selection of pe converters and components

Dr. Suvarun Dalapati

Assistant Professor,

Department of Electrical Engineering

Indian Institute of Engineering Science and Technology, Shibpur

Selection of Power Converters and

Components

Presentation Layout: Introduction

General guidelines regarding selection of PE

Converter for a given application

Rectifiers – Selection of Topology and Components

DC/DC Converters and SMPS – Selection of

Topology and Components

Inverters – Selection of Topology and Components

General Considerations

Case Study

Conclusions

Introduction

Scope of PE Converters in Today‟s

World: Today, Power Electronics Converters have diverse

application-areas and modes

From small toys to large multi-megawatt conversion processes, power electronic converters are being applied

Converters vary in topologies and control-strategies

Even for a given converter, its design can be approached differently, for different applications

A properly designed converter focuses on optimizing the „most-desirable‟ features, as stated by the user-specifications, while „compromising‟ on other aspects

Usually, efficiency factor is never compromised with

Criticality of Proper Selection in PE

Converters and Components: Choice of PE Converters implies selection of a PE

Converter-topology and control strategy for a given application (as per user‟s specifications)

A proper choice of topology and control-strategy may yield optimum results for the given application at a minimum complexity and cost

An improper choice may not only increase cost, but also worsen reliability and efficiency

Once the topology and control-strategy are selected, the system must be fabricated with the „correctly chosen‟ components

An improper component selection may imply that the converter does not work as per specs, or even worse, may not work at all !

Objective of this Presentation: This presentation tries to guide the listener to make

the proper choice of PE converter and components

The critical issues, which decides upon such a choice are discussed briefly

Case studies are presented, illustrating the choice of (i) rectifiers, (ii) dc/dc converters and (iii) inverters

Some general considerations, regarding any power electronic system design, are also discussed briefly

After attending this presentation, the listener should be able to decide upon the PE Converter topology, broad control strategy and the type of major components (outline) to be used in a converter for a given application

Selection of PE Converters

- Some General Guidelines

Information required (to be obtained

from user‟s specification / datasheet):

Input and output range and type

Input electrical constraints (e.g. power factor) and output constraints (type of load; stability etc.)

Size and weight and space constraints

Environmental aspects (decides packaging / controller type etc.)

Level of criticality of the application (i.e. how reliable it should be; what if it fails? etc.)

Protections required and the logic

Additional features required, if any (e.g. data-logging, self-diagnostic abilities etc.)

Input and Output Range and Type:

Sl. No. Parameter Information / Data

1. Input supply type (a)DC / sinusoidal or non-sinusoidal AC.

(b) Range of variation

2. Output required (a) DC / Sinusoidal or non-sinusoidal AC

(b) If AC, then frequency range

(c) Percentage regulation required

(d) Isolation required between input and

output?

Input and Output Constraints:


1. Input Current (a) THD, (b) Power Factor

2. Output Load (a) Type: Linear / Non-Linear / Motor /

Arcing type etc.

(b) Range: Intermittent / Near full-load etc.

(c) Stability of magnitude of current and /

or voltage, stability of frequency

3. Start-up condition Soft-start, V/f start etc.

4. Stop condition Step, gradual, ramp down etc.

Size, Weight and Space Constraints:


1. Size (a) Study the mechanical drawing of the

converter box

(b) Note the dimensions

2. Weight This can be obtained from specification;

Usually lighter weight implies smaller size

3. Fitment Note the position, where this converter will

be installed / fitted / commissioned; this

helps to decide the packaging / enclosure

type etc.

4. Terminations Note the points of cable entry and exit, if

specified in the drawing; this helps in

component placement decisions

Environmental Aspects:


1. Temperature Note the range

2. Humidity As above

3. Salinity This decides upon the materials to be

used; some materials do not last in saline

environment; judicious selection is reqd.

4. Presence of Air

Flow

This helps to decide upon designing the

cooling scheme (natural / forced air,

forced water etc.)

5. Presence of

EM/ES surges

Presence of such surges affects the

reliability of electronic circuits; proper

precaution are then required

Level of Criticality:


1. Application Type OT, Crane etc. (judge the level of

criticality)

2. Analysis (a) Possible causes of failures (to be

minimized)

(b) Reliability issues of electrical /

electronic components used

(c) Two or three levels of redundancy (if

price is not an issue!)

(d) Highest grade of components (MIL)

Protections Required:


1. Protection reqd. Judge the protections required as per

application

2. Protection for OV

/ UV, OC, Phase

Outage,

Regulation

Failure etc.

Can be applied to Input and / or output;

speed of response

3. Interlocks

required

Judge by the application

4. Priority and

Speed

Decide upon the priority and speed of fault

tripping

5. Display /

Indication

Decide upon the display / indication

required

Additional Features Required:


1. Fault Indication Display through alpha-numeric / graphical

LCD / simple LED display / alarm etc.

2. Normal

operational state

Displaying input and output parameters

3. Data Logging Recording of electrical data and / or faults

at finite time intervals; accessible via a PC

for analysis

4. User Interface Can be touch panels, keyboards etc.

5. Connectivity Can be communicated through Ethernet

or wireless data network

Rectifiers

- Selection of Topology and Components

Basic Rectifier Topologies: Level of

Control

Single Phase / Two

Pulse

Three Phase / Six Pulse Six Phase / 12 Pulse

Uncontrolled

Semi-

controlled

Full-

controlled

1PUC

1PSC

1PFC

3PUC

3PSC

3PFC

6PFC

Rectifier Topologies: Performance

Comparison Name of the Parameter 1PUC 1PSC 1PFC 3PUC 3PSC 3PFC 6PFC

DC value of O/P voltage 3 2 1 6 5 4 7

Peak-to-Peak Ripple 5 6 7 2 3 4 1

Vdc/Vac 3 2 1 6 5 4 7

THD of input current 7 5 7 6 4 6 3

DPF of input current 7 6 5 7 6 5 5

Filter Requirement 5 6 7 2 3 4 1

Power Handling

Capability

1 1 1 2 2 2 3

Bidirectional Power Flow NO NO YES NO NO YES YES

Size / Weight (for same

power output)

4 5 6 1 2 3 7

Note:

1. Ranked as per higher value; thus, 1 implies least value, while 7 implies highest value

2. Continuous conduction of inductor current has been assumed

3. Many parameters are dependent on firing angle value; major trends have been assumed

General Points Regarding Selection

of Rectifiers: For low to medium power applications, controlled single phase

rectifiers can be used if the output voltage/current can contain appreciable amount of ripple

Single phase uncontrolled rectifiers are usually used as front end converters for SMPS applications or, for low power uncontrolled rectification processes

If three phase supply is available 3-phase rectifiers are the automatic choice

Usually, because of isolation requirement or the need for much lower / higher dc voltage, transformer is used; its weight adds to the system weight

Filters also add to the weight of the system

12 pulse rectifiers are only used for very high power applications (usually above 30 kW)

Chief Components for Rectifiers:

Semiconductor Devices – Diodes and/or SCRs

Input Transformer – If required

Filtering Components – Inductor and Capacitors

Snubbers for Solid-State-Devices

Auxiliary components – Switches, relays, heat

sinks etc.

Selection of Solid-State-Devices for

Rectifiers: Depending on the topology chosen, a number of diodes

and/or SCRs (rectifier grade) may be chosen

For selection of such devices, (i) PIV and (ii) Forward current rating (average and RMS) are the most crucial factors

The power loss (worst-case) for the devices in the given application must be calculated and heat sinks be designed with sufficient safety margins

Stud type SCRs / Diodes can handle large power but are body-connected (anode/cathode); thus, heat sinks form part of the current-flow-path

Two-in-one packages are also available without this feature

In case of arcing type load (e.g. welding), surge current rating of the device must be noted


Rectifiers:

Stud type diodes

Stud type SCR

Two-in-One Package

SCR module

Entire 3-ph Semi-Converter

in one module

Stud type diode, mounted

on heat sink

Selection of Transformers for

Rectifiers: Based upon the maximum output power, delivered by

the rectifier, the transformer kVA rating should NOT be decided

Transformer kVA rating should be decided by knowing the maximum current at rectifier output at the maximum phase angle;

Input current contains huge quantity of harmonics; hence must be over-rated than required

Good cooling arrangement is a must for reliable operation over a long period

For very high power application (particularly low voltage, high current type), water cooled copper conductor-based transformers may have to be employed

Selection of Transformers for

Rectifiers:

Selection of Filtering Components for

Rectifiers:

Primarily L-C low pass filters are used in rectifiers

Judicious choice must be made while deciding

upon the L-C values

Higher L-C values will lead to a lower cut-off

frequency, but slower response; lower L-C values

may not „smoothen‟ the DC output sufficiently

Filters, particularly inductors, add to the size and

weight of the system

Causes unwanted voltage and power loss

Cooling filter inductor is an additional issue


Rectifiers: Inductors Higher ripple voltage implies

higher Iac

Inductor current rating must be decided as a combined RMS of Iac and Idc

For the same cut-off frequency, if L is small and C is large, then Iac is large, leading to more loss in the inductor

If L is very large, and C is small, then size and weight rises

Upto about 800 Hz, steel-plate based cores are to be used, while beyond 1.5 kHz, ferrites are better

Preferably one inductor on +ve and one on –ve bus, wound on the same core

Paths of circulating ac and dc currents in a rectifier


Rectifiers: Capacitors

Usually to meet large filtering C-value requirements, electrolytic capacitors are used; they have large ESR and ESL

Large nos. of lower-value electrolytic capacitors in parallel is preferred (to a single large-value capacitor)

Each capacitor may be supported by an AC polyester capacitor in parallel (close proximity)

Connection between parallel capacitors must be done carefully so that stray inductances of wires are minimized – use bus-bar structure, if possible

Always connect a discharging-resistor across the capacitor bank to allow de-energizing of the capacitors


Rectifiers: Overall Scheme

Selection of Snubbers for Rectifiers:

Usually, R-C series snubbers are used

Usually, polyester / polypropylene capacitors are used

Resistors are MFR (most preferred), if not then CFR (intermediate) and if not then wire-wound (least preferred)

Carefully select the method of fitting these snubbers „close‟ to the corresponding thyristors / diodes

Reduce wire-length to a minimum

An SCR with R-C Snubber

Selection of Auxiliary Components

for Rectifiers:

Auxiliary components include switches, relays, contactors, start-up circuitry etc.

Usually, rectifiers draw high current at start-up; in addition, high inrush current may be drawn by the transformer; this necessitates the use of a „soft-start‟ circuit

Usually, the dc bus capacitors are charged through a resistor, which is then cut off by a contactor

Fuses can be used to protect the rectifier; this should be chosen as per I2t rating of the SCR (fuse should have a lower I2t rating to melt first)

In addition, usually MCBs (and not ordinary switches) are used as primary side switches, which give an additional degree of protection; these should be based on maximum input RMS current ratings

Possible Protection Scheme for a

Rectifier:

Selection of Heat Sinks for Rectifiers:

Heat sink size should be carefully calculated for the maximum load and worst case temperature, occurring simultaneously

For air cooled systems (forced or natural), copper heat-sinks are usually chosen

Heat sinks should have a lot of fins (to have a large surface area within a small size), but should be mechanically robust

Black anodized versions are preferred (greater emissivity)

Surface of heat sinks should be smooth and even (to have good contact); if needed heat sink paste be used

Should be sufficiently thick to bore a threaded hole for fitting stud type SCRs

For reducing size and weight, one may opt for water-cooled copper heat sinks (increases cost for piping, water treatment plant etc.)

Selection of Gate Drivers for

Rectifiers:

Judiciously designed gate drivers, capable of turning

on the device at worst-case scenario, should be

used

Often, manufacturers of solid state devices, also

manufacture and market „optimized‟ gate drivers for

the specific device model no.

There are manufacturers, who specialize in

designing SCR-firing circuits

Place gate drivers at suitable positions to avoid long

wire lengths and flashover due to accidental contact

DC/DC Converters


DC/DC Converters:

Perhaps the most widely used category of PE Converters

Large no. of hard and soft / semi-soft switching topologies are available

Selecting a particular topology for a given application may not be easy

Some standard topologies will be discussed and the merits / demerits of only these topologies will be discussed

Most applications are of the „Buck‟ type

„Boost‟ type converters are usually more difficult to design

Isolation is almost always required

Efficiency is, as usual, a big issue

DC/DC converters have a wide range of power handling capability from a few mW to hundreds of kW

DC/DC Converters: TopologiesType Non-Isolated (Hard

Switched)

Isolated (Hard Switched)

Buck

Boost

Buck-

Boost

Buck

Boost

Buck-Boost

Forward

Flyback

Push-Pull Full-Bridge

DC/DC Converters: Topologies

Isolated series resonant full-bridge topology (FB-SRC)

DC/DC Converter Topologies:

Performance Comparison Name of the

Parameter

Buck Boost Buck-

Boost

Forward Flyback Push-

Pull

Full-

Bridge

FB-SRC

Efficiency 7 6 6 6 5 6 6 8

Filtering / Energy

Storage

Requirement

2 3 3 5 2 4 4 1

Size / Weight 2 3 3 4 2 4 5 1

Transformation

Ratio

7 4 3 8 8 8 8 8

Drive Requirement Easy Very

Easy

Very

Easy

Very

Easy

Very

Easy

Easy Slightly

Complex

Slightly

Complex

Power Handling

Capability

5 2 3 4 1 6 7 8

Cost 1 2 3 6 4 7 8 5

Note:

1. Ranked as per higher value; thus, 1 implies least value, while 8 implies highest value

2. Continuous conduction of inductor current has been assumed

3. Many parameters are dependent on schemes used; major trends have been assumed


of DC/DC Converters: If isolation is not required, one should try to stick with the three

basic topologies (Buck, Boost and Buck-Boost)

If isolation is required, then for lowest power buck operation, one may use flyback; for medium power – forward, higher power –push-pull, highest power – full-bridge

For any given application, resonant topologies are always smaller and lighter; hence preferred if operated near full load (best efficiency)

If the SMPS encounters wide range of load variation, then hard-switched forward, push-pull or full-bridge are preferred

In case the load is of fluctuating type (i.e. jumps from near full load to no-load), do NOT use Flyback or Boost converter

Never use resonant converter if the load is mostly near no-load or less than 10% of full load

Almost all isolated SMPS topologies are used mostly in Buck applications

Chief Components for DC/DC

Converters:

Semiconductor Devices – MOSFETs / IGBTs

Switch-mode Transformer – If required




sinks etc.


DC/DC Converters: Depending on the topology chosen, a number of

MOSFETs / IGBTs and/or diodes (fast / ultra-fast) may be chosen

For selection of diodes, (i) PIV (ii) Forward current rating (average and RMS) and (iii) Reverse Recovery Time are the most crucial factors

For selection of MOSFETs (lower current but higher speed) / IGBTs (higher current but lower switching speed), the following parameters are crucial: (i) Collector / Drain Current, (ii) Vce or Vds, (iii) Surge current rating and duration, (iv) Gate-charge characteristics (for driver design) and (v) Rise and Fall times of voltages and currents

Some packages have collector / drain shorted to the metallic tab; this tab is used to fit with the heat sink; thus heat sink become a part of the current path


DC/DC Converters: Other packages have bodies isolated; these contain

multiple devices in a pack and are costlier

For low voltage, high / medium current applications, on-state losses are crucial; hence look out for Rds or on state Vce values for devices

While calculating device losses, consider both switching and conduction losses (this enables correct selection of heat sink)

MOSFETs work satisfactorily with a uni-polar pulse (e.g. 15V / 0), but IGBTs usually work best with +15V / -5V pulses

Usually, for compact SMPS, with power output in the range of 25 W to 200 W, MOSFETs are the automatic choice


DC/DC Converters:

MOSFET – Package

(single); D shorted to

TAB

Single MOSFET

isolated from body

Two-in-One Package IGBT

module (1200 V, 200 A)

Two-in-One Package IGBT

module (1200 V, 900 A)

MOSFET – Package

(single); D shorted to TAB

Selection of Transformers for DC/DC

Converters: In SMPS applications, transformers are used for

isolation and voltage step-up / down

Almost always made of soft-ferrites

Based upon no. of turns needed and the cross-sectional area of the conductor, choose the window area

Based upon the window area and the cross-sectional area of the core, select U-U / U-I / E-I / E-E type ferrite cores

Note the value of the allowable flux density in the core; keep sufficient margin

Use a bobbin of sufficient mechanical strength, made up of a non-magnetic material

Selection of Transformers for DC/DC

Converters: Usually, resin-coated copper wires are used for windings

For larger current ratings, multiple thin wires are wound in parallel (rather than using one thick wire)

Paper / cotton tape-based isolation is used between layers

Sometimes, multi-core wires (Litz-wires) are used –intention is to maximize coupling between windings

Careful marking of „start‟ and „finish‟ of windings must be made to avoid confusion during connections

It is better to measure the leakage inductance of any transformer, before using it in the actual circuit

Often covered with a metal plate / sheet – which is grounded to provide magnetic shielding to the adjoining electronic hardware


DC/DC Converters: Filtering components crucial for correct operation of any

DC/DC converter

Usually passive L-C filters are employed

They add to the weight of the system

Large switching frequency reduce filtering requirement and the values of L and C – reduction of size and weight

For large current inductors, magnetic shielding is to be applied

Ideally, use of electrolytic capacitor is to be avoided

When large C is required, some electrolytic and polyester capacitors are stacked in parallel

It is to be ensured that all magnetic components (transformer and inductor) are having their cores isolated from the windings

Sectional View of a Transformer: Insulating layer should

be present between successive conductor layers

Thicker insulation between windings

Air gap usually achieved by using mil paper or carefully measured insulator blocks

Tight mechanical binding between the two core-pieces

Case Study: Stray Inductances in a

Push-Pull Converter Leakage inductances

cause voltage spikes during switching

Semi-conductor devices are exposed to dangerously high voltages

Such „spikes‟ are partially transmitted through filters and appear at the output

Sometimes, these spikes may be „strong‟ enough to damage the drivers and/or control circuit

Can cause spurious tripping / abnormal behavior from control circuits

Leakage inductances (shown in RED)

in a push-pull converter

Eliminating Stray Inductances in a

DC/DC Converter Minimize leakage inductance of all magnetic components

Use careful layout

Cables must be running close by to minimize loop-area of circulating currents – may use sandwiched bus-bars

Use large no. of ac capacitors in parallel with electrolytic capacitors

Use Kelvin Point grounding principle

Use a properly designed PCB

Place magnetic components and electronic cards away from each other or magnetically shielded from each other

Use twisted wires wherever possible; else use shielded cables for data / signal transmission – this can eliminate spurious behavior of control circuits to a great extent

Use properly designed snubbers for solid-state switches (to eliminate / minimize switching spikes)

Snubbers for Solid State Devices in

DC/DC Converters:

Snubbers essential for hard-switched converter in most cases

Popularly R-C-D type (turn-off) snubbers are used

Optimized snubber selection is essential

Minimizing stray inductance by proper layout design will also minimize the need for snubber in many cases

Usually turn-on type snubbers are rarely used

In resonant converters, separate snubbers are rare; often, the device output capacitance itself is sufficient to serve the purpose

Auxiliary Components for Solid State

Devices in DC/DC Converters: For DC/DC converters, only fast-acting fuse may be used

Fuse protection alone is not sufficient; current sensing based tripping must be electronic in nature

Voltage based tripping may also be electronic (preferred)

In addition, relay-contactors may be used for starting or interlock-failure based issues

Properly chosen aluminum anodized heat sinks are preferred; in some cases copper heat sinks may also be used

Usually, the cooling scheme is forced-air; hence, place small fans at suitable location within the package, if space is available

In case of very compact designs, selection of heat-sink and cooling method must be optimized

Some Common Protections for

DC/DC Converters:

Over-current (input and / or output)

Over-voltage (input and / or output)

Regulation failure (output)

Fan failure / over-temperature interlock (other similar interlocks may also be provided with)

Earth-fault failure

Reference brown-out failure

Shoot through or any device failure

Current-unbalance failure (for parallel current paths and/or windings etc.)

DC/DC Converters: Improved Push-

Pull Converter Power Circuit

Reverse polarity relay at input

Snubbers for MOSFETs and diodes

DC bus capacitors close to transformer

Transformer close to MOSFETs

AC capacitor in || with electrolytic capacitor at output

Discharge resistor used

Inverters


Inverters:

Most important category of PE Converters

Although different „types‟ of inverters are known of,

almost all of them are all derived from the basic „bridge-

topology‟

Resonant inverters (SRC, PRC etc.) are also derived

from the „bridge topology‟

Some exceptions are there (e.g. Class-E)

Among such inverters, PWM inverters are most common

This presentation will restrict itself to the „bridge-

topology‟-based voltage-source inverters of the hard-

switching type

Some Common Inverter Topologies:

Single Phase Full Bridge Inverter

Three Phase Full Bridge Inverter

Three Phase Multi-Level Inverter

Inverters: Some Standard

Waveforms

Single Phase Full Bridge VSI (2-Level)

Waveforms

Single Phase Full Bridge VSI (3-Level)

Waveforms


of Inverters: Generally full-bridge type PWM inverter (2-Level) is suitable for

most applications

3 or higher level inverters are used for higher power / higher input dc voltage etc.

Generally, such inverters are all „buck‟ type; i.e. output RMS value < input DC value; hence to boost up output voltage either DC bus voltage is to be boosted (by a DC/DC Converter) or AC output is to be boosted (via transformer)

Sine PWM / Space Vector PWM / SHE PWM are commonly used for generating the output AC

Filters are employed where sinusoidal current and/or voltage is required

Usually, fsw is lower (3 – 8 kHz), but current may be high (50 –100 A is quite common); hence mostly, inverters are made up of IGBTs (and not MOSFETs)

For sine-wave inverters, a THD of around 3% is expected for Vo

Chief Components for Inverters:

Semiconductor Devices – IGBTs

Switch-mode / Power Frequency Transformer – If

required




sinks etc.


Inverters:

IGBTs are chosen based upon (i) amplitude of output current, (ii) peak value of input DC bus voltage, (iii) allowable value of hard-switching speed

In case of welding type load, surge current limit and duration are also noted

Devices are accompanied by properly designed heat sinks (based on device conduction and switching losses)

Gate drive requirements are also carefully noted

Some common IGBT packages have been discussed previously

Selection of Switch-Mode / Power

Frequency Transformer for Inverters:

Transformers are employed for primarily two

purposes: (a) isolation and (b) output voltage

boosting

Switch-mode transformer is usually employed at

the DC bus stage (via a DC/DC converter); its

design is based upon the SMPS topology chosen

(usually Push-Pull or Full-Bridge)

Power Frequency Transformer is usually placed

at the output of the bridge-inverter; it directly

boosts up the available ac output to higher levels

Switch-Mode vs. Power Frequency

Transformer for Inverters:Sl. No. Switch-Mode Transformer Power Frequency Transformer

1. Lighter; hence system is

compact

Heavier – makes the system bulky;

but more robust

2. EMI / EMC problem; proper

shielding is often mandatory

EMI / EMC problem can be

minimized by placing low-pass

filters at the transformer input

3. Core made of soft-ferrite;

hence fragile; proper

placement and packing is a

must

Much more strong mechanically;

4. Efficiency is very high Relatively lower due to slightly

higher core losses

5. Designing is relatively

difficult for higher power

level (above 10 kW)

Designing is much simpler for

higher powers; works reliably


Inverters: Various topologies of L-C low-pass filters are used

Usually all filters are designed with respect to 50-Hz

Hence, size cannot be reduced below a particular limit (without increasing frequency)

Frequency is not pushed beyond 15 kHz (normally)

Filters are normally designed for optimum performance at full-load

Performance generally worsens as no-load condition is approached

Usually ferrite cores are used (since attempts are made to keep the L-value small – a few hundred micro-henries)

Good isolation strength between winding and core is a must


Inverters: Inductor is designed keeping the worst case circulating

current in mind – avoid core-saturation

Capacitors for filters are normally AC capacitors, having low values of ESR and ESL

Multiple capacitors may be stacked up in parallel to get a higher net value of capacitance

Cut off frequency is usually chosen mid-way between the output frequency and the switching frequency

L is chosen to produce a voltage drop of around 4 – 5% (max.) from no-load to full-load

C value is calculated by knowing the L value

Ensure that the cut-off frequency does not match with any intermediate switching harmonic – may amplify the same by resonance

Filtering Components for Inverters:

Issues

Single Phase Full-Bridge Inverter

Three Phase Bridge Inverter

Snubbers and Auxiliary Components

for Inverters: Snubbers for inverters can be selected by employing

techniques, which are similar to the case of DC/DC converters

Heat sinks can be chosen on similar lines

Relay-contactors may be used for soft-start and reverse-polarity-protection

Hard-switched PWM inverter generates lot of heat and EMI; proper cooling and shielding is essential

A common practice is to place the heat sink, so that it forms a wall of the cabinet; fins are cooled by external air; fans cool from inside

Cable entry and exit must be proper to avoid earth-fault / accidents – cables of reliable standards be used – connect by lugging to ensure proper contact

Inverter Design: Some General

Points Large radiation expected; keep the low-power electronic

controller well-shielded from the power-section –preferably in a separate box

For feedback of analog / digital signals, always use shielded cables to avoid the entry of spurious noise

Keep the controller ground floating w.r.t. all other grounds and physical earth

Usually start the inverter by either V/f mode (for motor drives) or const. f variable V mode (for static loads); NEVER start directly, unless explicitly specified

Starting to steady state – should take about 2- 5 seconds (at least); increase if necessary and permitted by the user

Provide discharge resistors for all internal capacitors, so that they discharge within 1 minute (preferable); this avoids shock to personnel working with the set

Protections for Inverters: Almost all faults, as discussed for DC/DC converters,

also apply here

In addition, usually phase outage protection is provided

Protection for severe output current / voltage unbalance may also be given

Protection against large circulating current (in filters) are also desirable

Spurious trip due to regenerative power feedback from AC motors (which shoots up the DC bus voltage) may be avoided by judiciously selecting the protection logic

These protection-features are shown in the next slide

An Inverter with a few protective

schemes (Block Diagram): Stray inductance -

nullified by sandwich bus-bars or ac capacitors

Input reverse polarity protection by relay-contactor (do not use diode)

Place ac capacitors close to devices

Feedback signals for protection to controller

Instantaneous / averaged time-delay protection may be applied

PE Converter and Components

Some General Considerations

Some General Considerations: Careful selection of control strategy

Minimization of stray inductance and capacitance –

Use proper layout

Avoid contact drop – use bus bars for currents above

150 A

Keep controller and power circuits in separate

chambers – magnetic and electrical isolation

Careful packaging – should be compact while offering

adequate cooling

Higher the switching frequency, smaller the size – but

more critical the selection of components and design

Some General Considerations: The body of the cabinet of the converter is usually

always earthed

Often, all constraints are not met optimally

In such cases, more critical constraints are stressed

upon

Take special care in magnetic-design; it is the area

where most people fail

Test each required protection individually at various

load levels before finally approving the design

Being a good designer takes time and experience –

learn from your faults

Case Study

Specification for an On-Line UPS:

Input: 415 V, ± 10 %, 50 Hz ± 3 %, 3-phase

Output: 415 V, ± 0.5 %, 50 Hz ± 0.1 %, 3-phase,

10 kVA at a power factor of 0.6 to 0.8 (lag)

Weight: Not a huge constraint

Reliability: Highest

Back Up Source: A battery-bank of 144 V

Input power factor: Unity with a good sinusoidal

profile-current

Output type: 3-phase 3 wire

Selection of PE Converter Topology: Precision output control implies the use of digital controller

– it is also useful for display, data-logging etc.

Input power factor is unity with good current profile – use a PWM rectifier based front-end

Input = Output = 415 V – Boosting of DC bus or output required – Weight is not an issue, but reliability is high; hence use transformer at the input – this also provides isolation

Battery should be charged or discharged through this DC bus – bi-directional DC/DC converter required

Harmonic attenuating low-pass filters are placed at the output terminals of the inverter

Current level is rather high (for discrete MOSFETs); hence use IGBTs and keep the frequency down to around 15 kHz – 20 kHz

Online UPS Topology:

Conclusion

Conclusion:

This presentation provides the listener some hints

on converter topology and component selection

Judicious selection can make the design easy as

well as reliable

A selection is merely an optimization of several

constraints, as found out from the customer‟s

specification

In case, all constraints are not equally optimized,

stress must be put on those, which occupy the

higher priority

Reference: 1. A. Pressman, “Switching Power Supply Design,” 2nd

Edition, McGraw-Hill

2. N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics: Converters, Applications, and Design,” 2nd Edition, John Wiley and Sons

3. M. H. Rashid, “Power Electronics Handbook,” Academic Press

4. R. W. Erickson and D. W. Maksimovic, “Fundamentals of Power Electronics,” 2nd Edition, Kluwer Academic Publishers, 2004

5. V. Ramanarayanan, “Course Material on Switch Mode Power Conversion,”

selection of pe converters and components

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