Loop Compensation Voltage-Mode Buck Converters major challenge op

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Loop Compensation Voltage-Mode Buck Converters major challenge op

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Application Note
Loop Compensation Voltage-Mode Buck Converters
major challenge optimization dc/dc power conversion solutions today feedback loop compensation. laymen dc/dc power conversion circuits, this concern only difficult understand, highly intimidating matter deal with. Various effects feedback loop stability occur with application feedback compensation, which, properly calculated, cause instability regulation failure occur. This application note helps clarify more advanced Type-III feedback loop compensation considerations voltage-mode buck converter applications, which viewed inherently more stable when compared current-mode conversion topologies. Most designers believe application ceramic output capacitors good design decision, both their cost, abundance suppliers, inherently ESR. Ceramic capacitors indeed good choice converter output filtering, where relatively capacitance required. Ceramic capacitors offer Equivalent Series Resistance (ESR) that reduces output ripple. However, inherently typical ceramic output capacitor necessitates Type-III compensation network. Type-III compensation network, which more complicated than Type-II, will explained following text. Buck Converter System Block Diagram system block diagram Buck-Converter shown figure where VOUt converter input output voltage respectively. Error Amplifier accompanying passive components comprise compensation network (compensation). focus this application note proper selection these passive components order meet compensation goals. Output compensation network analog control signal Pulse-width-Modulator (Modulator) generates duty-cycle that proportional Duty-cycle control power switches conjunction with filter produce desired voltage VOUT from VIN.
VREFERENCE Compensated Error Amplifier (Compensation) Pulse-Width Modulator (Modulator) Power switches output filter (Power stage) VOUT
Feedback
Figure System Block Diagram Buck-Converter
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Open-Loop Response System response from input Modulator output power stage called "Open-Loop Response". shown figure output filter gives rise "Double-Pole" that -180 degrees phase shift. Double-Pole frequency given
output capacitor gives rise "ZERO" that degree phase shift. ZERO frequency fESR given
fESR .C.ESR
Figure shows plots. plot representative Open-Loop gain lower plot shows relevant phase. When output capacitor small ceramic type, fESR significantly larger than fLC. this case, phase open-loop reaches -180 degrees before Zero brings phase degrees (see figure
Gain (dB)
20log(Vin/Vramp)
-40dB/dec
(dB)
-20dB/dec
(deg)
(deg) -90deg/dec Phase (deg) -180 (deg) +45deg/dec
Figure Gain/Phase Open-Loop Response with ceramic output capacitor
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Goals Compensation goal compensation design feedback system such that converter will stable will quickly regulate output against changes input voltage load conditions. Quick response requires that Loop cross-over frequency "fc" (also known bandwidth) high practical. general, compensation designed such that (fs/10)<fc<(fs/5); where switching frequency converter. Stability criterion requires that phase margin corresponding "fc" greater than degree where Phase Margin degree phase Loop Gain essence have shape Gain/Phase Error Amplifier such that when combined with Gain/Phase Open-Loop figure satisfies above requirements. Type-III Compensation Type-III compensation realized connecting resistors/capacitors controller's integral Error Amplifier shown figure nomenclature consistent with Sipex datasheet used. Transfer function Type-III "Zeros" "Poles" frequencies shown figure combined effect Zeros results degree phase boost. This phase boost necessary counter degree phase output filter double-Pole shown figure generate required phase margin. order simplify solution frequency Zero Pole, components must chosen that CZ2>>CP1 R1>>RZ3. Further simplification made making frequency Zeros coincide. stated above, goal locate Poles Zeros compensation such that desired crossover frequency corresponding phase margin obtained.
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Vout
Vcomp Vreference
Conditions: CZ2>>CP1, R1>>
1/(6.28 CZ2) 1/(6.28 CP1)
1/(6.28 CZ3)
1/(6.28 RZ3CZ3) 20log(RZ2/RZ3)
Gain (dB)
20log(RZ2/R1) frequency (Hz)
Maximum boost possible degree
Phase (degree) frequency (Hz)
Figure Type-III compensation associated gain/phase plots. resistors capacitors, when connected Error Amplifier shown, create type-III compensation network. Component nomenclature same commonly used Sipex datasheets. frequency second "Zero" first "Pole" simplified solutions based choosing CZ2>>CP1, R1>>RZ3.
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Procedure Calculating Type-III Components mentioned, when ceramic output capacitor applied, open loop phase usually drops -180 degrees close order achieve required phase margin degrees greater (i.e., phase greater than -135 degrees), type-III compensation needed provide sufficient phase boost. Let's assume that phase open-loop system gain lowest possible, i.e., degrees. minimum required closed-loop phase-margin degrees compensation must provide degree phase margin (i.e., boost degrees). order maximize boost, Poles Zeros must placed apart possible. outline step-by-step procedure calculating component values, follows: R1=68.1k This value generally provides satisfactory solution helps meet requirement R1>>RZ3 Place second Zero output filter's double-Pole frequency solve CZ3: Where output inductance capacitance respectively Zero scale factor desired value following equation calculate from:
Vramp
Where VRAMP ramp amplitude converter's input voltage typically 1/10 switching frequency first Zero coincide with second Zero calculate from: first Pole switching frequency converter solve CP1: second Pole also solve RZ3:
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Example Design compensation Buck converter with following specification: Note: Loop Compensation component calculations discussed VRAMP 1.1V this application note quickly iterated with Type 900kHz Loop Compensation Calculator 2.2uH 22uF fESR (calculated from above) 22.9kHz 2.4MHz respectively. Since fESR/fLC=105, clearly Type-III compensation used. Following above procedure letting fc=fs/9, get: 68.1k 170pF 17.2k 673pF 10.2pF 1.04k Figure plots actual SPICE simulation supporting these correct values Type-III compensation network.
Figure Spice simulation showing gain/phase zsf=0.6, cross-over frequency just over 100kHz corresponding phase margin degrees
Oct11-06 Loop Compensation Voltage-Mode Buck Converters Page 2006 Sipex Corp.
Figure Step load response corresponding conservative compensation, 0A-2.5A, transient response 75us
Practical Considerations (adjusting system response) starting point above procedure locating Zeros (i.e.,zsf=0.6). This, general, provides conservative solution. seen figure phase margin nearly degrees quite acceptable. However tradeoff between system response system stability apply. seen figure transient response about 75us, impressive 900kHz converter. more aggressive compensation (i.e., faster transient response) locate Zeros closer slightly above (i.e., fLC). instance desired faster response design example zsf=1.2. Recalculating components Example get: 68.1k 85pF 34.4k 168pF 2.08k
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Gain/phase zsf=1.2 shown figure compared original solution. seen, mid-frequency gain increased 10dB phase margin decreased degrees with minimum phase about degrees. Step load response shown figure seen, response time been reduced (improved) much faster 20us.
Figure Step load response corresponding aggressive compensation, transient response been reduced (improved) 20us
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.
Part number SP6132/H SP6133 SP6134/H SP6136 SP6137 SP6138 SP6139
Ramp amplitude
Figure Ramp amplitude Sipex controllers
Conclusion With half dozen simple, low-cost discrete components, some creative `positioning', Type-III compensation greatly improve circuit response while maintaining loop stability. best part this compensation case allowed cost ceramic output capacitors solution.
further assistance: Email: Support page: Live Technical Chat: Type Loop Compensation Calculator: Sipexsupport@sipex.com
Oct11-06
Loop Compensation Voltage-Mode Buck Converters Page
2006 Sipex Corp.

Loop Compensation Voltage-Mode Buck Converters major challenge op

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