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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. 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 Single-ended forward converter typical step-down regulator output filter Ip T IS D2 Up UIN t T Uw Zp ZS CIN L D1 C R0 U0 Za Zp Up n ZS US turns ratio Transformer model Ip Up rp Llp Lls Lp FM rS rSn2 I*= I /n w w ideal transformer U* p = USn US IM n Assumptions: Llp, Lls = 0 rp, rs = 0 IS Forward converter equivalent circuit 0<t<t I cycle transistor T - ON , diode D2 - ON, diode D1 - OFF T Ip = I T UIN IM FM IS* IS D 2 Up US Lp IL D1 L R0 U0 C IM n IT IT ILmin n IL IM I*w IMmax ILmax IMmax t n IMmax ILmax is (t) ILmin ILmin FM IT I*s IM FMmax UIN t Lp Us U0 L t II cycle t <t <T Transistor T - OFF , diode D2 - OFF, diode D1 - ON Da UIN D2 dUS dUa Za Z F UIN p M Za Lp UT I M Ua=UIN IM IT IT ILmin I*w IM n ILmax IL IMmax ILmax IMmax t Up= Uwe Zp Za n T I0 ILmin F M IM FMmax current in Zp t UT IMmax IS=0 I*s 0 Zp dUp T Ip = 0 current in Za t1 UIN UT UIN Up UIN Zp Za Up n IL D1 L C R0 U0 US=Up/n 1. When T switches off the overvoltage dUp appears across Zp and is transformed as dUS to the secondary side 2. Diode D2 switches off , overvoltage is transformed to the winding Za and diode Da switches on 3. The voltage Ua acrosss Za approaches the value of UIN and can not rise any more 4. Voltage Ua=UIN is transformed to the primary winding Zp and is limited on the value Up 5. The core of the transformer is being demagnetised during t1 (core reset) Transformer core reset (magnetizing energy recovery) – detailed analysis UT UT iZa(t) iZp(t) T UIN UIN dULL UT UIN Zp Za t1 UIN Zp Lp Uwe Za Zp t Zs Za UIN dULL real overvoltage UIN dULL LL Zp T IZp IMmax Za To keep dULL low, LL should be small enough - it requires very good magnetic coupling between the windings Zp and Za IZa IMmax Zp Za In practice Zp = Za and both windings are bifilary wound hence UTmax = 2UIN IMmax FM FMmax UIN U t IN t1 t1 t Lp La Full demagnetization of the transformer is possible under the following condition: Τ t t1 t 0,5 Typical magnetizing energy recovery circuits Da Zp=Za D2 Zp CIN ZS L D1 C R0 U0 Za UIN Cs T Ds Rs overvoltage dumping circuit snubbar circuit Disadvantages: necessity of placing two bifilar windings , difficult construction, high transformer cost, problems with insulation, duty cycle limited to < 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 D2 Cs Rs Up Zp ZS L D1 C R0 U0 UIN CIN T UT=Up+UIN 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 ( > 0,5) 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 T1 D1 D2 Ip Up Z p UIN CIN US ZS D1 Up=UIN < 0,5 T2 D2 L C R0 U0 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 Magnetic core behaviour, magnetising curve , core saturation B T B T Bs Bs H A M H H I Z lg Core saturation in case of improper transformer design M BS -Bs -Bs iM(t) F M IM FMmax magnetising current t demagnetising current IMmax t1 A M Ip FM IM t iS*(t) t Core saturation as a result of incomplete core reset (transformer demagnetisation) FM B T Bs S BS S t H A M Ip H I Z lg M BS -Bs t Output choke (inductance) saturation B T IL Bs B0 I1 DB I0 DH H H0(I0) t A M H1(I1) Ip H0 -Bs I0 ZL lg t