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TNY274-280 Datasheet(PDF) 10 Page - Power Integrations, Inc. |
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TNY274-280 Datasheet(HTML) 10 Page - Power Integrations, Inc. |
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10 / 24 page  Rev. N 12/21 10 TNY274-280 www.power.com value (10 W to 47 W) resistor in series with the bias winding diode and/or the OVP Zener as shown by R7 and R3 in Figure 14. The resistor in series with the OVP Zener also limits the maximum current into the BP/M pin. Reducing No-load Consumption As TinySwitch-III is self-powered from the BP/M pin capacitor, there is no need for an auxiliary or bias winding to be provided on the transformer for this purpose. Typical no-load consumption when self-powered is <150 mW at 265 VAC input. The addition of a bias winding can reduce this down to <50 mW by supplying the TinySwitch-III from the lower bias voltage and inhibiting the internal high-voltage current source. To achieve this, select the value of the resistor (R8 in Figure 14) to provide the data sheet DRAIN supply current. In practice, due to the reduction of the bias voltage at low load, start with a value equal to 40% greater than the data sheet maximum current, and then increase the value of the resistor to give the lowest no-load consumption. Audible Noise The cycle skipping mode of operation used in TinySwitch-III can generate audio frequency components in the transformer. To limit this audible noise generation the transformer should be designed such that the peak core flux density is below 3000 Gauss (300 mT). Following this guideline and using the standard transformer production technique of dip varnishing practically eliminates audible noise. Vacuum impregnation of the transformer should not be used due to the high primary capacitance and increased losses that result. Higher flux densities are possible, however careful evaluation of the audible noise performance should be made using production transformer samples before approving the design. Ceramic capacitors that use dielectrics such as Z5U, when used in clamp circuits, may also generate audio noise. If this is the case, try replacing them with a capacitor having a different dielectric or construction, for example a film type. TinySwitch-lll Layout Considerations Layout See Figure 15 for a recommended circuit board layout for TinySwitch-III. Single Point Grounding Use a single point ground connection from the input filter capacitor to the area of copper connected to the SOURCE pins. Bypass Capacitor (C BP) The BP/M pin capacitor should be located as near as possible to the BP/M and SOURCE pins. EN/UV Pin Keep traces connected to the EN/UV pin short and, as far as is practical, away from all other traces and nodes above source potential including, but not limited to, the BYPASS and DRAIN pins. Primary Loop Area The area of the primary loop that connects the input filter capacitor, transformer primary and TinySwitch-III together should be kept as small as possible. Primary Clamp Circuit A clamp is used to limit peak voltage on the DRAIN pin at turn off. This can be achieved by using an RCD clamp or a Zener (~200 V) and diode clamp across the primary winding. In all cases, to minimize EMI, care should be taken to minimize the circuit path from the clamp components to the transformer and TinySwitch-III. Thermal Considerations The four SOURCE pins are internally connected to the IC lead frame and provide the main path to remove heat from the device. Therefore all the SOURCE pins should be connected to a copper area underneath the TinySwitch-III to act not only as a single point ground, but also as a heat sink. As this area is connected to the quiet source node, this area should be maximized for good heat sinking. Similarly for axial output diodes, maximize the PCB area connected to the cathode. Y-Capacitor The placement of the Y capacitor should be directly from the primary input filter capacitor positive terminal to the common/ return terminal of the transformer secondary. Such a placement will route high magnitude common mode surge currents away from the TinySwitch-III device. Note – if an input π (C, L, C) EMI filter is used then the inductor in the filter should be placed between the negative terminals of the input filter capacitors. Optocoupler Place the optocoupler physically close to the TinySwitch-III to minimizing the primary-side trace lengths. Keep the high current, high-voltage drain and clamp traces away from the optocoupler to prevent noise pick up. Output Diode For best performance, the area of the loop connecting the secondary winding, the output diode and the output filter capacitor, should be minimized. In addition, sufficient copper area should be provided at the anode and cathode terminals of the diode for heat sinking. A larger area is preferred at the quiet cathode terminal. A large anode area can increase high frequency radiated EMI. PC Board Leakage Currents TinySwitch-III is designed to optimize energy efficiency across the power range and particularly in standby/no-load conditions. Current consumption has therefore been minimized to achieve this performance. The EN/UV pin undervoltage feature for example has a low threshold (~1 mA) to detect whether an undervoltage resistor is present. Parasitic leakage currents into the EN/UV pin are normally well below this 1 mA threshold when PC board assembly is in a well controlled production facility. However, high humidity conditions together with board and/or package contamination, either from no-clean flux or other contaminants, can reduce the surface resistivity enough to allow parasitic currents >1 mA to flow into the EN/UV pin. These currents can flow from higher voltage exposed solder pads close to the EN/UV pin such as |
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