A Low-Cost Adaptive Multi-Mode Digital Control Solution

Maximizing AC/DC Power Supply Efficiency

Feb 24, 2010

Yong Li & Jerry Zheng

iWatt Inc.Los Gatos, CA 95032

Outline

Review of existing AC/DC control solutions for energy efficiency

Proposed new low-cost high-efficiency digital control solution

• System architecture• Operational principal• Experimental results• Cost analysis

Summary

Efficiency Issues in Conventional PWM Power Supplies

Energy efficiency standards (EPA 2.0, etc.) specify the average efficiency by testing at 100%, 75%, 50%, and 25% of rated output current, and then computing the arithmetic average of these four values

At most times power supplies operate at medium to light loads

Improving light-load efficiency is the key for green power

Poor efficiency at light load

Conventional PWM/PFM Controls to Address Efficiency Issue

PFM Control

• Audible noise issue when switchingfrequency FSW drops to human audio rangebut still significant energy remains

~16kHz

PFM

IO

IO

VO

FSW

Audible Noise

PWM/PFM Control

100%

PWM

IO

IO

VO

FSW

PFM

25%

•Transition far below 25% load•No help on average efficiency

PFM Control w/ Frequency Clamp

•Voltage regulation and ripple issues at lighter load

•Suffers on no-load standby power

~20kHz

PFM

IO

IO

VO

FSW

Clamped

New Digital Green-Mode AC/DC Solution

iW1696

No feedback/regulationcomponents at secondary side

Extremely Low Cost

BJT

No RCD Clamp Needed for <3W due to BJT slow di/dt

Simple Windings No Y-Cap DesignSimple EMI filter

High Efficiency by Design

Top side

Bottom side

Example Circuit Board

SecondaryPrimary

Digital Controller iW1696

SOT-23, 5 pin

BJT

Cell-Phone Charger Application

+Vin

VSENSE

VCC

ipri

isec

iW1696 Series Controller

Ipri

isec

System Architecture• True digital peak-current mode control • Primary-side feedback eliminates opto-isolators and simplifies design• Tight 2% CV and 3% CC regulation w/ primary-side-only feedback

Built-in Multi-Layer Protections- OCP, OVP, output short - Current sense resistor short - Auto-restart current limit

High Efficiency by Design- Dynamic base current control ensures

BJT always works in optimal switching conditions - Adaptive multi-mode PWM/PFM control- Unique valley-mode switching

Multiple U.S. Patents Granted/Pending

Rsense

Adaptive Multi-Mode PWM/PFM ControlUnlike conventional approaches, digital controllers are well suited for multi-mode PWM/PFM

operation in response to varying load

As load changes from 100% to 0%, the controller goes through multiple PWM and PFM modes

U.S. Patent Pending

VO

Fsw

IO100%

b) PFM--- For medium to light load efficiency improvement

PFM

PFM

c) DPWM (Deep PWM)---i) Avoid audible noiseii) Provide smooth transition to very-light load

DPWM

DPWM~20kHz

d) DPFM (Deep PFM)---i) Energy has been reduced to insignificant level

when the switching frequency drops to audio band ii) Smoothly transition to no-load operationiii) Minimize no-load standby power consumption

DPFM

DPFM

IO

a) PWM--- For heavy load operationPWM

PWM 40kHz-65kHz

Conventional Valley Mode Switching (VMS)

Used in flyback to reduce switching loss and EMIi) After a switch has been turned off, if the switch is turned on at a Valley, the switching loss is reduced--- “quasi-resonant” or “valley mode” switching

ii) EMI can be reduced due to lower dv/dt

BJT VCE

Issues with Conventional VMS schemesi) Typically switch turns on at the First Valleyii) Work with variable switching frequency only iii) Can be challenging for EMI filter designiv) Switching frequency can be as high as 130kHz,

not suitable for BJT’s

First Valley

Unique Valley-Mode Switching

• No modification to the basic main control algorithms • Always turn on at the “local” valleys across the entire load range• Work with constant frequency PWM, variable frequency PFM, and/or their combinations• No frequency clamping or other performance compromise

Light Load Heavy Load

Seamlessly integrated with the adaptive multi-mode PWM/PFM controlin iW1696 series digital controllers

U.S. Patent Pending

Measured Operational Waveforms

PWMPFMDPWMDPFM

CC

IO

VO

Measured Switching Frequency vs. Load Changes

• No audible noise heard during the entire load range

• Lower switching frequency at high line, and higher switching frequency at low line----further reduces power losses

• Digital control enables flexible and easy configuration for different applications

PWM

PFM

DPWM

DPFM

PWM

PFM

DPWM

DPFM

Adaptive Digital Control Flattens Efficiency Curve

Meet EPA 2.0 Efficiency Spec.with lots of Margin

EPA2.0 (Final) for Low Voltage Model (Pno=2.85W) 0.075 (2.85 ) 0.561 64.0%Ln W× + =

Measured iW1696-Controlled 5V/550mA Power Supply Efficiency

Digitally-Controlled BJT SolutionsSimplify EMI Design

• BJT switching--- Inherent slower di/dt and dv/dt

• Valley-mode switching--- Further reduces dv/dt --- Spreads frequency spectrum with

natural frequency jittering

• Digital frequency white noise generation

Net Effects

(Persistence Mode)

Vsense

Ipri

CM Noise Comparison

BJT Drive MOSFET Drive

(Measured between transformer primary and secondary windings)

2.7V pk-pk 3.6V pk-pk• Dynamic base drive current control

--- Optimizes BJT switching conditions

230VAC/50Hz,Live 230VAC/50Hz,Neutral

QP

AV AV

QP

Conducted EMI

Measured EMI Performance--- No Y-Cap, No CM Choke, Simple Transformer

• Measured on typical iW1696 demo board• Individual designs may vary

230VAC/50Hz,Vertical 230VAC/50Hz, HorizontalRadiated EMI

Lowest Total System Bill-of-Material Cost

Typical 5V/550mA DesignActual Circuit

• Estimated based on typical 5V/550mA design• Individual designs may vary

Summary

• 5-pin primary-side digital controller with built-in BJT base driver• Dynamic BJT base drive current control• Adaptive multi-mode PWM/PFM control • Unique valley mode switching • No audible noise, voltage regulation or other performance compromise

iW1696 series digital control solutions offer the lowest total system BOM cost, and achieve the highest efficiency and lowest EMI for low-power (< 5W) AC/DC power supplies available in the industry.

Low-Cost Green Power by Design

World-wide energy standards are being tightened. iWatt innovative digital control platforms will enable development of new solutions that meet the emerging requirements.