Appliance Design - March 2008 - (Page 27)

POWER a wide range of frequency variation as the load changes. Another drawback is the increased conduction loss compared to the CCM operation, since the critical conduction mode operation requires a relatively small inductor. The conduction loss becomes more severe when applied to a universal input voltage application, since a smaller inductor is required to guarantee the critical conduction mode operation over the entire input-voltage range. The proposed hybrid control scheme allows the converter to operate in the conventional hard-switched pulse-width-modulated (PWM) mode when the converter operates in CCM, and operates in the quasi-resonant mode when operating in DCM. Thus, the proposed control scheme can enable quasi-resonant operation only when the converter operates at a mediumor light-load condition. This permits a relatively large inductor from the conventional PWM converter design and improves efficiency by reducing the switching loss at a light- to medium-load condition without increased conduction loss. posed control method for different load conditions. When the converter operates in CCM, the auxiliary winding voltage is clamped at Vo× Na /Ns until T1, and no oscillation of the auxiliary winding voltage is observed within the minimum switching period (Tsmin). Then, the controller performs the conventional PWM control by turning on the MOSFET at T1 (Operation A). Meanwhile, as the load decreases and the converter enters into DCM, oscillation of the auxiliary winding voltage is observed within the minimum switching period (Tsmin). Then, the controller allows the converter to operate in valley switched DCM (quasi-resonant operation). In DCM operation, the controller finds the first minimum drain voltage within the detection time window (TW) by monitoring the auxiliary winding voltage and turns on the MOSFET with minimum voltage (Operations B and C). Thus, the switching frequency is limited regardless of the operation mode as Fig. 2. Operation waveforms of the proposed method. 1 T min S Hybrid controller The quasi-resonant flyback converter topology can be derived from a conventional squarewave, PWM, flyback converter by operating the converter in the critical conduction mode. Because the MOSFET is turned on when the drain-source voltage reaches its minimum value by resonance, quasi-resonant operation reduces electromagnetic interference (EMI) while increasing power-conversion efficiency. However, the CCM operation undergoes a wide range of frequency variation as the load changes. As the load decreases, the switch’s “ontime” should decrease to reduce the peak drain current and therefore increase the switching frequency. This results in severe switching losses as frequency increases at a light-load condition. Another drawback is the increased conduction loss compared to CCM operation, since the critical conduction mode operation requires a relatively small inductor. The conduction loss becomes more severe when applied to a uni- fS 1 T min S TR versal input voltage application, since a smaller inductor is required to guarantee critical conduction mode operation over the entire input voltage range. To solve the drawbacks of the conventional quasi-resonant operation, the proposed hybrid control scheme combines the conventional PWM operation and quasi-resonant operation. The operation mode is adaptively selected, which allows the converter to operate in the conventional hard-switched PWM mode when the converter operates in CCM, while in quasi-resonant operation when operating in DCM. Fig. 1 shows the simplified schematic of the flyback converter employing the proposed control method. In order to monitor the drain voltage waveforms, the auxiliary winding voltage is used. Fig. 2 shows operation waveforms of the pro- Where, TR is the resonance period between the primary side inductance and MOSFET output capacitance. The above equation illustrates another benefit of the proposed controller. Normally, when the switching frequency decreases, a bigger core is needed in the Switch Mode Power Supply to avoid core saturation. Therefore, it is not easy to select an optimal core size when switching frequency varies widely like a conventional QR converter, and sometimes the selection of a bigger core is inevitable. Because the switching frequency variation is quite narrow due to the proposed scheme as described in Fig. 3, the transformer design becomes quite simple and cost effective, which is the advantage of a fixedfrequency flyback converter. Test results In order to demonstrate the validity of the proposed method, three 90~265 VAC/5 V, 30 W Table 1. Energy efficiency regulation for active mode. www.applianceDESIGN.com Table 2. Experimental flyback converter specifications. (Vin = 90~265 VAC, Vo = 5 V/6 A). applianceDESIGN March 2008 27 http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - March 2008

Appliance Design - March 2008 Contents Editorial Shipments/Forecasts News Watch A New Appliance is Set to Break into Homes Soon - the Micro CHP Unit, which Generate both Heat and Power. Thermally Enhanced Varistors Help Protect Low-Power Systems Against Damage Caused by Over-Current, Over-Temperature and Over-Voltage Faults. Hybrid Controller Reduces Standby Power Consumption and Improves Active-Mode Efficiency. Battery-Management ICs Solve Design Challenges for Cordless Appliances Using High-Voltage, Lithium-Ion Battery Technology. A Semiconductor Solution Protects the Relay in a Temperature Controller for a Cooking Appliance. New Polyurethane Foam Insulation System Optimizes both Insulation Performance and Productivity. Innovations in Decorative, Pre-Finished Metals Expand Range of Design Options for Appliance Designers. Design Marts Association Report: AHAM Advertiser's Index

Appliance Design - March 2008

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