Download Problem Set 7 for Resonant Techniques in Power Electronics | ECEN 5817 and more Assignments Electrical and Electronics Engineering in PDF only on Docsity! Problem Set 7 ECEN 5817 Spring, 2006 1 Snubber circuit in the flyback converter. The flyback converter illustrated below is a conven- tional hard-switched PWM converter. The transformer is modeled as illustrated: the model contains an ideal 1: n transformer, a magnetizing inductance L M , and a leakage inductance L l . The MOSFET contains an output capacitance C ds that can be modeled as a conventional linear capacitance having constant value C ds . The converter operates in the continuous conduction mode with small ripples in the magnetizing current and output voltage. You may also assume that V and I M are related to V g , R , and D by the usual ideal CCM PWM converter equations, which neglect losses, ringing, and other nonidealities. When the transistor turns off, the leakage inductance L l causes a voltage spike to be observed across Q 1 . This voltage spike is controlled and limited by a snubber circuit modeled here by diode D 2 and voltage source V s . Typical observed waveforms are illustrated in Fig. 1. (a) Sketch the normalized state plane trajectory for the intervals 1 to 5, and label the + – LM + v – Vg Q1 D11:n C Transformer model iM R L l i l Cds + vds – + – D2 Vs i l vds Conducting devices: Q1 D1 D2 D1X 1 2 3 4 5 D1 Vg + V/n IM Vs Fig. 1 Flyback converter with voltage-clamp snubber, and observed waveforms. important features. You should normalize all voltages by dividing by V base = V g . Use a base current of I base = V g / R 0 . (b) Sketch the waveform of the current i D 2 flowing through diode D 2 , and derive an expression for its average value 〈 i D 2 〉 . Hence find an expression for the power flowing into the snubber: 〈 P snubber 〉 = V s 〈 i D 2 〉 . Your approach should involve solution of the normalized state plane from part (a); no credit will be given for methods that do not employ state plane analysis. Express your answer in terms of V g , R 0 , and the following normalized quantities: Next, this converter is constructed and operated with the following element values and specifications: V g = 400 V V = 42 V V s = 700 V D = 1/3 C ds = 100 pF Load power: P = 100 W f s = 250 kHz The transformer is constructed so that the magnetizing current ripple ∆ i M is equal to 20% of the dc com- ponent of magnetizing current I M . It is also found that the leakage inductance L l is equal to 2% of the magnetizing inductance L M . (c) For these operating conditions, compute the numerical value of 〈 P snubber 〉 . (d) The snubber is realized as illustrated in Fig. 2. Capacitor C s is large, and has negligible switching ripple. Determine the value of R s that causes V s to be equal to 700 V. F = fs f0 M s = Vs Vg M = VnVg Qe = R n2R0 + – LM + v – Vg Q1 D11:n C Transformer model iM R L l i l Cds + vds – D2 + Vs – Rs Cs Fig. 2 Realization of snubber using capacitor and resistor.
