Experiment 4 – Flyback Feedback Loop | ECEN 4517, Study notes of Electrical and Electronics Engineering

Download Experiment 4 – Flyback Feedback Loop | ECEN 4517 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! ECEN 4517 1 Lecture 7 ECEN 4517/5517 Step-up dc-dc converter with isolation (flyback) Feedback controller to regulate HVDC Experiment 4 part 2: flyback feedback loop DC-AC inverter (H-bridge) 12 VDC HVDC: 120 - 200 VDC AC load 120 Vrms 60 Hz Battery DC-AC inverter H-bridge DC-DC converter Isolated flyback +– d(t) Feedback controller Vref Digital controller d(t) + vac(t) – ECEN 4517 2 Due dates Right now: Prelab assignment for Exp. 4 Part 1 (one from every student) Due within five minutes of beginning of lecture This week in lab (Feb. 26-28): Definitely finish Exp. 3, and begin Exp. 4 Next week in lecture (Mar. 4): Prelab assignment for Exp. 4 Part 2 (one from every student) Next week in lab (Mar. 4-6): Exp. 3 final report due ECEN 4517 5 Negative feedback: a switching regulator system + – + v – vg Switching converterPower input Load –+ Compensator vref Reference input HvPulse-width modulator vc Transistor gate driver δ Gc(s) H(s) ve Error signal Sensor gain iload ECEN 4517 6 Transfer functions of some basic CCM converters Table 8.2. Salient features of the small-signal CCM transfer functions of some basic dc-dc converters Converter Gg0 Gd0 0 Q z buck D V D 1 LC R C L boost 1 D' V D' D' LC D'R C L D' 2R L buck-boost – D D ' V D D' D' LC D'R C L D' 2 R D L where the transfer functions are written in the standard forms Gvd(s) = Gd0 1 – sωz 1 + s Qω0 + sω0 2 Gvg(s) = Gg0 1 1 + s Qω0 + sω0 2 Flyback: push L and C to same side of transformer, then use buck-boost equations. DC gains Gg0 and Gd0 have additional factors of n (turns ratio). ECEN 4517 7 Bode plot: control-to-output transfer function buck-boost or flyback converter example f 0˚ –90˚ –180˚ –270˚ || Gvd || Gd0 = 187 V ⇒ 45.5 dBV || Gvd || ∠ Gvd 0 dBV –20 dBV –40 dBV 20 dBV 40 dBV 60 dBV 80 dBV Q = 4 ⇒ 12 dB fz 2.6 kHz RHP ∠ Gvd 10-1/2Q f0 101/2Q f0 0˚ 300 Hz 533 Hz –20 dB/decade –40 dB/decade –270˚ fz /10 260 Hz 10fz 26 kHz 1 MHz10 Hz 100 Hz 1 kHz 10 kHz 100 kHz f0 400 Hz ECEN 4517 10 Example: a loop gain leading to a stable closed-loop system T(j2 fc) = – 112˚ m = 180˚ – 112˚ = + 68˚ fc Crossover frequency 0 dB –20 dB –40 dB 20 dB 40 dB 60 dB f fp1 fz || T || 0˚ –90˚ –180˚ –270˚ ϕm ∠ T ∠ T|| T || 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz ECEN 4517 11 Example: a loop gain leading to an unstable closed-loop system T(j2 fc) = – 230˚ m = 180˚ – 230˚ = – 50˚ fc Crossover frequency 0 dB –20 dB –40 dB 20 dB 40 dB 60 dB f fp1 fp2 || T || 0˚ –90˚ –180˚ –270˚ ∠ T ∠ T|| T || ϕm (< 0) 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz ECEN 4517 12 Transient response vs. damping factor 0 0.5 1 1.5 2 0 5 10 15 ωct, radians Q = 10 Q = 50 Q = 4 Q = 2 Q = 1 Q = 0.75 Q = 0.5 Q = 0.3 Q = 0.2 Q = 0.1 Q = 0.05 Q = 0.01 v(t) ECEN 4517 15 Compensator circuit f 0˚ –90˚ –180˚ –270˚ || T || 0 dB –20 dB –40 dB 20 dB 40 dB fz ∠ T 1 MHz10 Hz 100 Hz 1 kHz 10 kHz 100 kHz f0 || T || ∠ T Obtain crossover frequency having good phase margin ECEN 4517 16 Effect of transformer leakage inductance + – LM + v – Vg Q1 D11:n C Transformer model iig R L l + v l – + vT(t) – • Leakage inductance L l is caused by imperfect coupling of primary and secondary windings • Leakage inductance is effectively in series with transistor Q1 • When MOSFET switches off, it interrupts the current in L l • L l induces a voltage spike across Q1 t Vg + v/n vT(t) iRon DTs {Voltage spikecaused byleakageinductance vl = L l dil dt If the peak magnitude of the voltage spike exceeds the voltage rating of the MOSFET, then the MOSFET will fail. ECEN 4517 17 Protection of Q1 using a voltage-clamp snubber + – + v – Vg Q1 D11:n C Flyback transformer ig R + vT(t) – CsRs – vs + Snubber{ • Snubber provides a place for current in leakage inductance to flow after Q1 has turned off • Peak transistor voltage is clamped to Vg + vs • vs > V/n • Energy stored in leakage inductance (plus more) is transferred to capacitor Cs, then dissipated in RsUsually, Cs is large Decreasing Rs decreases the peak transistor voltage but increases the snubber power loss See supplementary flyback notes for an example of estimating Cs and Rs