12Vdc – 120Vac Emergency Power System

Jim MosleyTA: Wayne Weaver

Introduction

ac power is taken for granted Most dc powered communication systems

are charged by ac systems Back-up systems are rarely capable of

extended operation Economical alternative to stand-alone ac

generation is needed

Batteries

First option to be suggested

Good source for clean dc power

Limited amount of energy storage

Usually charged by an ac source

Charging

Typical ac powered charger Alternative power such as solar and wind Alternators and generators

Power Source

All methods of recharging a battery require a power source

Source must be:

Reliable

Available at all times

Maintenance free

Not an expensive “just in case” item

People Power

People are always around

Reliable, although intermittent

Not sitting in storage waiting to be used

Don’t have a shelf life

Power Transfer

Person + Bicycle + Alternator =

Charged Battery + Tired Person

Next time I’m taking the bus!

Muscle to Electrons

Modified commercial bike stands

Home-made stands

What is feasible?

Typical person has sustainable output of around 100 watts

Power directly from a human powered source is too intermittent to be reliable for most electrical devices

Main power source would be the battery with a person recharging the battery

Complexity of Simplicity

Design standardization is required for use by the general public

There are many different alternators, each with their own mounting and wiring quirks

There are just as many different bicycles, each with their own gearing, tire dimensions, and crank lengths

Determining the range of operation

Bench test several different alternators to find any similarities in operation– Efficiency– Minimum speed required to output at least 35

watts– Torque requirements at 35 watts– Calculate normal operating speeds from pulley

dimension

Determining the range of operation

Record data from a variety of bicycles to find a common gear ratio

Perform tests to determine comfortable range of cycling

Perform tests to estimate the power a human can comfortably produce

Data

Only one suitable alternator was found

Machine shop was unable to complete mount in time

Bicycle data was collected, but not analyzed

100-120 watts is practical

dc Voltage to ac Voltage

How to get the “readily available” dc source to power ac chargers and emergency communication equipment

Converter is necessary

Converter Components

Two components are needed– Push-pull forward converter to step up 13.4Vdc to 120Vdc– Inverter to produce 120V square wave

Push-Pull Forward Converter

To achieve a high gain necessary, the push-pull forward converter uses a dc bus with MOSFETs Q1 and Q2 switching at 50kHz to apply an ac current across the high frequency transformer T1

The diodes rectify the signal back to dc while L1 and C2 help to clean up the signal

Inverter

MOSFETs T1, T4 provide the positive pulse of the output while T2, T3 provide the negative pulse

Deadtime between the switching events eliminates the current spikes that would result from the short circuit

Unitrode UC2526 PWM Modulator

Factors in Choosing the Unitrode Chip

Low supply current Soft-start Over-current protection Under-voltage protection Thermal protection Shut-down input for other external protective

circuits

Testing the Unitrode 2526A

After several attempts to operate the converter with the Unitrode chip, it was replaced with the TL494 PWM modulator

TL494 has less features, but was chosen because of extensive use in the Dept

Lessons learned implementing the TL494 provided potential solutions for applying the UC2526A

Key Requirements for PWM control

Error amplifiers are non-inverting Un-used amplifier inputs should not be left

floating Reference input should be kept 2V below

Vref Oscillator frequency easily adjusted with an

RC circuit

TL494 Operation

50 kHz oscillator signal used by comparitor

TL 494 Outputs at Different Feedback Voltages

MIC4424 MOSFET Driver

To protect the output of the TL494, a line driver was used

Higher current capacity Cheaper and easier to

replace Two inputs and two

outputs so only one chip is needed

Other Converter Components

MOSFETs were chosen to meet voltage and current requirements

Center-tapped transformer wound to provide the widest operating range

Large capacitor on supply to reduce switching noise

High current diodes to rectify the output Large capacitor to smooth the output voltage

Output Waveforms

MOSFET gate signal and output voltage before the diodes

MOSFET gate signal and output voltage after the diodes and capacitor

Inverter Components

MC78L00 voltage regulator to provide 5Vdc control power

LM555 timer for 50% duty cycle 60Hz oscillator

SN74LS75 latch to provide complimentary outputs

IR2113 high/low side MOSFET driver

LM555

The versatile LM555 timer has been a reliable industry work-horse for many years

Simple RC circuit sets frequency

Difficulties Implementing LM555

Original design incorporated an 74LS14 Schmitt trigger inverter to provide complimentary inputs to the MOSFET driver

Unable to achieve dead-time at MOSFET driver due to only one rising edge from the LM555

SN74LS75 latch with complimentary outputs used to provide two outputs

Solving Dead-time Issues

IR2113 has Schmitt trigger input Dead-time easily controlled by RC circuit

Difficulties in Inverter Operation

Dead-time During testing, line driver was configured for

low/low side operation Jumper was not removed to allow high/low

side operation By-pass caps not installed to reduce

switching noise

Tests Performed

Operation with varying input voltages Effect on small ac charger output Efficiency

Commercial “brick” with dc Output

Output waveform on commercial power

Output waveform on inverter

Voltage spikes are more pronounced

Commercial “brick” with ac Output

Output wave form on commercial power

Output wave form on inverter

Efficiency

Preliminary results show an overall efficiency of 3% at no load and 56% at full load

The converter is more efficient with a no load efficiency of 53% and 58% at full load

The main reason for the difference is that the converter requires a minimum load to operate, therefore the actual output is 0 at no load, but the converter is still consuming power

Future Modification

Safety features such as:– Floating the input and referencing the output to

earth ground– Overcurrent protection– Short-circuit protection– Under-voltage shutdown

Credits

Professor Swenson Professor Chapman Wayne Weaver Brett Nee Jonathan Kimball Dustin Kramer