High-Density Ac-Dc Power Supplies using Active-Clamp Flyback Topologу

Август 2, 2022

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High-Density Ac-Dc Power Supplies using Active-Clamp Flyback Topologу

Содержание

2. Agenda Introduction to active-clamp flyback operation (ACF) ACF light-load efficiency challenge Introduction to the NCP1568 –
3. Introduction to Active-Clamp Flyback Operation (ACF)
4. Active-Clamp Flyback The clamp diode in a standard flyback converter is replaced by a switch hence
5. Why Active-Clamp Flyback? Zero-Volt Switching of the FETs with Fixed-Switching Frequency Results in high switching frequency,
6. Energy-Storage Mode The energy-storage mode is similar to that of a classical flyback converter: when the
7. Transition from Energy-Storage Mode to Power-Delivery Mode When FET turns off, the lump capacitor on the
8. Power-Delivery Mode In this mode, Lleak resonates with clamp capacitor (Cclamp). The resonant frequency is given
9. Transition from Power-Delivery Mode to Energy-Storage Mode When the clamp FET turns off, Lleak resonates with
10. Leakage Inductance Needed for ZVS For universal design, leakage inductance needed to get ZVS increases in
11. ZVS Phenomenon – 1 During Tdis1 (shaded region), Lleak resonates with Cclamp. The time it takes
12. ZVS Phenomenon – 2 During Tdis2 (shaded region), negative magnetizing current starts to discharge the clamp
13. Fixed-Frequency Operation Magnetizing current in ACF is in CCM. As the load current decreases, the valley
14. Variable-Frequency Operation As the load current decreases, increasing the frequency minimizes Imag and reduces the conduction
15. Light-Load Efficiency & Standby Power Challenge
16. Light-Load Efficiency Requirements European Code of Conduct, Ver. 5, Tier 2 poses stringent efficiency standards at
17. Standby Power Standard US DoE standards are equally stringent Most of the brand name OEMs require
18. ACF Specific Light-Load Challenges Magnetizing current is in CCM. Frequency modulation results in high-frequency operation at
19. DCM Operation Holding active-clamp FET off, DCM operation can be implemented in ACF. This allows magnetizing
20. Introduction to NCP1568 Ac-Dc PWM Controller for ACF
21. Introduction to NCP1568 Control Scheme Adaptive ZVS frequency modulation allows variable Vout operation Integrated adaptive dead-time
22. Frequency Movement vs. Load NCP1568 NCP1568 features a combination of nonlinear & linear foldback schemes The
23. Clamp Capacitor Challenge VCLAMP_DCM>VCLAMP_ACF Leakage energy is not recycled in DCM and is dissipated in the
24. Transition from DCM to ACF Active-clamp FET can be soft-started to discharge the clamp capacitor slowly.
25. DCM Operation Determination NCP1568 can be configured to operate in pure ACF mode and pure DCM
26. Key Components Selection Transformer Design & Key Equations
27. Design Specifications
28. Turns Ratio Selection Turns ratio can be calculated by the following formula assuming Dmax=0.5. Turns ratio
29. Minimum On-Time Minimum on-time needs to be calculated at worst case duty ratio to ensure that
30. Valley Current for ZVS In order to determine the inductance value, valley current is needed. To
31. Inductance Calculation Inductance can be calculated as follows: Where Dmin is the minimum duty cycle given
32. Inductance vs. Required Valley Current for ZVS As the required valley current for ZVS decreases, the
33. Core Selection Assuming a 200-mT Bmax operating at 400 kHz results in a core loss of
34. Primary and Secondary Turns The primary and secondary turns can be calculated from the following formulae:
35. Clamp Capacitor Selection Clamp capacitor should be selected at worst-case off-time i.e., lowest frequency and minimum
36. RMS Current Formulae The primary and secondary FET selection criterion is no different than with standard
37. 60-W UHD-Board Performance
38. Simplified Schematic Secondary side is similar to any standard flyback topology.
39. Frequency Modulation w. Load As the load current decreases, the negative current is minimized & kept
40. Fixed Frequency vs. Frequency Modulation 115 V rms, 1.5-A load Fixed Fsw of 231 kHz 115
41. Frequency Movement w. Vout & Vin Frequency movement is similar to QR flyback switching in 1st
42. Frequency vs. Load Current Active-Clamp Mode Discontinuous Mode
43. DCM SW waveforms 90 V rms 265 V rms
44. NCP1568 USB PD 65-W UHD Demonstration Board Featured Devices: NCP1568 ACF Controller NCP51530 Half-Bridge Driver NCP4305
45. UHD Board Performance Achieving a full-load efficiency of 93.5% at a 60-W output Primary FETs running
46. NCP1568 Demonstration Board Efficiency
48. Скачать презентацию

Слайд 2

Agenda
Introduction to active-clamp flyback operation (ACF)
ACF light-load efficiency challenge
Introduction to the

NCP1568 – Ac-Dc ACF PWM IC.
Light load and standby solution
Design equations for transformer selection of the ACF
Primary and secondary component selection considerations
Performance data of ultra-high density active-clamp flyback board.

Слайд 3

Introduction to Active-Clamp Flyback Operation (ACF)

Слайд 4

Active-Clamp Flyback
The clamp diode in a standard flyback converter is replaced

by a switch
hence the name Active-Clamp Flyback or ACF.

Standard Flyback Converter w. RCD clamp

Active-Clamp Converter

Слайд 5

Why Active-Clamp Flyback?
Zero-Volt Switching of the FETs with Fixed-Switching Frequency
Results in

high switching frequency, improves efficiency and EMI.
Soft Increase in Secondary Current
Good for EMI
Clean Drain Waveforms Without Any Ringing
Better efficiency as the leakage energy is recycled.
Better EMI
Single-Ended Topology
Relatively simple design of magnetics compared to LLC.
Single switch/diode in the secondary.

Слайд 6

Energy-Storage Mode
The energy-storage mode is similar to that of a classical

flyback converter: when the main FET is on, energy is stored in the transformer.

ACF works in continuous conduction mode. Its input-to-output relationship is given by:

Слайд 7

Transition from Energy-Storage Mode to Power-Delivery Mode
When FET turns off, the

lump capacitor on the SW node is linearly charged at a rate given by Tcharge

Слайд 8

Power-Delivery Mode
In this mode, Lleak resonates with clamp capacitor (Cclamp). The

resonant frequency is given by:

The primary resonant current is given by:

The magnetizing current during the (1-D) phase is given by:

The difference between the primary resonant current and the magnetizing current flows in the secondary

Слайд 9

Transition from Power-Delivery Mode to Energy-Storage Mode
When the clamp FET

turns off, Lleak resonates with Cclamp. For the main FET to get ZVS, following condition has to be satisfied

Слайд 10

Leakage Inductance Needed for ZVS
For universal design, leakage inductance needed

to get ZVS increases in a parabolic fashion.
Increasing leakage & tightly controlling the spread add cost
Additional resonant inductor is an alternative, but inductor adds cost & volume

Assuming Clump = 220 pF, constant Ipeak = 1 A,
85 V to 265 V rms (universal input)

Слайд 11

ZVS Phenomenon – 1
During Tdis1 (shaded region),
Lleak resonates with Cclamp.

The time it takes for the resonance between leakage inductance and lump capacitance to reach its valley point is 1/4th of a resonant period. Therefore:

Слайд 12

ZVS Phenomenon – 2
During Tdis2 (shaded region), negative magnetizing current starts

to discharge the clamp capacitance
The time it takes to discharge the lump capacitance is given by:

Слайд 13

Fixed-Frequency Operation
Magnetizing current in ACF is in CCM.
As the load current

decreases, the valley point of the magnetizing current decreases.

Слайд 14

Variable-Frequency Operation
As the load current decreases, increasing the frequency minimizes Imag

and reduces the conduction losses.
Ideally, the valley of the magnetizing current needs to be maintained constant for ZVS.

Слайд 15

Light-Load Efficiency & Standby Power Challenge

Слайд 16

Light-Load Efficiency Requirements
European Code of Conduct, Ver. 5, Tier 2 poses

stringent efficiency standards at light-load condition

For a 60-W design, 4-point average (25%, 50%, 75%, and 100% average) efficiency needs to be > 88% for full load and 78% for 10% load.

Слайд 17

Standby Power Standard
US DoE standards are equally stringent
Most of the brand

name OEMs require to pass
stringent Tier-2 standard

DoE: Department of Energy

Слайд 18

ACF Specific Light-Load Challenges
Magnetizing current is in CCM.
Frequency modulation results in

high-frequency operation at light load
Classical frequency foldback is not possible to implement when magnetizing current is in CCM

Слайд 19

DCM Operation
Holding active-clamp FET off, DCM operation can be implemented in

ACF.
This allows magnetizing current to enter DCM: frequency foldback can be implemented

Слайд 20

Introduction to NCP1568
Ac-Dc PWM Controller for ACF

Слайд 21

Introduction to NCP1568
Control Scheme
Adaptive ZVS frequency modulation allows variable Vout operation
Integrated

adaptive dead-time
Peak-current-mode control
DCM & Light-Load Operation
Optional transition to DCM mode
Frequency foldback with 31-kHz minimum frequency clamp
Quiet skip eliminates audible noise
Standby power < 30 mW
HV Startup
700-V HV startup JFET
Integrated sensing of HV SW node for optimum ZVS
Brownout and X2 discharge inbuilt.

Слайд 22

Frequency Movement vs. Load
NCP1568
NCP1568 features a combination of nonlinear & linear

foldback schemes
The lower the frequency at light load, the higher the efficiency

Слайд 23

Clamp Capacitor Challenge
VCLAMP_DCM>VCLAMP_ACF
Leakage energy is not recycled in DCM and is

dissipated in the clamp resistor (RClamp)

Слайд 24

Transition from DCM to ACF
Active-clamp FET can be soft-started to discharge

the clamp capacitor slowly.
Leading-edge modulation of active-clamp FET allows the main FET to achieve ZVS

Слайд 25

DCM Operation Determination
NCP1568 can be configured to operate in pure ACF

mode and pure DCM mode.
Efficiency can be plotted in both ACF and DCM to determine optimal transition points.
NCP1568 uses the feedback information to transition from ACF to DCM or vice-versa.

Слайд 26

Key Components Selection
Transformer Design & Key Equations

Слайд 27

Design Specifications

Слайд 28

Turns Ratio Selection
Turns ratio can be calculated by the following formula

assuming Dmax=0.5.

Turns ratio should be calculated at the lowest input voltage while delivering maximum power
For this design, rounded turns ratio is 6.

Слайд 29

Minimum On-Time
Minimum on-time needs to be calculated at worst case duty

ratio to ensure that the controller can deliver the pulses

NCP1568 has a minimum on-time of 200 ns. The calculated on-times of the above equations are 600 ns and 700 ns respectively.
If the min on-time is < 200 ns, the turns ratio needs to be adjusted and the process iterated

Слайд 30

Valley Current for ZVS
In order to determine the inductance value, valley

current is needed.
To calculate the valley current, the capacitance lumped at the SW node can be expressed as follows:

Main FET
Output capacitance

Active Clamp FET
Output capacitance

Synchronous Rectifier FET

The above capacitances can be approximated from the FET datasheet.
Considering an ac-dc power supply, a 600-V FET for primary and a 120-V type for secondary have been selected resulting in a Clump of 220 pF

Co(er) is the nonlinear capacitance averaged along a given ΔVDS

Слайд 31

Inductance Calculation
Inductance can be calculated as follows:
Where Dmin is the minimum

duty cycle given by:

For this design, the above formula results in a magnetizing inductance of 120 µH

Слайд 32

Inductance vs. Required Valley Current for ZVS
As the required valley current

for ZVS decreases, the inductance falls.

Слайд 33

Core Selection
Assuming a 200-mT Bmax operating at 400 kHz results in

a core loss of 1.8 W.

A RM8LP core has been selected for this low-profile and high-density design.

Слайд 34

Primary and Secondary Turns
The primary and secondary turns can be calculated

from the following formulae:

This results in a primary turns of 23.
Since turns ratio is 6, 24 turns are selected for primary turns and 4 for secondary turns
A flux density, ∆B, of 0.2 T & Ae of 65 mm2 have been assumed for this design.

Слайд 35

Clamp Capacitor Selection
Clamp capacitor should be selected at worst-case off-time i.e.,

lowest frequency and minimum D
Clamp capacitor should be selected such that it resonates 1/4th of the resonant period at worse case off-time.
Ceramic capacitors are selected for clamp capacitors. Standard derating should be followed (voltage and rms current).

The above equation results in 330 nF.
After derating, a 660 nF is selected

Слайд 36

RMS Current Formulae
The primary and secondary FET selection criterion is no

different than with standard flybacks.
The active-clamp FET voltage rating is same as main FET.
The clamp and secondary FETs see different current waveforms than standard flyback. Their formulae are noted below

Слайд 37

60-W UHD-Board Performance

Слайд 38

Simplified Schematic
Secondary side is similar to any standard flyback topology.

Слайд 39

Frequency Modulation w. Load
As the load current decreases, the negative current

is minimized & kept constant leading to low conduction losses

2.25 A

1.7 A

1 A

Слайд 40

Fixed Frequency vs. Frequency Modulation
115 V rms, 1.5-A load
Fixed Fsw

of 231 kHz

115 V rms, 1.5-A load
Frequency modulation, Fsw = 260 kHz

Слайд 41

Frequency Movement w. Vout & Vin
Frequency movement is similar to QR

flyback switching in 1st valley

90 V rms

265 V rms

Слайд 42

Frequency vs. Load Current
Active-Clamp Mode
Discontinuous Mode

Слайд 43

DCM SW waveforms
90 V rms
265 V rms

Слайд 44

NCP1568 USB PD 65-W UHD Demonstration Board
Featured Devices: NCP1568 ACF Controller

NCP51530 Half-Bridge Driver
NCP4305 SR Controller
Full Load Efficiency: 93.4% @ 115 V rms (20 V/3.0 A)
93.6% @ 230 V rms (20 V/3.0 A)
Transformer Type: RM8 LP
Power Density: 30 W/in3 or 17 W/cm3
Board Dimensions: 1.66” x 1.78” x 0.70” or
4.2 cm x 4.5 cm x 1.7 cm

Слайд 45

UHD Board Performance
Achieving a full-load efficiency of 93.5% at a 60-W

output
Primary FETs running at 83 °C

Слайд 46

High-Density Ac-Dc Power Supplies using Active-Clamp Flyback Topologу

Содержание

AgendaIntroduction to active-clamp flyback operation (ACF)ACF light-load efficiency challengeIntroduction to the

Introduction to Active-Clamp Flyback Operation (ACF)

Active-Clamp FlybackThe clamp diode in a standard flyback converter is replaced

Why Active-Clamp Flyback?Zero-Volt Switching of the FETs with Fixed-Switching FrequencyResults in

Energy-Storage ModeThe energy-storage mode is similar to that of a classical

Transition from Energy-Storage Mode to Power-Delivery ModeWhen FET turns off, the

Power-Delivery ModeIn this mode, Lleak resonates with clamp capacitor (Cclamp). The

Transition from Power-Delivery Mode to Energy-Storage Mode When the clamp FET

Leakage Inductance Needed for ZVS For universal design, leakage inductance needed

ZVS Phenomenon – 1During Tdis1 (shaded region), Lleak resonates with Cclamp.

ZVS Phenomenon – 2During Tdis2 (shaded region), negative magnetizing current starts

Fixed-Frequency OperationMagnetizing current in ACF is in CCM.As the load current

Variable-Frequency OperationAs the load current decreases, increasing the frequency minimizes Imag

Light-Load Efficiency & Standby Power Challenge

Light-Load Efficiency RequirementsEuropean Code of Conduct, Ver. 5, Tier 2 poses

Standby Power StandardUS DoE standards are equally stringentMost of the brand

ACF Specific Light-Load ChallengesMagnetizing current is in CCM.Frequency modulation results in

DCM OperationHolding active-clamp FET off, DCM operation can be implemented in

Introduction to NCP1568Ac-Dc PWM Controller for ACF

Introduction to NCP1568Control SchemeAdaptive ZVS frequency modulation allows variable Vout operationIntegrated

Frequency Movement vs. LoadNCP1568NCP1568 features a combination of nonlinear & linear

Clamp Capacitor ChallengeVCLAMP_DCM>VCLAMP_ACFLeakage energy is not recycled in DCM and is

Transition from DCM to ACFActive-clamp FET can be soft-started to discharge

DCM Operation DeterminationNCP1568 can be configured to operate in pure ACF

Key Components SelectionTransformer Design & Key Equations

Design Specifications

Turns Ratio SelectionTurns ratio can be calculated by the following formula

Minimum On-TimeMinimum on-time needs to be calculated at worst case duty

Valley Current for ZVSIn order to determine the inductance value, valley

Inductance CalculationInductance can be calculated as follows:Where Dmin is the minimum

Inductance vs. Required Valley Current for ZVSAs the required valley current

Core SelectionAssuming a 200-mT Bmax operating at 400 kHz results in

Primary and Secondary TurnsThe primary and secondary turns can be calculated

Clamp Capacitor SelectionClamp capacitor should be selected at worst-case off-time i.e.,

RMS Current FormulaeThe primary and secondary FET selection criterion is no