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Introduction to DC-DC Converter – Cont. EE174 – SJSU Tan Nguyen SWITCHING MODE POWER SUPPLY (SMPS) • The switching-mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, inductors, and transformers -- and the use of switches that are in one of two states, on or off. • It offers high power conversion efficiency and design flexibility. • It can step down or step up output voltage. • The term switchmode was widely used for this type of power supply until Motorola, Inc., who used the trademark SWITCHMODE TM for products aimed at the switching-mode power supply market, started to enforce their trademark. Switching-mode power supply or switching power supply are used to avoid infringing on the trademark. • Typical switching frequencies lie in the range 1 kHz to 1 MHz, depending on the speed of the semiconductor devices. • Types of SMPS: • Buck converter: Voltage to voltage converter, step down. • Boost Converter: Voltage to voltage converter, step up. • Buck-Boost or FlyBack Converter: Voltage-Voltage, step up and down (negative voltages) • Cuk Converter: Current-Current converter, step up and down These converters typically have a full wave rectifier front-end to produce a high DC voltages SIMPLE SWITCHING MODE POWER SUPPLY 42% 58% PULSE WIDTH MODULATION (PWM) • The switch control signal, which controls the on and off states of the switch, is generated by comparing a signal level control voltage vcontrol with a repetitive waveform. • The switching frequency is the frequency of the sawtooth waveform with a constant peak. • The duty ratio D can be expressed as t on v control D ^ Ts V st THE BUCK CONVERTER • The buck converter is known as voltage step-down converter, current step-up converter, chopper, direct converter. • The buck converter simplest and most popular switching regulator. The Buck Converter Circuit Diagram DC-DC Buck Converter Module 4.5-14V to 0.8-9.5V 6A Adjustable Set-Down Regulator Size:30mm(L)*18mm(W)*14(H) mm THE BUCK CONVERTER During switch is close Vi = Vo, when switch is open at T1, Vo starts to discharge, with higher capacitor C value slower discharge rate (improve ripple). If switch VSW is close and open as shown below. Observe output Vo and current spike at ISW every time the switch is close. Use high capacitor C value to improve the output voltage Vo ripple, but still have issue with huge current spike ISW when switch is close can easily burn out the switch. THE BUCK CONVERTER The current spikes ISW can be controlled by adding an inductor (L) between the switch (SW) and the capacitor (C ). Since Inductor acts as a storage energy. When the switch is close, the inductor will absorb the energy and when the switch is open, the inductor will supply that energy to the capacitor result in smooth out ISW. However, there is another issue with inductor current path. The inductor current must have continuous to flow through the inductor but during the switch open, there is no current path for the inductor current. To fix this issue, adding a free-wheeling diode to the circuit below it, this will keep the inductor current continues to flow. Freewheeling Diode THE BUCK CONVERTER SUMMARY Two Mode of Operations: 1. Continuous Conduction Mode: Inductor current IL does not reach zero, when output current IO is very large. 2. Discontinuous Conduction Mode: Inductor current IL will reach zero, when output current IO is very small. • • • • • • LC low-pass filter: to pass the DC component while attenuating the switching components. diode is reversed biased during ON period, input provides energy to the load and to the inductor energy is transferred to the load from the inductor during switch OFF period Interchange of energy between inductor and capacitor is referred as flywheel effect. in the steady-state, average inductor voltage is zero in the steady-state, average capacitor current is zero THE BUCK CONVERTER CONTINUOUS MODE 1. Continuous Conduction Mode: Inductor current IL does not reach zero, when output current IO is very large. THE BUCK CONVERTER DISCONTINUOUS MODE 2. Discontinuous Conduction Mode: Inductor current IL will reach zero, when output current IO is very small. When the switch is ON (short), Diode reversed bias (open): VL = Vi – VO = constant > 0 When the switch is OFF (open), Diode forwarded bias: VL = – VD – VO ≈ – VO = constant < 0 Calculate IL,max and relationship of Vo and Vi: THE BUCK CONVERTER EXAMPLE Given a buck converter design with fsw = 200 kHz (TS = 5 μsec), L = 33 μH, C = 10 μF, I0 = 1 A and D = 50% duty cycle. Find: a) VO if Vi = 10 V in continuous mode b) Output current and voltage ripples c) Current IL,max d) VO if Vi = 10 V in discontinuous mode Solutions: a) VO = D Vi = 0.5 x 10 = 5V b) c) d) VO = 3.32 V = (33 μH)-1 (10 – 5) x 0.5 x 5 μsec = 0.38 A Buck Converter Design Example For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio and “on” time of the switch. D = V0 /Vd = 5/40 = 0.125 = 12.5% Ts = 1/fs = 1/4000 = 0.25 ms = 250 μs Ton = DTs = 31.25 μs Toff = Ts – ton = 218.75 μs When the switch is “on”: VL = Vd - V0 = 35 V When the switch is “off”: VL = -V0 = - 5 V I0 = IL = V0 / R = 5 A Id = D I0 = 0.625 A Power Losses in a Buck Converter There are two types of losses in an SMPS: • DC conduction losses. • AC switching losses. DC conduction losses in Buck converter • The conduction losses of a buck converter primarily result from voltage drops across transistor Q1, diode D1 and inductor L when they conduct current. • A MOSFET is used as the power transistor. The conduction loss of the MOSFET = IO2 x RDS(ON) x D, where RDS(ON) is the on-resistance of MOSFET Q1. • The conduction power loss of the diode = IO • VD • (1 – D), where VD is the forward voltage drop of the diode D1. • The conduction loss of the inductor = IO2 x RDCR, where RDCR is the copper resistance of the inductor winding. Power Losses in a Buck Converter Therefore, the conduction loss of the buck converter is approximately: PCON_LOSS = (IO2 x RDS(ON) x D) + (IO • VD • [1 – D]) + (IO2 x RDCR) Considering only conduction loss, the converter efficiency is: Example: For 12V input buck supply 3.3V/10AMAX output buck supply. • Use 27.5% duty cycle provides a 3.3V output voltage. Vout = Vin x D = 12 x 0.275 = 3.3 V • MOSFET RDS(ON) = 10 mΩ • Diode forward voltage VD = 0.5V (freewheeling diode) • Inductor RDCR = 2 mΩ Conduction loss at full load: PCON_LOSS = (IO2 x RDS(ON) x D) + (IO x VD x [1 – D]) + (IO2 x RDCR) = (102 x 0.01 x 0.275) + (10 x 0.5 x [1 – 0.275]) + (102 x 0.002) = 0.275W + 3.62W + 0.2W = 4.095W Buck converter efficiency: AC Switching Losses in Buck Converter 1. MOSFET switching losses. A real transistor requires time to be turned on or off. So there are voltage and current overlaps during the turn-on and turn-off transients, which generate AC switching losses. 2. Inductor core loss. A real inductor also has AC loss that is a function of switching frequency. Inductor AC loss is primarily from the magnetic core loss. 3. Other AC related losses. Other AC related losses include the gate driver loss and the dead time (when both top FET Q1 and bottom FET Q2 are off) body diode conduction loss. Basic Nonisolated DC/DC SMPS Topologies BUCK COVERTER Basic dc-dc converters and their dc conversion ratios M(D) = V/Vg. Mobile Device Using Linear versus Switch-Mode Regulator DC-DC Converter Technology Comparison Parameter Linear Regulator Switching Regulator Efficiency Low High EMI Noise Low High Output Current Low to Medium Low to High Boost (Step up) No Yes Buck (Step down) Yes Yes Size Small Large Cost Inexpensive High cost Sample of Linear and Switch-Mode Regulator Output References: http://en.wikipedia.org/wiki/DC-to-DC_converter https://www.jaycar.com/images_uploaded/dcdcconv.pdf Linear Technology - Application Note 140 buck converter tutorial abuhajara http://www.smpstech.com/tutorial/t03top.htm#SWITCHINGMODE Notes from Fang Z. Peng Dept. of Electrical and Computer Engineering MSU https://www.google.com/webhp?sourceid=chromeinstant&rlz=1C1OPRB_enUS587US587&ion=1&espv=2&ie=UTF8#q=picture+of+noise+on+buck+output https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCQQFjABa hUKEwj329J4YvIAhVLy4AKHZiyADY&url=http%3A%2F%2Fusers.ece.utexas.edu%2F~kwasinski%2F_6_EE 462L_DC_DC_Buck_PPT.ppt&usg=AFQjCNH1PIzP73b3t11mgGhnUBBg-sVNXg&cad=rja http://ecee.colorado.edu/ecen4517/materials/Encyc.pdf https://www.valuetronics.com/Manuals/Lambda_%20linear_versus_switching.pdf