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Switch-Mode Power Converters: Design and Applications

Learn about the application of switching transformers in switch-mode power converters for efficient and safe power supply systems.

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Switch-Mode Power Converters: Design and Applications

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  1. SWITCH-MODE POWER SUPPLIES AND SYSTEMS Lecture No 6 Silesian University of Technology Faculty of Automatic Control, Electronics and Computer Sciences Ryszard Siurek Ph.D., El. Eng.

  2. Switch-Mode Power Converters Application of the switching transformer - ensures galvanic isolationbetween output and input circuit (safety regulations, output voltage polarity not restricted) - small transformer dimensions according to high switching frequency - output voltage higher or lower than the input voltage independently of switching regulator configuration - possibility of efficient operation (with optimum duty cycle) in presence of high difference between input and output voltage (mains input voltage - very low output voltage) - several output voltages easily available

  3. Single-ended forward converter typical step-down regulator output filter Ip T IS D2 L Up UIN Uw t T D1 C R0 Zp ZS U0 CIN Za turns ratio Transformer model ideal transformer * rp Llp Lls Ip Iw = Iw /n IS FM Lp * Up US Up = USn IM n Assumptions: Llp, Lls = 0 rp, rs = 0

  4. Forward converter equivalent circuit I cycle 0 < t <t transistor T - ON , diode D2 - ON, diode D1 - OFF L T Ip = IT IS IL D2 FM Up US UIN IM R0 U0 C Lp D1 IM n IT IT IMmax IM IMmax t IL ILmax ILmin FM FMmax

  5. II cycle t < t < T Transistor T - OFF , diode D2 - OFF, diode D1 - ON Da L IS=0 IL T Ip = 0 Zp dUp D2 dUa UT FM dUS IM R0 U0 UIN Za C D1 Lp Up Ua=UIN n IM US=Up/n IT Up= IT IMmax When T switches off the overvoltage dUp appears across Zp and is transformed as dUS to the secondary side Diode D2 switches off , overvoltage is transformed to the winding Za and diode Da switches on The voltage Ua acrosss Za approaches the value of UIN and can not rise any more Voltage Ua=UIN is transformed to the primary winding Zp and is limited on the value Up The core of the transformer is being demagnetised during t1 (core reset) IM IMmax t T ILmax IL I0 ILmin FM IM FMmax current in Za current in Zp IMmax t t1 UT UIN

  6. Transformer core reset (magnetizing energy recovery)– detailed analysis UT dULL UT iZa(t) iZp(t) real overvoltage T UIN dULL LL UIN t1 UIN Lp t T Zs IZp IMmax Zp Za To keep dULL low, LL should be small enough - it requires very good magnetic coupling between the windings Zp and Za In practice Zp = Za and both windings are bifilary wound hence UTmax = 2UIN IZa FM FMmax Full demagnetization of the transformer is possible under the following condition:

  7. Typical magnetizing energy recovery circuits Da Zp=Za D2 L D1 C R0 Zp ZS U0 CIN Za UIN Cs overvoltage dumping circuit - snubbar circuit T Rs Ds Disadvantages: necessity of placing two bifilar windings , difficult construction, high transformer cost, problems with insulation, duty cycle limited to g < 0,5, snubbar circuit required to avoid voltage stress across the switching transistor Advantages: most of transformer magnetizing energy is recovered (higher efficiency), only one switching transistor, simple transistor gate drive circuit

  8. D2 L Rs Cs D1 C R0 Zp ZS U0 Up UIN CIN When Uo rises (higher value of Rs), core reset time t1 decreases and may be shorter than t. That is why the duty cycle may be higher (g > 0,5) T UT=Up+UIN Disadvantages: magnetising energy is dissipated in Rs, lower efficiency, high power resistor (resistors) are required, heating of some components, high coltage stress across the tswitching transistor ( UT > 2UIN) Advantages: simple and cheap transformer, duty cycle not limited to 0,5, no extra voltage spikes across the transistor – no influence of leakage inductance This configuration is not used in practice due to excesive power losses

  9. T1 D1 D2 L Ip US Up D1 C R0 Zp UIN ZS U0 CIN T2 Up=UIN g< 0,5 D2 Disadvantages: two switching components, complicated drive circuits, higher cost Advantages: simple transformer, no excesive voltage spikes – problem of leakage inductance does not exist, transistor voltahe does not exceed UIN This configuration often used in power supplies with higher output power – usually over 300 – 500W

  10. Magnetic core behaviour, magnetising curve ,core saturation B B Bs Bs H H Core saturation in case of improper transformer design -Bs -Bs iM(t) Ip FM IM demagnetising current FM magnetising current FMmax * iS(t) IM IMmax t t t t1

  11. Core saturation as a result of incomplete core reset (transformer demagnetisation) FM B Bs t H Ip t -Bs

  12. Output choke (inductance) saturation B IL Bs I1 DB B0 I0 t DH H H0(I0) H1(I1) Ip -Bs t

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