Design and Measurement of Oscillator Built in a PWM IC for the Flyback Converter

Transcription

1 Design and Measurement of Oscillator Built in a PWM IC for the Flyback Converter Min-Sung Kim 1, Hyoung-Woo Kim 2, Kil-Soo Seo 2, Nam-Kyun Kim 2, Pan-Bong Ha 1 and Young-Hee Kim 1 1 Changwon National University, Changwon, Republic of Korea 2 Power Semiconductor Research Center, Korea Electrotechnology Research Institute {minsungkim@changwon.ac.kr, hwkim@kere.re.kr, ksseo@keri.re.kr, nkkim@keri.re.kr, pha@changwon.ac.kr, youngkim@changwon.ac.kr} Abstract. In this paper, a PWM (Pulse width modulation) IC with stand-by power control function to reduce stand-by power of the AC-DC flyback converter is designed and measured. Also, an oscillator of current increase and decrease feedback structure which is insensitive to PVT (Process, voltage, and temperature) variation to enhance the accuracy of driving frequency when the flyback converter is operating in either no load or light load condition is proposed. It is confirmed by measurements that the proposed oscillator has an error of 1.80% and a mean of 49.10kHz according to charge in the capacitor used in setting the driving frequency. Keywords: Oscillator, PWM, Flyback Converter 1 Introduction Due to recent global energy crisis and intensified environment regulations, eco-friendly and power-saving green energy technology with reduced stand-by power consumption function has been the topic in the power semiconductor industry. Power ICs controlling power supplies have been developed in the direction of multi function, high efficiency, low power, and low loss. Hereupon, an oscillator enhancing the accuracy of driving frequency is proposed and included in the designed PWM IC for the flyback converter [1] to reduce its stand-by power and power consumption. There is a disadvantage of a decreased accuracy in driving frequency due to ringing phenomenon occurring in switching and comparing operations with the reference voltage for an essential oscillator driving and controlling switching devices, process parameter variation, and a thermal characteristic variation [2]. To solve this problem, an oscillator of current increase and decrease feedback structure which is insensitive to PVT variation to enhance the accuracy of driving frequency and also to reduce unnecessary power consumption occurring in switching operations is proposed. CES-CUBE 2013, ASTL Vol. 25, pp , 2013 SERSC

2 Proceedings, The 3rd International Conference on Circuits, Control, Communication, Electricity, Electronics, Energy, System, Signal and Simulation 2 Circuit Design 2.1 Design of driving frequency generator with its reliability increased In general, a Schmidt trigger type oscillator circuit of current mirror type has been used to generate a low frequency such as 50kHz [3]. In contrast, the driving frequency of an on-chip oscillator is changing due to such variations as currents, capacitances, and propagation delays from process and temperature changes [4]. Also, there is a disadvantage that it is very hard to secure the reliability of a control IC in the system requiring a fixed frequency because of different on or off time of the driving frequency. To solve these problems, an oscillator circuit of current increase and decrease feedback structure is proposed in this paper as shown in Fig. 1. The proposed oscillator can change the time position of each generated pulse by comparing the reference voltage with the increased or decreased current from the nominal position of 50kHz. Also, the current change by process and temperature is overcome by current source circuits generating currents insensitive to PVT variation. The operating principle of the proposed oscillator is in the following. Fig. 1. Oscillator of current increase and decrease feedback structure. An oscillator driving signal is generated by an inverter operation in case that Vramp reaches a set-up threshold through a charging and discharging of a capacitor connected to CF node. Then, Comparator_1 increases or decreases the amount of current repeatedly by sensing changes in capacitor charge and Vramp slope. Vramp voltage is kept constant by this feedback operation and the accuracy of the oscillator output with driving frequency of 50kHz is secured by a comparison of Vramp with a set-up voltage Vref_2. Table 1 shows simulation results of driving frequencies for the proposed oscillator. The proposed oscillator limits the maximum operating frequency to kHz in case that charge in the capacitor is abundant. Also, the operating frequency is limited to kHz by reducing the charge in the capacitor through a feedback of Comparator_1 sensing capacitor charge variation and Vramp slope. 291

3 Design and Measurement of Oscillator Built in a PWM IC for the Flyback Converter Table 1. Simulated result of driving frequencies for the reliability enhanced oscillator. Node_A mean Node_B Time[us] Frequency[kHz] Error[%] It is verified by simulations that the proposed oscillator has a driving frequency of kHz on the average and a frequency variation width of ±2.029kHz. 3 Measurement Results The proposed PWM control IC is applied to a flyback converter and drives the power MOSFET. Thus, it is very important to secure the accuracy of the oscillator driving signal determining the turn-on duty ratio of the power MOSFET. Fig. 2 shows measured waveforms of the proposed oscillator with a driving frequency of 50kHz. Fig. 3. Measured waveforms of reliability increased oscillator. Driving frequency variation is limited to a nominal time location of 50kHz by a feedback 292

4 Proceedings, The 3rd International Conference on Circuits, Control, Communication, Electricity, Electronics, Energy, System, Signal and Simulation operation to keep Vramp constant and simulation results of oscillator driving frequencies are shown in Table 2. It is confirmed by measurements that the proposed oscillator has a driving frequency range from 48.05kHz to 50.75kHz according to charge in the capacitor and the driving frequency has an error of 1.80% and a mean of 49.10kHz through continuous feedback operation. Table 2. Measured driving frequencies. Node_A mean Node_B Time[us] Frequency[kHz] Error[%] Conclusion This research was done to reduce stand-by power consumption [5] which has been highlighted due to global energy crisis and environment regulations recently. To this end, a PWM control IC for SMPSs applied to 10W-class AC-DC flyback converters and an oscillator of current increase and decrease feedback structure which is insensitive to PVT variation to enhance the accuracy of driving frequency, that is the major technology of the SMPSs, was designed and measured. Fig. 3. Bare chip image of the PWM control IC. 293

5 Design and Measurement of Oscillator Built in a PWM IC for the Flyback Converter Fig. 3. shows the bare chip image of the PWM control IC including the pad with ESD (Electrostatic discharge) protection function. Its size is 2,700um 2,700um. Also, the control IC was designed with an X-Fab 1.0um modular DIMOS 650V process and verified with Cadence Spectre verification tool. It was confirmed by measurements that the proposed oscillator had a driving frequency range from 48.05kHz to 50.75kHz and a good driving frequency variation from 0.14kHz to 1.28kHz. The mean value was 49.10kHz. Soon, stand-by power consumption will be measured with the PWM control IC applied to a 10W-class AC-DC flyback converter application board. Acknowledgement. This work was supported by Industrial Strategic Technology Development Program funded by the Ministry of Knowledge Economy of Korea ( , Development of Power Management System SoC Supporting Multi- Battery Cells and Multi-Energy-Sources for Smart Phones and Smart Devices ). References 1. M.S Kim, H.W Kim, J.H Jang, K.H Kim, K.S Seo, N.K Kim, Y.H Kim, Advanced burst mode control to reduce the standby power of flyback converter, Engineering, Vol. 5, No.1B, pp , January Behzad Razavi, A study of phase noise in CMOS oscillators, IEEE Journal of Solid-State Circuits, Vol. 31. pp , Mar S.F, Al-Sarawi, Low Power Schmitt Trigger Circuit, Electronics Letters, Vol. 38, pp , 29 Aug Sekedi Bomeh Kobenge, Huahong Yang, Power Optimized Digitally Programmable Delay Element, WSEAS international conference on Robotics, control and manufacturing technology, Vol. 9, pp , Laurence McGarry, Standby Power Challenge, International IEEE Conference on the AGEC, pp.56-62,