2.1 Proposed control topology
The control topology is shown in Fig. 6. From Fig. 6, we can find that the controller has: (a) the ON/OFF time of the switches S1 and S2 are controlled by two modules, “Adaptive TON control” and “Adaptive TOFF control”. (b) the voltage Vo feeds into the “Adaptive TON control” and “Adaptive TOFF control” modules by two paths (loop A and loop B). (c) the constant frequency mechanism module can regulate the ON/OFF time of the switches S1 and S2 to keep the switching frequency of S1(S2) constant. (d) the control signals PH1 and PH2 are inverted to each other.
2.2 Circuits implementation
Based on the control topology in Fig. 6, the circuit implementation is shown in Fig. 7. From Fig. 7, we can find the following points: (a) the circuits are simple, (b) no special processes are required, (c) the constant frequency mechanism is mainly based on the PLL design concept, and its modules include a phase frequency detector, a charge pump, and a low pass filter, (d) the controller mainly detects the voltage Vo to control the ON/OFF state of the switches.
2.3 Controller operation principle
Before the description, we define the turn-on time of the switch S1 as TON, and the turn-off time as TOFF. In addition, whenever the switch S1 is ON, then the switch S2 must be OFF and vice versa. Each module operates as described below:
a. Adaptive TON control:
The internal signal Vramp is an important signal in deciding the TON. Once the slope of the Vramp is higher, the TON will be shorter. Moreover, the slope of the Vramp is affected by the voltages Vo and VCTR. Briefly, the TON is related to the Vo and VCTR as follows: (1) The higher Vo is, the shorter TON is, and (2) The higher VCTR is, the longer TON is. Besides, another control signal VCMP also affects TON, and its correlation is that the higher VCMP is, the longer TON is.
b. Adaptive TOFF control:
Similar to the adaptive TON control module, the voltage Vramp2 in the adaptive TOFF control module (Fig. 7) is used to decide the TOFF. The TOFF is regulated by the Vo, VCMP and VCTR. The relations between Vo, VCMP and VCTR are as follows: (1) The higher Vo is, the longer TOFF is, (2) The higher VCMP is, the shorter TOFF is, and (3) The higher VCTR is, the longer TOFF is.
c. Constant frequency mechanism:
The module mainly keeps the switching frequency of the controller constant. In this paper, the REF_CLK is set to 1MHz, so the switching frequency will also be maintained at 1MHz. Why should the switching frequency of the converter be constant in this paper? This is because all SMPS (switch-mode power supply) converters produce switching noise that affects the surrounding electrical equipment through the power lines. Therefore, an EMI filter is usually added to the front of the converter to suppress the effects of switching noise. The in/out relationship of this module is such that if the FB_CLK frequency is higher than the REF_CLK frequency (i.e., FB_CLK frequency > 1MHz), then VCTR is increased and vice versa (Fig. 7).
d. EA (error amplifier):
The EA senses the voltage Vo to control TON/TOFF. The control loop formed by EA will cause Vo to follow VREF. Finally, the voltages VFB and VREF will be equal in steady state (Fig. 7).
Summarizing (a) to (d), we illustrate the key waveforms in Fig. 8.