2 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.
3 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 downThese converters typically have a full wave rectifier front-end to produce a high DC voltages
4 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.Vout = Vin x D where D = Ton / (Ton + Toff) : Duty cycle
5 The Buck ConverterThe 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.It has two operating modes, depending on if the transistor Q1 is turned ON or OFF.
6 Buck Converter Assume large C so that Vout has very low ripple Flywheel circuitAssume large C so that Vout has very low rippleSince Vout has very low ripple, then assume Iout has very low rippleInterchange of energy between inductor and capacitor is referred as flywheel effect.
7 Buck ConverterWhat 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 zerothe average voltage across an inductor operating in periodic steady state is zero
8 Switch closed for DT seconds Buck Converter+ (Vin – Vout) –Switch closed for DT secondsWhere D = Duty CycleT = Switching period
9 Switch open for (1 − D)T seconds Buck Converter– Vout +Switch open for (1 − D)T secondsWhen 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.
10 Buck Converter If D is duty cycle average output voltage is Since the average voltage across L is zeroThe input/output again becomesFrom power balance,
11 Power Losses in a Buck Converter There are two types of losses in an SMPS:DC conduction losses.AC switching losses.
12 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.
13 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:
14 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 = 3.3 VMOSFET RDS(ON) = 10 mΩDiode forward voltage VD = 0.5V (freewheeling diode)Inductor RDCR = 2 mΩConduction 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 W + 0.2W = 4.095WConverter efficiency:
15 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.