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   1N5820, 1N5821, 1N5822 http://onsemi.com 4 NOTE 3 — DETERMINING MAXIMUM RATINGS Reverse power dissipation and the possibility of thermal runaway must be considered when operating this rectifier at reverse voltages above 0.1 VRWM. Proper derating may be accomplished by use of equation (1). TA(max) = T J(max) R JAPF(AV) where TA(max) = Maximum allowable ambient temperature RJAPR(AV)(1) TJ(max) = Maximum allowable junction temperature (125°C or the temperature at which thermal runaway occurs, whichever is lowest) PF(AV) = Average forward power dissipation PR(AV) = Average reverse power dissipation RJA = Junction-to-ambient thermal resistance Figures 1, 2, and 3 permit easier use of equation (1) by taking reverse power dissipation and thermal runaway into consideration. The figures solve for a reference temperature as determined by equation (2). TR = T J(max) R JAPR(AV) (2) Substituting equation (2) into equation (1) yields: TA(max) = T R R JAPF(AV) (3) Inspection of equations (2) and (3) reveals that TR is the ambient temperature at which thermal runaway occurs or where TJ = 125 °C, when forward power is zero. The transition from one boundary condition to the other is evident on the curves of Figures 1, 2, and 3 as a difference in the rate of change of the slope in the vicinity of 115°C. The data of Figures 1, 2, and 3 is based upon dc conditions. For use in common rectifier circuits, Table 1 indicates suggested factors for an equivalent dc voltage to use for conservative design, that is: VR(equiv) = V (FM) F (4) The factor F is derived by considering the properties of the various rectifier circuits and the reverse characteristics of Schottky diodes. EXAMPLE: Find TA(max) for 1N5821 operated in a 12-volt dc supply using a bridge circuit with capacitive filter such that IDC = 2.0 A (I F(AV) = 1.0 A), I (FM)/I(AV) = 10, Input Voltage = 10 V(rms), RJA = 40°C/W. Step 1. Find VR(equiv).Read F = 0.65 from Table 1, VR(equiv) = (1.41) (10) (0.65) = 9.2 V. Step 2. Find TR from Figure 2. Read T R = 108 °C @ VR = 9.2 V and R JA = 40 °C/W. Step 3. Find PF(AV) from Figure 6. **Read P F(AV) = 0.85 W @I(FM)I(AV) 10 and IF(AV) 1.0A. Step 4. Find TA(max) from equation (3). TA(max) = 108 (0.85) (40) = 74 °C. **Values given are for the 1N5821. Power is slightly lower for the 1N5820 because of its lower forward voltage, and higher for the 1N5822. Variations will be similar for the MBR-prefix devices, using PF(AV) from Figure 6. Table 1. Values for Factor F Circuit Half Wave Full Wave, Bridge Full Wave, Center Tapped*? Load Resistive Capacitive* Resistive Capacitive Resistive Capacitive Sine Wave 0.5 1.3 0.5 0.65 1.0 1.3 Square Wave 0.75 1.5 0.75 0.75 1.5 1.5 *Note that VR(PK) 2.0 V in(PK). ?Use line to center tap voltage for Vin. |