratic slope compensation scheme is presented. The use of a quadratic slope instead of a conventional linear slope makes a damping factor and a frequency bandwidth of a current feedback loop independent of the converter's output voltage. Further designing the coefficient of the quadratic slope to be fully dependent of the input voltage, the damping factor and the frequency bandwidth become completely independent of both the input and output voltages. A test chip of a buck converter in a 5 MHz operation which uses a quadratic slope compensation scheme has been fabricated by using a 0.35 mum CMOS process. The evaluation results show that with a current capability of up to 500 mA the frequency characteristics of the total loop are constant when the input and output voltages change from 3.3 V to 2.5 V and from 2.5 V down to 0.5 V, respectively, and also that the recovery time is 50 mus with a peak voltage deviation of less than 50 mV for load current changes from 20 mA to 200 mA and vice versa.