IC资料-AN1680.pdf
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1、 1Motorola Applications Data ? ? ? ? ? ? ? ? ? ? ? ? ? ? Christophe BASSO, MOTOROLA SPS BP1029, Le Mirail, 31023 Toulouse France email: R; Tel.: 33 5 61 19 90 12 In the large family of SwitchMode Power Supply (SMPS) components, the recently introduced highvoltage monolithic switch- ers start to play
2、 an important role. First of all because they provide an easy mean to instantaneously build an efficient offline supply but also because their internal structure offers everything a designer needs: internal clock, pulsebypulse limitation, Leading Edge Blanking (LEB) etc. However, the internal MOSFET
3、 exhibits a lowenergy capability bodydiode which no longer protects the device against accidental avalanche. This element thus needs an adequate protection network against the electromagnetic leakage energy. This paper details what network is best adapted to the protection of these devices and how t
4、o predict its efficiency in the application. The Leakage Inductance Figure 1 shows a transformer wound across a standard magnetic material. The primary side made of Np turns creates the nec- essary force F which gives birth to two components: ?m who links both windings, but also ?l1 which does not c
5、ouple to the second- ary and corresponds to a leakage path through the air. Thanks to ?m, a current Io circulates in the secondary, but this current also gives birth to another leakage flux ?l2 whose polarity is opposite of that of ?m. It is important to note that ?m produces Io while ?l2 is a conse
6、quence of it. Figure 1. A Twowinding Transformer Showing the Leakage Paths Ns Np Ip Io Vo Vexcitation ?m ?l1 ?l2 Magnetic material with a linear reluctance m Order this document by AN1680/D ? SEMICONDUCTOR APPLICATION NOTE Motorola, Inc. 1999 ? 2Motorola Applications Data Figure 2. A Simple FLYBACK
7、Configuration Implementing a Clamping Network C3 150 ?F Vout Rp 100 m 1 16 14 Lleak 4 ?H 4 VDCmains 330 V + MC 3337X Rload 5 Vclamp 250 V Dclamp C6 10 nF R6 3.6 C1 50 ?F Lp 250 ?H C5 10 ?F D1 1N4146 Aux Int C2 1 mF MOC8101 R12 270 UP TI Aux R8 15 k C4 100 n R7 75 k L3 5 ?F UP TI X2 MBR20100 MC33374
8、VCC GND CTL MOS FB RATIO_POW = 0.183 RATIO_AUX = 0.142 Int 12 15 16 2 3 7 17 6 13 10 + As one can see from the picture, l1 and l2 close through the air. As any (magnetic) medium, the air is affected by a Reluctance , or its inverse, the permeance P. These permeances create in the primary and seconda
9、ry two leakage inductance with a value of: Lleak = N2 Pair, with N being the primary or secondary turns. As an effect, these parasitic leakage elements degrade the energy transfer between the primary and secondary (ies). In a FLYBACK converter, the presence of the leakage element will a) generate a
10、voltage spike at turnoff and b) divert a portion of the primary current into the clamping network. Point a) implies the use of protec- tion network to prevent a lethal drain voltage excursion while point b) explains the root of a degraded openloop gain: the peak current needs to be higher than theor
11、etically calculated to deliver the full rated output power. The Principle of a Protection Network The goal of the clamping network is to prevent the drain voltage to exceed a given limit. For instance, in the new MOTOROLA MC3337X family, the maximum voltage shall stay within 700 V. Worse case arises
12、 when the mains is a its highest level, e.g. 285 VAC in a universal mains application. To prevent the drain from reaching this value, Figure 2 shows how a perfect network would work: when the MOSFET closes, the current buildsup in both primary and leakage coils. When the ON periods stops, the MOSFET
13、 opens and interrupts its current. Since no current discontinuities can take place in an inductor, both magnetic fields col- lapse and the voltage across the inductances reverses in an attempt to keep the ampsturn constant: Lp energy is thus coupled to the secondary and gives birth to the output cur
14、rent charge. Since Lleakage cannot find a circulating path, it pullsup 2 the drain voltage until Dclamp starts to conduct and protects the switcher at a maximum theoretical level of: Vmains + Vclamp = 650 V. Figure 2b shows the results of an INTUSOFTs IsSpice4 (SanPedro, CA) simulations. When all th
15、e leakage energy is released, a short parasitic oscillation takes place involving Lleakage and all the parasitic capacitive elements present in the circuit (transformers pri- mary capacitance, MOSFETs Coss etc.) CLIPPING EFFECT Figure 2b. The Drain is Safely Clipped Below 700 V at High Mains Fosc? 1
16、 2 ? ? ?Lleak? Clump ? VDS 300 100 100 500 700 210.20U211.20U212.20U213.20U214.20U X = 212.29U Y = 650.84 ? 3Motorola Applications Data Figure 2c. Waveforms at Turnoff: the Leakage Coil Prevents an Immediate Transfer ton ?t Vclamp? Vo? N Lleak Vo? N Lp Ip Isecondary ILleak Vin/Lp Iprimary Ipx Iprima
17、ry ILleak Diverting the Primary Current Figure 2 is interesting because it helps understanding how the reflected secondary voltage resets the leakage energy and how much of primary current this leakage inductance “steals away” by diverting it into the clamp. Everything is detailed on Figure 2c graph
18、. When the MOSFET turnsoff, a reset voltage is applied to the leakage inductance. This reset voltage depends on the clip- ping voltage but also on the FLYBACKs. The higher this level, the faster the leakage energy drops to zero and authorizes the sec- ondary current to take place. The time t needed
19、to complete the energy transfer is easily defined by: ?t ? Lleak? Ip V clamp ? (V out ? Vfsec) ? N, where Ip is the final primary current, N the transformer ratio secondary to primary, Vfsec the secondary diode forward drop and Lleak the primary leakage inductance. Estimating the percentage of diver
20、ted current tells you the real peak current you will actually put in the primary to deliver the rated power. Figure 2cs Ipx point shows where the secondary diode catchesup with the primary current. The slope of the decreasing primary current is simply N ? (Vout ? Vfsec) Lp , but this equation can al
21、so be written as: Ip ? Ipx ?t ? N ? (Vout? Vfsec) Lp . Replacing t and solving for Ipx gives: Ipx Ip ? 1 ? Lleak Lp ? Vclamp (Vout?Vf)?N ? 1? . This last equation gives you the effective percentage of primary current stolen by the leakage inductance. Applying Figure 2 numerical values gives: Ipx = 9
22、8.4% of Ip. Since Ip grows up to 2.73 A, then the theoretical peak secondary current establishes at: 0.984 2.73 12.5 = 33.58 A. Figure 2d validates the calculation. x = 202.37U y = 33.600 Ip = 2.73 A Ileakage Iprimary Figure 2d. IsSpice4 Simulation of Figure 2s Circuit 10 20 30 0 200.60U201.79U202.9
23、8U204.17U205.37U Isec. peak Vout turnoff = 10 V Protecting the MOSFET with an Active Element The easiest way to clamp at a known level is to replace the nullimpedance Vclamp source by a zener diode or a transient sup- pressor. Figures 4 and 4b detail the connections and their associated waveforms. S
24、ince we have two diodes in series, we have ? 4Motorola Applications Data to care about both dissipated powers. Diodes can be modeled by a voltage source V (which equals Vzener or Vforward) in series with a dynamic resistance Rd. The total average conducted power dissipated is therefore: Pavg = V Iav
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