Electronics Guide

Pulse Width Modulation

PWM control operates at a fixed switching frequency while varying the duty cycle to regulate output voltage. The duty cycle, defined as the ratio of switch on-time to the total switching period, directly determines the voltage conversion ratio. In a buck converter, for example, the output voltage ideally equals the input voltage multiplied by the duty cycle.

Fixed-frequency operation offers several advantages. The predictable switching harmonics simplify EMI filter design since interference occurs at known frequencies. Synchronization with other system clocks becomes straightforward. The inductor current ripple remains relatively constant across load conditions, simplifying component selection. PWM controllers typically achieve tight voltage regulation and excellent transient response, making them the dominant choice for moderate to heavy loads.

The control loop in a PWM converter compares the output voltage to a reference, generating an error signal that modulates the duty cycle. Various modulation schemes exist: trailing-edge modulation uses a fixed rising edge and variable falling edge, leading-edge modulation does the opposite, and dual-edge modulation varies both edges symmetrically around the pulse center.

Pulse Frequency Modulation

PFM control maintains a fixed on-time or fixed energy per pulse while varying the switching frequency to regulate output voltage. At light loads, PFM reduces switching frequency dramatically, sometimes to just a few kilohertz, minimizing switching losses that would otherwise dominate at low power levels.

This characteristic makes PFM particularly valuable in battery-powered applications where light-load efficiency directly impacts standby time. Modern controllers often implement hybrid PWM/PFM schemes, operating in PWM mode at moderate to heavy loads for tight regulation and low ripple, then transitioning to PFM mode at light loads for efficiency optimization.

The variable frequency nature of PFM spreads switching harmonics across a frequency range, potentially simplifying EMI compliance in some cases but complicating filter design in others. PFM mode typically exhibits higher output voltage ripple than PWM mode, requiring careful specification of acceptable ripple ranges for the intended application.

Advanced Modulation Techniques

Beyond basic PWM and PFM, advanced modulation techniques address specific performance requirements. Constant on-time control offers fast transient response with inherent current limiting. Constant off-time control simplifies inductor selection and provides stable operation across wide input ranges. Hysteretic control, operating between upper and lower voltage thresholds, offers the fastest possible transient response at the cost of variable frequency operation.

Operating Principles

In voltage mode control, an error amplifier compares the sensed output voltage to an internal reference, producing an error signal that feeds a PWM comparator. This comparator compares the error signal to a sawtooth or triangular waveform synchronized to the switching clock, generating the control pulses for the power switches.

The loop dynamics depend on the power stage transfer function from duty cycle to output voltage, the output filter characteristics, the feedback network, and the error amplifier compensation. The power stage of a buck converter, for instance, contains a complex pole pair formed by the output inductor and capacitor, followed by an ESR zero from the output capacitor.

Compensation Design

Achieving stable operation with adequate phase margin and crossover frequency requires careful compensation network design. Type II compensation, using an integrator with a single pole-zero pair, suffices for converters with high-ESR output capacitors where the ESR zero provides phase boost near crossover. Type III compensation adds an additional pole-zero pair, enabling higher bandwidth with low-ESR capacitors such as ceramic types.

The compensation design process involves analyzing the uncompensated loop gain, selecting a target crossover frequency (typically one-tenth to one-fifth of the switching frequency), and placing compensation poles and zeros to achieve the desired gain slope and phase margin at crossover. A minimum phase margin of 45 degrees ensures stable operation without excessive ringing.

Advantages and Limitations

Voltage mode control offers simplicity, noise immunity, and straightforward loop analysis. The single feedback loop requires only output voltage sensing, and the duty cycle varies smoothly with load changes. However, voltage mode control exhibits slow response to input voltage variations since the control loop must first detect the resulting output change before correcting.

The double pole in the power stage transfer function also complicates compensation, particularly with low-ESR output capacitors that eliminate the stabilizing ESR zero. These limitations led to the development of current mode control for applications requiring faster transient response and simpler compensation.

Peak Current Mode Control

In peak current mode control, the switch turns on at the beginning of each switching cycle and turns off when the sensed current reaches a threshold set by the error amplifier output. This inner current loop effectively eliminates the inductor from the small-signal model, converting the double-pole power stage to a single-pole system that requires only simple Type II compensation.

Current sensing typically occurs through a low-value resistor in series with the power switch or by measuring the voltage across the switch during conduction. Resistive sensing offers accuracy and temperature stability but dissipates power. Switch voltage sensing eliminates sensing losses but requires careful timing and compensation for the switch on-resistance temperature coefficient.

Benefits of Current Mode Control

Current mode control provides cycle-by-cycle current limiting, inherently protecting the converter against overloads and short circuits. The inner current loop responds to input voltage changes within a single switching cycle, dramatically improving line transient response. Current sharing in parallel converters becomes automatic since all units track the same voltage loop error signal.

The simplified transfer function makes compensation design more straightforward, with the single pole at the output capacitor ESR zero frequency. This allows higher loop bandwidths with simpler compensation networks, improving overall transient response.

Average Current Mode Control

Average current mode control uses a current error amplifier to regulate the average inductor current rather than the peak value. This approach eliminates the need for slope compensation and provides more accurate current regulation, making it particularly suitable for power factor correction circuits and battery chargers where precise current control matters.

The current loop bandwidth in average current mode control can approach the switching frequency, enabling extremely fast current response. However, the additional amplifier increases complexity and requires careful design to maintain stability.

Understanding Subharmonic Instability

The instability arises from the interaction between the fixed-frequency clock, the peak current threshold, and the inductor current slopes. When the duty cycle exceeds 50%, the falling current slope during the off-time becomes steeper than the rising slope during on-time. A current perturbation at the beginning of a cycle produces a larger opposite perturbation at the beginning of the next cycle, leading to growing oscillation.

Adding Slope Compensation

Slope compensation eliminates subharmonic oscillation by adding an artificial ramp to either the current sense signal or the error amplifier output. The compensation ramp reduces the effective control gain at high duty cycles, ensuring that perturbations decay rather than grow.

The minimum compensation slope required for stability equals half the inductor current downslope during the off-time. In practice, designers often use the full downslope to provide margin and eliminate the dependence of transient response on duty cycle. Some controllers integrate fixed slope compensation, while others allow external adjustment for optimization.

Implementation Considerations

Adding slope compensation affects the current loop gain and bandwidth, requiring adjustment of the voltage loop compensation for optimal overall response. Excessive slope compensation reduces the benefits of current mode control, eventually making the system behave more like voltage mode control. The designer must balance stability margin against transient performance.

Soft-Start Mechanisms

Soft-start circuits gradually increase the reference voltage or current limit during power-up, allowing the output voltage to rise smoothly to its final value. This controlled ramp prevents the large inrush currents that would otherwise flow to charge output capacitors through low-impedance paths, protecting fuses, input capacitors, and power switches from stress.

Implementation approaches include gradually increasing an internal reference voltage, ramping the error amplifier output clamp, or progressively increasing the current limit. The soft-start time constant depends on the application requirements, typically ranging from hundreds of microseconds to tens of milliseconds.

Power Supply Sequencing

Many electronic systems require multiple supply voltages that must power up and down in specific sequences to prevent latch-up, excessive current draw, or undefined logic states. Sequencing controllers coordinate the enable signals to multiple regulators, ensuring proper ordering and timing.

Common sequencing strategies include sequential start-up where each supply reaches regulation before the next begins, ratiometric tracking where all supplies ramp proportionally, and simultaneous start where all supplies begin together but reach their final values at different times. The choice depends on the requirements of the powered circuitry.

Tracking and Margining

Voltage tracking ensures that a supply follows another reference, maintaining a fixed ratio or offset during transients. This capability proves essential for systems with voltage constraints between power domains, such as certain FPGA configurations. Margining allows adjustment of the output voltage above or below nominal for testing purposes, verifying system operation across the expected voltage tolerance range.

Control Timing Requirements

Proper synchronous rectification requires precise timing to prevent shoot-through, where both high-side and low-side switches conduct simultaneously, creating a short circuit across the input. Dead-time intervals between the turn-off of one switch and turn-on of the other prevent shoot-through but must be minimized to reduce the interval when body diode conduction occurs.

Adaptive dead-time control adjusts the non-overlap interval based on switch characteristics and operating conditions, minimizing body diode conduction while maintaining shoot-through protection. Some controllers measure switch voltage to detect zero-voltage switching conditions and optimize timing accordingly.

Light-Load Efficiency Optimization

At light loads, synchronous rectification can reduce efficiency because the synchronous switch continues conducting even when the inductor current reverses, drawing energy back from the output. Discontinuous conduction mode (DCM) operation, where the synchronous switch turns off when inductor current reaches zero, improves light-load efficiency by preventing this reverse current flow.

Detecting the zero-current condition requires either direct current sensing or indirect methods such as monitoring the switch node voltage. Zero-current detection enables efficient DCM operation while maintaining the benefits of synchronous rectification during continuous conduction at heavier loads.

Diode Emulation Mode

Diode emulation mode operates the synchronous switch to mimic an ideal diode, conducting only when forward-biased and blocking when reverse-biased. This mode eliminates negative inductor current at light loads, improving efficiency and enabling stable parallel operation of multiple converter phases. Modern controllers implement diode emulation with minimal transition delays to capture the full efficiency benefit.

Interleaving Benefits

When multiple phases operate with equal phase spacing, their ripple currents partially cancel at both input and output. For N phases operating at 360/N degree phase shifts, ripple current cancellation reaches maximum at duty cycles of 1/N and multiples thereof. This cancellation reduces the RMS current stress on input and output capacitors, allowing smaller and less expensive capacitors while improving efficiency through reduced I2R losses.

Multi-phase operation also distributes thermal dissipation across multiple components and enables modular power scaling. Adding phases increases power capability while maintaining the same transient response characteristics. The effective switching frequency at the output appears multiplied by the number of phases, simplifying filter design.

Current Sharing and Balancing

Achieving equal current sharing among phases prevents individual phase overloading and maximizes efficiency. Active current sharing compares phase currents and adjusts individual duty cycles to equalize loading. This requires current sensing in each phase and a sharing control loop that operates slower than the main regulation loop.

Passive current sharing relies on matched components and careful layout but cannot correct for manufacturing variations. Active sharing ensures balanced operation even with component tolerances and temperature differences between phases.

Phase Shedding for Efficiency

At light loads, operating all phases reduces efficiency because switching and gate drive losses in each phase remain significant even with minimal power transfer. Phase shedding disables unnecessary phases at light loads, concentrating current in fewer phases operating at higher efficiency points.

The controller monitors load current and progressively enables or disables phases as load conditions change. Hysteresis prevents rapid phase switching near transition thresholds. Some systems disable phases completely while others maintain them in a ready state for fast response to load increases.

Resonant Topology Fundamentals

Unlike hard-switched PWM converters where switches transition abruptly between on and off states, resonant converters use the natural oscillation of LC circuits to bring switch voltage or current to zero before switching occurs. Zero-voltage switching (ZVS) and zero-current switching (ZCS) eliminate the overlap of voltage and current during transitions that causes switching losses in conventional converters.

Common resonant topologies include LLC resonant converters widely used in high-efficiency power supplies, phase-shifted full-bridge converters for high-power applications, and quasi-resonant flyback converters for cost-sensitive designs. Each topology offers different trade-offs between complexity, efficiency, and operating range.

Frequency Control in Resonant Converters

Resonant converters typically regulate output voltage by varying the switching frequency relative to the tank resonant frequency. Operating above resonance enables ZVS in LLC converters, while the gain characteristic varies with frequency according to the tank circuit parameters.

The control loop must account for the nonlinear relationship between frequency and output voltage, which differs significantly from the linear duty-cycle relationship in PWM converters. Compensation design requires careful analysis of the small-signal transfer function, which varies with operating point.

Maintaining Soft-Switching Conditions

The efficiency advantage of resonant conversion depends on maintaining soft-switching conditions across the operating range. Load and line variations can push the converter out of the soft-switching region, causing hard switching that increases losses and stress. Advanced controllers monitor operating conditions and adjust frequency or timing to ensure continuous soft-switching operation.

Digital PWM Generation

Digital controllers generate PWM signals using high-resolution counters and comparators rather than analog ramps and comparators. Resolution requirements are demanding: achieving 1% duty cycle precision at 10-bit output requires 14 or more bits of effective PWM resolution. Techniques such as sigma-delta modulation and hybrid analog-digital approaches extend effective resolution beyond what simple digital counters provide.

Digital Compensation Implementation

The compensation function in digital controllers uses digital filter algorithms, commonly implementing PID (proportional-integral-derivative) control or more sophisticated structures. Coefficient values stored in registers allow easy adjustment of loop dynamics without hardware changes, enabling adaptive control that optimizes performance across operating conditions.

Sampling and computation delays add phase lag that must be accounted for in stability analysis. At high switching frequencies, these delays can limit achievable bandwidth unless compensated through prediction algorithms or other advanced techniques.

PMBus and Communication Standards

PMBus (Power Management Bus) provides a standardized I2C-based interface for digital power supply control and monitoring. Through PMBus commands, a system controller can adjust output voltage, read current and temperature telemetry, set fault thresholds, and coordinate sequencing across multiple supplies.

This standardization simplifies system design by allowing interchangeable power modules from different manufacturers. Non-volatile storage preserves configuration through power cycles, and on-chip calibration compensates for component tolerances. The combination of precision regulation and comprehensive monitoring makes digital power solutions particularly attractive for telecommunications, server, and industrial applications.

Advanced Digital Techniques

Digital control enables sophisticated algorithms impractical with analog implementations. Predictive control anticipates load transients and pre-adjusts duty cycle. Model-based control uses mathematical representations of the power stage for optimal response. Autotuning algorithms characterize the power stage and adjust compensation automatically. These capabilities continue to expand as digital controllers become more powerful and cost-effective.

Noise and Signal Integrity

Switching converters generate significant electrical noise that can corrupt feedback signals and degrade regulation. Kelvin sensing connections minimize errors from high-current traces. Proper grounding separates power and signal returns. Filtering on feedback inputs reduces noise coupling while maintaining adequate loop bandwidth.

Thermal Considerations

Control circuit behavior varies with temperature, affecting loop stability and protection thresholds. Components such as current sense resistors and references should be selected for temperature stability appropriate to the application. Thermal protection circuits prevent damage from overtemperature conditions.

Component Selection

Control loop behavior depends critically on component values and tolerances. Compensation networks require precision resistors and stable capacitors. Current sense resistors must handle continuous power dissipation and peak currents. MOSFET gate drivers must provide adequate current for fast, clean switching transitions. Careful component selection ensures consistent performance across production variations and operating conditions.