Power integrations InnoSwitch3 Application Note

Application Note AN-72

InnoSwitch3 Family

Design Guide

Introduction

InnoSwitch™3 devices combine a high-voltage power MOSFET switch,
with both primary-side and secondary-side controllers, an innovative
high-speed magneto-coupling communications technology and a

synchronous rectication driver into one isolated, safety-rated device.
The incorporation of Fluxlink™, which transmits information safely and
reliably across the isolation barrier, eliminates the need for an
optocoupler - used in the feedback loop of conventional power

conversion circuits. This reduces component count and eliminates the

lifetime and reliability limitations inherent in opto-feedback devices.
The InnoSwitch3 integrated circuits feature a variable frequency,
variable peak-current control scheme which together with quasi­resonant switching and synchronous rectication ensure very high
conversion efciency across the load range. The family can be used to

create power supplies up to 65 W output, including CV/CC chargers

that easily meet average-power-supply-efciency requirements and
offers very low no load input power and outstanding standby
performance. Power Integrations’ EcoSmart™ technology used in
InnoSwitch3 ICs enables designs that consume as little as 15 mW of
no-load power and makes the family ideal for applications that must
meet energy efciency standards such as the United States
Department of Energy DoE 6, California Energy Commission (CEC) and
European Code of Conduct.

The primary-side yback controller in InnoSwitch3 can seamlessly

transition between DCM, QR and CCM switching. The primary

controller consists of start-up circuitry, a frequency jitter oscillator, a

receiver circuit that is magnetically coupled to the secondary side, a
current limit controller, audible noise reduction engine, overvoltage
detection circuitry, lossless input line sensing circuit, over-temperature
protection and a 650 V or 725 V power MOSFET.

The InnoSwitch3 secondary controller consists of a transmitter circuit

that is magnetically coupled to the primary-side, a constant voltage

(CV) and constant current (CC) control circuit, synchronous-rectier­MOSFET driver, QR mode circuit, and a host of integrated protection
features including output overvoltage, overload, power limit, and

hysteretic thermal overload protection.

At start-up the primary controller is limited to a maximum switching
frequency of 25 kHz and 70% of the maximum programmed current
limit. An auto-restart function limits power dissipation in the switching
MOSFET, transformer, and output SR MOFET during overload,
short-circuit or open-loop fault conditions.

www.power.com October 2018

Application Note AN-72

Basic Circuit Conguration

The circuit in Figure 1 shows the basic conguration of a yback
power supply designed using InnoSwitch3. Different output power
levels may require different values for some circuit components, but
the general circuit conguration remains similar. Advanced features

such as line overvoltage and undervoltage protection, primary or
secondary sensed output overvoltage protection and constant current

limit programming are implemented using very few passive components.

L

F

R

SN

R

LS1

BRD

F

R

L

C

R

T

N

O

t

C

1

F

2

B

C

SN

R

S

C

BIAS

InnoSwitch3

Primary FET

and Controller

power supplies is shown in Figure 1, which also serves as the

reference circuit for component identication used in the description

throughout this application note.

In addition to this application note, there is the InnoSwitch3 reference

design kit (RDK) containing an engineering prototype board as well
as device samples that provides an example of a working power
supply. Further details on downloading PI Expert, obtaining an RDK
and updates to this document can be found at www.power.com.

CY

R

FB(UPPER)

C

C

D

SN

D

BIAS

R

FWD

R

LS2

D V

S IS

BPP

R

BP

C

BPP

C

R

FB(LOWER)

FB

VOUT

Secondary
Control IC

FB

C

SR

R

SR

OUT

SR FET

C

BPS

GND

BPS

SR

FWD

PH

R

PH

V

OUT

R

IS

PI-8465-041818

RTN

Figure 1. Typical Adapter Power Supply Schematic using InnoSwitch3 with Line Undervoltage Lockout, Line Overvoltage Shutdown, Constant Output Current Limit

and Quasi-Resonant Synchronous MOSFET Rectier and Integrated Output Overvoltage Protection.

Scope

This application note is intended for engineers designing an isolated
AC-DC yback power supply or charger using the InnoSwitch3 family
of devices. It provides guidelines to enable an engineer to quickly
select key components and also complete a suitable transformer
design. To help simplify the task, this application note refers directly
to the PIXls designer spreadsheet that is part of the PI Expert™
design software suite available online (https://piexpertonline.power.
com/site/login). The basic conguration used in InnoSwitch3 yback

Quick Start

Readers familiar with power supply design and Power Integrations
design software may elect to skip the step by step design approach
described later, and can use the following information to quickly
design the transformer and select the components for a rst prototype.
For this approach, only the information described below needs to be
entered into the PIXls spreadsheet, other parameters will be
automatically selected based on a typical design. References to
spreadsheet line numbers are provided in square brackets [line
reference].

2

Rev. A 10/18

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Application NoteAN-72

• Enter AC input voltage range and line frequency, VAC_MIN [B3],
VAC_MAX [B4], LINEFREQ [B6]

• Enter input capacitance, CAP_INPUT [B7]

• 3 µF / W for universal (85-265 VAC) or single (100/115 VAC) line.

A more aggressive value of 2 µF / W can be used for many
charger designs that do not need to meet hold up time require-

ment

• Use 1 µF/W for 230 VAC or for single (185-265 VAC) line. If this

cell is left blank then the capacitance value for a VMIN of 70 V
(universal input) or 150 V (single 230 VAC) is calculated. Often
this will lead to an optimal input lter capacitance value

• Enter nominal output voltage, VOUT [B8]

• Enter desired cable drop compensation, PERCENT_CDC [B9]

• “0%” for no cable compensation

• “1% - 6%” for featured H-code trim

• Enter continuous output current, IOUT [B10]

• Enter efciency estimate, EFFICIENCY [B12]

• 0.83 for universal input voltage (85-265 VAC) or single 100/115

VAC (85-132 VAC) and 0.85 for a single 230 VAC (185-265 VAC)
design. Adjust the number accordingly after measuring the
efciency of the rst prototype-board at max load and VACMIN

• Select power supply enclosure, ENCLOSURE [B14]

• Select current limit mode, ILIMIT_MODE [B19]

• Two current limit congurations are available, STANDARD or

INCREASED

Output Power Table

230 VAC ± 15% 85-265 VAC

Product

INN3162C 10 W 12 W 10 W 10 W

INN3163C 12 W 15 W 12 W 12 W

INN3164C 20 W 25 W 15 W 20 W

INN3165C 25 W 30 W 22 W 25 W

INN3166C 35 W 40 W 27 W 36 W

INN3167C 45 W 50 W 40 W 45 W

INN3168C 55 W 65 W 50 W 55 W

Notes:

1. Minimum continuous power in a typical non-ventilated enclosed typical size
adapter measured at 40 °C ambient. Max output power is dependent on the

design. With condition that package temperature must be < 125 °C.

2. Minimum peak power capability.

3. Package: InSOP-24D.

3

Adapter

1

Open

Frame

2

Adapter

1

Open

Frame

• Select InnoSwitch3 from drop-down list or enter directly [B20]

• Select the device from Table 1 according to output power, input
voltage and application

• InnoSwitch3-CE for CV/CC yback application

• InnoSwitch3-EP for CV/CC yback application with 725 V

MOSFET

• Enter desired maximum switching frequency at full load, FSWITCH-

ING_MAX [B34]

• Enter desired reected output voltage, VOR [B35]

• Enter core type (if desired), CORE [B63] from drop down menu

• Suggested core size will be selected automatically if none is

entered [B63]

• For custom core, enter CORE CODE [B64], and core parameters
from [B65] to [B72]

• Enter secondary number of turns [B88]

If any warnings are generated, make changes to the design by
following instructions in spreadsheet column D.

• Build transformer as suggested in “Transformer Construction” tab

• Select key components

• Build prototype and iterate design as necessary, entering measured

values into spreadsheet where estimates were used (e.g. efciency,

V

). Note that the initial efciency estimate is very conservative.

MIN

Output Power Table

Product

2

3

INN3672C 12 W 10 W

INN3673C 15 W 12 W

INN3 674 C 25 W 20 W

INN3675C 30 W 25 W

INN3676C 40 W 36 W

INN3677C 45 W 40 W

Notes:

1. Minimum continuous power in a typical non-ventilated enclosed typical size
adapter measured at 40 °C ambient. Max output power is dependent on the

design. With condition that package temperature must be < 125 °C.

2. Minimum peak power capability.

3. Package: InSOP-24D.

230 VAC ± 15% 85-265 VA C

Peak or

Open Frame

1,2

Peak or

Open Frame

1,2

Table 1. Output Power Tables of InnoSwitch3-CE and EP.

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3

Rev. A 10/18

Application Note AN-72

APPLICATION VARIABLES

265

7 CAP_INPUT 40.0 uF Input capacitor

5.00

0%

Percentage (of output voltage) cable drop
10 IOUT 4.00 4.00 A Output current

AC-DC efficiency est imate at full load given that

Step-by-Step Design Procedure

This design procedure uses the PI Expert design software (available
from Power Integrations), which automatically performs the key
calculations required for an InnoSwitch3 yback power supply design.
PI Expert allows designers to avoid the typical highly iterative design
process. Look-up tables and empirical design guidelines are provided
in this procedure where appropriate to simplify the design task.

Iterate the design to eliminate warnings. Any parameters outside the

recommended range of values can be corrected by following the

guidance given in the right hand column. Once all warnings have

been cleared, the output transformer design parameters can be used
to create a prototype transformer.

Step 1 ‒ Application Variables

Enter: VIN_MIN, VIN_MAX, LINEFREQ, CAP_INPUT, VOUT,
PERCENT_CDC, IOUT, EFFICIENCY, FACTOR _Z, and
ENCLOSURE

Minimum and Maximum Input Voltage, V_MIN, V_MAX (VAC)

Determine the input voltage range from Table 2 for a particular
regional requirement.

Line Frequency, LINEFREQ (Hz)

50 Hz for universal or single 100 VAC, 60 Hz for single 115 VAC input.
50 Hz for single 230 VAC input. These values represent typical line
frequencies rather than minimum. For most applications this gives
adequate overall design margin. For absolute worst-case or based on
the product specication reduce these numbers by 6% (47 Hz or 56 Hz).

Total Input Capacitance, CAP_INPUT (

µF)

Enter total input capacitance using Table 3 for guidance.

2

3 VIN_MIN 85 85 V Minimum AC input voltage

4 VIN_MAX

5 VIN_RANGE UNIVERSAL Range of AC input voltage

6 LINEFREQ 60 Hz AC Input voltage frequency

8 VOUT

9 PERCENT_CDC

265 V Maximum AC input voltage

5.00 V Output voltage at the board

0%

Design Title

compensation desired at full load

11 POUT 20.00 W Output power

12 EFFICIENCY 0.89 0.89

13 FACTOR_Z 0.50 Z-factor estimate

14 ENCLOSURE ADAPTER ADAPTER Power supply enclosure

Figure 2. Application Variable Section of InnoSwitch3-CE Design Spreadsheet with Gray Override Cells.

Region

Nominal Input

Voltage (VAC)

Minimum Input

Voltage (VAC)

the converter is switching at the valley of the
rectified minimum input AC voltage

Maximum Input

Voltage (VAC)

Japan 100 85 132 50 / 60

United States, Canada 120 90 132 60

Australia, China, European Union Countries,

India, Korea, Malaysia, Russia

230 185 265 50

Indonesia, Thailand, Vietnam 220 185 265 50

115, 120, 127 90 155 50 / 60

Rest of Europe, Asia, Africa, Americas

and rest of the world

220, 230 185 265 50 / 60

240 185 265 50

Visit: https://en.wikipedia.org/wiki/Mains_electricity_by_country

Table 2. Standard Worldwide Input Line Voltage Ranges and Line Frequencies.

Nominal Line

Frequency (Hz)

4

Rev. A 10/18

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Application NoteAN-72

TotalLosses

Secondary Losses

Total Input Capacitance per Watt of

Output Power (µF/W)

AC Input Voltage (VAC) Full Wave Rectication

Adapter with hold-up time requirement

100 / 115 3 2

230 1 1

85-265 3 2

Table 3. Suggested Total Input Capacitance for Different Input Voltage Ranges.

The capacitance is used to calculate the minimum and maximum DC
voltage across the bulk capacitor and should be selected to keep the
minimum DC input voltage, VMIN > 70 V.

Nominal Output Voltage, VOUT (V)

Enter the nominal output voltage of the main output at full load.
Usually the main output is the output from which feedback is derived.

Cable Compensation, PERCENT_CDC (%)

Select the appropriate cable compensation depending on the choice

of cable for the design. If this power supply is not supplied with a
cable, use the default 0%. (For InnoSwitch3-EP, this feature is not
available)

Power Supply Output Current, IOUT (A)

This is the maximum continuous load current of the power supply.

Output Power, POUT (W)

This is a calculated value and will be automatically adjusted based on

cable compensation selected.

Power Supply Efciency, EFFICIENCY (η)

Enter the estimated efciency of the complete power supply
measured from the input and output terminals under peak load
conditions and worst-case line (generally lowest input voltage). The
table below can be used as a reference. Once a prototype has been
constructed then the measured efciency should be entered and
further transformer iteration(s) can be performed if required.

Power Supply Loss Allocation Factor, FACTOR_ Z

This factor describes the apportioning of losses between the primary
and the secondary of the power supply. Z factor is used together
with the efciency to determine the actual power that must be
delivered by the power stage. For example losses in the input stage
(EMI lter, rectication etc) are not processed by the power stage
(transferred through the transformer) and therefore although they
reduce efciency the transformer design is not effected.

For designs that do not have a peak power requirement, a value of

0.5 is recommended. For designs with a peak power requirement

enter 0.65. The higher number indicates larger secondary side
losses.

Enclosure

Power device selection will also be dependent on the application
environment. For an open frame application where the operating
ambient temperature is lower than in an enclosed adapter, the PIXls
will suggest a smaller device for the same output power.

Efciency is also a function of output power, low power designs are
most likely around 84% to 85% efcient, whereas with a synchronous
rectier (SR) the efciency would reach 90% typically.

Total Input Capacitance per Watt of

Output Power (µF/W)

Open Frame or Charger/Adapter without

hold-up time requirement

Z =

Nominal Output

Voltage (VOUT)

5 0.84 0.87 0.84 0.88 0.87 0.89

12 0.86 0.90 0.86 0.90 0.88 0.90

Table 4. Efciency Estimate Without Output Cable .

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Typical Low-Line Range Typical Universal Range Typical High-Line Range

85 VAC - 132 VAC 85 VAC - 265 VAC 185 VAC - 265 VAC

Schottky Diode

Rectier

Synchronous

Rectier

Schottky Diode

Rectier

Synchronous

Rectier

Schottky Diode

Rectier

Synchronous

Rectier

5

Rev. A 10/18

Application Note AN-72

18 PRIMARY CONTROLLER SELECTION

19 ILIMIT_MODE STANDARD STANDARD Device current limit mode

Auto

21 DEVICE_CODE INN3165C Actual device code

Power capability of the device based on thermal
23 RDSON_100DEG 3.47 Ω

Primary MOSFET on time drain resistance at 100
degC

24 ILIMIT_MIN 0.88 A Minimum current limit of the primary MOSFET

26 ILIMIT_MAX 1.02 A Maximum c urrent limit of the primary MOSFET

27 VDRAIN_BREAKDOWN 650 V Device breakdown voltage

Peak drain voltage on the primary MOSFET

Step 2 – Primary Controller Selection

Enter: Device Current Limit mode, ILIMIT and Generic Device
Code, DEVICE_GENERIC

20 DEVICE_GENERIC

22 POUT_MAX 22 W

25 ILIMIT_TYP 0.95 A Typical current limit of the primary MOSFET

28 VDRAIN_ON_MOSFET 0.87 V Primary MOSFET on time drain voltage

29 VDRAIN_OFF_MOSFET 508.4 V

Figure 3. Primary Controller Selection of InnoSwitch3-CE Design Spreadsheet with Current Limit Mode Selection.

Generic Device Code, DEVICE_GENERIC

The default option is automatically selected based on input voltage
range, maximum output power and application (i.e. adapter or open
frame).

For manual selection of device size, refer to the InnoSwitch3 power
table in the data sheet and select a device based on the peak output

power. Then compare the continuous power to adapter column

numbers in the power table, (if the power supply is of fully enclosed
type), or compare to the open-frame column (if the power supply is
an open-frame design). If the continuous power exceeds the value
given in the power table (Table 1), then the next larger device should
be selected. Similarly, if the continuous power is close to the
maximum adapter power given in the power table, it may be

necessary to switch to a larger device based on the measured

thermal performance of the prototype.

Device Current Limit Mode, ILIMIT_MODE

For designs where thermals are not as challenging (such as open
frame applications) and lowest cost is a critical requirement, ILIMIT
MODE allows the choice of an INCREASED current limit mode, this

INN31X5 Generic device code

performance

during turn-off

will set the peak current of the device equivalent to the next bigger
device’s current limit and allow higher output power. By default,
ILIMIT is set to STANDARD.

On-Time Drain Voltage, VDRAIN_ON_MOSFET (V)

This parameter is calculated based on RDSON_100DEG and primary

RMS current.

Drain Peak Voltage, VDRAIN_OFF_MOSFET (V)

This parameter is the assumed Drain voltage seen by the device

during off-time. The calculation assumes 10% minimum margin from
the breakdown voltage rating of the internal MOSFET and gives a
warning if this is exceeded.

VDRAIN < (VIN_MAX * 1.414) + VOR + VLK

VLK

is the voltage induced by the leakage inductance of the

PRI

transformer when MOSFET turns off.

PRI

– (BV

× 10 %).

DSS

Other electrical parameters are displayed based from the data sheet,

RDSON_100DEG, ILIMIT_MIN, ILIMIT_TYP, ILIMIT_MAX,
VDRAIN_BREAKDOWN.

6

Rev. A 10/18

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WORST CASE ELECTRICAL

34 FSWITCHING_MAX 80000 80000 Hz

Maximum s witching frequency at full load and
valley of the rectified minimum AC input voltage

Seconday voltage reflected to the primary when
36 VMIN 85.95 V

Valley of the rectified minimum AC input voltage
at full power

Measure of continuous/discontinuous mode of
39 DUTYCYCLE 0.433 Primary MOSFET duty cy cle

42 LPRIMARY_MIN 805.6 uH Minimum primary inductance

3.0

45 LPRIMARY_MAX 855.4 uH Maximum primary inductance

48 IPEAK_PRIMARY 0.95 A Primary MOSFET peak currrent

51 IRIPPLE_PRIMARY 0.76 A Primary MOSFET ripple current

54 SECONDARY CURRENT

Step 3 – Worst-Case Electrical Parameters

Enter: FSWITCHING_MAX, VOR and LPRIMARY_TOL, or VMIN

33

PARAMETERS

Application NoteAN-72

35 VOR 65.0 V

37 KP 0.66

38 MODE_OPERATION CCM Mode of operation

40 TIME_ON 7.46 us Primary MOSFET on-time

41 TIME_OFF 7.09 us Primary MOSFET off-time

43 LPRIMARY_TYP 830.5 uH Typical primary inductance

44 LPRIMARY_TOL

46

47 PRIMARY CURRENT

49 IPEDESTAL_PRIMARY 0.30 A Primary MOSFET current pedestal

50 IAVG_PRIMARY 0.25 A Primary MOSFET average current

52 IRMS_PRIMARY 0.41 A Primary MOSFET RMS current

53

55 IPEAK_SECONDARY 12.24 A Secondary winding peak current

56 IPEDESTAL_SECONDARY 3.79 A Secondary winding current pedestal

57 IRMS_SECONDARY 6.44 A Secondary winding RMS current

Figure 4. Worst-Case Electrical Parameters Section of InnoSwitch3-CE Design Spreadsheet with Gray Override Cells.

3.0 % Primary inductance tolerance

the primary MOSFET turns off

operation

Switching Frequency, FSWITCHING_MAX (Hz)

This parameter is the switching frequency at full load at minimum
rectied AC input voltage. The maximum switching frequency of
InnoSwitch3 in normal operation is 100 kHz, and the typical overload
detection frequency of is 110 kHz. In normal operating condition, the
switching frequency at full load should not be close to the overload
detection frequency.

The programmable switching frequency range is 25 to 95 kHz, but it
should be continued that the average frequency accounting for
primary inductance and peak current tolerances does not result in
average frequency higher than 110 kHz as this will trigger auto­restart due to overload. Pushing frequency higher to reduce

InnoSwitch3 Family Maximum Switching Frequency

INN3xx2C and

INN3xx3C

INN3xx4C and

INN3xx5C

INN3xx6C 75 kHz

INN3xx7C 70 kHz

INN3xx8C 65 kHz

Table 5. Suggested Maximum Switching Frequency.

transformer size is advisable, but Table 5 provides the suggested
frequency based on the size of the internal high-voltage MOSFET, and
represents the best compromise to balance overall device losses (i.e.
conduction and switching losses).

Reected Output Voltage, VOR (V)

This parameter is the secondary winding voltage during the diode /

Synchronous Rectier MOSFET (SR FET) conduction-time reected
back to the primary through the turns ratio of the transformer. Table
6 provides suggested values of VOR. VOR can be adjusted to achieve

SR FET while simultaneously achieving sufciently low Drain-Source
voltage of the primary side MOSFET. VOR can be adjusted as

necessary to ensure that no warnings in the spreadsheet are

triggered. For design optimization purposes, the following factors

should be considered,

• Higher VOR allows increased power delivery at VMIN, which

minimizes the value of the input capacitor and maximizes power
delivery from a given.

a design that does not violate design rules for the transformer and

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85 - 90 kHz

80 kHz

7

Rev. A 10/18

Application Note AN-72

• Higher VOR reduces the voltage stress on the output diodes and SR
FETs, which in some cases may allow a lower voltage rating for
higher efciency.

• Higher VOR increases leakage inductance which reduces power
supply efciency.

• Higher VOR increases peak and RMS current on the secondary-side
which may increase secondary side copper, diode and SR FET losses

thereby reducing efciency.

It should be noted that there are exceptions to this guidance
especially for very high output currents where the VOR should be
reduced to obtain highest efciency. Higher output voltages
(above 15 V) should employ a higher VOR to maintain acceptable
peak inverse voltage (PIV) across the output SR FET.

Optimal selection of the VOR value depends on the specic
application and is based on a compromise between the factors

mentioned above.

Output

Voltage

Suggested VOR

Value

Suggested

Range

5 V 55 V 45 V - 60 V

9 V 85 V 80 V - 90 V

12 V - 20 V 110 V 100 V - 120 V

Table 6. Suggested Values for VOR.

Mode of Operation, KP

KP is a measure of how discontinuous or continuous the mode of
switching is. KP > 1 is said to be in discontinuous operation (DCM),
while KP < 1 denotes continuous operation (CCM).

Ripple to Peak Current Ratio, K

Below 1 (indicating continuous conduction mode), KP is the ratio of

P

ripple to peak primary current (Figure 5).

I

KP ≡ KRP =

I

R

Primary

(a) Continuous, K

Primary

R

(b) Borderline Continuous/Discontinuous, K

P

< 1

R

I

P

I

P

I

PI

= 1

P

PI-2587-103114

Primary

Secondary

Primary

Secondary

D × T

(a) Discontinuous, K

D × T

KP ≡ KDP =

> 1

P

T = 1/f

t

T = 1/f

Figure 5. Continuous Mode Current Waveform, K

(1-D) × T

t

S

(1-D) × T

S

(1-D) × T = t

P

≤1.

(b) Borderline Discontinuous/Continuous, K

Figure 6. Discontinuous Mode Current Waveform, KP≥1.

8

Rev. A 10/18

= 1

P

PI-2578-103114

www.power.com

Application NoteAN-72

I

I

P

DT

VV D

VD

#

-

^

^

h

h

KK

/ =

PRP

Above a value of 1, indicating discontinuous conduction mode, KP is
the ratio of primary MOSFET off time to the secondary SR_FET

conduction time.

/ =

KK

PDP

#

OR MAX

=

MIN DS MAX

The value of KP should be in the range of 0.5 < KP < 6. Guidance is
given in the comments cell if the value of KP is outside this range.

Experience has shown that a KP value between 0.8 and 1 will result in
higher efciency by ensuring DCM or critical mode operation (CRM)
which is desirable for most charger designs.

The spreadsheet will calculate the values of peak primary current,

primary RMS current, primary ripple current, primary average current,

and the maximum duty cycle for the design based on the selection of

the these parameters.

Typical Primary Inductance, LPRIMARY_TYP (µH)

This is the typical transformer primary inductance target.

R

-

1

#

t

^

1

h

-

Primary Inductance Tolerance, LPRIMARY_TOL (%)

This parameter is the assumed primary inductance tolerance. A value

of 7% is used by default, however if specic information is provided
from the transformer vendor, then this may be entered in the grey
override cell. A value of 7% helps to reduce unit-to-unit variation and
is easy to meet for most magnetics vendors. A value of 3% will help
improve production tolerance further but will be more challenging to

vendors.

The other important electrical parameters are automatically calculated
by the spreadsheet. These can used to appropriatley select the other

components in the circuit, such as input fuse (FR) and EMI lter (LF),
bridge rectiers (BRD), output rectiers (SR

as described in Figure 1.

PRIMARY CURRENT

IPEAK_PRIMARY − Peak primary current
IPEDESTAL_PRIMARY − Primary MOSFET current pedestal in CCM mode
IAVG_PRIMARY − Primary MOSFET average current
IRIPPLE_PRIMARY − Primary MOSFET ripple current
IRMS_PRIMARY − Primary MOSFET RMS current

SECONDARY CURRENT

IPEAK_SECONDARY − Peak secondary current
IPEDESTAL_SECONDARY − Secondary winding current pedestal
IRMS_SECONDARY − Secondary winding RMS current

Minimum Rectied Input Voltage, VMIN

Valley of the rectied minimum AC input voltage at full power is
calculated based on input capacitance (CAP_INPUT).

) and capacitors (C

FET

OUT

),

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9

Rev. A 10/18

Application Note AN-72

TRANSFORMER CONSTRUCTION
PARAMETERS

CORE SELECTION

63 CORE RM6 Info RM6

The transformer windings may not fit: pic k a

Parameters tab for fit calculations

66 LE 29.20 mm Core magnetic path length

Safety margin width (Half the primary to
74 PRIMARY WINDING

77 BMAX 2844 Gauss Maximum flux density

80 LG 0.310 mm Core gap length

4

Primary winding wire outer diameter with

Primary winding wire outer diameter without
85 CMA_PRIMARY 248 Cmil/A Primary winding wire CMA

SECONDARY WINDING

88 NSECONDARY 6 6 Secondary turns

Secondary winding wire outer diameter with

Secondary winding wire outer diameter without

BIAS WINDING

Step 4 – Transformer Construction Parameters

Enter: CORE, AE, LE, AL, VE, BOBBIN, AW, BW, MARGIN

Choose Core and Bobbin based on maximum output power.

61

62

bigger core or bobbin and refer to the Transformer

64 CORE CODE PC95RM06Z Core code

65 AE 37.00 mm^2 Core cross sectional area

67 AL 2150 nH/turns^2 Ungapped core effective inductance

68 VE 1090.0 mm^3 Core volume

69 BOBBIN B-RM06-V Bobbin

70 AW 15.52 mm^2 Window area of the bobbin

71 BW 6.20 mm Bobbin width

72 MARGIN 0.0 mm

73

75 NPRIMARY 77 Primary turns

76 BPEAK 3125 Gauss Peak flux density

78 BAC 933 Gauss AC flux density

79 ALG 140 nH/turns^2 Typical gapped core effective inductance

81 LAYERS_PRIMARY

82 AWG_PRIMARY 30 AWG Primary winding wire AWG

83 OD_PRIMARY_INSULATED 0.303 mm

84 OD_PRIMARY_BARE 0.255 mm

86

87

89 AWG_SECONDARY 19 AWG Secondary winding wire AWG

90 OD_SECONDARY_INSULATED 1.217 mm

91 OD_SECONDARY_BARE 0.912 mm

92 CMA_SECONDARY 216 Cmil/A Secondary winding wire CMA

93

94

95 NBIAS 15 Bias turns

4 Number of primary layers

secondary creepage distance)

insulation

insulation

insulation

insulation

Figure 7. Transformer Core and Construction Variables Section of InnoSwitch3 PIXLs Spreadsheet.

Core Type, CORE

By default, if the core type cell is left empty, the spreadsheet will
select the smallest commonly available core suitable for the
continuous (average) output power specied. Different core types

and sizes from the drop-down list are available to choose from if a
user-preferred core is not available, the grey override cells (AE, LE,
AL, VE, AW & BW) can be used to enter the core and bobbin
parameters directly from the manufacturer’s data sheet.

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Rev. A 10/18

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Application NoteAN-72

Core and Bobbin Table

Core Bobbin

Output

Power at

Core Code

75 kHz

0 W ‒ 10 W EE10

PC47EE10-Z

0 W ‒ 10 W EE13 PC47EE13-Z 17.1 30.2 1130 517 B-EE13-H 18.43 7.60

0 W ‒ 10 W EE16 PC47EE16-Z 19.2 35.0 1140 795 B-EE16-H 14.76 8.50

0 W ‒ 10 W EE19 PC47EE19-Z 23.0 39.4 1250 954 B-EE19-H 29.04 8.80

10 W ‒ 20 W EE22 PC47EE22-Z 41.0 39.4 1610 1620 B-EE22-H 19.44 8.45

10 W ‒ 20 W EE25 PC47EE25-Z 41.0 47.0 2140 1962 B-EE25-H 62.40 11.60

20 W ‒ 50 W EE30 PC47EE30-Z 111.0 58.0 4690 6290 B-EE30-H 13.20

0 W ‒ 10 W RM5 PC95RM05Z 24.8 23.2 2000 574 B-RM05-V 4.90

10 W ‒ 20 W RM6 PC95RM06Z 37.0 29.2 2150 1090 B-RM06-V 6.20

20 W ‒ 30 W RM8 PC95RM08Z 64.0 38.0 5290 2430 B-RM08-V 30.00 8.80

30 W ‒ 50 W RM10 PC95RM10Z 96.6 44.6 4050 4310 B-RM10-V 10.00

Table 7. Commonly Available Cores and Power Levels at Which These Cores Can be used for Typical Designs.

AE LE AL VE

2

) (mm) (nH/T2) (mm3) (mm2) (mm)

(mm

Code

AW BW

12.1 26.1 850 300 B-EE10-H 12.21 6.60

Safety Margin, MARGIN (mm)

For designs that require safety isolation between primary and
secondary, but are not using triple insulated wire the width of the
safety margin to be used on each side of the bobbin should be
entered here. Typically for universal (85 – 265 VAC) input designs a
total margin of 6.2 mm is required, and a value of 3.1 mm should be

entered into the spreadsheet. For vertical bobbins the margin may

not be symmetrical; however if a total margin of 6.2 mm is required
then 3.1 mm would still be entered even if the physical margin was
only present on one side of the bobbin. For designs using triple

insulated wire it may still be necessary to enter a small margin in

order to meet the required safety creepage distances. Typically
several bobbins exist for each core size and each will have different
mechanical spacing. Refer to the bobbin data sheet or seek guidance
to determine what specic margin is required.

Margin reduces the available area for the windings, marginated
construction may not be suitable for small core sizes. If after
entering the margin more than 3 primary layers are required, it is

suggested that either a larger core be selected or that the design is

switched to a zero margin approach using triple insulated wire.

Primary Turns, NPRIMARY

This is the number of turns for the main winding of the transformer

calculated based on VOR and Secondary Turns.

Peak Flux Density, BPEAK (Gauss)

A maximum value of 3800 gauss is recommended to limit the peak
ux density at max current limit and 132 kHz operation. Under an
output-shorted condition the output voltage is low and little reset of
the transformer occurs during the MOSFET off-time. This allows the
transformer ux density to “staircase” beyond the normal operating
level. A value of 3800 gauss at the max current limit of the selected
device together with the built in protection features of InnoSwitch3
provides sufcient margin to prevent core saturation under output

short-circuit conditions.

Maximum Flux Density, BMAX (Gauss)

The low frequency operation resulting from a light load condition can
generate audible frequency components within the transformer,
especially if a long core is used. To limit audible noise generation,
the transformer should be designed such that the maximum core ux
density is below 3000 gauss (300 mT). Following this guideline and
using the standard transformer production technique of dip varnishing
practically eliminates audible noise. A careful evaluation of the
audible noise performance should be made using production
transformer samples before approving the design.

AC Flux Density, BAC (Gauss)

The BAC value can be used for calculating core loss.

Gapped Core Effective Inductance, ALG: (nH/N

2

)

Used to specify the CORE GAP [LG].

Primary Layers, LAYERS_PRIMARY

By default, if the override cell is empty, a value of 3 is assumed.
Primary layers should be in the range of 1 ≤ L ≤ 3, and in general it
should meet the current capacity guideline of 200 – 500 circular mils/
ampere for designs without forced air cooling. Primary winding wire
gauge AWG_PRIMARY is calculated in cell [E82]. Values above 3
layers are possible but the increased leakage inductance and physical
t of the windings should be considered. A split primary construction
may be helpful for designs where leakage inductance clamp
dissipation is too high. In this approach half of the primary winding is
placed on either side of the secondary (and bias) windings in a

sandwich arrangement.

Primary Winding Wire Guage, AWG_PRIMARY (AWG)

By default, if the override cell is empty, double insulated wire is

assumed and a standard wire diameter is chosen. The grey override

cells can be used to enter the wire gauge directly by the user, or if
the wire used is different from the standard double insulated type.

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Rev. A 10/18