Introduction to Feedback - Power Electronics I | ECE 562, Study notes of Electrical and Electronics Engineering

Download Introduction to Feedback - Power Electronics I | ECE 562 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 LECTURE 11 Introduction to Feedback I. Feedback on PWM Converters A. Why Employ Feedback? 1. Improved Stability 2. Lower Zout for Stiffer V(out) vs. I(out) 3. Faster Frequency Response 4. BUT Danger of Oscillation is introduced by feedback B. How to implement feedback 1. Voltage Feedback 2. Current Feedback C. Various Semiconductor Control Chips and Switch Device Components III. Transient Effects A. Start Up B. Other Black Box Calculations i Transformer Dein i Ovipt ier Recfer Design ' Power Swit & Drver Desin H Controle Dein } Output Fehr Desi ! requrements chose a sitchin ea opp Datemine second at oa any tule pt Desn tafe, ie ages, Desion outa slr relies an cop, Design der ces Cts contol mde LC, Dein bas nets, Das Volage feck & cr regulation cui 5 |AOLB|(dB) ω 40 dB/decade ω |Aβ | → 1 or 0 dB and φ = 180° Recall from op amp design and control theory, one designs the feedback loop carefully such that undesired loop oscillation does not occur at any frequency. In some server computer power supplies or system tape drives, safe reliable operation is as important as speed - ultrasafe case. Ultra-safe case: cross unity gain of AOL only at a slope of 20 dB/octave due to a single pole only. Only one pole in AOL converters are made by design. Discontinuous mode and current programmed mode converters are examples of one pole transfer functions we can design for. See Chapter 10 and 11 of Erickson respectively. We will see second semester that for an optimum feedback design we need to hit a specific value of phase margin for the open loop gain. This value gives the fastest response without any danger of oscillation. 6 B. How to implement feedback There are several feedback schemes: • Voltage Feedback • Current Feedback • Frequency Feedback Below we will focus on voltage and current feedback only. We will leave frequency feedback , which is employed in resonant switching converters, for second semester. 7 1. Voltage Feedback (Chapter 8 and 9 of Erickson) Feedback itself, in PWM dc-dc converters, can operate in two circuit modes: continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The former has well orchestrated control of switches while the later has intervals controlled by the circuit and not the switch drivers. A v DOL out= ∆ ∆ Loop gain with respect to duty cycle We will find later that for the same feedback loop on the same converter operating either in continuous conduction mode (CCM) or operating in discontinuous conduction mode (DCM) will have two very different closed loop gains and dynamic conditions: a. CCM has two poles and we need to design carefully for phase margin of 76° to avoid oscillation. b. DCM has only one pole in transfer function. It is unconditionally stable and will never oscillate. 10 In summary for current feedback we have: • Characterized by a comparator fed by the difference between the error voltage and the instantaneous power switch current. Modern switch devices have on board current sensors to protect the switch from over current •Now peak currents are sensed immediately and switches protected in a more direct and faster responding manner. This reduces costly field replacement of switches. • C. Various Semiconductor Control and Switch Device Components 1. Overview The three major categories of PWM converter parts, for the PWM parts bill of lading, are given below. a. Cheap IC controller chips exist with many on-board capabilities: 11 •timing components •current sensing •PWM with variable D •switch drivers (b) Power devices for switching: See Chapter 5 of text •MOSFET’s •IGBT’s •diodes •GTO (Gate turn-off Thyristor) •MCT (MOS-Controlled Thyristor) (c) Reactive elements: •Capacitors •Inductors on cores In practice parasitic R, L, and C components often make up half the circuit model components though they do not appear on the bill of lading. 2. Commercial Controller Chips The controller chip is available from integrated circuit manufacturers at very low cost, yet, featuring a host of capabilities. Two types of control chips are listed on the next page. Features on board the chips include: • Power MOSFET Drive Circuits for the power switch • Multiple Output Sensing with Weighting of Each Output • Over-current Shutdown circuits • Over-voltage Protection Circuits • Under-voltage Protection Circuits a. Commercial Control Chips 12 b. MOSFET Driver The gate of the power MOSFET must be driven by 10 V at VGS to reach full ID. The gate capacitance is usually 2nF, so large peak currents are drawn. 15 We assume that the feedback loop has opened or the load current on one output has gone to zero causing the voltage to rise above the maximum specification. In this case we need separate hard wired output sensors and a separate comparator to activate override of the error amplifier as shown below via three approaches f. Undervoltage Shutdown 16 Here we assume that brownout conditions occur at the input which could inadvertently cause the duty cycle to latch up to unity and lose control. A simple comparator sensing the line input will avoid this case as shown below. If a logic or microprocessor chip as well as a hard disc drive is driven by a power supply we may also need a POWER ABOUT TO FAIL signal be generated to allow a sufficient time to institute a orderly shutdown. As much warning time as possible is desired. This is beyond today’s discussion. III. Transient Effects There are two separate effects we will consider. One is the isolated turn-on of the converter which has a long transient time to reach steady-state output. During this time the control chip and driver circuits may not be powered up in time. If this occurs, we may not be able to drive the switch properly and we can destroy the expensive power switch. The second is the fast switching at each Ts which causes losses as we try tomaintain the output. A. Slow turn on vs. steady state 17 SwitchVg RC output Vout Boost Buck Buck-Boost Consider buck case: Apply Vg switch at fsw. Turn-on requires: @ t = 0, iL = 0; @ t = ∞ , iL = Iout (1) Turn on: Up-ramp slope @ t = 0: s V Lu g= − 0 Up-ramp slope @ t = ∞ : s V V Lu g out= − 20 vout ∼ -cos ω t How could we get a sinusoid centered about zero volts? (2) Bridge-inverter case: voltage fed, not current fed In a fixed D operation we find Vout = M(D)Vin. V V Dout g = −2 1 D M(D) 1 1 -1 0.5 Noting that the output is symmetric about 0.5. We set D=0.5 and Vo=0. Add a time varying component D = 0.5 - ∆dcos ω t to achieve sinusoidal output around zero volts. 21 Later we will model the two synchronized SPDT switches by a switch averaged two port model. Two port model i1 i2 vg vs (b) DC-DC converter with feedback To better stabilize DC-DC converters, we use feedback that looks at a fixed Vref compared to the changing Vout, which sets the proper D for desired Vo dynamically. If Vo varies for whatever reason then the on duty cycle D varies to stabilize Vo back to the desired value. D will become a function of time rather than a constant and the transfer function of the inverter becomes the output voltage divided by the duty cycle V d o $ will be valid. On the following page is a full schematic for a flyback converter. FOR PRACTICE look through the schematic to find the peripheral circuitry in a PWM: • Input filter and rectifier circuit block • Various Outputs • Control and PWM Circuits HI N dS INSO004s 400V ik r ov 00V) Moy 100UF 1m, /2w H2 2504 o t Los 190uF Je 1M,1 720 aK ky 250 Earth PWR GND GND + To output Gnd Ti MUR420 W : pt 2a 22k i 4.7K, (72 11BR370 +12 MaRz40 = Ot 22K 4.1K, 1/20 Soubber pry MPSASa element ++ 7—0/P GND a 1NS148 4 “12¥ Iv Vauxe SMBR370 INS2414 4 lou pT aes 20) 7 gndl s}-—_1 IRF720 1000 pF Lane fis ucsza4san “TW 470 39K 5 +12 424 +Vin 2 1 sax SIKE240K 320.0k it sak 1K " h P andi 3), 32pF mcs40 1 PWR C " i ¥ FAL 205K) = omit 39K # MOCBIO2 Ay 20 TL4310P ser 2KE E - C t fay 0/P GND PWR GND Nate) (Cy 65 W, off-line flyback converter.