Comparison of PSFB and FB-LLC

for high power DC/DC conversion

Adam Lin

1
Agenda
1/ Introduction to High Power Battery Chargers
• A Typical Battery Charger Specification
2/ Introduction to PSFB and FB-LLC
• How the PSFB works
• How the FB-LLC works
3/ Comparison
• The basic problem – how to choose between PSFB and FB-LLC in a given application
Much of what I will say about the FB-LLC is applicable to the HB-LLC too.
4/ Conclusions
• Conclusions
• Further Reading, TI Components, Reference Designs, Acknowledgements
2
Introduction to High Power Battery Chargers

3
Typical high power DC/DC system: (EV charger)
DC/DC: Phase Shifted Full Bridge (PSFB) or Full Bridge LLC (FB-LLC)?

Cable
400Vdc
Grid EVSE* AC/DC DC / DC Battery

External Vehicle
Power Transfer Isolation Barrier
Signalling This system is characterised by:
Charge rate, metering etc High Power levels
Fault Detection PFC
Proximity Sensor, GFCI* etc
Dangerous voltages
Dangerous currents
Charger power levels (SAE) Harsh environment
Level 1: Single phase: AC power, 1.92kW *EVSE – Electric Vehicle Service Equipment
Level 2: Split phase: AC power, 19.2 kW *GFCI – Ground Fault Current Interruptor
Level 3: DC power, 240kW (EVSE) Reference Design, tidub87

4
Batteries
Li-Ion:
• Charged to 4.2V/cell, discharged to 2.5V/cell
• 400V battery discharged to 230V,
• 1.75:1 ratio 85% 100%

Lead Acid:
• Charged to 2.35V/cell, discharged to1.9V/cell 3%
Lithium Ion
• 12V battery, 14.1V charged, fully discharged to 11.4V
• 1.25:1 ratio

Considerations:
• Tight voltage tolerances and maintaining charged state
• Temperature rise during charging → OTP ≈70% 98%
• Battery pack cell balancing – not considered here 100%
• Charge Rate vs Charge Time vs Battery Life
• Charger must operate in CI and CV modes
Lead Acid

5
 CI / CV operation
Two feedback paths
Lowest error ‘wins’ and
• One measures output current
controls the output
• Compare to reference
• Output error signal (power demand)
• One measures output voltage
• Compare to reference
• Output error signal (power demand)
• Diode ‘or’ errors – lowest error ‘wins’
– Automatic CV / CI transition

Low side sense Float signal

at 400Vout from MCU
High side sense - Lead Acid
- +
at 12Vout is only
possible

6
What do we care about – and why
• Cost
• Reliability and Lifetime
• Component count
• Efficiency – Thermal design, Size, Liquid or Air cooling
• Switching losses
• Component stresses
• Light Load operation
• CI/CV operation – Battery Charging, Compliance limits
• Constant or Variable Frequency operation
• Synchronisation, EMI
• Parallel operation - Current Sharing
• Isolation
7
A brief word about efficiency

Is 99% efficiency really so much better than 98% efficiency ?

8
A brief word about efficiency

Is 99% efficiency really so much better than 98% efficiency ?

YES – of course it is TIDA-00705 (480W/in^3)
Better to think in terms of power loss
2kW at 99% efficiency => 20W of loss
2kW at 98% efficiency => 40W of loss – twice as much heat to shed
At constant power, as product gets smaller – surface area reduces – temperature
rises
Best solution to reducing temperature rise is to reduce losses –
This eases thermal design
Allowing replacement of expensive liquid cooling with lower cost air cooling !

9
Introduction to the PSFB and FB-LLC

10
Introduction to the PSFB
PSFB Full bridge configuration for high power handling
• Constant Frequency
• Wide conversion range Phase Shift control allows ZVS and much lower
• Easily Synchronised losses than a Hard Switched PWM full bridge.
• Easy to parallel for current sharing
• ZVS ZVS gives lower
• High Efficiency • Switching losses,
• dv/dt
• EMI

Widely used for high power DC/DC applications (kW)

PSFB
Introduction to the FB-LLC
FB-LLC: Full bridge variant of the ‘usual’ half bridge LLC Increased complexity is justified
• Primary voltage doubled
Reposition CR and add two switches, A’ and B’ • Primary current halved
• More power for same size
Same drive signals can be used as on half bridge LLC • The same arguments as for
• Applied to Diagonal Pairs all Full Bridge vs Half Bridge
topologies

OUTA OUTA OUTB

OUTB OUTB OUTA

HB-LLC FB-LLC
PSFB and FB-LLC: Side by side
PSFB FB-LLC
• Constant Frequency • Variable Frequency
• Wide conversion range • Limited conversion range
• Can be easily Synchronised • Difficult to Synchronise
• Easy to parallel for current sharing • Difficult to force current sharing
• ZVS • ZVS
• High Efficiency • Highest Efficiency

Component Count – similar

Component stresses – different
Method of operation - different
Which to choose
How to choose

PSFB FB-LLC
How the PSFB works

14
Phase Shifted Full Bridge PA Leg AP Leg
Left Leg Right Leg
QA, QB QC, QD

𝑁𝑆
Buck Derived topology 𝑉𝑂𝑈𝑇 = 𝐷 𝑉𝐼𝑁
OUTA, OUTB – reference pair 𝑁𝑃
D controlled by phase shifting OUTC & OUTD
QE, QF are SRs, Diode rectification is possible Mouse over the waveforms to play the animation

15
 Energy Transfer
Timing Diagram: 1
• QA, QD, QF are ON: others are OFF
• First energy transfer interval
• I_PRI is Iout /N* + Imag.
• QF current is Iout
• Current flow in red (pri) and blue (sec) paths

0A

0V
Currents at end
of interval,
solid red / blue
0V

0V
I_LOUT: increasing

*N is the turns ratio >0A

16
 ZVS (AP transition)
Timing Diagram: 2
QD turns off
• QA, QF are ON: QC is OFF
• QD turns OFF DELCD** – Delay for Node B transition
QC turns on at 0V (ZVS)
• Node B charges to Vin as I_PRI current
moves out of QD and into QC Body Diode*
• QC: turns ON

0A
Leakage
Inductance Current in QD
0V
L_lk goes to zero
during interval
0V
Uses L_lk + I_Mag + I_Lout
energy.
0V Faster than Node A transition,
because I_LOUT is at maximum
*ZVS transition >0A
and more energy is available

17
 Freewheeling
Timing Diagram: 3
• QA, QC, QE, QF are ON: others are OFF
• T1 Primary is short circuited, VXFMR = 0V
• T1 Sec is short circuited by QE & QF
QE turns on
• Output current supplied by Lout
• Current flows asymmetrically in T1 Sec !

0A

0V
½ ΔI_Lout
½ ΔI_Lout + Iout 0V

QE turns ON 0V
I_LOUT: decreasing
Secondary is
shorted >0A

18
 ZVS (PA transition)
Timing Diagram: 4
QA turns off DELAB – allows time for node A
• QA, QC, QE are ON: QB is OFF transition
• QA turns OFF
– Node A charges to GND as I_PRI current
moves out of QA and into QB Body Diode
• QB: turns ON DELAF
QF turns off
after DELAF

0A

Leakage
0V
Inductance
QB turns on at 0V (ZVS)

0V

Uses L_lk energy only.

QF turns OFF 0V Slower than Node B
Removes sec transition, because
short less energy available.
>0A

19
 Energy Transfer
Timing Diagram: 5
• QB, QC, QE are ON: others are OFF
• Second energy transfer interval
• I_PRI is Iout /N* + Imag
QC turns off
• QE current is Iout
• Current flow in red (pri) and blue (sec) paths

0A

0V

0V

0V
*N is the I_LOUT: increasing
turns ratio >0A

20
 ZVS (AP transition)
Timing Diagram: 6
QC turns off
• QB, QC, QE are ON: QD is OFF
• QC turns OFF DELCD –allows time for
node B transition
• Node B charges to Gnd as I_PRI current
moves out of QC into QD Body Diode*
• QD: turns ON

0A
Leakage
Inductance 0V

0V

0V
QD turns on at 0V (ZVS)
*ZVS transition >0A

21
 Freewheeling
Timing Diagram: 7
• QB, QD, QE, QF are ON: others are OFF
• T1 Primary is short circuited, VXFMR = 0V
• T1 Sec is short circuited by QE & QF
• Output current supplied by Lout
• Current flows asymmetrically in T1 Sec QF turns on

0A

0V

½ ΔI_Lout ½ ΔI_Lout
+ Iout 0V

QF turns ON 0V
Secondary is
shorted >0A

22
 ZVS (PA transition)
Timing Diagram: 8 DELAB - allows time for
QB turns off Node A transition
• QB, QD, QF are ON: QA is OFF
• QB turns OFF
– Node A charges to Vin as I_PRI current
DELBE
moves out of QB into QA Body Diode QE turns off after DELBE
• QA: is turned ON

0A
Leakage
Inductance 0V
QA turns on at 0V (ZVS)

0V

Uses L_lk energy only.

QE turns OFF 0V Slower than Node B
Removes sec transition, because
short less energy available.
>0A

23
 SR Transitions: PA
PA = Passive/Active
OUTE OUTF QE turns off
• Current has to transfer out of one SR into the other.
after DELBE
• This takes time, di/dt is set by leakage inductance
• SRs always switch with zero volts. V_PRI V_XFMR QB turns off
• SR turns off before current goes negative 0V

• Body diode conduction interval after SR turned off

• DELBE associated with positive transition at PA leg
I_LOUT I_OUT
• DELAF associated with negative transition at PA leg I_QE I_QF
0A
• DELBE = DELAF
VDS_QE QE trr
SR current will go negative* and be carried in 0V VDS_QF losses
Channel if SR not turned off in time.
Destructive voltage spike if SR is turned off with
negative* current Note unequal current QE body diode
Very important to avoid this sharing , I_QE and I_QF conduction
See: Timing Diagram: 3
* Negative current means from drain to source.

24
 SR Transitions: AP
AP = Active/Passive
OUTE OUTF
• SRs see ZVS at AP transition
• SR turn on is co-incident with primary side switch
OUTD/OUTF and OUTC/OUTE V_PRI V_XFMR
The transformer secondary becomes short circuited 0V
when the second SR turns on (OUTD in this diagram)

I_LOUT I_OUT
I_QE I_QF
0A

VDS_QE
VDS_QF
0V

OUTC OUTD

25
 PSFB: Other Features
The time needed to achieve ZVS for both PA and AP legs is a function of the transformer
current. Some controllers allow the user to change the delay times of the primary and
secondary switches as a function of the current, UCC28950, UCC28951, UCC3895.

SR disable – the ability to disable the SRs and revert to diode rectification at light loads. This
prevents reverse currents in the resonant tank and improves light load efficiency.

Bi-Directional operation
The PSFB isn’t well suited to bi-directional operation but we do have some examples -

PMP5726 This is a slow drain modulation power converter – not truly bi-directional but it allow
SRs to operate right down to zero load for improved transient response.
TIDA-00653 A 48V/12V bidirectional battery charger. PSFB in forward direction. Push-Pull in
reverse direction

26
Phase Shifted Full Bridge PA Leg
Reminder !
AP Leg
Left Leg Right Leg
QA, QB QC, QD

Buck Derived topology 𝑁𝑆

𝑉𝑂𝑈𝑇 = 𝐷 𝑉𝐼𝑁
𝑁𝑃
OUTA, OUTB – reference pair
D controlled by phase shifting OUTC & OUTD
QE, QF are SRs, Diode rectification is possible

27
How the FB-LLC works

28
LLC: Introduction
• LLC is popular because - • CR, LR and LM form a resonant tank.
• ZVS (Zero Voltage Switching) reduces • RL is the load resistance
switching losses
• The gain of the LLC stage is Vout/Vin
• It can achieve good efficiency
• The gain is a function of frequency
• Low EMI
• Regulate Vout by changing fsw

Basically the LLC is

a Potential Divider

29
LLC: First Harmonic Approximation
• LLC Stage Analysis is difficult
– No easy analytical solution
• Approach used here is FHA
– Assumes that only the first harmonic of the switching waveform is significant
– Reasonably accurate close to resonance
– Increasingly inaccurate as system operates away from resonance
• Most efficient close to resonance.

2
LN f
 
Gain f Q
2  2  
LN f  f  1 1  jf LN Q

An Alternative LLC Design Process is described in slua733

30
LLC: Gain vs Frequency Characteristic
• LN = 5 Peak Gain Curve Vs Q ( LN = 5)
which is fairly typical LM LR
2
LN
• RE = 8 NT2 RL / LR CR
QE
RE
(NT = Turns Ratio, RL = Load)
• QE = (√(LR/CR))/RE
1
• Resonant frequency is f0
2  LR CR

FHA Calculation
2
LN f
 
Gain f Q
2  2  
LN f  f  1 1  jf  LN Q

LLC stage gain vs frequency with QE as a

parameter (normalised to f0, resonant frequency )

31
LLC: Gain vs Frequency Characteristic
Graph connecting Peak Gain points
• QE = (√(LR/CR))/RE ΔG/ ΔF is positive
• Resonant Tank peak gain increases as Q
Inductive_2 Inductive_1
decreases – ie. as load decreases
QE = 0.4
ΔG/ ΔF is negative
• ΔG/ ΔF slope changes as switching
frequency crosses from Inductive to
Capacitive region – AVOID this Capacitive

• Loss of ZVS and Control Law Reversal

• ZVS is possible in Inductive regions QE = 1

– Possible ≠ Guaranteed
• Operate in Inductive regions LLC stage gain vs frequency with QE as a
parameter (normalised to f0, resonant frequency )

32
LLC: Gain range
• Can get the same gain range across different frequencies
• Complex tradeoff between best efficiency at resonance
• Increased core losses at higher frequency
• Above resonance: Loss of rectifier ZCS
Gain vs Freq
• Possible loss of ZVS at higher frequency LN = 5
QE = 0.4
A
• Summary:
• Design optimisation is very difficult

• QE = (√(LR/CR))/RE where
• RE = 8 NT2 RL / 2 f0

33
LLC: Gain range
Determine required gain MG(min) = 0.83: 1.5 times f0 = 180kHz
• MG(max) = 1.3; at maximum Vout MG(max) = 1.3: 0.6 times f0 = 72kHz
Operates in INDUCTIVE Region
• MG(nom) = 1; at nominal Vout
• MG(min) = 0.83; at min Vout
Gain = 1.3 Gain vs Freq
LN = 5 at 0.6f0 LN = 5
Gain = 1 QE = 0.4
at f0

Gain = 0.83
0.4 at 1.5f0

LN = 5, QE = 0.4, Peak Gain is 1.4 f0

34
Effect of Resonant Parameter Variations
f f
Cr=22 nF Cr=22 nF
Lr=150 μH Lr=60 μH
Lm=250 μH Lm=250 μH

– Re=1 kΩ
– Re=500 Ω
– Re=100 Ω

Lr/Cr f Lm/Lr f

Cr=10 nF Cr=10 nF
Lr=150 μH Lr=60 μH
Lm=250 μH Lm=250 μH
 Timing Diagram: Resonance Energy Transfer
I_MAG increases – energy to drive ZVS
• QA, QA’, turn ON: QB, QB’ are OFF
• VPRI = VIN
• Output current is AVG of I_SEC
• Voltage across CR and LR sum to 0V
– (At Resonance only)

OUTB
OUTA

OUTB OUTA

Reverse voltage across diode is 2 x Vout

(For Centre tapped secondary)

36
 Timing Diagram: Resonance ZVS Transition

• QA, QA’, turn OFF: QB, QB’ are OFF I_MAG increases – energy to drive ZVS

• VPRI Reverses, driven by I_MAG OUTA

OUTB
• VSEC Reverses.
I_PRI
• Secondary current transfers into other I_MAG
winding V_PRI
V_CRES
• Dead time – allows time for ZVS V_LRES
V_SEC1
V_SEC2
ZVS_1 ZVS_2

I_COUT
I_SEC

Reverse voltage across

diode is 2 x Vout
(For Centre tapped
secondary)
Dead Time

37
 Timing Diagram: Below Resonance ZVS Transition
I_MAG increases – energy to drive ZVS
• QA, QA’, turn OFF: QB, QB’ are OFF
• VPRI Reverses, driven by I_MAG OUTA
OUTB
• VSEC Reverses.
I_PRI
• Secondary current transfers into other I_MAG
winding V_PRI
V_CRES
• Dead time – allows time for ZVS V_LRES
V_SEC1
V_SEC2

I_COUT
I_SEC

Reverse voltage
across diode is 2 x
Vout
(For Centre tapped
secondary)

38
 Timing Diagram: Above Resonance ZVS Transition
I_MAG increases – energy to drive ZVS
• QA, QA’, turn OFF: QB, QB’ are OFF
• VPRI Reverses, driven by I_MAG OUTA
OUTB
• VSEC Reverses.
I_PRI
• Secondary current transfers into other I_MAG
winding V_PRI
V_CRES
• Dead time – allows time for ZVS V_LRES
V_SEC1
V_SEC2

I_COUT
I_SEC

Reverse voltage
across diode is 2 x
Vout
(For Centre tapped
secondary)

39
 LLC – Below, At and Above Resonance
• Above Resonance, ZVS achieved, CCM on sec, Rectifiers not soft
switched. Lower RMS currents for given power
• At Resonance, ZVS achieved, BCM on sec, Rectifiers are soft
switched (ZCS), Optimum efficiency 0A

• Below Resonance, ZVS achieved, DCM on sec, Rectifiers are soft

switched (ZCS), RMS currents higher for given power.
Q = 0.4
‘Sweet Spot’ at Resonance,
Frequency almost
independent of load 0A

BUT – the difficulty is

Wide gain range requires

Gain vs Freq large frequency variation
Q=1 For various Q
0A

40
 FB-LLC: Other Features
The time needed to achieve ZVS is a function of the magnetizing current. Some controllers
change the delay times as a function of the current.

SR disable – the ability to disable the SRs and revert to diode rectification at light loads. This
prevents reverse currents in the resonant tank and improves light load efficiency.

Bi-Directional operation
The FB-LLC (like the normal LLC) isn’t well suited to bi-directional operation but some
examples have been published in the literature, I’m not aware of any production ready design

Seamless Operation of Bi-Directional LLC Resonant Converter for PV System

Abe et al, APEC 2014
Bidirectional LLC Resonant Converter for Energy Storage Applications
Jiang et al, Apec 2013

41
Comparison

42
PSFB and FB-LLC: Comparison
PSFB FB-LLC Comment
ZVS on PA leg more difficult
than AP leg (PSFB) and
ZVS* Yes Yes
more difficult at higher
frequency (FB-LLC)
ZCS reduces reverse
recovery switching losses
Rectifier ZCS turn off No Yes – below f0
n/a if SiC or Schottky
Diodes used.
Use Burst Mode to
Use Burst Mode to prevent Fsw FB_LLC losses will be
Light Load Operation
maintain ZVS increasing higher in this condition
unreasonably
It is easier to achieve ZVS with the LLC than the PSFB
But the magnetizing current reduces as switching frequency increases
PSFB and FB-LLC: Comparison
PSFB FB-LLC Comment
Good, best at FB-LLC best at ‘Sweet
Efficiency Good
Resonance Spot’

At nominal point (output voltage and current), LLC is more efficient than PSFB. This
partly because the LLC has ZCS in the secondary side and ZVS in primary. The PSFB
has ZVS on primary but no ZCS in sec. The difficulty is that battery charger
applications require sustained operation over a wide Vo range.
Paralleling and Current Sharing: PSFB
Easy on the PSFB, difficult on the FB-LLC

Paralleling is used to increase system level power in Current Sharing PSFB

manageable steps. A 5kW system may be built from three With (optional) SYNC
1.7kW systems in parallel. We also want the three sub-
systems to share the load equally.

Redundancy, n+1

Expansion to meet future expected load growth

45
Paralleling and Current Sharing: FB-LLC
Easy on the PSFB, difficult on the FB-LLC
Reference design tiduct9 is a current sharing, paralleled,
synchronised HB-LLC. It uses a C2000 processor to control
the system and modulate the duty cycle.

PWM modulation for

synchronisation
and current sharing

46
PSFB and FB-LLC: Comparison
PSFB FB-LLC Comment
Difficult –
FB-LLC needs
Paralleling and Current Share Simple especially if
Microcontroller
SYNC needed
Synchronous Rectification
Easy on the PSFB, less so on the FB-LLC

The SR signals for the PSFB are synchronous with the

primary signals.

A timing based approach to SR control won’t work in the FB-

LLC, especially when it is operating below resonance.
Vds sensing must be used on the FB-LLC.

SR gives significant efficiency improvement for low output

voltages – 12V, 24V, 48V
SR is marginally useful for medium output voltages – 130V
SR is not worth while for high output voltages – 400V

SRs are controlled rectifiers, if you don’t turn them off they will
conduct current in either direction.

48
PSFB and FB-LLC: Comparison
PSFB FB-LLC Comment
Diode rectification used on
high Vo designs. Low di/dt
Synchronous Rectification Easy Needs Care
rates in FB-LLC makes SR
drive tricky.

SR Controller
UCC27424 SR Driver

UCC24612 UCC24610
A note on transformer secondary circuits
Applicable to both PSFB and FB-LLC

The Centre Tapped secondary is popular for low output voltages.

• Rectifier Voltage stresses are at least 2 x Vout
• Diodes for low currents (<50A), low cost and low efficiency !
• SRs for high currents – with ground referenced drives

Single winding secondary with full bridge diode rectification

• Popular for high Vout.
• Rectifier stresses are lower than CT
• SRs not often used because of high side drive requirement
• On a 400V output the 1.5V Diode Vf is not very significant

Single winding secondary with current doubler

• Exists but not used
• Link to Current Doubler article.

50
Dithering
Dithering is the deliberate changing the switching frequency and is sometimes used as a
method to reduce the conducted EMI signature of a product

In a PWM system changing the switching frequency does not change the on time so the duty
cycle changes and so does Vout
The duty cycle has to be corrected by the operation of the control loop otherwise it will cause
Vout variations. Depends on relative speeds of dithering and loop bandwidth

In the LLC the switching frequency is the control variable. Dithering is not possible without
changing Vout – the control loop cannot correct for this.
• True for both DFC (Direct Frequency) and HHC (Hybrid Hysteretic) control methods

Dithering is a complex subject. slup269 Understanding Noise-Spreading Techniques and

their Effects in Switch-Mode Power Applications gives some insight.

51
Dithering
PSFB FB-LLC Comment
Dithering is of
Dithering Simple Very Difficult
marginal benefit.

Jitter No Jitter

52
Component Stresses: General
The component stresses in the following
400Vin, 250A PSFB FB-LLC
slides are all based on simulations of A RMS A RMS
3kW PSFB and FB-LLC designs
10.8V 8.8A 9.2
operating at constant 250V output. The
LLC is resonant at 100kHz and its 12V 9.2A 9.3A
transformer turns ratio was set so that
14V 10A 10.7A
Vout was 12V at resonance. Stresses
were calculated for three output
voltages, corresponding to a typical
Lead-Acid battery charging application. I_IN_PSFB I_IN_LLC

Please note that the relative levels of stress are correct and these are used for the basis
of comparison
The absolute values of stress are less accurate

53
Component Stresses: Input Current
Input current in FB-LLC is about 5%
400Vin, 250A PSFB FB-LLC
higher than the PSFB A RMS A RMS
10.8V 8.8A 9.2A*
This isn’t really significant
12V 9.2A 9.3A**
Input current wave shapes are different.
14V 10A 10.7A***
PSFB has more HF harmonics than
LLC.

I_IN_PSFB I_IN_LLC
*above Resonance, **At Resonance, ***below Resonance

I_IN_LLC

I_IN_PSFB

54
Component Stresses: Primary Switches
At a constant 250A output current
400Vin PSFB FB-LLC
Currents in 4 LLC switches are equal A RMS A RMS
Currents in 4 PSFB are equal 10.8Vo, 2.7kW 7.87A 6.52A *

RMS current higher in PSFB than FB-LLC 12Vo, 3kW 7.69A 6.59A **

Difference reduces as Vout and Pout increase 14Vo, 3.4kW 7.72A 7.52A ***
This assumes constant output current

Estimate a 40% higher dissipation in PSFB

for a given Rds_on

Use lower Rds_on switches

to reduce losses. I_QA_PSFB I_QA_LLC

I_QC_PSFB I_QC_LLC
*above Resonance, **At Resonance, ***below Resonance

55
Component Stresses: Resonant and Blocking Caps
CR is required in the LLC topology.
Current is output reflected current, 400Vin, PSFB (Blocking) FB-LLC (CR)
IOE plus magnetizing current, Imag 12Vo, 3kW
P-P Voltage ±2.8V ±300V

RMS current 11A 9.4A

Part Rating 600V 600V

C 7.5uF 75nF
A DC blocking capacitor is needed for Voltage Mode
Control in the PSFB – CBLK 7.5uF at 3kW. Sinusoidal Current in CR
Not needed in Peak Current Mode Control - At resonance

Capacitors perform different functions.

Both are non-polarised I_Pri_LLC = I_Cres

CBLK larger and more expensive than CR

V_Cres_LLC

56
Component Stresses: Resonant and Shim Inductors
LR is required in the LLC topology.
Current is output reflected current, IOE plus magnetizing 400Vin, PSFB (LSHIM) FB-LLC
current, Imag , Same as in the resonant capacitor 12Vo, 3kW (LR)
Current 11A RMS 9.4A RMS

L 5uH (Lshim+Lleak) 34uH

A Shim inductor may be needed to store energy for ZVS

in the PSFB – LSHIM 5uH at 3kW. – depends on L_Leak

Possibility to use transformer leakage inductance

in both circuits
Sinusoidal Current in LR , At resonance
Inductors perform different functions Trapezoidal current in LSHIM
but similar cost
I_Lshim_PSFB,
I_LR_LLC

57
Component Stresses: Transformer – Primary
A transformer is required in both topologies.
Safety isolation barrier usually built into 400Vin, PSFB FB-LLC
transformer 12Vo, 3kW
IPRI 11A RMS 9.4A RMS
Vpri is a square wave in both cases – modulated
by duty cycle in the PSFB LMAG 550uH 172uH

Primary currents are higher in the PSFB I_XFMR_Pri_PSFB I_XFMR_Pri_LLC

• Copper losses will be greater

Generally, a FB-LLC transformer will be larger

than an equivalent PSFB transformer. V_LRES_LLC V_CRES_LLC
• Must operate at lower frequencies – below
CR + LR voltage sums to zero
resonance.

LLC transformers in general can be low profile – V_XFMR_Pri_PSFB V_XFMR_Pri_LLC

increased leakage inductance is not a problem

One reason LLC is very popular in DTV

58
Component Stresses: Transformer – Secondary
There are a number of secondary winding options
400Vin PSFB FB-LLC
At low output voltage the usual choice is (A RMS) (A RMS)
Centre Tapped with Two rectifiers
10.8Vo, 2.7kW 251 276*
At high output voltage the usual choice is
Single winding with Full Bridge. 12Vo, 3kW 248 278**

RMS currents are lower in PSFB – 14Vo, 3.4kW 252 315***

lower Cu and SRRDSON losses

Average currents are the same – I_Sec_PSFB I_Sec_LLC

Diode Vf losses are the same.

For the Centre Tapped Secondary 0A

LLC VSEC is 2 x VOUT
PSFB Vsec is Vin/NT
0V
*above Resonance, **At Resonance, ***below Resonance V_Sec_PSFB V_Sec_LLC

59
Component Stresses: Output Rectifiers
This is a comparison of the currents in the CT
400Vin PSFB FB-LLC
secondary. (A RMS) (A RMS)

To a first approximation
10.8Vo, 2.7kW 177 195
RMS current determines I2R losses in SRs
Average current determines Vf*If losses in Diodes 12Vo, 3kW 175 197

14Vo, 3.4kW 178 223

FB-LLC has higher secondary currents than the
PSFB, expect higher conduction losses ZVS on both PSFB and FB-LCLC
CCM, no ZCS (PSFB) ZCS (FB-LLC)
Voltage stresses (Centre Tapped Secondary)
2xVout (FB-LLC)
2 xVin Ns/NP (PSFB) I_Cout_PSFB I_Cout_LLC
0A

*above Resonance, **At Resonance, ***below Resonance 0A

60
Component Stresses: Output Capacitors
COUT ripple current is MUCH higher in the LLC
400Vin, PSFB FB-LLC
Cap Ripple rating usually determines C needed* 12Vo, 3kW
Less of a problem at high Vout (400V)
ICOUT
Ratio of currents in the output capacitors of the 14A (RMS) 125A (RMS)
(200kHz)
PSFB and FB-LLC is about 1:9 at 20% pp ripple
5x8200uF, 16V 30x8200uF, 16V
Example
4.2A 4.2A
Ripple current causes heating in the capacitor
• ESR of Cap, Lifetime and allowed temp rise Limiting Ripple current Ripple current
Factor spec Spec
3kW, 12V PSFB needs about 40000uF Cost 1 6
3kW, 12V FB-LLC needs about 240000uF
Nippon Chemi Con type EKY series includes an 8200uF part,
Ripple current rating = 4.2A

I_Cout_PSFB I_Cout_LLC
*Batteries may behave like capacitors in some respects
but they are sensitive to ripple currents. Therefore it is
0A
not possible to use them to absorb the ripple currents
at the output of the power stage

61
Comparison - Summary
PSFB FB-LLC Comment
Switching Frequency Fixed Variable Fixed frequency is nice to have
Frequency dependent losses in FB-
Control Variable PWM Switching Frequency
LLC, difficult to predict
Current Mode, Direct Frequency,
Control Methods
Voltage Mode Hybrid Hysteretic
FB-LLC: Significantly more
Vo = D Vin, CCM Complex, approximate complex. FHA analysis is popular
Conversion Ratio mode only expression but not very accurate
Component Count Similar
Primary switches 4 4 2 High Side, 2 Low Side
Sec Rectifier 2 2 Same rectifier circuits can be used
Cap in xfmr Pri DC Blocking, VMC Resonant Cap FB-LLC CR is part of topology
Low pri/sec capacitances give high
Yes, tolerant of Yes, Very tolerant of
Isolation leakage inductance.
leakage inductance leakage inductance

62
Comparison - Summary
PSFB FB-LLC Comment
Design Optimisation Difficult Very Difficult
PSFB: ZVS on PA leg more difficult
Yes but difficult at
Primary side ZVS Yes than AP leg, FB-LLC: ZVS more
light loads
difficult at higher frequencies
Reverse recovery losses. Unless
Rectifier ZCS No, high di/dt Yes if fsw < f0
SiC or Schottky diodes used.
Good, minimise Body Good, best at
Efficiency FB-LLC best at ‘Sweet Spot’
Diode Conduction. Resonance
FB-LLC uses Fsw as the Vout
Synchronisation Simple Difficult
control variable.
Paralleling and
Simple Difficult FB-LLC needs Ext Ckt
Current Share
Synchronous Low di/dt rates in FB-LLC makes
Easy Needs Care
Rectification SR drive tricky

63
Comparison - Summary
PSFB FB-LLC Comment
Dithering Simple Very Difficult Dithering is a complex subject
Ripple current in Cout is about 9
times greater in the FB-LLC than
Large C, Very High
Output Capacitor Small, Low Ripple the PSFB. Low ESR high quality
Ripple
capacitors needed in the LLC.
Limiting factor for FB-LLC
RMS primary switch currents are
Primary Switches Vin Vin
higher in the PSFB.
Generally, a FB-LLC transformer
Fixed frequency Operates over a wider
Transformer will be larger than an equivalent
operation frequency range
PSFB transformer
Use Burst Mode to
Light Load Use Burst Mode to FB_LLC losses will be higher in
prevent Fsw increasing
Operation maintain ZVS this condition
unreasonably
Transient Response Good Good Not relevant for a battery charger

64
Comparison - Summary
PSFB FB-LLC Comment
Achieving required gain
Maintaining ZVS over
range with minimum
Main Design full Vout/Iout range. FB-LLC: resonant tank design is
frequency variation
considerations Switch timing. System both difficult and critical.
around the resonant
noise
frequency.
Can be a limiting factor for FB-
Vo range Wide Medium
LLC, makes design more difficult
Complexity High High
Component stresses are roughly
Component Stress Medium High (Cout) equivalent with the exception of
Cout
Can be done but neither of these
Bi-Directional Difficult Difficult
topologies is ideal

65
Conclusions

66
Conclusions
PSFB FB-LLC

• Design is easier • Design is difficult

• Flexible all rounder • Best efficiency at ‘sweet spot’
• ‘Systems Friendly’ features – • Full ZVS at each load
parallelability, current sharing, • Not suited to high power low Vout
synchronisation are all easy applications – Cout Ripple
• Suited to high power low and high
output voltage
Which to choose
How to choose

67
Further Reading
1. Design and Optimization of a High-Performance LLC Converter; B McDonald, J Freeman: slup306 Note_1
2. Designing an LLC Resonant Half-Bridge Power Converter; H. Huang: slup263
3. LLC Design for UCC29950: J Leisten: (note: despite the title this covers LLC design in general.) slua733
4. A current sharing, paralleled, synchronised HB-LLC, using a C2000 processor: tiduct9
5. LCC Converter Small Signal Modeling: McDonald.: Texas Instruments Power Supply Design Seminar, SEM2100, 2014. Note_2
6. A Design Review of a Full-Featured 350-W Offline Power Converter.: Marjanovic et al Texas Instruments Power Supply Design Seminar 2012.*
7. Zero Voltage Switching Resonant Power Conversion: Andreycak:. Unitrode Power Supply Design Seminar 700, 1990.
8. UCC28950 600-W, Phase-Shifted, Full-Bridge Application Report.: O’Loughlin, M. Texas Instruments Application Report, June 2011. slua560c
9. Phase Shifted Full Bridge, Zero Voltage Transition Design Considerations: Texas Instruments Application Report, August 2011. slua107a
10. Seamless Operation of Bi-Directional LLC Resonant Converter for PV System: Abe et al. APEC 2014
11. Bidirectional LLC Resonant Converter for Energy Storage Applications: Jiang et al. APEC 2013
12. Understanding Noise-Spreading Techniques and their Effects in Switch-Mode Power Applications, Rice et al. slup269

Note_1: A Google search for ‘slup306’ for example should find these TI papers
Note_2: TI power supply design seminar archive at http://www.ti.com/ww/en/power-training/login.shtml?DCMP=pwr-psds-archive

68
Texas Instruments Components
• UCC27714 , High-Speed, 4-A, 600-V High-Side Low-Side Gate Driver
• UCC24610 Secondary Side Synchronous Rectifier Controller
• UCC24612 High-Frequency Multi-Mode Synchronous Rectifier Controller
• UCC256301 Wide Vin LLC Resonant Controller With High-Voltage Start Up Enabling Ultra-Low Standby
Power (one of a family of UCC25630x HHC controllers)
• UCC28950 Green Phase-Shifted Full-Bridge Controller with Synchronous Rectification. Automotive
qual version is available (-Q1)
• UCC28951-Q1 Phase-Shifted Full-Bridge Controller for Wide Input Voltage Range
• UCC2895 BiCMOS Advanced Phase Shift Resonant Controller. Automotive version is available (-Q1)
• UCC25600 8-Pin High-Performance Resonant Mode LLC Controller
• UCC21520 4A/6A, 5.7 kVrms Isolated Dual Channel Gate Driver
• UCC27424 Dual, Low side, 4A MOSFET Driver

69
Reference Designs
(EVSE) Reference Design, tidub87 is a reference design for Level 1 and Level 2 Electric Vehicle Service
Equipment
PMP20657B This is is a compact, efficient unidirectional 48V to 12V @ 400W power converter.
PMP6712 is a 1600W DCDC converter using the UCC28950 controller in a dual phase master-slave
configuration
Existing Reference Design PMP8880, 12V, 460W with SRs

PMP5726 This is a slow drain modulation power converter – not truly bi-directional but it allows SRs to
operate right down to zero load for improved transient response.

TIDA-00653 A 48V/12V bidirectional battery charger. PSFB in forward direction. Push-Pull in reverse
direction
TIDA-00705 480-W, 97% η, Ultra-Compact (480W/in3), Bi-Directional DC-DC (Low voltage, half bridge)
Reference Design Library The full TI reference design library

70
Acknowledgements
• A note to say a big ‘Thank You’ to the following people who gave me valuable inputs for
this presentation. Of course any errors are my responsibility alone.

• Roberto Scibilia
• S.Ramkumar
• Jose Gomez
• Bing Lu
• Joe Leisten
• Yalong Li

71
Thank You

72
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