How to use the DC/DC loop stability calculator
- Load the closest preset first. A preset is a known working starting point, not a rule for your final design.
- Select the topology, control mode and conduction mode: voltage mode or peak current mode, CCM or DCM. You can see on results section an estimate of the conduction mode margin and the minimum inductance for CCM.
- Enter the operating point and power-stage values: input voltage, reference voltage, switching frequency, load current, inductance, output capacitor, ESR and transformer ratio where the topology uses one.
- Set output voltage through Vref, R1 and Rbot. R1 is also part of the compensator, so changing R1 affects both the setpoint and the compensator plot.
- When useful, enable "Show loop without compensator" to inspect the feedback divider times the selected control-to-output model. Use the plant pole and zero markers to identify the dynamics the compensator must handle.
- Select Type I, Type II or Type III compensation and tune the component values. You can type exact values or move the sliders to see the compensator and loop Bode plots update live.
- Inspect the complete loop-gain curve, crossover frequency, phase margin, gain margin, warnings and every 0 dB crossing. You should aim for a plot that crosses 0 dB one time and with as much downward slope as possible.
- Use Freeze curves to keep the current gain and phase traces on the plots, then change values or presets to compare alternatives. Unfreeze removes the stored traces. This is very useful to get a sense of how the loop shape changes with different compensator types or plant parameters.
- Validate the design across input voltage, load, tolerances and operating modes with switching simulation and hardware loop-gain measurement. ALWAYS MEASURE THE LOOP OF THE FINAL DESIGN.
Video tutorial
Learn how to configure the converter, tune the compensator, interpret the Bode plots, compare designs, and run a tolerance sweep.
Using the tolerance sweep
Enable Tolerance sweep after the nominal design is working, then enter the allowed ± variation for input voltage, output current, power-stage parts and compensator parts. Components vary independently. The calculator evaluates the nominal case, one-at-a-time minimum and maximum cases, and a bounded deterministic set of combined corners; it is not an exhaustive sweep or a Monte Carlo simulation.
Input voltage, load and component values never remain exactly nominal. Their combined variation can move poles and zeros, shift crossover frequency, reduce phase or gain margin, increase ripple, and move the converter toward a CCM/DCM or subharmonic boundary. Tolerance analysis reveals designs that look stable at nominal values but lose robustness at realistic corners, helping you preserve adequate margin before switching simulation and hardware validation.
How to interpret the Bode plot and stability results
The calculator reports the first 0 dB loop-gain crossing, but a robust design is judged by the whole loop shape. Treat these results as loop-shaping guidance before simulation and measurement.
Crossover frequency
Crossover frequency is where the complete loop-gain magnitude first crosses 0 dB. It is a practical estimate of control-loop bandwidth. Crossover should be well below the switching frequency. Aim at about fs/10. The reason for that is that averaged models used in this calculator become less reliable as crossover approaches switching frequency. Without going into too much detail, average models loose some of the information of what happens inside the switching cycle. This lost information has high frequency content. Conseuently, the model cannot predict it well. That being said, try to make your converter crossover frequency as high as possible as long as it works reliably. A higher crossover frequency means a faster control loop and better rejection of disturbances.
Phase margin
Phase margin is reported as 180 degrees plus the loop phase at the first 0 dB crossing. A phase margin above about 45 degrees is a common minimum starting point, and about 60 degrees is a common practical target. Those numbers are guidelines, not universal pass/fail limits. In some books you may see different conventions for phase margin depending on whether they include or not the minus sign in the phase. Phase margin describes how agressive the loop is. Low phase margins will produce a fast response but may cause overshoot and ringing. High phase margins will produce a slower response but with less overshoot and ringing. The ideal phase margin is a trade-off between speed and overshoot.
Gain margin and multiple crossings
Gain margin is evaluated at the first -180 degree phase crossing found in the plotted range. If the magnitude curve crosses 0 dB more than once, the first reported crossing is not enough to prove robustness; inspect every crossing and the phase near each one.
Right-half-plane zeros
Boost and flyback converters in CCM include a right-half-plane zero. It adds phase lag while the gain still rises, so crossover normally needs to stay well below that marker. Practically speaking, don't use those topologies in VM if they operate in CCM. For your electronics intuition, this is what happens: topologies that have the inductor between Vin and the switch do not transfer energy to the output during the switch on-time (inductor is shorted to ground). The inductor current is rising, but the output voltage is falling as only the capacitor can supply charge to the load. When the switch turns OFF, the energy in the inductor is then transferred to the output (and rechargint the capacitor). If the control demands more duty cycle, the output voltage will fall even more before (more duty on-time) it rises again (during duty off-time). If this happens slowly (slow duty variation), the loop can still regulate. If it happens at high frequency, duty increases, Vo falls, controller sees Vo fall and increases duty even more, Vo falls even more, and the loop can oscillate. The right-half-plane zero is a mathematical representation of this behavior.
CCM/DCM and subharmonic warnings
The CCM/DCM warning tells you when the selected conduction mode disagrees with the calculated boundary. In peak current mode, the subharmonic margin is a slope-compensation indicator for CCM designs; it is useful for intuition near fs/2, but it is not a perfect model. For buck-derived PCM designs in DCM, the calculator also warns when the linearized output pole is unstable. Check every warning with switching simulation and hardware.
Choosing Type I, Type II or Type III compensation
The upper plot shows the compensator by itself and the lower plot shows what happens after that compensator is multiplied by the selected converter model. Choose the compensator type from the plant you need to shape and the crossover you are trying to reach.
Type I
Type I provides integral action and high low-frequency gain. It is suitable when only modest bandwidth and little phase shaping are required. In practice, Type I is not often used by itself, but it is the basis for Type II and Type III compensators.
Type II
Type II adds a zero and a high-frequency pole. It is commonly useful for lower-order plants, many current-mode cases and conservative designs where moderate phase boost is enough.
Type III
Type III provides a broader phase-boost region. It is often used with voltage-mode LC double-pole plants, especially when the target crossover is near the output-filter resonance.
These are starting points. The actual plant, ESR zero, right-half-plane zero, target bandwidth, noise sensitivity and transient requirements determine the final choice.
How the calculator models the converter
The diagrams show the signal path used to build the Bode plots. Start at Vref, follow the error through the compensator and controller to Vout, and then follow the R1/Rbot feedback path back to the comparison point. The selected topology, conduction mode and control mode determine how the blocks respond with frequency.
Voltage-mode feedback loop
Peak-current-mode feedback loop
From entered values to an operating point
The calculator first establishes the converter condition described by your inputs. Vref, R1 and Rbot set Vout; Iout sets the equivalent load; and Vin, topology and transformer ratio determine the required duty cycle. The frequency response is valid around this operating point, so changing input voltage or load can move the gain, poles and zeros even when L, C and the compensator stay unchanged.
What the control-to-output transfer function tells you
This transfer function answers a practical question: if the controller changes its command slightly, how strongly and how quickly will Vout respond at each frequency? Buck-derived converters share the familiar output-filter behavior, including the LC resonance in CCM. Boost and flyback converters have different energy-transfer dynamics and, in CCM, a right-half-plane zero that limits useful bandwidth. In DCM the current returns to zero each cycle, so the response changes with the operating point and is modeled differently. Capacitor ESR changes damping and high-frequency behavior.
How voltage mode and peak current mode differ
In voltage mode, VCOMP is compared with the PWM ramp to change duty cycle; this is why PWM ramp amplitude changes loop gain. In peak current mode, VCOMP commands a peak current instead. Current-sense sensitivity, COMP-to-current-sense gain and slope compensation then form an inner current loop that reshapes the power-stage response. The calculator also estimates sampled-current behavior near half the switching frequency, but that region deserves extra validation in a switching simulation.
The first diagram shows voltage-mode control; the second shows peak-current-mode control.
How the compensator becomes complete loop gain
The compensator shapes the voltage error before it reaches the converter. The first Bode plot isolates this contribution, making its poles, zeros and phase boost easy to inspect. The second plot combines the compensator with the selected control-to-output response and shows the loop that determines crossover frequency and stability margins. When Show loop without compensator is enabled, the plot instead shows the R1/Rbot divider acting with the control-to-output response, which helps reveal the uncorrected power-stage behavior.
Model assumptions and limitations
- This is an averaged small-signal model for loop-shaping and design intuition; it is not a replacement for switching simulation or bench measurement.
- Only output capacitor ESR is modeled as a non-ideal component. Inductor DCR, capacitor ESL, transformer leakage, optocoupler/TL431 dynamics, PWM delay, current-sense filters and current-limit behavior are not included.
- Peak current mode uses an equivalent current-programmed model. The sampled behavior near fs/2 is approximate and should be validated carefully if the design is close to subharmonic oscillation.
- Half-bridge peak current mode does not model split-bus capacitor midpoint-balance dynamics; verify capacitor-voltage balance with switching simulation and hardware.
- For flyback, the inductance field is primary magnetizing inductance.
- Confidence decreases as crossover approaches switching frequency. Treat the fs/10 warning as a sign that the averaged model needs extra validation.
- Stability should be checked across input voltage, load, component tolerances, temperature and possible CCM/DCM mode transitions.
Ready-to-run LTspice simulations
Use the LTspice files below as a practical starting point for your own converter designs. Each simulation is configured to match its named calculator preset, making it easy to open a known case, compare the loop-gain result, and then replace the power-stage and compensation values with your own.
These are switching transient simulations that use the LTspice FRA block to measure loop response. You can tailor the power stage, feedback network, compensator, control parameters and FRA settings to your design. If a simulation is too slow, narrow the FRA frequency span by increasing fstart and/or reducing fend so it covers only the region you need, while retaining enough range to inspect the relevant poles, zeros and 0 dB crossings.
In the included FRA plots, the phase trace is already expressed as phase margin rather than loop phase: a reading of 60° therefore means a 60° phase margin. To display the compensator response in the same plot, right-click the plot pane, choose Add Traces and add the Verror trace.
- Buck CCM voltage mode
- Buck DCM voltage mode
- Buck CCM peak current mode
- Buck DCM peak current mode
- Forward CCM voltage mode
- Half-bridge CCM voltage mode
- Half-bridge DCM voltage mode
- Flyback CCM voltage mode
- Flyback DCM voltage mode
- Flyback CCM peak current mode
- Flyback DCM peak current mode
If you want to shape the error amplifier by itself first, use the control loop compensator calculator. If the LC resonance or ESR zero is the part you want to build intuition for, the low-pass filter calculator is a useful companion.
Frequently asked questions
-
What does this DC/DC loop stability calculator calculate?
It calculates compensator response, complete loop gain, crossover frequency, phase margin, gain margin, output-current related margins and ripple estimates for the selected converter model. -
Why is there no direct output-voltage input?
Output voltage is calculated from Vref, R1 and Rbot. R1 is both the upper feedback resistor and part of the compensator network, so changing it can affect the operating point and the compensator response. -
What phase margin should I target?
A phase margin above about 45 degrees is a common minimum starting point, and about 60 degrees is a common practical target. These are guidelines, not universal guarantees of robustness. -
How far below switching frequency should crossover be?
Crossover should normally be safely below switching frequency. The calculator warns above about fs/10 because averaged models become less reliable as crossover approaches switching frequency. -
Which compensator type should I choose?
Type I is for simple integral action, Type II adds moderate phase shaping, and Type III gives broader phase boost for harder voltage-mode LC plants. The plant and target crossover decide the final choice. -
Why do CCM boost and flyback converters require special care?
In CCM, boost and flyback converters include a right-half-plane zero. It adds phase lag while gain is still rising, so crossover usually needs to stay well below it. -
What do multiple 0 dB crossings mean?
They mean the loop may regain or lose gain margin at more than one frequency. Inspect all crossings and their phase; the first reported crossing alone is not enough to establish robustness. -
Does this replace SPICE simulation or loop-gain measurement?
No. Use it to choose and understand compensation, then validate the design with switching simulation, component tolerances and hardware loop-gain measurement. -
Does the calculator support peak current mode?
Yes. Peak current mode uses current-sense sensitivity, COMP-to-CS gain and slope compensation. The sampled behavior near fs/2 is approximate and should be validated for final designs.
References
This calculator is based on published books, papers, courses, seminars and technical material by Robert W. Erickson, Dragan Maksimović, Christophe Basso, Ray Ridley and Sam Ben-Yaakov. Together, their work provides the foundations for the converter-averaging methods, small-signal models, loop-compensation techniques, current-mode control models and practical validation approach used here.
If you want to really learn this subject, these authors are the real heroes. Seek out their books, courses, papers, seminars and videos: their original material goes far beyond what any calculator can summarize.
- Erickson and Maksimović: Fundamentals of Power Electronics
- Erickson and Maksimović: Power Electronics courses on Coursera
- Maksimović: Modeling and Control of Power Electronics on Coursera
- Christophe Basso: PowerSimTof
- Sam Ben-Yaakov: power-electronics video lectures
- Ray Ridley: Ridley Engineering
The LTspice simulations included with this calculator were created by Miguel P..