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SWITCH-MODE POWER SUPPLIES AND SYSTEMS Lecture No 10 Switching transformer design rules. Power losses analysis in switching regulators Silesian University of Technology Faculty of Automatic Control, Electronics and Computer Sciences Ryszard Siurek Ph.D., El. Eng. Flyback converter transformer UIN IT iT (t) t D1 I0 Lp IC Ipmax UIN UIN C R0 Zp ZS n U0 t B IT BS T energy storing during cycle I B UIN zp d H B Se dt he nce: UIN t UIN zp Se dB zp Se B zp dt t B Se Minimum number of zP turns assuming t = tmax, B = Bs, UIN = UINmax: UINmax tmax zpmin Bs Se Assuming required output power equal to P0 zpmin – is set for the chosen core 2 LpIpmax 2 LpIpmax E P0 lg Lp P0 2 2T U2 P0 we t U 2Lp Imax we tmax Certain air-gap is necessary to Lp achieve required output power Cycle II - transistor T is off ID U0 iD (t) IDmax t ID D1 I0 IDmax LS IC T ZS U0 C R0 t t’ B U0 BS I2 LS Dmax E2 Energy stored in the core is trans- 2 fered to the output during cycle II H E 2 U0 U2 2 LSIDmax U I P IDmax t' LS 0 t'2 2T 00 0 2P0 T T LS 2 Lp 2 UIN t 2 zp 2 UIN t 2 zp U t U t' 2 2 IN zS zp 0 L S U0 t'2 z 2 U0 t'2 z S U0 t' UIN t S Selecting t’ < T-t for the maximum output power Po one decides to work with the discontinuous magnetic flux flow in the whole range of load changes. To increase t’ one must also increase LS, and it is related to higher number of turns of the secondary winding zS. When t’ = T-t transformer starts to operate with continuous magnetic flux flow. 2 U IN 2 For discontinuous n For continuous U0 IN flux flow U0 flux flow U 2LpfI0 n 1 Flyback transformer design simplified procedure 1. Select maximum (nominal) output power Po 2. Select switching frequency – basing on specifications of available magnetic material, semiconductors etc. 3. Calculate tmax, current Imax and required value of Lp 4. Select core dimensions accordind to Hahn diagrams or using „AP” method (same as in inductor design procedure) 5. Calculate (find from diagrams) the air-gap 6. Calculate minimum number of primary turns, calculate required number of turns zP 7. Select operating pronciple (continuous or discontinuous flux flow) 8. Calculate secondary number of turns zS Usually discontinuous flux flow is observed in flyback converters due to the following reasons : - lower number of winding turns (lower „copper” power losses) - lower level of EMC disturbances (transistor switches on with current equal to 0) - self-oscillating converter is very easy to design (low-cost solution) Forward converter transformer Ip ΙS Ip IS n IM UIN Zp ZS US Lp US n transformer equivalent circuit UIN ΙS UIN t IM t 02 Lp Lp n I 0,2 S n 1. Selection of the core - basing on diagrams (nomograms etc.) relating core dimensions to total power for certain converter topology 2. Calculation of minimum number of turns for the primary winding to avoid saturation in most unfavourable operating conditions UINmax tmax Equation identical for any zpmin Bs Se converter topology 3. Selection of wire cross section (diameter) taking into account primary current RMS value and calculation of number of turns for required winding inductance Lp (using Al constant for selected core) – the following condition must be performed: zp > zpmin 4. Calculation of secondary winding (windings) number of turns zS U0 1 U0 UIN z S zp zp UIN 5. Calculation of wire cross section area (copper strip, litz wire) for secondary winding resulting from secondary current RMS value ISrms=nIprms 6. Checking if it is enough space to place windings in the core (bobbin) window area – required isolation and winding arrangement according to safety standards must be considered secondary bobbin windind magnetic core leakage distance (6 mm) Safety insulation (3 layers) primary winding functional insulation 3 mm (between winding layers) General notes 1. Core power loses are higher when frequency and flux density amplitude increase - that is why the high value of primary inductance Lp is desirable 2. High Lp value is related to more primary turns – more trouble with placing the winding in the bobbin and higher „copper” losses - look for optimum settlemet! 3. Chose the magnetic core with best available performance – high saturation flux density Bs, lowest power losses, smallest dimensions 4. Small air-gap in transformer core may be considered (forward converter) – better utilisation of the core may be achieved by lowering magnetic remanence B B - without air-gap B - with small air -gap H 5. Remember that Bs value decreases with temperature – at 100oC it is lower by 20% – 25% in comparison to the value specified at 25oC Switching regulator power losses analysis 1. Switching power losses (dynamic) UT LL L IL I0 IL ILmax IT I0 ID T UIN D C Ro t T Ucontr 0 t ILmax dIT IT IRmax 2QR dt ILmin ts QR - diode reverse charge [mC] td tf ILmax , ID t1 t1 ILmin overvoltage due to leakage UT -IRmax inductance UIN ITrmsrds Discharging of transistor capacitances CBCand CBE (bipolar transistors) Eloss Swith-mode power supply power losses - review 1. Power losses in passive components - winding resitances (skin and proximity effects) - capacitor series resitance (ESR) – output filer electrolytic capacitors - magnetic core losses (hysteresis and eddy currents) - power losses in snubbar circuits 2. Static power losses in semiconductors: - related to ON- resistance of MOSFET transistor or saturation voltage drop across bipolar transistor - related to voltage drop across rectifier diodes (mains input rectifier) and fast swiching output diodes IMPORTANT! for bipolar transistors and diodes for MOSFET trnasistors Ploss Iav Uces Ploss Irms rdson 2 3. Switching (dynamic) power losses - related to semiconductor switching times, reverse charges - depandant on base (gate) drive circuits How to minimize power losses – general rules 1. Power losses in passive components - select proper wire diameter, use copper stripes or litz wire - select low ESR capacitors (for switching applications), as big (dimensions) as possible, connected in parallel, - make wide and thick copper paths on the PCB - select modern ferrite cores with best performance at specified operating frequency and smallest dimensions - avoid high amplitude of flux density changes - recover the magnetising energy – do not dissipate it - use converter topologies with low overvoltages – decrease the influence of leakagae inductances (eg. two-transistor „forward” topology) 2. Static power losses in semiconductors: - select MOSFET trnasistors with low on-resistance - in high power and high voltage applications use IGBT modules (simple MOSFET drive circuits, low saturation voltage drop as for bipolar transistors) - use Shottky diodes if possible (voltage drop below 0,5V) - use synchronous rectification technique After switching on the internal body diode the transistor with very low on-resistisance switches on – the voltage drop across the conducting transistor is much lower than across the diode 3. Transistor dynamic power losses - select fast transistors (low tr anf tf times) - use special converter topologies with zero-current or zero-voltage switching (eg. resonant topologies) - design carefully snubbar circuits Voltage UT rise without snubbar circuit IT Zp CIN UT Charging of the capacitor delays voltage rise across the UIN switching transistor Cs decreasing significantly IT UT transistor power losses T t Ds It is possible to select such value of the capacitor Cs, that the overall power loss in the transistor and snubbar circuit reaches minimum.
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