Phase-Shifted Full-Bridge, Zero-Voltage
Transition Design Considerations

Application Report

Literature Number: SLUA107A
September 1999 – Revised August 2011

Application Report
SLUA107A– August 2011

PHASE SHIFTED FULL BRIDGE, ZERO VOLTAGE
TRANSITION DESIGN CONSIDERATIONSABSTRACTThis Application Note will highlight the design considerations incurred in a high frequency power
supply using the Phase Shifted Resonant PWM control technique. An overview of this switching
technique including comparisons to existing fixed frequency non-resonant and variable frequency
Zero Voltage Switching is included. Numerous design equations and associated voltage, current andtiming waveforms supporting this technique will be highlighted. A general purpose Phase Shifted
converter design guide and procedure will be introduced to assist in weighing the various design
tradeoffs. Considerations will be given to the details of the magnetic and power switching areas. A
summary of comparative advantages, differences and tradeoffs to other conversion alternatives is
included.
UCC28950 CONTROL CIRCUIT SCHMATIC
Bias supply
VO

UCC28950

CREF
R2

C1

1

R1

R4

C3

GND 24
CVDD

R3
R5
R6

VREF

2

EA+

VDD 233EA-

OUTA 22

4COMP

OUTB 21

5

SS/EN

OUTC 20

6DELAB

OUTD 19

7DELCD

OUTE 18

8DELEF

OUTF 17To
synchronousrectifierdrivers

9

TMIN

SYNC 16

Synchronization

C2

To
bridgedrivers

CSS

RAB
RCD
REF

VREF
RTMIN
RRT

10 RT

CS 15

RA1

RSUM
RDCM1

Primary current sense

11 RSUM
12 DCM
RDCM2

RAEF1

ADEL 14RCSADELEF 13
RA2

RAEF2

Figure 11

Application Report
SLUA107A– August 2011

INTRODUCTION
The merits of lossless transitions using Zero Voltage Switching techniques have already been
established in power management applications, [1-5]. Effects of the parasitic circuit elements areused advantageously to facilitate the resonant transitions as opposed to being dissipatively snubbed.
This resonant tank functions to position zero voltage across the switching device prior to turn-on,
eliminating any power loss due to the simultaneous overlap of switch current and voltage at eachtransition. High frequency converters operating from high voltage input sources stand to gainsignificant improvements in efficiency with this technique. The full bridge topology as shown in figure
2 will be the specific focus of this presentation, with an emphasis placed on the fixed frequency,
phase shifted mode of operation.
Full Bridge Topology – General Circuit

+

QA

QC

T1

A

LR

B

D1

VIN

D2

+

LO
CO

VO

QB

QD

Figure 2
SWITCH DRIVE COMMANDS
The diagonal bridge switches are driven together in a conventional full bridge converter whichalternately places the transformer primary across the input supply, Vin, for some period of time, tON
as shown in figure 3.
Power is only transferred to the output section during the ON times of the switches whichcorresponds to a specific duty cycle when operated at fixed frequency. Additionally, the complete
range of required duty cycles is unique to the application, and can be estimated from the power
supply input and output voltage specifications.

2Application Report
SLUA107A– August 2011

Conventional Full Bridge PWM Waveforms

Figure 3
Rather than driving both of the diagonal full bridge switches together, a deliberate delay will be
introduced between their turn-on commands with the Phase Shifted approach. This delay will be
adjusted by the voltage loop of the control circuitry, and essentially results as a phase shift between
the two drive signals. The effective duty cycle is controlled by varying the phase shift between theswitch drive commands as shown in figure 4.
Unique to this Phase Shifted technique, two of the switches in series with the transformer can be
ON, yet the applied voltage to the transformer is zero. These are not diagonal switches of the fullbridge converter, but either the two upper or two lower switches. In this mode the transformer
primary is essentially short circuited and clamped to the respective input rail. Primary current ismaintained at its previous state since there is no voltage available for reset to take place. This deadband fills the void between the resonant transitions and power transfer portion of the conversion
cycle. Switches can be held in this state for a certain period of time which corresponds to therequired off time for that particular switching cycle.
When the correct one of these switches is later turned off, the primary current flows into the switch
output capacitance (Coss) causing the switch drain voltage to resonate to the opposite input rail. Thisaligns the opposite switch of the particular bridge “leg” with zero voltage across it enabling Zero
Voltage Switching upon its turn ON.

3Application Report
SLUA107A– August 2011

Phase Shifted PWM Control Waveforms

VERR
SYNC
FREQ
OUT_A
OUT_B
DLY A/B
OUT_C
OUT_D
DLY C/DPWM A/DPWM B/C
Figure 4
ZVS FUNDAMENTALS
An intentional dead-time can be introduced in the power conversion cycle whereby the switch
remains off and is clamped at zero voltage by the resonant tank. Rather then turn the switch on
instantly when zero voltage is attained, the switch is held off while the primary current circulates into
the shorted primary through the body diode and the opposite leg switch, which is still on. This offtime is used to fill in the voids between the point where zero voltage has been reached where theswitch needs to turned on to achieve fixed frequency operation.
Fixed frequency operation is obtainable over an identified range of input voltages and output
currents. For reference purposes, the variable frequency ZVS technique has similar limitations forproper operation which occur at minimum output load and maximum input line as shown in figure 5.

Figure 54

Application Report
SLUA107A– August 2011

ZVS LimitationsZVS Occurs

Non ZVS

Turn ON atzero voltage
switch point
0
Figure 6
PHASE SHIFTED FUNDAMENTALS
Switches within the Phase Shifted full bridge converter will be utilized differently than those of its
non-resonant counterpart. Instrumental to this technique is the use of the parasitic elements of the
MOSFET switch’s construction. The internal body diode and output capacitance (Coss) of eachdevice (in conjunction with the primary current) become the principal components used toaccomplish and commutate the resonant transitions.
CIRCUIT SCHEMATIC AND DESCRIPTION
Detailed operation of the Phase Shifted Converter operation will begin following a description of thecircuit elements. The circuit schematic of this technique is shown in figure 7 including voltage andcurrent designations.

5Application Report
SLUA107A– August 2011

Phase Shifted PWM Switch Orientation

+

QA

DA

CA

QC

T1

LR

A

DC

CC

B

D1

VIN

+

LO

D2

CO

VO

QB

DB

CB

QD

DD CD

Figure 7
The basic circuit is comprised of four switches labeled QA through QD and is divided up into two“legs”, the right and left hand legs. Each switch is shown shunted by its body diode (DA through DD)
and parasitic output capacitance, (CA through CD). These have been identified separately to clarify
the exact elements and current paths during the conversion interval.
A detailed model of the transformer primary section is presented which separately indicates theleakage and magnetizing inductances and currents of the primary. The reflected secondary
contributors to primary current are also shown for completeness, and divided into two components.
The DC primary current (IP) is the secondary DC output current divided by the transformer turns ratio(N). The secondary AC current should also be accounted for by multiplying the output inductance by
the turns ratio squared (N2), or dividing the secondary AC ripple current ISEC(ac) by the turns ratio (N)as shown in figure 8.
Primary Magnetic Components

LLKG

T1

LMAG

LO x N 2

1
N1

Figure 86

Application Report
SLUA107A– August 2011INITIAL CONDITIONS
Time: t = t(0)The description of the Phase Shifted operation will begin with the conclusion of one power transfer
cycle. This occurs when the transformer had been delivering power to the load and two of diagonalswitches of the converter were conducting. The initial current flowing in the primary can be
designated as IP(t(0)).
INITIAL CONDITIONS:
time t<t(0)QA = ON, QD = ON

+

QA

DA

CA

T1

A

QC

LR

DC

CC

B

D1

VIN

D2

+

LO
CO

VO

QB

DB

CB

QD

DD CD

Figure 9
RIGHT LEG RESONANT TRANSITION INTERVAL
Time: t(0) < t < t(1)The primary current flowing at time t(0) is equal to IP(t(0)) and was being conducted through the
diagonal set of transistors QA in the upper left hand corner of the bridge and transistor QD in thelower right. Instantly, at time t(0) switch QD is turned off by the control circuitry which begins theresonant transition of the right hand leg of the converter.
The primary current flowing is maintained nearly constant at IP(t(0)) by the resonant inductance ofthe primary circuit, often referred to as the transformers leakage inductance. Since an external
series inductance can be added to alter the effective leakage inductance value, this presentation willrefer to the lumped sum of these inductors as the resonant inductance, LR. In a practical application
it may be difficult to accurately control the transformers leakage inductance within an acceptable
ZVS range, necessitating an external “shim” inductor to control the accuracy. It’s also possible thatthe transformer leakage inductance can be too low to provide the desired transition times for theapplication so an external inductor can be introduced to modify the resonant inductance.

7

Application Report
SLUA107A– August 2011

RIGHT LEG TRANSISTION
time = t(0) < t < t(1)QA = ON, QD = OFF, CC = ↓, CD = ↑

+

QA

DA

CA

T1

A

QC

LR

DC

CC

B

D1

VIN

D2

+

LO
CO

VO

QB

DB

CB

QD

DD CD

Figure 10
With switch QD turned off, the primary current continues to flow using the switch output capacitance,
CD to provide the path. This charges the switch capacitance of QD from essentially zero volts to theupper voltage rail, Vin+. Simultaneously, the transformer capacitance (Cxfmr) and the output
capacitance of switch QC is discharged as its source voltage rises from the lower to the upper rail
voltage. This resonant transition positions switch QC with no drain to source voltage prior to turn-on
and facilitates lossless, zero voltage switching.
The primary current causing this right leg transition can be approximated by the full load primary
current of IP(t(0)). The small change due to the barely resonant circuit contribution is assumed to be
negligible in comparison to the magnitude of the full load current.
During this right leg transition the voltage across the transformers primary has decreased from VIN tozero. At some point in the transition the primary voltage drops below the reflected secondary
voltage, VO x N. When this occurs the primary is no longer supplying full power to the secondary andthe output inductor voltage changes polarity. Simultaneously, energy stored in the output choke
begins supplementing the decaying primary power until the primary contribution finally reaches zero.
Once the right leg transition has been completed there is no voltage across the transformer primary.
Likewise, there is no voltage across the transformers secondary winding and no power transferred,assuming ideal conditions. Note that the resonant transition not only defines the rate of change inprimary and secondary voltages dV/dt, but also the rate of change in current in the output filter
network, dI/dt.
CLAMPED FREEWHEELING INTERVAL
Time: t(1) < t < t(2)Once the right leg transition is complete the primary current free wheels through transistor QA andthe body diode of switch QC. The current would remain constant until the next transition occurs

8Application Report
SLUA107A– August 2011

assuming that the components were ideal. Switch QC can be turned on at this time which shunts the
body diode with the FET Rds(on) switch impedance thus lowering conduction losses. Althoughcurrent is flowing opposite to the normal convention (source to drain) the channel of QC will conduct
and divide the current between the switch and body diode.
CLAMPED FREE WHEELING INTERVAL
time t(1) < t < t(2)QA = ON, QC = ON, DC = ON

+

QA

DA

CA

T1

A

QC

LR

DC

CC

B

D1

VIN

D2

+

LO
CO

VO

QB

DB

CB

QD

DD CD

Figure 11
LEFT LEG TRANSITION INTERVAL
Time: t(2) < t < t(3)At time t(2) a residual current was flowing in the primary of the transformer which is slightly less thanIP(t(0)) due to losses. Switch QC has been previously turned ON and switch QA will now be turned
OFF. The primary current will continue to flow but the path has changed to the output capacitance(CA) of switch QA instead of its channel. The direction of current flowing causes the drain to source
voltage of switch QA to increase and lowers its source from the upper to lower rail voltage. Just the
opposite conditions have occurred to switch QB which previously had the full input across its
terminals. The resonant transition now aligns switch QB with zero voltage across it, enabling lossless
switching to occur.
Primary current continues to flow and is clamped by the body diode of switch QB, which is still OFF.
This clamping into a short circuit is a necessary condition for fixed frequency, zero voltage switching.
Once switch QB is turned ON, the transformer primary is placed across the input supply rails since
switch QC is already ON and will begin to transfer power. Although zero voltage switching has
already been established, turning ON switch QB the instant it reaches zero voltage will cause
variable frequency operation.

9

Application Report
SLUA107A– August 2011

LEFT LEG TRANSITION
Time t(2) < t < t(3)QA = OFF, QC = ON, DC = ON, CB = ↓, CA = ↑

+

QA

DA

CA

T1

A

QC

LR

DC

CC

B

D1

VIN

D2

+

LO
CO

VO

QB

DB

CB

QD

DD CD

Note that this left leg transition will require more time to complete than the right leg transition.
Conduction losses in the primary switches, transformer winding and interconnections result in a net
DC voltage drop due to the flowing primary current. Energy stored in the series resonant inductor
and magnetizing inductance is no longer ideally clamped to zero voltage. This loss, in addition to thelosses incurred during the previous transition, reduce the primary current below its initial IP(t(0))
value, thus causing a longer left leg transition time than the right leg.
Unlike conventional power conversion, one transistor in the diagonal pair of the phase shifted fullbridge converter is ON just before power is transferred which simplifies the gate drive. An additional
benefit is realized by designating these commutating switches as the high side switches of theconverter, usually far more difficult to drive than their lower side counterparts.
POWER TRANSFER INTERVAL
Time: t(3) < t < t(4)This interval of the phase shifted cycle is basically identical to that of conventional square wave
power conversion. Two diagonal switches are ON which applies the full input voltage across thetransformer primary. Current rises at a rate determined by VIN and the series primary inductance,
however starts at a negative value as opposed to zero. The current will increase to a DC level equal
to the output current divided by the turns ratio, IO/N. The two time variant contributors to primary
current are the magnetizing current (IMAG) and the output inductor magnetizing contribution reflectedto the primary, LO x N2. The exact switch ON time is a function of VIN, VO and N the transformer turns
ratio, just as with conventional converters.

10Application Report
SLUA107A– August 2011

POWER TRANSFER INTERVALTime t(3) < t < t(4)QB = ON, QC = ON

+

QA

DA

CA

T1

A

QC

LR

DC

CC

B

D1

VIN

D2

+

LO
CO

VO

QB

DB

CB

QD

DD CD

Figure 13
SWITCH TURN OFF
Time: t(4)One switching cycle is concluded at time t(4) when QC the upper right hand corner switch is turned
OFF. Current stops flowing in QC’s semiconductor channel but continues through the parasitic output
capacitance, CC. This increases the drain-to-source voltage from essentially zero to the full input
supply voltage, VIN. The output capacitance of the lower switch in the right hand leg (QD) issimultaneously discharged via the primary current. Transistor QD is then optimally positioned for zerovoltage switching with no drain-to-source voltage. The current during this interval is assumed to beconstant, simplifying the analysis. In actuality, it is slightly resonant as mentioned in the left leg
transition, but the amplitude is negligible in comparison to the full load current. The power
conversion interval is concluded at this point and an identical analysis occurs as for the opposite
diagonal switch set which has thoroughly been described for the switch set QA and QD.

11

Application Report
SLUA107A– August 2011

OPERATIONAL WAVE FORMS

VA
0

VB
0

VPRI
0

IPRI
0

ID1
0

ID2
0
t=

0 1 2 3 4

Figure 14
RESONANT TANK CONSIDERATIONS
The design of the resonant tank begins with the selection of an acceptable switching frequency; one
selected to meet the required power density. Second, the maximum transition time must also beestablished based on achievable duty cycles under all operating conditions. Experience may provide
the best insight for acceptable results.

12Application Report
SLUA107A– August 2011

The maximum transition time will occur during the converters left leg transition operating at
the minimum output load current.
RESONANT CIRCUIT LIMITATIONS
Two conditions must be met by the resonant circuit at light load, and both relate to the energy stored
in the resonant inductor. One, there must be enough inductive energy stored to drive the resonantcapacitors to the opposite supply rail. Two, this transition must be accomplished within the allocatedtransition time. Lossy, non-zero voltage switching will result if either, or both are violated. The first
condition will always be met when the latter is used as the resonant circuit limitation.
Designers can argue that some switching loss may be of little consequence in a practical application
at very light loads - especially considering that there is a significant benefit at heavy loads. While thismay be a pragmatic approach in many applications, and a valid concern, this presentation willcontinue using the fully lossless mode as the ultimate design goal.
The stored inductive energy requirement and specified maximum transition time have also defined
the resonant frequency (ωR) of the tank circuit. Elements of this tank are the resonant inductor (LR)and capacitor (CR), formed by the two switch output capacitors, also in parallel with the transformer
primary capacitance Cxfmr. The maximum transition time cannot exceed one-fourth of the self
resonant period, (four times the self resonant frequency) to satisfy the zero voltage switchingcondition.
The resonant tank frequency, ωR:

ωR =1
LR × CRt (max) transistion =π
2 × ωR

Coss, the specified MOSFET switch output capacitance will be multiplied by a 4/3 factor toaccommodate the increase caused by high voltage operation. During each transition, two switch
capacitances are driven in parallel, doubling the total capacitance to 8/3 x Coss. Transformer
capacitance (Cxfmr) must also be added as it is NOT negligible in many high frequency applications.
The resonant capacitance, CR:

8
CR = × Coss + Cxfmr
3
The capacitive energy required to complete the transition, W(CR) is:

W (C R ) =1
× C R × V Pr i 22This energy can also be expressed as:

1
⎛4
⎞
2
W (C R ) = ⎜ × Coss + Cxfmr ⎟ × VIN
2
⎝3
⎠
13

Application Report
SLUA107A– August 2011

STORED INDUCTIVE ENERGY
The energy stored in the resonant inductance must be greater than the energy required to charge
and discharge the FET output and transformer capacitances of the leg in transition within the
maximum transition time.
Inside the transformer, all of the energy is stored in the leakage inductance since the secondary
current has clamped the transformers primary voltage to essentially zero. This causes highcirculating primary current (as shown in figure 8) in the physical winding but has no effect on thestored energy used to perform the ZVS transition. More detail about the tradeoffs and design
optimization is presented in the Design Procedure.
The energy stored in the resonant inductor, LR:

W ( LR ) =1
2
× LR × I PRI
2RESONANT CIRCUIT SUMMARY
There are several ways to arrive at the solutions for the resonant inductor value and minimum
primary current required for any application. Each of these is based upon the following fundamental
relationships. The resonant tank frequency must be at least four times higher than the transition time
to fully resonate within the maximum transition time tMAX at light load.

TRES = 4 × t MAX
f RES =

1
TRES

or

where ω R = 2 × π × f RES

ωR =

2×π
TRES

Reorganizing and combining these relationships;

ωR =
ωR =

2 ×π
4 × t MAXπ
2 × t MAXThe resonant radian frequency (ωR) is related to the resonant components by the equation:

ωR =1
LR × CR

14

Application Report
SLUA107A– August 2011

Both sides of this can be squared to simplify the calculations and reorganized to solve for the exact
resonant inductor value.

LR =1
ω × CR
2
R

Previously outlined relationships for ωR and CR can be introduced to result in the following specificequation.

LR =

1
2

⎛ π ⎞ ⎛8
1⎞⎜⎜⎟⎟ × ⎜ × Coss + Cxfmr ⎟2⎠⎝ 2 ×t MAX ⎠ ⎝ 3

Note that this figure indicates the exact resonant inductor value required to satisfy only the task ofresonant transitions. This resonant inductor is in series with the transformer primary hence also
defines the maximum primary current slew rate, dI/dt as a function of input voltage.

dI PRI VIN=
dt
LR
If the resonant inductor value is too large it may take too long to reach the necessary load current
within the conversion cycle. The calculated inductor value satisfies the light load condition, however
full load operation must also be evaluated. Details of possible solutions to this are highlighted in the
Practical Applications section of this paper.
STORED ENERGY REQUIREMENTS
As detailed, the energy stored in the resonant inductor must be greater than the capacitive energy
required for the transition to occur within the allocated transition time. The governing equations are
summarized below.

1
1
2
× LR × I PRI× CR × VIN2 ( MAX ) or
( MIN ) =22
2
2
LR × I PRI( MIN ) > C R × VIN ( MAX )Since CR and VIN are known or can be estimated for a given application, this term becomes aconstant and LR has been quantified.
MINIMUM PRIMARY CURRENT
The minimum primary current required for the phase shifted application can now be determined byreorganizing the previous equation.

I PRI ( MIN ) =CR × VIN2 ( MAX )LR

15

Application Report
SLUA107A– August 2011

This value can be supported by the calculating the average current required to slew the resonantcapacitor to the full rail voltage. Although this figure will be lower that IP(MIN) it can be used as aconfirmation of the mathematics.

I R ( AVERAGE ) = C R ×

VIN
t MAX

Obtaining the necessary amount of primary current can be done in several ways. The most direct
approach is to simply limit the minimum load current to the appropriate level. One alternative,
however, is to design the transformer magnetizing inductance accordingly. Also assisting themagnetizing current is the reflected secondary inductor current contribution which is modeled inparallel. Any duty cycle variations modifying the peak charging current must also be taken into
account.
Generally the magnetizing current alone is insufficient in many off-line high frequency converters.
The transformer is usually core loss limited which means numerous primary turns and a highmagnetizing inductance. Shunting the transformer primary with an external inductor to develop theright amount of primary current is one possibility. Incorporating the output filter inductor magnetizing
current to assist resonance on the primary side is also an alternative.
SUMMARY
The fixed frequency phase shifted control technique of the full bridge converter offers numerous
performance advantages over the conventional approach. Switching losses due to the simultaneous
overlap of voltage and current disappear along with the dissipative discharge of the FET output
capacitance. EMI/RFI is significantly lower, also due to the "soft" switching characteristics which
incorporate parasitic elements of the power stage advantageously. For most applications, there islittle reason to consider the traditional square wave counterpart of this phase shifted PWM techniquefor future designs.
Very high frequency operation of this technique, beyond 500 KHz, is probable above the optimal
operating point. Transition times quickly erode the usable duty cycle to a point where the transformer
turns ratio has been compromised. This could result in unreasonably high primary currents andpower loss in the switches. Any incremental gains in cost or power density by reducing the size ofthe output filter are probably nullified by the need for larger MOSFETs and heatsinks. This phaseshifted PWM technique does excel in the overall majority of mid to high power, off-line applications.
Peak efficiency will be obtained in applications with moderate load ranges, however excellent results
can also be obtained in most designs with load ranges of ten-to one. A subgroup of applications may
exist where non ZVS operation extremely light loads is acceptable, especially when the advantages
under all other operating conditions are considered.

16

Application Report
SLUA107A– August 2011

REFERENCES
1. STEIGERWALD, R. and NGO, K; “Full Bridge Lossless Switching Converter”; United States
Patent #4,864,479
2. DALAL, D.; “A 500 KHz Multi-Output Converter programmed by theft start interval minimizes
power dissipation with Zero Voltage Switching”; IEEE 1990
3. FISHER, NGO and KUO; “A 500 KHz, 250 W DC-DC Converter with Multiple Outputs Controlled
by Phase Shifted PWM and Magnetic Amplifiers ” HFPC 1988"; HFPC 1988
4. ANDREYCAK, W. “Zero Voltage Switching Resonant Power Conversion”; UNITRODE Power
Supply Design Seminar SEM-700, 1990
5. MWEENE, WRIGHT and SCHLECHT, “A 1 KW, 500 KHz Front-End Converter for a Distributed
Power Supply System”; IEEE 1989
6. ANDREYCAK, W. “Controlling Zero Voltage Switched Power Supplies”; HFPC Proceedings 1990
7. SABATE′, VLATKOVIC′, RIDLEY, LEE and CHO; “Design Considerations for High Voltage, HighPower, Full Bridge, Zero Voltage Switched PWM Converter; IEEE APEC 1990
8. LOFTI, SABATE′, and LEE; “Design Optimization of the Zero Voltage Switched PWM Converter”;
VPEC Seminar 1990
9. CHEN, LOFTI and LEE; “Design Tradeoffs in 5V Output Off-line ZVS PWM Converters”;
Proceedings of the International Telecommunications Energy Conference; Kyoto, Japan 1991

17

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