diode applications chapter 2. overview power supply half-wave rectifiers full-wave rectifiers line...

Click here to load reader

Post on 31-Mar-2015




2 download

Embed Size (px)


  • Slide 1

Diode Applications Chapter 2 Slide 2 Overview Power Supply Half-wave rectifiers Full-wave rectifiers Line regulation Limiter Clamper Slide 3 Basic components Transformer (not shown) Rectifier Filter Regulator DC Power Supply Or Full-wave rectifier Slide 4 Sine Wave The sine wave is a common type of alternating current (ac) and alternating voltage. The time required for a sine wave to complete one full cycle is called the period (T). Frequency ( f ) is the number of cycles that a sine wave completes in one second. The more cycles completed in one second. The higher the frequency. Frequency is measured in hertz (Hz) Relationship between frequency ( f ) and period (T) is: f = 1/T Slide 5 Peak-to-Peak / Average / RMS The peak-to-peak value of a sine wave is the voltage or current from the positive peak to the negative peak. The peak-to-peak values are represented as: V pp and I pp Where: V pp = 2V p and I pp = 2I p The rms (root mean square) value of a sinusoidal voltage is equal to the dc voltage that produces the same amount of heat in a resistance as does the sinusoidal voltage. V rms = 0.707V p I rms = 0.707I p Slide 6 Half-wave Rectifiers Forward biased Slide 7 Half-wave Rectifiers Reverse biased Output result Slide 8 Half-wave Rectifier Note that the frequency stays the same Strength of the signal is reduced Vavg = Vp(out)/ = 0.318 x Vp(out) [31.8 % of Vp] Vp(out) = Vp(in) VBar For Silicon VBar = 0.7 V Half-wave Rectifier Vp(out) Vp(in) Vavg 22 Slide 9 Half-wave Rectifier - Example Output: Peak Inverse Voltage (PIV) The peak voltage at which the diode is reverse biased In this example PIV = Vp(in) - Hence, the diode must be rated for PIV = 100 V Draw the output signal Vp(out) = Vp(in) 0.7 Vavg = 99.3/ What happens to the frequency? Slide 10 Transformers (Review) Transformer: Two inductors coupled together separated by a dielectric When the input magnetic field is changing voltage is induced on the second inductor The dot represents the + (voltage direction) Applications: Step-up/down Isolate sources Turns ratio (n) n = Sec. turns / Pri. turns = Nsec/Npri Vsec = n. Vpri depending on value of n : step-up or step-down Center-tapped transformer Voltage on each side is Vsec/2 Slide 11 Half-wave Rectifier - Example Example: Assume that the input is a sinusoidal signal with Vp=156 V & T = 2 msec; assume Nsec:Npri = 1:2 Draw the signal Find turns ratio; Find Vsec; Find Vout. n = = 0.5 Vsec = n.Vpri = 78 V Vout = Vsec 0.7 = 77.3 V 78-0.7 Slide 12 Full-wave Rectifier Note that the frequency is doubled Vavg = 2Vp(out)/ = 0.637 x Vp(out) Slide 13 Full-wave Rectifier Circuit Center-tapped full-wave rectifier Each half has a voltage = Vsec/2 Only one diode is forward biased at a time The voltages at different halves are opposite of each other Slide 14 Full-wave Rectifier Circuit Center-tapped full-wave rectifier Each half has a voltage = Vsec/2 Only one diode is forward biased at a time The voltages at different halves are opposite of each other Slide 15 Full-wave Rectifier Circuit Vout = Vsec /2 0.7 Peak Inverse Voltage (PIV) PIV = (Vsec/2 0.7)- (-Vsec/2) = Vsec 0.7 Vout = Vsec/2 0.7 Assuming D2 is reverse-biased No current through D2 Slide 16 Full-wave Rectifier - Example Assuming a center-tapped transformer Find the turns ratio Find Vsec Find Vout Find PIV Draw the Vsec and Vout What is the output freq? Vsec n=1:2=0.5 Vsec=n*Vpri=25 Vout = Vsec/2 0.7 PIV = Vsec-0.7=24.3 V Slide 17 Full-wave Rectifier - Multisim XFrmr can be virtual or real Use View Grapher to see the details of your results The wire-color can determine the waveform color Make sure the ground is connected to the scope. Slide 18 Bridge Full-wave Rectifier Uses an untapped transformer larger Vsec Four diodes connected creating a bridge When positive voltage D1 and D2 are forward biased When negative voltage D3 and D4 are forward biased Two diodes are always in series with the load Vp(out) = Vp(sec) 1.4V The negative voltage is inverted The Peak Inverse Voltage (PIV) PIV=Vp(out)+0.7 Slide 19 Bridge Full-wave Rectifier - Example Assume 12 Vrms secondary voltage for the standard 120 Vrms across the primary Find the turns ratio Find Vp(sec) Show the signal direction when Vin is positive Find PIV rating n=Vsec/Vpri = 0.1 10:1 Vp(sec) = (0.707) -1 x Vrms = 1.414(12)=17 V Vp(out) = V(sec) (0.7 + 0.7) = 15.6 V through D1&D2 PIV = Vp(out) + 0.7 = 16.3 V 120Vr ms Note: Vp-Vbr ; hence, always convert from rms to Vp Slide 20 Bridge Full-wave Rectifier - Comparison 12 0Vr ms Vp(2)=Peak secondary voltage ; Vp(out) Peak output voltage ; Idc = dc load current Make sure you understand this! Slide 21 Filters and Regulators Slide 22 Filters Slide 23 Slide 24 Slide 25 -Ripple voltage depends on voltage variation across the capacitor - Large ripple means less effective filter Filters peak-to-peak ripple voltage Slide 26 Too much ripple is bad! Ripple factor = Vr (pp) / VDC Vr (pp) = (1/ fR L C) x Vp(unfiltered) VDC = (1 1/ fR L C) x Vp(unfiltered) Filters Slide 27 Diode Limiting What is Vout? Vout + = Vin (RL)/(RL+R1) = 9.09 Vout - = -0.7 Forward biased when positive Reverse biased when negative, hence voltage drop is only -0.7 So how can we change the offset? Slide 28 Diode Limiting Changing the offset What if we mix these together? Positive limiter Negative limiter Remember: When positive voltage reverse biased No current no clipping! Slide 29 Diode Limiter When the input signal is positive D1 is reversed biased; acting as positive limiter Pos. Limiter -V BIAS -0.7 +V BIAS +0.7 Slide 30 Diode Clamper It adds a dc level When the input voltage is negative, the capacitor is charged Initially, this will establish a positive dc offset Note that the frequency of the signal stays the same RC time constant is typically much larger than 10*(Period) Note that if the diode and capacitor are flipped, the dc level will be negative Output: Slide 31 Diode Characteristics (VRRM & IF(AV) )