CCM
Understanding CCM
Inductor Current Continuity and Ripple Statistics
Switching regulators are widely used to supply power to RF components due to their high efficiency. In a switching regulator, the switch turns on and off at a high frequency $f_{\text{sw}}$. When the switch is closed, energy accumulates in the storage inductor, causing the inductor current to ramp up. When the switch opens, the inductor discharges its stored energy to the output, causing the current to ramp down. If the average load current is high enough, the inductor current remains positive throughout the entire switching cycle, which defines Continuous Conduction Mode (CCM).
If the load current drops below a critical threshold, the inductor current reaches zero before the next cycle begins, forcing the converter into Discontinuous Conduction Mode (DCM). The boundary between CCM and DCM is determined by the output inductance, the switching frequency, the input/output voltages, and the minimum load current. To maintain CCM across all operating conditions, designers must size the inductor to be larger than a calculated minimum critical inductance.
Advantages for Noise-Sensitive RF Circuitry
For RF power supplies feeding low-noise amplifiers (LNAs), mixers, or voltage-controlled oscillators (VCOs), CCM is the preferred mode of operation. In CCM, the switching frequency remains constant, and the output voltage ripple is low and predictable. This concentrates the electromagnetic interference (EMI) energy into discrete harmonics of the switching frequency, which are easy to filter using compact LC post-filters.
In contrast, DCM causes the inductor to ring with the parasitic capacitances of the switches during the dead-time (when the current is zero). This ringing produces broad-spectrum high-frequency noise that can easily couple into sensitive RF signal paths, causing unwanted spurs and degrading the system's spurious-free dynamic range (SFDR). Additionally, the duty cycle in CCM is independent of the load current, which simplifies the design of the feedback control loop.
Key Mathematical Relations
Technical Specifications Comparison
| Converter Parameter | Continuous Conduction Mode (CCM) | Discontinuous Conduction Mode (DCM) | Impact on RF Performance |
|---|---|---|---|
| Inductor Current | Always positive ($I_L > 0$) | Reaches zero with a dead-time | CCM avoids high-frequency parasitic ringing |
| Output Ripple Voltage | Low and load-independent | Higher and load-dependent | CCM simplifies output filter design |
| EMI Spectrum | Concentrated at $f_{\text{sw}}$ and its harmonics | Broadband noise from ringing | CCM harmonics are easy to trap/filter |
| Peak/RMS Currents | Lower peak currents | Higher peak currents for same load | CCM reduces I2R losses and component stress |
| Control Loop Dynamics | 2nd-order (has RHP zero in boost) | 1st-order (simpler to stabilize) | CCM boost requires careful compensation |
Frequently Asked Questions
How do you design a power supply to stay in CCM at light loads?
To maintain CCM at light loads, you must increase the value of the inductor ($L$) or increase the switching frequency ($f_{\text{sw}}$). Alternatively, some modern controllers support forced continuous conduction mode (FCCM), which allows the inductor current to go negative by using synchronous rectification, keeping the switching frequency constant at all loads.
Why does DCM cause parasitic ringing?
In DCM, when the inductor current falls to zero, both the main switch and the synchronous rectifier switch are open. The inductor forms a resonant tank circuit with the parasitic drain-to-source capacitance of the switches, resulting in decaying high-frequency voltage oscillations.
What is the impact of power supply ripple on an RF power amplifier?
Power supply ripple modulates the bias current of the RF power amplifier, creating sidebands around the RF carrier at offsets equal to the switching frequency and its harmonics. This amplitude modulation (AM) distortion degrades the EVM and causes spectral regrowth.