Roche resonator - a PWM signal generator for DC/DC converters

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Roche resonator - a PWM signal generator for DC/DC converters [Copy link]

The DC/DC boost converter family is widely used in computer hardware and industrial applications [1,2,3], such as: computer peripheral power supply, automotive auxiliary power supply, servo motor drive power supply and medical equipment. In recent years, DC/DC converter technology has developed rapidly. For example, the Roche converter family and the Cuk converter have been developed [1,4,5,6]. The static switch S of the DC/DC converter can be a bipolar power transistor, MOSFET or IGBT, which needs to be driven by a PWM pulse train with adjustable frequency f and on-duty cycle k. Usually this PWM pulse train is generated by a digital signal processor (DSP). This paper introduces a Roche resonator, as shown in Figure 1. It can replace the DSP to generate PWM pulse trains to drive the static switch S. The Roche resonator can be integrated into the application-specific integrated circuit (ASIC) of the buck and boost portable DC/DC converter series.

　　The Roche resonator has an efficient and simple circuit structure, and it is easy to change the pulse frequency f and the conduction duty cycle k. It consists of three operational amplifiers (OA) and auxiliary components. The three operational amplifiers are named OA1, OA2 and OA3, all of which are μA741 and integrated into a TL074 chip (containing four operational amplifiers). Two potentiometers are used to adjust the pulse frequency f and the conduction duty cycle k respectively.

　　Before analyzing the Roche resonator, we first assume that the op amp is ideal, then:

　　(1) The open-loop voltage gain is infinite;

　　(2) The input resistance is infinite and the output resistance is 0;

　　(3) The maximum positive and negative output voltage is equal to the power supply voltage.

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Circuit Description The μA741 op amp OA can operate with a power supply voltage between ± (3 ~ 18) V. The power supply terminal and the ground terminal are respectively denoted as V +, V - and G, where 2 = V +. In Figure 1, OA2 is an integrator, and its output voltage VC is a triangular wave, and its frequency f = 1/T can be adjusted by potentiometer R4. OA1 is a resonator, and its output voltage VB is a rectangular wave with a frequency of f. OA3 is used as a comparator, and its output voltage VD is a rectangular pulse, and its on-duty ratio k can be adjusted by potentiometer R7. First, assume that when t = 0, VB = V +, and is positively fed back to OA1 through resistor R2. This keeps the output voltage of OA1 VB = V +. At the same time, VB is input to OA2 through R4, so the output voltage VC of OA2 decreases to mV - at a slope of 1/(R4C). The voltage VC is negatively fed back to OA1 through R3. (Usually, R3 is slightly smaller than R2, and its ratio is defined as m = R3/R2). The voltage VA at point A changes from [2mV＋/(1＋m)] to 0 in the time period 2mR4C. Therefore, the voltage VA decreases to a negative value, resulting in the output voltage VB=V－ of OA1 at t=2mR4C, and the voltage VA jumps to [2mV－/(1＋m）]. Similarly, when t=2mR4C, VB=V－, and is positively fed back to OA1 through resistor R2. This keeps the output voltage of OA1 at VB=V－. At the same time, VB is input to OA2 through R4, so the output voltage VC of OA2 increases to mV＋ at a slope of 1/R4C. The voltage VC is negatively fed back to OA1 through R3. The voltage VA at point A changes from [2mV－/(1＋m)] to 0 in the time period 2mR4C. Therefore, the voltage VA increases to a positive value, resulting in the output voltage VB=V＋ of OA1 at t=4mR4C, and the voltage VA jumps to [2mV＋/(1＋m)]. VC is input to OA3 and compared with the offset signal Voff-set through R6. The offset signal can be adjusted by potentiometer R7. When Voff-set=0, the output voltage VD of OA3 is a series of pulses with a conduction duty cycle k=0.5. When Voff-set is positive, the intersection with the voltage VC moves up, so the output voltage VD of OA3 is a series of pulses with a conduction duty cycle k<0.5. Similarly, when Voff-set is negative, the intersection with the voltage VC moves down, so the output voltage VD of OA3 is a series of pulses with a conduction k>0.5, as shown in Figure 2. The conduction duty cycle k is controlled by Voff-set through potentiometer R7. The calculation formula is as follows:,, The PWM pulse train VD is provided to the switch of the DC/DC converter, such as a power transistor, MOSFET or IGBT, through a coupling circuit. The waveforms of voltages VA, VB, VC and VD are shown in Figure 2. Design example: The internal circuit of the Roche resonator is shown in Figure 1, and its component parameters are: R0 = 10kΩ; R1 = R2 = R5 = 100kΩ; R3 = R6 = 95kΩ; R4 = 510Ω ~ 5.1kΩ; R7 = 10kΩ; C = 5.1nF. Therefore, m = 0.95 frequency f = 100kHz ~ 10kHz, and on-duty ratio k = 0 ~ 1.0.