Introduction to DC-DC Conversion – 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.

SWITCHING MODE POWER SUPPLY (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 Heater: The heater turns on and off every several minutes to keep the room temperature constant. Examples: Vin = 12 Vdc and the load resistor R2 = 0.25 ohms. The objective is to open and close the switch so that the average voltage across R2 is 5 Vdc. The waveform of the voltage across R2 shown below. Vo = Vin x D where D = Ton / (Ton + Toff) : Duty cycle

Pulse-width Modulator (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

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. 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 Home > Electronics >Alternative Energy >Eco Gadgets > Shenzhen Shanhai Technology Ltd. > Item#: 9SIA5FB1ZE9203

The Buck Converter converts dc from one level to another the average output voltage is controlled by the ON-OFF switch PWM switching is employed lower average output voltage than the dc input voltage Vd depending on the duty ratio, D D=ton/Ts Average output: Applications: regulated switch mode dc power supplies dc motor drives 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 in the steady-state, average inductor voltage is zero in the steady-state, average capacitor current is zero

Buck Converter Inductor current iL flows continuously Average inductor voltage over a time period must be zero Assuming a lossless circuit

Buck Converter Assume large C so that Vout has very low ripple  Flywheel circuit Assume large C so that Vout has very low ripple Since Vout has very low ripple, then assume Iout has very low ripple Interchange of energy between inductor and capacitor is referred as flywheel effect.

Buck Converter in Continuous Conduction What do we learn from inductor voltage and capacitor current in the average sense? the average current through a capacitor operating in periodic steady state is zero the average voltage across an inductor operating in periodic steady state is zero

Buck Converter in Continuous Conduction Switch closed for DT seconds + (Vin – Vout) – Switch closed for DT seconds Where D = Duty Cycle T = Switching period

Buck Converter in Continuous Conduction – Vout + Switch open for (1 − D)T seconds When switch open VL = - Vout, diode is closed (forward biased) so iL continues to flow. This is the assumption of “continuous conduction” in the inductor which is the normal operating condition.

Buck Converter If D is duty cycle average output voltage is Since the average voltage across L is zero The input/output again becomes From power balance, 

Power Losses in a Buck Converter There are two types of losses in an SMPS: DC conduction losses. AC switching losses.

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

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:

Power Losses in a Buck Converter 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 Converter efficiency:

AC Switching Losses in Buck Converter 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. 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. 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.

An example of Linear Regulator versus Switch-Mode Regulator VBAT = 3.7 V nom, BIN_BB = 1.2 V Load Current = 600 mA Power delivered to load = 600 mA * 1.2 V = 720 mW Power converted to heat = 20 mW * ((3.7/1.2) 1) = 1,500 mW Total power consumed = 720 mW + 1,500 mW = 2,200 mW 32% goes to work, 68% goes to heating user hand and ear. Switch-mode regulator: VBAT = 3.7 V nom; BIN_BB = 1.2 V Load Current = 600 mA Converter efficiency = 90% Power delivered to load = 600 mA * 1.2 V = 720 mW Power converted to heat =720 mW * ((1/0.9) 1)=80 mW Total power consumed = 720 mW + 80 mW = 800 mW 90% goes to work, 10% goes to heating user hand and ear.

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 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=chrome-instant&rlz=1C1OPRB_enUS587US587&ion=1&espv=2&ie=UTF-8#q=picture+of+noise+on+buck+output https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCQQFjABahUKEwj329-J4YvIAhVLy4AKHZiyADY&url=http%3A%2F%2Fusers.ece.utexas.edu%2F~kwasinski%2F_6_EE462L_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