linear technology corporation · 2015-05-31 · © 2008 linear technology 1 solution for power...

© 2008 Linear Technology

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Solution for Power

Linear Technology Corporation

JM Park Field Applications Engineer

Power Business Unit

No-opto coupler Isolated

Flyback converter


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Introduction

No-opto isolated Flyback IC 특징 및 제품 line-up 소개

No-opto isolated Flyback IC 동작 원리

LT8302 Design 시 고려사항

Transformer 선택 시 주의사항

Coupled Inductor 선택 시 주의사항

Debugging시 유의사항

Copyright © 2008 Linear Technology. All rights reserved.

No-opto isolated Flyback IC 특징 및 제품 line-up 소개


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Traditional Flyback 1. 2nd winding is needed for

output feedback

2. Additional circuit for Internal Vcc

3. Lots of external components

4. External FET & Rsense


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startup / bias circuitry

secondary-side regul

ation and loop comp

ensation

Opto-coupler

Traditional Flyback


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1.No opto coupler, No 3rd winding is needed

2.No additional start-up /Vcc bias circuit needed

3.Easy on/off control

4.Integrated circuit makes very simple solution

No-opto Flyback converter


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7 Solution for Power

Monolithic No-Opto Isolated Flybacks

Part VIN IQ Power Switch POUT Package

LT8300 6V-100V 70uA/330uA 0.26A/150V DMOS up to 2.5W SOT23-5

LT8301 2.8V-42V 100uA/350uA 1.2A/65V DMOS up to 6W SOT23-5

LT8302 2.8V-42V 106uA/380uA 3.6A/65V DMOS up to 18W SO8E

LT3573 3V-40V 3.5mA 1.25A/60V NPN up to 6W MSE16

LT3574 3V-40V 3.5mA 0.65A/60V NPN up to 3W MS16

LT3575 3V-40V 4.5mA 2.5A/60V NPN up to 12W FE16

LT3511 6V-100V 2.7mA 0.25A/150V NPN up to 2.5W MS16(12)

LT3512 6V-100V 3.5mA 0.42A/150V NPN up to 4.5W MS16(12)

New!

New boundary mode flyback controllers offer simple, high performance isolated supplies for a variety of applications!


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36VIN to 72VIN Isolated 24W 12VOUT

Typical LT3748 – Typical Application

Isolated Flyback Controllers (External FET)


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LT3748 LT1725/37 LT3825/37

No Opto or 3rd

Winding? Yes Yes Yes

Synchronous No No Yes

Vin 6V – 100V * / 4.5V – 20V * / 9V – 36V

Boundary Mode

Control? Yes No No

Prog. ILIMIT /

Soft Start? Yes / Yes No / Yes No

Package MS-16 MS-16, QFN-16 TSSOP-16

New!

Isolated Flyback Controllers (External FET)

For higher power isolated flyback, consider our new boundary mode isolated flyback controller with external FET


No-opto isolated Flyback IC 동작 원리


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11 Solution for Power 4

LT8302 Features


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No opto- isolated flyback

SW SW

Red : Primary inductor current

Blue : Secondary inductor current

(= output diode current )

1. Same flyback operation as traditional flyback

2. Boundary Mode vs CCM(Continuous Conduction Mode)

3. Variable Frequency vs Fixed Frequency

4. Voltage feedback sampling at Zero current crossing

on 2nd side.


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N: the turns ratio of the transformer

VIN: the input voltage

VC: the maximum clamped voltage

Red : Primary inductor current

Blue : Secondary inductor current

(= output diode current )

How to regulate the Vout

The output voltage is accurately measured at the

primary-side switching node waveform during the off

time of the power transistor.

(We can neglect output diode forward voltage)

SW


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The load regulation is much improved in boundary

mode operation because the reflected output voltage

always samples at the diode current zero-crossing.

The LT8302 typically provides ±5% load regulation.

The down

slope is a

function of the

load current

Achieve a good load regulation


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Ringing should settle down before blanking time to sample Vout.

2% minimum Leakage inductance of primary inductance is

required.

Blanking time

Minimum switch off time

Sampling the voltage


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Operation Mode (Switching Frequency vs Load)

Boundary Conduction mode

Discontinuous Mode

Burst Mode

Minimum switching frequency at Burst mode = 12Khz

Maximum switching frequency at Discontinuous Mode = 380Khz

380KHz max


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Operation Mode (Switching Frequency vs Load)


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LT8302 Block Diagram


LT8302 Design시 고려사항


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peakI

D

minlim,

minoff,psfoutpri

I

tN)V(VL

350ns~t minoff,

minlim,

minon,maxin,pri

I

tVL

0ns02~t minon,

1. Select Turn Ratio

Design Example Should be lower than SW pin max voltage rating!


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Normally 12Vin to 5V

Np/Vp = Ns/Vs

Then Nps = 2.5 : 1


Nps Higher -> Duty cycle gets higher -> More Power

Whereas, Voltage stress gets higher


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7~36V to 18V/20mA

1:2 Transformer (87uH : 350uH)

Stable

Low primary turns ->

Low leakage inductance ->

SW pin waveform better

->Low voltage stress,

but less output current



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7~36V to 18V/20mA

2:1 Transformer (350uH : 87uH)

High primary turns ->

High leakage inductance ->

SW pin waveform not good

-> High voltage stress,

-> Requires Snubber

but much higher output

current

unstable



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2. Determine the Primary Inductance


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Concept of Inductor/transformer

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Concept of Inductor/transformer

N = 3, Primary Inductance = 9uH,

Then, Secondary Inductance = 1uH


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We can calculate switching frequency at Max load

Efficiency : expected 80%


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3. Choose output Diode

Isw(max) =4.5A Duty cycle


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3. Choose the best output Diode for efficiency

Minimizing output diode VF is the

best method to increase

application efficiency when the

output voltage is below 12V.

Output diodes are often resistive

at higher currents and peak

current may be 4 to 8 times higher

than the average output current .

SEVERAL RECOMMENDED OUTPUT DIODES • Microsemi UPS840 : 8A, 40V, VF @ 3A = 0.39V MAX, Powermite 3

• Vishay V8P10 : 8A, 100V, VF @ 3A = 0.5V typ, SMPC

• Diodes Inc. SBR10U200 : 10A, 200V VF @ 3A = 0.7V typ, Power DI5

Sources of Loss at 5V Out


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4. Choose output capacitor

If we want output voltage ripple less than 0.1V

We can choose 220uF 6.3V Ceramic capacitor.


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Clamps and Snubbers

Recommended for LT830X

5. Design the Snubbers Circuit


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DZ clamp

✔ Well-defined clamp voltage

✔ Lower power dissipation at light loads (lower leakage spike)

✘ Does not dampen ringing

✘ Customer must buy a zener

RC snubber

✘ Not well-defined clamp voltage

✘ Always dissipates power (even at light loads)

✔ Reduces ringing and Improves EMI

RCD clamp

✔ Fairly well-defined clamp voltage (with large C)

✘ Always dissipates power (even at light loads)

✔ Reduces some ringing to help EMI

5. Design the Snubbers Circuit – Pros and Cons


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For lower-power parts

LT8300 – DZ clamp or RC snubber (but typically not needed)

LT8301 – DZ clamp

Lower power levels, so typically less ringing

Higher efficiency at light loads (lower leakage spike)

For higher-power parts

LT8302 – both DZ clamp and RC snubber

LT8304 – both DZ clamp and RC snubber

RC damps ringing; DZ provides accurate SW pin clamp

DZ clamp better utilizes the SW pin ABS MAX rating

5. Design the Snubbers Circuit – Recommended


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LT8300 has 250ns

LT8302 has 250ns

• LT8301 has 350ns

SOT23 package

(no RREF pin)

S8E package

(has RREF pin)

5. Design the Snubbers Circuit – Blanking Window


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RREF pin could be used (but typically not needed)

10pF~15pF provides a little extra blanking and filtering if needed

no RREF pin

5. Design the Snubbers Circuit – Leakage blanking


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6. Choose Rfb feedback resistors

Rfb

Rref


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7. Choose Rtc resistors for diode temp

compensation.

Rtc

If high Vout application over 12V,

then output voltage change is only 0.2~0.3V.

So if it’s not an issue, doesn’t need Rtc.


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8. Set the UVLO if needed.


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1% - 2% of maximum load at maximum VIN – not nominal VIN!! This is very demanding for applications with very wide VIN range.

Lack of output load can cause the output voltage to run away

When output voltage runs away, it may damage downstream components or either output diode or MOSFET may avalanche (diode and MOSFET may fail if their avalanche energy is exceeded)

9. Set the minimum load requirement


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9. Set the minimum load requirement

Vz should be >Vout

Use zener If Low Iq current needed.

Zener diodes across the output are the preferred solution when

highest efficiency is required, but must be sized for sufficient

power dissipation

Vout R=500 ohm


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http://www.linear.com/designtools/software/#LTspice

10. Use LT Spice for simulation.


Transformers 선택 시 주의사항


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LT8300/01/02 & LT3748 Transformers

Recommended Transformers (from Applications)

Wurth

Sumida

BH Electronics

Pulse

Coilcraft

Wurth gives good service


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LT830X (and LT35XX/LT3748) on Wurth Website Wurth provides info for all datasheet transformers


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Wurth keeps stock at DigiKey All Wurth LT830X datasheet transformers are available from DigiKey


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Wurth Datasheets are a good “Starting Point”

Even if engineers will not use a Wurth transformer

Wurth datasheets are a good example for specifications

they specify LLEAKAGE

they specify turns ratio (to ±1%)

Both of these specs are very important.


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Make sure they specify:

LLEAK,typical to < (2% of LPRI)

N (turns ratio) accuracy to ±1%

Verify custom transformer

Get their transformer datasheet, or even better – get their transformer

Measure their leakage inductance

“Pin-compatible” transformer is not good enough

If engineers design custom Transformer

Short secondary side of transformer

0V at primary

Measure LLEAK with LCR meter


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Important to have Low Leakage Inductance

LT8300 LLEAK,typical< (2% of LPRI)

Higher Turns Ratios typically will have higher Leakage Inductance


Coupled Inductors 선택 시 주의사항


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LT8301 – Miniature Isolated Solution: 2.7-16VIN/5VOUT

VOUT is clamped by 5.6V Zener

13mm × 7mm × 3mm

When only Functional Isolation is Needed

coupled

inductor


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Always Check the Coupled Inductor Datasheet

LLeakage should be <(2% of Lprimary)

Coilcraft Coupled Inductor from Previous Page Schematic

Coilcraft Coupled Inductors LPD3015 and MSD1260 also good choices.


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Always Check the Coupled Inductor Datasheet A different Coilcraft Coupled Inductor

No specifications for LLeakage


Multiple Output Applications


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Regulating Multiple Outputs with LT830X

Looks at all outputs, reflected back through the transformer


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Multiple Outputs – Load Regulation

Three outputs heavily loaded;

one output swept

Three outputs lightly loaded;

one output swept


Other Stuff


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If Having Issues, Probe the SW Pin

The SW pin is the Feedback node – look there first

A quiz – Identify the “good” and “bad” transformer photos from this LT8300 customer production circuit

250ns 250ns


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SEVERAL RECOMMENDED MOSFETS

• Infineon BSC160N10 : 100V, 0.018W, 14nC, S-SO8

• Fairchild FDMS2572 : 150V, 0.04W, 25nC, Power56

• Vishay Si7464 : 200V, 0.21W, 9nC, PPAK SO8

Selecting a MOSFET is a compromise between QG, RDS,on and

VDS,max specifications. Unless losses in the MOSFET become

several percent of target output power, minimizing QG and

maximizing VDS,max – hopefully to the level that a snubber is not

required – usually results in an optimum solution.

VIN = 6V, VO = 5V VIN = 60V, VO = 5V

Example: Two MOSFETs in same 6-60V VIN to 5V, 2A out application (ILIM = 10A, no 3rd winding):

VDS,MAX = 200V

R DS, ON = 0.08W

QG = 12nC

VDS, MAX = 100V

R DS,ON = 0.006W

QG = 51nC

MOSFET Specs:

FET selection for controller like LT3748


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감사합니다.

linear technology corporation · 2015-05-31 · © 2008 linear technology 1 solution for power...

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