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DPA426R-TL Datasheet(PDF) 7 Page - Power Integrations, Inc. |
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DPA426R-TL Datasheet(HTML) 7 Page - Power Integrations, Inc. |
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7 / 20 page  AN-31 C 7/04 7 Ifpowerdissipationisaproblemwiththesmallestdevice,select the next larger device and program the current limit with the X pin to 10% above the peak primary current. This is done to limit overload power capability. Refer to the DPA-Switch data sheet to determine the value of the resistor on the X pin that corresponds to the desired current limit. Figure 3 illustrates how the efficiency is related to the selection of the DPA-Switch. Devices with lower R DS(ON) dissipate less power where resistive voltage drop dominates the loss. Thus, the efficiency is higher for larger devices at low input voltage. At higher input voltages the RMS current in the DPA-Switch decreases and the loss from capacitance on the drain increases, so the lower R DS(ON) has virtually no effect on efficiency. Clamp Circuit All applications must protect the DPA-Switch from excessive drain voltage. Figure 1 shows a simple and effective solution. A Zener diode from the drain to source provides a hard clamp. The 30 W prototype example (Table 1), uses a 150 V Zener to guarantee substantial margin from the breakdown voltage of 220 V. A small capacitor across the primary of the transformer may be necessary in conjunction with the Zener clamp (see Figure 4). The designer should put a placeholder for this capacitor on the initial prototype. In some designs there is sufficient stray capacitance on the primary of the transformer to remove the need for this clamp capacitor. Bench tests will determine whether the capacitor is required to maintain safe drain-to- source voltages. In normal steady-state operation, the capacitor C CP across the primary of the transformer absorbs energy from leakage inductance to keep the drain-to-source voltage below the Zener voltage. There is an optimum value for C CP that typically ranges between 10 pF and 100 pF for converters in the range of 10 W to 40 W. The value of C CP depends on the leakage inductance and the peak current. The proper value of capacitance will allow most of the energy in the leakage inductance to be recovered during the next switching cycle. Too little capacitance will cause the Zener diode to conduct. Dissipation in the Zener will reduce efficiency. Too much capacitance will also reduce efficiency because it will increase turn-on losses in the DPA-Switch and may also interfere with the reset of the transformer. The Zener diode does not conduct during normal steady-state operation, but it is required to limit the drain voltage during start-up, transient loading and overload conditions. At higher powers, the clamp capacitor value (C CP), becomes a limiting factor on the efficiency of the power supply. Different techniques can be used for these higher power applications (above approximately 40 W). Figures 5 and 6 show a non- dissipative clamp technique that also resets the transformer. See references [4] and [5] for a description of this technique. Transformer Reset Circuit The flux in the magnetizing inductance of the transformer must be reset in each switching cycle to maintain volt-seconds PI-2875-062204 D S C CONTROL CONTROL VR1 C S R S DC INPUT DC INPUT DPA-Switch U1 C CP + PI-3474-032603 D S CONTROL CONTROL DPA-Switch DC Input Voltage D2 L1 D1 C1 V1 VDS + Figure 4. Components of the Transformer Clamp and Reset Circuit. Figure 4. LC (Inductor Capacitor) Reset and Clamp. |
Similar Part No. - DPA426R-TL |
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Similar Description - DPA426R-TL |
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