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AN1453 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN1453 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 9 page APPLICATION NOTE 5/9 Without a snubber, in this example the diode is repeatedly in conduction because the oscillation is very strong. Furthermore, the voltage is close to the breakdown voltage. This means that the system is no longer reliable and a snubber circuit is necessary. On these 2 oscillograms, we can see that the value of the maximum reverse current (IRM) is defined when the reverse voltage rises (typical behaviour of a bipolar diode). At this time the voltage is not fixed by the diode. The curve Qrr,IRR versus dIF/dt and Tj is given in the datasheet. For example in Fig. 12, the evolution of IRM versus dIF/dt for a STPR1620CT can be observed. It can be also noticed, that the parameter IRM significantly increases with the temperature. In continuous mode the dIF/dt (few hundred A/µs) is fixed by the leakage inductance and the reverse voltage (VR): dI dt V L with V n n VV FR f R s p IN out == ⋅ + It is many time higher than in discontinuous mode (lower than 1A/µs): dI dt V LL withL L F out Sf Sf = + 〉〉 (LS: Secondary inductance) Thus, with this curve we can see that, in continuous mode (high dIF/dt), the bipolar diode must evacuate a non-negligible charge, which means a higher IRM. This is verified on oscillogram Fig. 11. With this value of IRM, an equivalent model at t0 with a snubber circuit can be established: Where: Vs: secondary voltage V n n V S s p IN =⋅ Lf: leakage inductance of the transformer Cj: junction capacitance CQrrb: equivalent capacitance modeling the reverse charge, necessary for the establishment of the potential barrier, which supports the reverse voltage. Vout: output voltage With the following initial conditions at t=t0: I I and V 0 LRM D f bipolar =≈ The equivalent schematic can be used to define VV DRmax = NB: 1) Without snubber, there is a L f, C circuit ( C=C j +C Qrrb) which lead to a second order differential equation: dV dt V V 0 and 1 / L C 2 C 2 CR f +⋅ +⋅ = = ⋅ ωω ω 0 2 0 2 0 2 with initial conditions at t=t0: I I and V V 0 LRM C D f0 == = In this equation, an approximation is made with C constant, because in reality Cj and CQrrb vary with the voltage applied. The solution of the differential equation gives us: VV V V I L C RD R R 2 RM f 2 max == + + ⋅ Therefore we can see that the VRmax depends the leakage inductance (Lf) and on recovery charge (IRM). Thus, V Rmax is very dependent on the temperature. STPR1620CG/CT 10 20 50 100 200 500 1 10 20 dIF/dt(A/µs) IRM(A) IF=IF(av) 90% confidence Tj=125°C Fig. 12: Peak reverse recovery current versus dIF/dt (per diode) V D V D V s L s n p L P R s C s C Qrrb C j C s R s L f I=I L f RM n s V= V + V R S o u t V o u t Fig. 13: Equivalent model at t0 for a bipolar diode |
Similar Part No. - AN1453 |
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Similar Description - AN1453 |
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