Managing PFC regulation, resonant DC-DC conversion, and load-adaptive efficiency across PSU power stages
Power controllers manage each PSU conversion stage - shaping input current through PFC, regulating resonant DC-DC conversion, and coordinating synchronous rectification at the output. Controller performance directly determines regulation accuracy, transient response, light-load efficiency, and protection robustness - constraints that become critical as designs target 80 PLUS Titanium efficiency levels.
Poor control strategy leads to hard-switching events, excessive standby power, acoustic noise, and thermal stress - limiting system efficiency across load and line conditions. At light load, switching losses can dominate total losses without proper frequency management - making light-load efficiency one of the hardest constraints in modern PSU design.
onsemi addresses these constraints with a coordinated controller portfolio:
- Totem Pole PFC (NCP1680, NCP1681) - CrM and CCM/multi-mode control; eliminates input diode bridge; optimizes efficiency across full load range without MCU firmware
- Current-mode LLC (NCP13994) - resonant half-bridge controller with integrated 600 V drivers; up to 750 kHz; quiet skip mode for acoustic noise reduction
- Secondary-side PFM LLC + SR (NCP4390) - charge-current technique for fast transient response; adaptive dead-time; integrated synchronous rectifier control across a 39–690 kHz operating range
- CrMPFC + QR Flyback Combo (NCP1945) - integrates CrMBoost PFC and quasi-resonant flyback control in a single IC; reduces component count and design complexity; high efficiency across load with VSFF, valley switching, frequency foldback, and skip mode
These controllers work alongside high-performance switching devices (e.g., SiC Cascode JFETs) and isolated gate drivers, forming a coordinated system across PSU power stages. The sections below map each controller to its PSU stage and detail the properties that drive system-level performance.
PSU Architecture and Controller Mapping
Modern PSUs use multi-stage architectures where each power stage requires a dedicated controller. The PFC stage shapes input current and regulates the DC bus; the LLC DC-DC stage provides isolated, regulated output. Controller selection directly impacts efficiency, EMI, transient behavior, and system protection.
Typical high-frequency PSU stages:
| PFC Stage (AC-DC) | LLC / DC-DC Stage |
|---|---|
| NCP1680 - CrM Totem Pole PFC; eliminates input diode bridge; constant on-time control with valley-synchronized frequency foldback for light-load efficiency - code-free, no MCU firmware required | NCP13994 / NCP13992 - Current-mode resonant half-bridge controller; integrated 600 V drivers; up to 750 kHz; quiet skip mode for acoustic noise reduction - eliminates external driver and level shifter |
| NCP1681 - CCM / Multi-mode Totem Pole PFC; fixed-frequency CCM at heavy load, CrM at medium/light load; near-unity power factor - seamless mode transitions without discontinuity | NCP4390 - Secondary-side PFM LLC controller with integrated SR control; charge-current technique for transient response; adaptive dead-time; 39-690 kHz - 100 W to 2 kW+ off-line and industrial PSUs |
| NCP1945 - CrMBoost PFC + quasi-resonant flyback combo controller: integrates both AC-DC stages in a single IC; reduces external components and design complexity; optimized efficiency with CrM-to-DCM transition (VSFF) and QR valley switching; frequency foldback and skip mode for light-load performance; integrated protection features | |
These controllers work alongside high-performance switching devices (e.g., SiC Cascode JFETs) and isolated gate drivers, forming a coordinated system across PSU power stages.
For LLC, QR flyback, and forward topologies, dedicated synchronous rectifier controllers (NCP4306, NCP4307) replace secondary-side diodes with MOSFETs - reducing conduction losses at the output stage.
onsemi Controller Core Properties and Value Drivers
| Core Property | What it Delivers | Example Orderable Part Numbers |
|---|---|---|
| Scalable PFC Topology Coverage | Boost PFC, follower boost, multi-mode (CCM/CrM/DCM), and Totem Pole PFC - spanning 65 W adapters to multi-kW server PSUs | NCP1623, NCP1618, NCP1680, NCP1655 |
| Code-Free Mixed-Signal PFC | CCM at heavy load, CrM at medium/light - valley-synchronized frequency foldback reduces light-load losses that can otherwise reach 10% | NCP1680, NCP1681, NCP1623 |
| Multi-Mode & Light-Load Efficiency | CCM at heavy load, CrM at medium/light - valley-synchronized frequency foldback reduces light-load losses where switching losses can otherwise dominate total system losses | NCP1681, NCP1680, NCP1623 |
| Interleaved PFC for Higher Power | 2/3-channel interleaved operation with 180°/120° phase shift - extends CrM power range, reduces input current ripple and bulk capacitor stress | NCP1632, FAN9673, FAN9672 |
| Current-Mode Control & Accurate Regulation | Pulse-by-pulse current limiting; charge-mode control compensates for load transients without waiting for voltage/frequency to respond | NCP13994, NCP13992, NCP4390, FAN9673 |
| Integrated Drivers & Simplified Layout | Integrated 600 V half-bridge drivers eliminate external driver and level shifter; adaptive dead-time minimizes body-diode conduction loss | NCP13994, NCP13992, NCP4390, NCP1618 |
| Synchronous Rectification | Self-driven SR replaces secondary-side diodes with MOSFETs for lower conduction loss - programmable min on/off time and light-load management for LLC, QR flyback, and forward topologies | NCP4306, NCP4307, NCP4390, NCP4318 |
| Comprehensive Protection | OVP, OTP, OCP, brown-out, cycle-by-cycle current limit, non-ZVS protection, active X2 discharge, and auto-restart - across PFC, LLC, and SR stages | NCP1680, NCP13994, NCP4390, NCP1618 |
These properties span onsemi's full PSU controller portfolio - from CrM boost PFC for USB-PD adapters to interleaved CCM PFC and current-mode LLC for multi-kW server and industrial power supplies - enabling designs targeting 80 PLUS Titanium-class efficiency.
Role of onsemi Controller in Power Stage
| Functional Role (Device Level) |
Operating Conditions (Power Stage Context) |
System Impact (Outcome) |
|---|---|---|
| Bridgeless Totem Pole PFC control (e.g., NCP1680, NCP1681) |
AC–DC stage; eliminates input diode bridge, removing ~1.7% input power loss at low line | Higher PFC stage efficiency; reduced component count |
| Multi-mode operation (e.g., NCP1681, NCP1618) |
Variable load conditions - CCM at heavy load, CrM at medium/light | Optimized efficiency from light to heavy load without mode-transition discontinuity |
| Quiet skip / frequency foldback (e.g., NCP13994, NCP1680) |
Light-load and standby conditions where switching losses dominate | Reduced standby power; minimized transformer acoustic noise |
| Current-mode resonant control (e.g., NCP13994, NCP13992) |
LLC half-bridge primary-side stage; integrated 600 V drivers | Stable frequency control; pulse-by-pulse current limiting; enables ZVS operation |
| Secondary-side PFM + SR control (e.g., NCP4390, NCP4306) |
LLC isolated DC-DC output stage; charge-current sensing; adaptive dead-time minimizes body-diode conduction | Improved load regulation and transient response without frequency-dependent delay - supporting 100 W to 2 kW+ off-line and industrial PSUs |
| Integrated protection (OVP, OCP, OTP, brown-out) |
Fault and abnormal operating conditions across PFC, LLC, and SR stages | Safe, reliable operation across all load/line conditions; auto-restart capability |
onsemi's PSU controller portfolio maps dedicated control to each power stage - from code-free Totem Pole PFC at the input, through current-mode LLC with integrated drivers at the primary, to secondary-side PFM with synchronous rectifier control at the output - forming a coordinated control architecture across the full PSU signal chain.
Technical resources to support your system design and evaluation
PFC Design & Optimization
- Meeting Challenging Efficiency Standards with Bridgeless Totem Pole Power Factor Correction
- Power Factor Correction − Optimization Options
- Totem Pole PFC Layout Considerations
PSU System & Topology Design
- 3 kW Totem−Pole PFC and Secondary−Side Regulated LLC Power Supply Using SiC MOSFETs
- Popular Topologies in Offline Power Supplies
- Meeting Ultra−High−Density Design Challenges with GaN−based 300 W Totem Pole PFC and LLC Power Supply
Reference Designs (spanning power ranges)
- 1 kW Universal Input 48 V Output Power Supply Reference Design
- 240 W UHD Power Supply - CrM Totem Pole PFC and Current Mode LLC HB Solution
- 140 W Two-Switch Combo PFC & QR Flyback for USB PD3.1 EPR Solution
System Solution Guides
Frequently Asked Questions (FAQs)
Conventional boost PFC uses an input diode bridge followed by a boost stage - the diode bridge alone accounts for approximately 1.7% of input power loss at low line. Totem Pole PFC eliminates the input diode bridge entirely by using active switches in both the fast leg (PWM-frequency) and slow leg (line-frequency), removing both bridge conduction losses and PFC diode losses. This enables PFC stage efficiencies exceeding 99%. onsemi's NCP1680 (CrM) and NCP1681 (CCM/multi-mode) implement Totem Pole PFC as code-free, mixed-signal controllers - no MCU firmware required.
CrM (Critical Conduction Mode) operates with variable frequency and zero-current turn-on, which eliminates reverse recovery losses in the boost diode or switch. It is best suited for lower-to-mid power applications (typically up to ~400-500 W single-phase) where simplicity and efficiency at lighter loads are priorities. NCP1680 implements CrM Totem Pole PFC with valley-synchronized frequency foldback for optimized light-load efficiency.
CCM (Continuous Conduction Mode) operates at fixed frequency with lower peak and RMS currents, making it better suited for higher-power designs where input current ripple, EMI filtering, and bulk capacitor stress must be minimized. NCP1681 implements CCM/multi-mode Totem Pole PFC - running fixed-frequency CCM at heavy load and transitioning to CrM at medium/light load for best full-range efficiency.
No. onsemi's NCP1680 and NCP1681 are mixed-signal controllers with proven PFC algorithms implemented in hardware - eliminating the need for MCU firmware development. This removes firmware development time, reduces code validation effort, and avoids the risk of software-related field failures. The controllers handle all PFC functions including zero-crossing detection, phase management, and frequency control without any programmable logic.
Audible noise in LLC converters typically occurs when the switching frequency drops into the audible range (below ~20 kHz) at light load, causing magnetostriction in the transformer core. NCP13994 addresses this with quiet skip mode - instead of allowing the switching frequency to decrease continuously, the controller skips switching cycles while maintaining the frequency above the audible threshold. This reduces switching losses at light load while preventing transformer acoustic noise. NCP4390 uses a complementary approach: hybrid PFM/PWM control where the controller transitions to PWM mode at light load, fixing the switching frequency and reducing the duty cycle instead - limiting the frequency while controlling power delivery.
In a conventional voltage-mode LLC controller, the error amplifier output directly controls the switching frequency. However, the LLC resonant converter's frequency response includes four poles whose positions shift with input voltage and load - making compensation design challenging.
Charge-mode control (used in NCP4390) integrates the primary-side switch current over each switching cycle and compares the resulting charge signal to the compensation voltage. Since the integrated current is proportional to average input current, this provides a fast inner control loop with inherent line feedforward and excellent transient response. The result is a simpler feedback loop design, true input power limiting, and faster response to load transients - without waiting for the voltage or frequency loop to respond.
onsemi offers two approaches to synchronous rectification for LLC output stages:
Integrated SR within the LLC controller: NCP4390 combines secondary-side PFM LLC control with built-in synchronous rectifier gate drives. It uses a dual-edge tracking adaptive method that predicts the SR current zero-crossing instant - automatically adjusting SR timing without external components. SR gate shrink and SR disable functions manage light-load conditions to prevent shoot-through.
Dedicated SR controllers: NCP4306 and NCP4307 are standalone self-driven SR controllers that replace secondary-side diodes with MOSFETs for lower conduction loss. They offer programmable minimum on/off time and light-load management, and support LLC, QR flyback, and forward topologies - giving engineers flexibility to pair them with any primary-side controller.
onsemi EliteSiC Solutions
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Simulation Tools
Building a complex electronic application and need some insights through system-level simulations?
Check out onsemi’s simulators:
Elite Power Simulator and Self Service PLECS Model Generator