Dc dc converters feedback and control

Design MPPT solar charge controller and understand that by manipulating the load impedance seen from the solar panel (reducing the duty cycle of the buck converter), the input voltage to the DCDC converter (from the solar panel) will increase as the load current will also increase with constant voltage output( the load being the battery). The charge controller will minatan the duty cycle where the load current stop increasing as this is to be the maximum point. The problem is I want the output voltage to be relatively controlled or constant while the load current increasing . How the output voltage is held constant while changing the duty cycle to increase the current? if the answer is by having a feedback loop that change the duty cycle, then this will interfere with reducing producer of the duty cycle to increase the current.

Dc-dc converters feedback and control presented by Christophe Basso Product Applications Engineering Director Pardon his French! Wild Bill Hickok Chris Basso – June 2008 Course agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating p g the converter  Current-mode converters  Automated pole-zero placement  Manual pole-zero placement  Compensating with a TL431  Watch the optocoupler!  Multi-output M lti t t converters t  Input filter  A real case example p  Conclusion Chris Basso – June 2008 Oooh, that looks so good! good!… Course agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating p g the converter  Current-mode converters  Automated pole-zero placement  Manual pole-zero placement  Compensating with a TL431  Watch the optocoupler!  Multi-output M lti t t converters t  Input filter  A real case example p  Conclusion Chris Basso – June 2008 What we expect from a dc-dc?  A stable t bl output t t voltage, lt whatever h t loading, l di iinput, t ttemperature t and aging conditions  A fast reaction to a incoming gp perturbation such as a load transient or an input voltage change  A quick settling time when starting-up or recovering from a transient state state A stable and noiseless dc source we can trust! Ambient temperature TA Input voltage Vin dc-dc Output voltage Vout Output current Iout Chris Basso – June 2008 What is feedback?  A target is assigned to one or several state state-variables, variables e e.g g Vout = 12 V  A dedicated circuit monitors Vout deviations  If Vout deviates from its target, an error is created and fed-back to the power stage for action  The action is a change in the control variable: duty-cycle duty cycle (VM), peak current (CM) or switching frequency Compensating for the converter shortcomings! Input p voltage g Vin d d dc-dc Output voltage Vout Input voltage Vin action control Chris Basso – June 2008 Rth Vth Vout The feedback implementation  Vout is permanently compared to a reference voltage Vref  The reference voltage Vref is precise and stable over temperature  The error,   Vref  Vout, is amplified and sent to the control input  The power stage reacts to reduce  as much as it can Power stage - H Vout Control variable d Error amplifier - G Rupper + - Vin  - Modulator - GPWM + Vp Vref Chris Basso – June 2008 Rlower Course agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating p g the converter  Current-mode converters  Automated pole-zero placement  Manual pole-zero placement  Compensating with a TL431  Watch the optocoupler!  Multi-output M lti t t converters t  Input filter  A real case example p  Conclusion Chris Basso – June 2008 Positive or negative feedback?  Do we want to build an oscillator? The « Plant » Vin(s) () +  Vout(s) () H(s)  G(s) Vout  s  Vin  s   H s 1 H sG s Open-loop gain T(s)   H s Vout  s   lim  Vin  s   Vin  s    G  s  H  s    Sign is neg for:  = -180° To sustain self-oscillations, as Vin(s) goes to zero, quotient must go infinite What Plant?? =1 Chris Basso – June 2008 Observing the dB point  Create phase lag: cascading RC networks R1 1k 80.0 plot1 vdb1 in db(volts) 40.0 AC = V1 H(s) R2 1k R3 1k C1 10n Vout C2 10n C3 10n -40.0 |H| = -29 dB -80.0 plot2 ph_v1 in degree es 100 38 kHz argH(s) -100 -200  = -180° 180 -300 100 1k f 10k i h Chris Basso – June 2008 100k 1Meg A constant gain with a 180° rotation  Add gain to obtain T(s) = dB where  = -180° 180° (38 kHz) R1 1k R2 1k R3 1k C1 10n C2 10n C3 10n K2 K1 SU UM2 X2 SUM2 K1 = K2 = parameters t Gfc=-29 G=10^(-Gfc/20) Ri 100k Ri=100k Rf=G*100k Vout R5 {Rf} Vin AC = E1 10k 10 R4 {Ri} Chris Basso – June 2008 Run the same circuit in SPICE X2x XFMR RATIO = -250m vc a p Vin 150 AC = D1A mbr20200ctp vint L1 2.2u R1 20m vout vout c duty y-cycle DC PWM sw witch CM  A simple dc bias is ok for H(s) alone X9 PWMCM L = Lp Fs = Fs Ri = Rsense Se = Se R10 15m L Lp {Lp} R15 85m (V(errXX)-1.2)/3 > ? : (V(errXX)-1.2)/3 B3 Voltage Rload 6.3 10 C5 2m C1 220u R2 47k X3 OP384X1 R3 47k 13 errXX CCM operation Low line voltage 15 UC384X gnd 2V5 V3 1.08 AC = Dc + ac modulation 206 Chris Basso – June 2008 Compare model and real board results simulated 207 Chris Basso – June 2008 Another simple method  H(s) alone can be measured without loop opening R19 47k R13 47k R3 47k C2 10n D5 MBR20100 400V C11 100p C4 100uF R7 10k 400V R6 6k R11 10k Vref CMP Ref FB Vcc CS Rt DRV GND U1 UC3843 C5a 1.2mF C5b 1.2mF 25V 25V C7 220uF D8 1N4937 R1 330 Gnd R8 1k R10 56k U3B U3A R5 1k R6a R15 4.7k C16 4.7nF B 20 log10    A R18 47k Vc(s) C12 220p A R12 10k SPP11N60S5 R16 10 Vref B R14 4.7k M1 Vout 25V C13 22 F 2.2nF Type = Y1 R17 47k D2 MUR160 ( ) Vout(s) L2 2.2u C10 10uF C6 100n R6b IC2 TL431 C3 220uF C15 10nF R9 10k Gnd Watch out for capacitor connection (short-circuit to GND when discharged) 208 Chris Basso – June 2008 Open-loop study, a possible solution?          Opening the loop offers several se eral advantages: ad antages Easy access to the output stage transfer function Works if the power supply is not stabilized Can almost pass dc if no ac capacitor is use It also brings g drawbacks: Possible runaway in high-gain systems Needs to adjust the bias point if Rload or Vin change Almost impossible to stabilize the error amp output Operate in closed loop and inject signal via a transformer! Floating ac source Ground referenced ac source 209 Chris Basso – June 2008 Transformer injection, a safer way  The loop is closed in dc but open in ac! Vout Gain decreases mV / div dB Vin = Vout Vin 1.5 kHz / V Vref R2 20 R20 4.7k High gain Vsource 2.13 210 6.38 10.6 14.9 19.1 Chris Basso – June 2008 A B ac source VB T  s   20 log10 VA Change the operating conditions easily simulated CCM operation, Rload = 6.3 Ω 211 Chris Basso – June 2008 Reduce the load to enter in DCM simulated DCM operation, Rload = 20 Ω 212 Chris Basso – June 2008 From the open-loop Bode plot, compensate 40.0 RLED = 270 Ω CZERO = 8.2 nF CPOLE = 10 nF 180 argT(fc) = -90° -20.0 90.0 ph_vout in degrees Plot1 vdbout in db(volts) 20.0 -90.0 |T(fc)| = -18 dB 21 -40.0 -180 10 213 100 1k frequency in hertz Chris Basso – June 2008 10k 20 100k Verify in the lab the open-loop gain  Sweep extreme voltages and loads as well! Simulated CCM operation, Rload = 6.3 Ω, Vin = 150 Vdc 214 Chris Basso – June 2008 Verify in the lab the open-loop gain Si l t d Simulated 100 1k 10k CCM operation, Rload = 6.3 Ω, Vin = 330 Vdc 215 Chris Basso – June 2008 100 Verify in the lab the open-loop gain Si l d Simulated DCM operation, Rload = 20 Ω, Vin = 330 Vdc 216 Chris Basso – June 2008 As a final test, step load the output  Good agreement between curves! Vin = 150 V CCM to A A/µs Simulated 217 Chris Basso – June 2008 As a final test, step load the output  DCM operation at high line is also stable Vin = 330 V DCM 0.5 to A A/µs Simulated 218 Chris Basso – June 2008 Agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating p g the converter  Current-mode converters  Automated pole-zero placement  Manual pole-zero placement  Compensating with a TL431  Watch the optocoupler!  Multi-output M lti t t converters t  A real case example  Conclusion 219 Chris Basso – June 2008 Conclusion  We now understand the origins of phase margin needs  The crossover frequency value is analytically derived Current mode technique simplifies the compensation  Current-mode  Operating mode transition is not a problem for CM  Type and type are also available with a TL431  The optocoupler brings a pole degrading the phase margin  Do NOT forget the influence of the EMI filter  SPICE eases the design with multi-output converters  A real-case example confirmed the validity of the approach! Merci ! Thank you! Xiè i ! Xiè-xie! 220 Chris Basso – June 2008 ...Course agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating... good! good!… Course agenda  Feedback generalities  Building an oscillator  Poles and zeros  Phase Ph margin i and d quality li coefficient ffi i  Undershoot and crossover frequency  Compensating... precise and stable over temperature  The error,   Vref  Vout, is amplified and sent to the control input  The power stage reacts to reduce  as much as it can Power stage - H Vout Control