Presentation on theme: "Kevin Johnson Minh Vo Lam Duong Wye-Chi Chok"— Presentation transcript:
1 Kevin Johnson Minh Vo Lam Duong Wye-Chi Chok DAC, Diodes, TriacsME 6405 – Intro to MechatronicsStudent LectureKevin JohnsonMinh VoLam DuongWye-Chi Chok
2 Outline DAC Diodes Triacs What is a DAC? Types of DAC Specifications Kevin JohnsonOutlineDACWhat is a DAC?Types of DACSpecificationsDiodesWhat are diodes?P-N Junction DiodeReal vs. IdealTypes of Diodes & ApplicationsTriacsWhat are thyristors?What are triacs?Applications
3 Principal components of DAC Kevin JohnsonPrincipal components of DACExplain picture in detail. 1 min
4 What is a DAC?Kevin JohnsonConvert digital signal (number) to analog signal (voltage or current)Either multiplying or non-multiplyingNon-multiplying contains its own referenceMultiplying takes external reference.Two main types: ladder and delta-sigma
5 Kevin JohnsonDAC ideal output.Each binary number sampled by the DAC corresponds to a different output level.Digital Input SignalAnalog Output SignalExplain picture in detail. 1 min
6 Ideally Sampled Signal Kevin JohnsonDAC real output.DACs capture a number and hold that value for a given sample interval. This is known as a zero-order hold and results in a piecewise constant output.Output typical of a real, practical DAC due to sample & holdIdeally Sampled SignalDACExplain graphs in detail min
7 Smoothing Used when a continuous analog signal is required. Kevin JohnsonSmoothingUsed when a continuous analog signal is required.Signal from DAC can be smoothed by a Low pass filterPiece-wise Continuous OutputAnalog Continuous OutputDigital Inputn bit DAC0 bitFilternth bit
8 Applications. Audio/Video Signal Generators Motor, valve, actuator Kevin JohnsonApplications.Audio/VideoMP3 playersCellphonesTelevision(well, old ones)Signal GeneratorsSine wave generationSquare wave generationTriangle wave generationRandom noise generationMotor, valve, actuatorRarely; usually PWM.
9 Types of DAC implementations Kevin JohnsonTypes of DAC implementationsBinary Weighted ResistorR-2R LadderPulse Width Modulator (not covered)Oversampling DAC, aka Delta Sigma (used internally in HCS12)
10 Binary Weighted Resistor Kevin JohnsonBinary Weighted ResistorAssume binary inputs B0 (LSB) to Bn-1 (MSB)Each Bi is 1 or 0 and is multiplied by Vref to get input voltageB5B4B3B2B1B0
11 Kevin JohnsonBinary weight theoryNeed to fill jars to a specific level using set of measuring cups.Cups are ½, ¼, 1/8, 1/16, etc.
12 BWR Pros and Cons Advantages Simple Fast Disadvantages Kevin JohnsonBWR Pros and ConsAdvantagesSimpleFastDisadvantagesNeed large range of resistor values (2048:1 for 12-bit) with high precision in low resistor valuesNeed very small switch resistancesOp-amp may have trouble producing low currents at the low range of a high precision DAC
13 R-2R ladder basic circuit Kevin JohnsonR-2R ladder basic circuitEquivalent resistance to ground at each top node is R.At each node, current gets split in two.Since nodes are cascaded, currents are ½, ¼, 1/8, etc.
14 R-2R Ladder results Final result is: Kevin JohnsonR-2R Ladder resultsFinal result is:Assuming Rf = R (and ignoring negative)Resolution is smallest step: i.e. B=1 in above equation.
15 R-2R Ladder Advantages: Disadvantages Only 2 resistor values Kevin JohnsonR-2R LadderAdvantages:Only 2 resistor valuesLower precision resistors acceptableDisadvantagesSlightly slower conversion rateOp-amp must still handle very small currents at high bit numbers.
16 Kevin JohnsonDelta-sigma DACNow all cups are the same size (or more precisely, he uses the same cup over and over).Cup size is1/(2^n).He must add this amount the proper number of times(pulse-count modulation).
17 Delta-sigma Pros and Cons Kevin JohnsonDelta-sigma Pros and ConsPros:Very accurateHigh bit-depth possibleReduced aliasingCons:Requires very fast oversampling clock.At least 2^n times faster than sampling rateComplicatedSensitive to clock jitter
18 General comments Circuits as shown produce only unipolar output Kevin JohnsonGeneral commentsCircuits as shown produce only unipolar outputReplacing ground with –Vref will allow Vout to be positive or negative
19 Specifications of a DAC Minh VoSpecifications of a DACReference VoltageResolutionSampling RateSettling TimeLinearityErrorsImportant in selecting a DAC
20 Reference Voltage Vref Minh VoReference Voltage VrefDetermines the output voltage rangeNon-multiplying DACFixed Vref set internally by manufacturerMultiplying DACVref is set externally and can be vary during operationFull-scale voltage VfsVoltage when all digital inputs are 1’sMax voltage if not amplifiedVREF also defines the voltage step by which the output changes in response to a 1-LSB transition at the input.Multiplying DAC voltage can be changed during operationFull-scale voltage – slightly smaller than the reference voltage
21 Minh VoResolutionThe resolution is the amount of output voltage change in response to a least significant bit (LSB) transition.Smaller resolution results in a smoother outputA common DAC has a bit resolutionResolution corresponds to the voltage of the LSBFiner resolution = smoother voltage change
22 Sampling Rate fsampling Minh VoSampling Rate fsamplingRate of conversion of a single digital input to its analog equivalentWhen the input changes rapidly, fmax, the DAC conversion speed must be highNyquist Criterion:Limited by the clock speed of the input signal and the settling time of the DACTypical computer sound cards 48kHz sampling freq
23 Minh VoSettling TimeDAC needs time to reach the actual expected analog output voltageThe time required for the output voltage to settle within +/- ½ of VLSB of the expected voltageIdeally a DAC would instantaneously change its output value when the digital input would change. However, in a real DAC it takes time for the DAC to reach the actual expected output value.
24 Minh VoLinearityThe difference between the desired analog output and the actual output over the full range of expected valuesLinear (Ideal)Non-LinearThe linearity is the relationship between the output voltage and the digital signal input.
25 Errors Gain Error Offset Error Full Scale Error Non Linearity Minh VoErrorsGain ErrorOffset ErrorFull Scale ErrorNon LinearityNon-MonotonicResolution ErrorsSettling Time and Overshoot
26 Minh VoGain ErrorDeviation in the slope of the ideal curve and with respect to the actual DAC outputHigh Gain Error: Step amplitude is higher than the desired outputDigital InputDesired/Ideal OutputAnalog Output VoltageLow GainHigh GainLow Gain Error: Step amplitude is lower than the desired outputGain Error is adjustable to zero using an external potentiometer
27 Minh VoOffset ErrorOccurs when there is an offset in the output voltage in reference to the ideal outputThis error may be detected when all input bits are low (i.e. 0).Digital InputDesired/Ideal OutputOutput VoltagePositive OffsetNegative Offset
28 Minh VoFull Scale ErrorCombination of gain and offset error
29 Differential Non-Linearity Minh VoDifferential Non-LinearityVoltage step size changes vary with as digital input increases. Ideally each step should be equivalent.Digital InputIdeal OutputAnalog Output VoltageVLSB2VLSBDiff. Non-Linearity = 2VLSB
30 Integral Non-Linearity Minh VoIntegral Non-LinearityOccurs when the output voltage is non linear. Basically an inability to adhere to the ideal slope.Digital InputIdeal Output1VLSBInt. Non-Linearity = 1VLSBAnalog Output Voltage
31 Minh VoNon-MonotonicOccurs when the an increase in digital input results in a lower output voltage.Analog Output VoltageDigital InputDesired OutputMonotonicNon-Monotonic
32 Minh VoResolution ErrorsDoes not accurately approximate the desired output due large voltage divisions.VoutDesired Analog signalApproximate output2 Volt. LevelsDigital Input1Poor Resolution(1 bit)
33 Settling Time and Overshoot Minh VoSettling Time and OvershootAny change in the input time will not be reflected immediately due to the lag time.Overshoot occurs when the output voltage overshoots the desired analog output voltage.Settling time generally determines maximum operating frequency of the DAC
34 What is a Diode?Lam DuongA diode is a two terminal electric component which conducts current more easily in one direction than in the opposite direction.The most common usage of a diode is as an electronic valve which allows current to flow in one direction but not the opposite direction.
35 Lam DuongA bit of historyDiodes were known as rectifiers until 1919, when a physicist by the name of William Eccles coined the term diode, which from its Greek roots means “through-path.”In 1873 Fredrick Guthrie discovered thermionic diodes (vacuum tube diodes) . Heating the cathode in forward bias permitted electrons to be transmitted into the vacuum, but in reverse bias the electrons were not easily release from the unheated anode.
36 Lam DuongA bit of historyIn 1874 Karl Braun discovered the first solid state diode (crystal diode). It consists of using Galena crystals as the semiconducting material.In 1939 Russell Ohl discovered the first P-N junction at Bell Labs.Today, the majority of diodes are made of semiconductor silicon P-N junctions.
37 Lam DuongP-N Junction DiodeA P-N junction diode consists of a p-type semiconductor (silicon) joined with an n-type semiconductor.P-type – A semiconductor doped with impurities to create positive charge carriers (holes).N-type – A semiconductor doped with impurities to create negative charged carriers.A depletion region is created when negative charge carriers from the N-type region diffuse into the P-type region, and vice versa.npDepletion RegionMajority carriers
38 Lam DuongP-N Junction DiodeForward BiasednpifDepletion RegionThe behavior of a diode depends upon the polarity of the supply voltage.Under forward bias the depletion region is reduced in size and less energy is required for the charged majority carriers to cross the depletion region.This decrease in energy requirement results in more charged majority carriers to cross the depletion region which induces a current.
39 P-N Junction DiodeLam DuongUnder reverse bias the depletion region is greatly increased in size and requires significantly more energy from the majority carriers in order to cross.Most majority carriers won’t be able to cross the depletion region and thus are unable to induce a current.npReverse BiasedDepletion RegionirV
40 Real vs. IdealLam DuongIdeal P-N Diode – no resistance to current in forward bias and infinite resistance in reverse bias. (Similar to a switch)In reality there is resistance to current flow in forward bias. It requires a certain voltage to be reached before the depletion region is eliminated and full current flow is permitted.Likewise, in reverse bias there is a small reverse (leakage) current induced by the flow of minority carriers. At a certain voltage (break down voltage) the reverse current will increase significantly. This is called the Avalanche current.VIconductionregionnon-conductionIdeal Curve
41 Lam DuongSchottky DiodeUnlike P-N junction diodes, Schottky diodes are based on a metal and semiconductor junction.An advantage of Schottky diodes over P-N junction diodes is that Schottky diodes have no recovery time when switching from conducting to non-conducting state and vice versa.The main disadvantage of Schottky diodes are that they operate in low voltage compare to P-N junction diodes (up to 50V).Another significant difference is that the “on-voltage” for a Schottky diode is around .3V while it is .7V for a P-N junction diode.MetalN-Type
42 Lam DuongFlyback DiodeSchottky diodes are often used as Flyback diodes due to their quick recovery and low forward voltage drop.A Flyback diode is a diode used to eliminate the sudden voltage spike that occurs across an indicutive load when voltage is abruptly reduced or removed.Lenz’s law - if the current through an inductance changes, this inductance induces a voltage so the current will go on flowing as long as there is energy in the magnetic field.Flyback diodes are important in mechatronics applications where one may want to vary the voltage of an inductive load to control its operation.
43 Lam DuongOther Types of DiodesLight Emitting Diodes (LEDs) - A diode formed from a semiconductor such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side.Photodiode – Exploits the fact that all semiconductors are subject to charged carrier generation when they are exposed to light. Photodiodes are often used to sense light such as in an Opto-isolator.Zener Diode – Allows current in forward bias like a regular diode, but also in reverse bias if the voltage is larger than designed voltage, called the Breakdown voltage.10/31/ ME 6405: Introduction to Mechatronics43
44 What are TRIACS? In order to know, we must first look at thyristors… Wye-Chi ChokIn order to know, we must first look at thyristors…
45 What are Thyristors?Wye-Chi ChokClass of semiconductor components that can only go in 1 direction.Wide range of devices, SCR (silicon controlled rectifier), SCS (silicon controlled switch), Diacs, Triacs, and Shockley diodesUsed in high power switching applicationsi.e. hundreds of amps / thousands of watts
46 How do Thyristors work? PNPN (4-layer) device: Wye-Chi ChokPNPN (4-layer) device:PNP and NPN transistor back-to-back.With forward voltage, small gate current pulse turns on device.once on, each transistor supplies gate current for the other, so no need for gate inputonly way to turn it off is to stop current (i.e. bring voltage to zero)
48 …now then, what are TRIACS? Wye-Chi Chok…now then, what are TRIACS?A TRIAC (TRIode for Alternating Current) is a 3-terminal AC semiconductor switch.Composed of 2 thyristors facing opposite directions such that it can conduct current in either direction.MT1 and MT2 are current carrying terminals while the Gate terminal is used for triggering by applying a small voltage signal.Once triggered, it continues to conduct current until the current falls below a threshold value.
49 Triac Operation Wye-Chi Chok 5 layer device Region between MT1 and MT2 are parallel switches (PNPN and NPNP)Allows for positive or negative gate triggering
51 Triac Characteristic Curve Wye-Chi Chok1st quadrant - MT2 is (+) with respect to MT1VDRM is the break-over voltage of the Triac and the highest voltage that can be blockedIRDM is the leakage current of the Triac when VDRM is applied to MT1 and MT2IRDM is several orders of magnitude smaller than the “on” rating
52 TriacsWye-Chi ChokPros:Better than a transistor as it has much better current surge rating – it can handle more current as it simply turns on moreInexpensive compared to relaysCons:Can't manually control turn-off with the gate; must turn off by stopping current through the device via the terminals.Specs to buy one:Gate signal requirementsVoltage dropSteady-state/holding current (continuously handle)Peak current (maximum amount to handle surge)
53 Triac Applications High Power TRIACS Wye-Chi ChokHigh Power TRIACS• Switching for AC circuits, allowing the control of very large power flows with milliampere-scale control currents• Can eliminate mechanical wear in a relayLow Power TRIACS• Light bulb dimmers (done by applying power later in the AC cycle aka PWM of AC wave)• Motor speed controls for electric fans and other AC motors, and heaters• Modern computerized control circuits in household appliances
54 Triac Applications Simple Triac Switch Small control current/voltage Wye-Chi ChokSimple Triac SwitchSmall control current/voltageEliminates Mechanical wear in a RelayMuch Cheaper
55 Real World Triacs Come in various shapes and sizes Wye-Chi ChokReal World TriacsCome in various shapes and sizesEssentially all the same operationallyDifferent mounting schemes
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