INELECT

INELECTPOWER SUPPLY - SMPSEngr. Francis A. MalabananECE, InstructorFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES11Power SupplyFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES2

Linear or Switching regulator2What is a Switching Regulator?Converts an input voltage into desire output voltage.

The power transistor operates as a switch, completely on or off.

An energy storage part (inductor) is used in the architecture

FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3Switching Regulator

3Choosing Between Linear and Switching RegulatorsWhen possible, most designers would prefer to use a linear voltage regulator rather than a switching voltage regulator

Linear regulators are usually lower in price

Linear regulators are usually simpler to implement

Linear regulators do not have associated noise/ripple problems apparent in switching regulators

FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES44Choosing Between Linear and Switching RegulatorsWhen to use a switching regulator #1:

When the minimum input voltage is at or below the desired output voltage

Linear regulators cannot provide an output voltage greater than the input voltage

VIN < VOUT

FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES55Choosing Between Linear and Switching RegulatorsWhen to use a switching regulator #2:The heatsinking of a linear regulator is prohibitive in price or space

Under most situations, linear regulators have a much lower efficiency than switching regulators. Low efficiency means a lot of power can be lost as heat.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES66The desired output voltage is greater than the input voltageLinear regulators cannot provide an output voltage greater than the input voltage

The desired output voltage is opposite polarity than the input voltageLinear regulators cannot invert an input voltage

1.5 VBattery

Power Supply5 VRequiredPower Supply

12 VBattery-12 VRequired

Why are switching regulators needed?Types of Switching Regulators AC-DC, AC-AC, DC-AC, and DC-DC ConvertersAC-DC

DC-ACDC-DC

12 Vdct110 Vact12 Vdct12 Vdct5 Vdct

AC-AC110 Vact

220 Vact110 VactPower supplies can be classified into a number of different categories. Two such categories are AC/DC converters and DC/DC converters.

In an AC/DC converter, the input power is delivered to the power supply as a true AC signal. The AC/DC power supply then creates a DC output voltage. We will not examine this type of power supply in this module.

Rather, we will address the DC/DC converter category of power supplies. With rare exception, DC/DC converters are used as the power supplies for automotive applications.Types of DC-DC Converters Step Down, Step Up and InvertingStep Down Buck

VtVtVin = 12 VVout = 5 VStep Up Boost

VtVin = 5 VVtVout = 12 VInvertingBuck-Boost

VtVin = 5VVtVout = -10 VThe DC/DC converter category of power supplies can be sub-divided by the ratio of input voltage to output voltage.

If the input power is delivered to the power supply at a voltage (VIN) which is less than the output voltage of the power supply (VOUT), the DC/DC converter must boost or step-up the voltage and the power supply is called a Boost Converter or a Step-Up Converter.

If the input power is delivered to the power supply at a voltage which is greater than the output voltage of the power supply (VOUT), the DC/DC converter must step down or buck the voltage and power supply is called a Step-Down Converter or a Buck Converter.

We will examine both buck and boost converters in this training module.Basic Circuit ConfigurationVOUTVINVMVGATELCISWILVOUTVINVMCVGATELILISWVOUTVINVMCVGATELILISWBuck-Boost VIN < -VOUT < VINBoost VIN < VOUTBuckVIN > VOUTAll topologies consists of the same basic components but are arranged differently

Buck ConfigurationThe input voltage is always greater than the output voltage

VOUTVINVMVGATELCISWILVINtime20V15V10V5V0VVOUTtime7.5V5V2.5V0V10VBoost ConfigurationThe input voltage is always less than the output voltage

VOUTVINVMCVGATELILISWVINtime20V10V5VVOUTtime10V0V0V20V5V15V15V24VBuck-Boost ConfigurationThe input voltage is always not constrained by the output voltage

VOUTVINVMCVGATELILISWVINtime20V15V10V5V0VVOUTtime-10V-20V0V-15V-5VVINSwitching RegulatorDuty Cycle ControllerOutputMonitorVOUTtime5VVoltageOK50%Filter NetworkVOUTHow a Switching Regulator WorksHow a Switching Regulator WorksVINVoltage RegulatorDuty Cycle ControllerOutputMonitorVOUTtime5VVoltageOK50%Filter NetworkVOUTHow a Switching Regulator WorksVINVoltage RegulatorDuty Cycle ControllerOutputMonitorVOUTtime5VVoltageOK50%Filter NetworkVOUTHow a Switching Regulator WorksVIN 1VVoltage RegulatorDuty Cycle ControllerOutputMonitorVOUTtime5VVoltageLow60%Filter NetworkVOUTHow a Switching Regulator WorksVINSwitching RegulatorDuty Cycle ControllerOutput MonitorVOUTtime5VVoltageOk50%Filter NetworkVOUTSwitching Power Supply Block DiagramVINVOUTSwitching Power SupplySwitchError AmplifierPWMControllerNetworkNetworkExternal NetworkAn external network (consisting of an inductor, capacitor, and diode) transforms the energy from the PWM controlled power switch into a desired output voltage

NetworkSwitchVINVOUTVIN = 12 VVOUT = 5 VPWM ControllerIn a switching voltage regulator, the pass transistor is used as a switch - it is either on or offThe output voltage, however, is an analog valuePWM controller senses error in VOUT via the error amplifierPWM controller updates the duty cycle (D) of the of transistor adjusting the output voltageErrorAmplifierPWMController0-100%VOUTThe Buck Converter (step down)Neglecting circuit losses, the average voltage at the input side of the inductor is VinD, while Vo is the output side voltage.

D is the transistor switch duty cycleOutput voltage regulation is provided by varying the duty cycle of the switch.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES2222The Buck Converter (Step down)OperationWhen the transistor is turned on, the input voltage is applied to inductor L1 and power is delivered to the output. Inductor current also builds up according to Faradays law.When the transistor is turned off, the voltage across the inductor reverses and freewheel diode becomes forward biased. This allows the energy stored in the inductor to be delivered to the output. This continuous current is then smoothed by output capacitor CoutFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES2323Step Down Switching RegulatorSteady State OperationVOUTVINVMVGATE+ VL -COUTISWILVGATE goes high

VM ~ VIN

VL = VM VOUT

tVMtVGATEtILVOUTtISWtRLOAD-VF-VF+Step Down Switching RegulatorSteady State OperationVOUTVINVMVGATECOUTISWILVL ConstanttVMtVGATEtILVOUTtISW

IL and ISW increaset

RLOADCOUT is charged by ILandVOUT increases-VF-VF++ VL -VOUTVINVMVGATECOUTISWILVGATE = 0VThe pass transistoris turned offISW = 0AtVMtVGATEtILVOUTtISWtRLOAD

IL cannot go to 0A instantly:VM goes negativeVL = VM VOUT

-VF-VF++ VL -Step Down Switching RegulatorSteady State OperationStep Down Switching RegulatorSteady State OperationVOUTVINVGATECOUTISWILBut, VM is clampedto -VFand IL decaysthrough the diode tVMtVGATEtILVOUTtISWtRLOADCOUT stabilizesthe output voltageso VOUT will only slowly decay-VFVM = -VF-VF++ VL -Step Down Switching RegulatorSteady State OperationVOUTVINVGATECOUTISWILThe MOSFET isturned on and offto repeatthe sequence RLOADtVMtVGATEtILVOUTtISWt-VFVM = -VF-VF++ VL -VOUTVINVMVGATECOUTISWILVOUTRLOADtVLtVGATEVIN - VOUTt+ VL --VOUTSINSGNDVOUT Increases with DVOUT = DVINVOUTtVLtVGATEVIN - VOUTt-VOUTVOUTVINVMVGATECOUTILRLOAD+ VL -SINSGNDISWVOUT Decreases with DVOUT = DVINThe Boost Converter (step up)Operation of another fundamental regulator, the boost is more complex than the buck.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES31

31The Boost Converter (step up)OperationWhen the switch is on, diode D1 is reverse biased, and Vin is applied across inductor, L1. Current builds up in the inductor to a peak value, either from zero current in a discontinuous mode, or an initial value in the continuous mode.When the switch turns off, the voltage across L1 reverses, causing the voltage at the diode to rise above the input voltage. The diode then conducts the energy stored in the inductor, plus energy direct from the supply to the smoothing capacitor and load. Hence, Vo is always greater than Vin, making this a step up converter.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3232The Boost Converter (step up)

FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3333The Boost Converter (step up)boost dc equation

D is the transistor switch duty cycleThe boost is a step-up type, where the output voltage is greater than the input.Output voltage regulation is provided by varying the duty cycle of the switchFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3434The Buck-Boost Regulator(Non-isolated Flyback)The flyback only delivers stored inductor energy during the switch off-time. The flyback is actually based on a combined topology of the previous twoFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES35

35The Buck-Boost Regulator(Non-isolated Flyback)OperationWhen the switch is on, the diode is reverse biased and the input is connected across the inductor, which stores energy as previously explained.At turn-off, the inductor voltage reverses and the stored energy is then passed to the capacitor and load through the forward biased rectifier diode.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3636The Buck-Boost Regulator(Non-isolated Flyback)Observation shows that the value of the switch duty ratio, D can be selected such that the output voltage can either be higher or lower than the input voltage.

This gives the converter the flexibility to either step up or step down the supply.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3737Selection of the power semiconductorsBipolarMOSFETSwitch SpeedSlowFastDrive MethodCurrentVoltageDrive CircuitComplexSimpleESD RobustnessHighLowCollectorEmitterBaseDrainSourceGateSelection of the power semiconductorsThe Power TransistorBipolar transistorlimited to use at frequencies up to 30kHz, due to switching loss. it has very low on-state losses and is a relatively cheap device, making it the most suitable for lower frequency applicationsMOSFEThigh frequency operation because of its very fast switching speeds, resulting in low (frequency dependent) switching losses.The on-state losses of the MOSFET are far higher than the BipolarFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES3939Selection of the power semiconductorsRectifiersSchottkyFor very low output voltages below 10V it is necessary to have an extremely low rectifier forward voltage drop, VF,They have very low VF values (typically 0.5V).Fast recovery epitaxial diode (FRED)For higher voltage outputs the most suitable rectifier is the fast recovery epitaxial diode (FRED).This device has been optimized for use in high frequency rectification.Its characteristics include low VF (approx. 1V) with very fast and efficient switching characteristics.FIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES4040Inductor TechnologyThere are a number of inductor technologies to choose fromDrum coreFlat coilToroidBeadWirewoundPlanar

In addition to inductance and saturation current, the inductor technology will also affect:Inductor resistance and impedanceSize (length, width, height)CostTHANK YOU!Prepared by:Engr. Francis A. MalabananECE, InstructorFirst Asia Institute of Technology and HumanitiesFIRST ASIA INSTITUTE OF TECHNOLOGY AND HUMANITIES4242