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126

CHAPTER 6

FULL BRIDGE BUCK CONVERTER WITH

SECONDARY RESONANCE

6.1 INTRODUCTION

This chapter introduces the full bridge buck converter with

secondary resonance. The soft switching can be extended to the secondary of

a transformer. One such buck converter design employing a soft switching in

the output side is presented. Operation of the circuit is explained. Design

procedure is presented. Simulation and experimental results are presented to

validate the design procedure.

Full bridge buck converter with secondary resonance is described in

section 6.2. Operating principle is explained in section 6.3. Design procedure

is explained in section 6.4. Simulation results are presented in section 6.5.

Proposed half bridge secondary resonance converter with C filter is presented

in Section 6.5.2. Proposed half bridge secondary resonance converter with pi

filter is presented in Section 6.5.3 and experimental results of half bridge

secondary resonance converter with pi are presented in section 6.6.

6.2 FULL BRIDGE CONVERTER WITH SECONDARY

RESONANCE

The block diagram of full bridge converter with secondary

resonance is shown in Figure 6.1. The DC-DC converter consists of a DC

source, followed by an inverter, a high frequency transformer, a rectifier, a

127

series resonance circuit, a half wave rectifier load, a driver and a

microcontroller. The constant DC source voltage is inverted by means of a

voltage source inverter whose output voltage, current and frequency are

controlled by PWM. The desired pulses for an inverter are obtained with the

help of a PIC microcontroller. The pulses from PIC microcontroller are given

to the inverter switches through the driver circuit which is used for

amplification of the pulses from control circuit and also used for isolation of

pulses from control circuit and power circuit. The output of the inverter is fed

to the high frequency transformer. The output of the transformer is passed to

half wave rectifier through series resonating circuit rectifier and the controlled

output is used to drive the load. A capacitor is shunted to maintain a constant

load voltage.

Figure 6.1 Block diagram of Full bridge converter with secondary

resonance

The schematic diagram of Full Bridge converter with secondary

resonance is shown in Figure 6.2. The main components of the converter are:

full bridge converter in the transformer primary circuit, a capacitor in the

secondary circuit Cr to achieve resonance, C0 to drive the load and two diodes

D1, D2 in the secondary circuit. Llks is the transformer secondary leakage

inductance.

128

Figure 6.2 Full bridge converter with secondary resonance

The switching devices MOSFETs (S1 and S3, S2 and S4) in each leg

of the converter conduct alternately in a switching cycle. It is assumed that the

converter is under steady state operation and the output capacitor Co is large

enough to be considered as a voltage source. The converter has four operation

modes during a switching period.

6.3 OPERATING PRINCIPLE

Figure 6.3 shows the operation principle wave forms of Full Bridge

converter with secondary resonance. The primary side consists of four

switches, one inductor and one capacitor.

The input is the DC supply which is inverted using the full bridge

inverter and is fed to the transformer primary. The current in secondary flows

through Llks, resonating capacitor Cr,, diode D1, load and back to secondary.

Diode D2 is reverse biased due to the transformer secondary voltage. The load

voltage ripples are removed by the capacitor C0. Since Llks and Cr are in series,

a series resonance condition will be established. At that instant D1 will be

turned off and D2 is on. The energy stored in the secondary winding and the

leakage inductor is freewheeled through diode D2. The stored energy in the

capacitor C0 drives the load. Once the inductor energy is depleted the cycle

repeats.

129

Figure 6.3 Operation principle waveforms of FB with secondary

resonance

Mode-1 [t0,t1]

Switch S2 is turned on. Current flows from supply to primary of

transformer through S2 and back to source through body diode of S2. Diodes

D1 and D2 in secondary are reverse biased. Therefore the capacitor Co will

take care of load. No current flows through the secondary circuit.

Mode-2[t1,t2]

Switches S1 and S3 are turned on. Current flows from source to S1,

primary winding, S4 and back to source. Diode D1 in secondary is forward

biased. Therefore secondary current flows through the load and charges the

capacitor shunted across the load.

130

Mode-3[t2,t3]

Switches S1, S3 and S2 are turned off. S4 conducts along with the

body diode of S3. Resonance is established in the secondary. Diode D1

conducts in the secondary. Load is taken care by the transformer’s secondary.

After dead time, S3 is turned on at zero voltage. As the voltage across

secondary is zero, secondary current goes to zero.

6.4 DESIGN PROCEDURE

Design is done with the following parameters:

Resonant capacitor Cr = 1.1µF

Resonant Frequency fr = 38 kHz

rrr CL

f2

1

6

3

101.12

11038

rL

Lr = 15.7 µF

sTPulsewidth r 132

RVI o

o

= 3A

131

Lm = 350 µF

mm Lf ..2

m = 85 k

6.5 SIMULATION RESULTS

6.5.1 Full Bridge Secondary Resonance Converter

Simulink model of Full Bridge Converter with Secondary

Resonance is shown in Figure 6.4. Switching losses in converters increase

with the increase in the switching frequency; whereas inductor volume

decreases with the increase in high frequency operation. To have a trade off

between these two, if the switches are turned on/off at Zero Voltage/Zero

Current, the switching losses can be alleviated. To achieve this, resonance has

to be established in the circuit with the help of a capacitor and an inductor.

Therefore additional components are required. If the isolation transformer is

present, instead of using an additional Inductor for resonance, the transformer

leakage inductance can be used to achieve the resonance. This will minimize

the inductance requirement and also reduce the losses. The transformer’s

secondary leakage inductance is used to achieve resonant, since post

regulation offers better efficiency. Without an auxiliary circuit, zero-voltage

switching on and zero-current switching off are achieved in the entire

operating range. The simulation results for DC-DC converter where

transformer leakage inductance is used for soft switching. DC input voltage

and current are shown in Figure 6.5.

132

Figure 6.4 Simulink model of full bridge converter with secondary

resonance

Simulation waveforms of FB secondary resonance converter are

shown in Figure 6.6. Secondary resonance is used to turn on and turn off the

switches in the primary side. As observed from simulated results, there are no

transients in transformer primary and secondary voltages; hence the switches

are not subjected to voltage stress. It is also observed that resonance is

established in the secondary circuit without additional inductor that is with the

help of leakage inductance. The switches are turned on/off at resonant

condition. Hence the switching losses are reduced which is evident from the

output voltage and current. Therefore the converter has less loss, higher

efficiency. The overall cost and size are less. DC output voltage is 12 V and

output current is 3A. Technical specifications are shown in Table 6.1.

Table 6.1 Simulation parameters of FB with secondary resonance

DC input voltage 48V Llkg 15.7 µHSwitching frequency 38.4kHz Lm 350 µH Cs1 6.8 Pf Cs2 6.8 pF Ci 1000 µF Cr 1.1 µF C0 470 µF R0 4

133

Figure 6.5 (a) DC input voltage (b) Input current

Figure 6.6 (a) Gate pulse, (b) Drain current and (c) Drain source

voltage of switch T1

Figure 6.7 (a) Gate pulse, (b) Drain current and (c) Drain source voltage

of switch B2

134

The switches T1 and B1 are turned on while the current flows

through their body diode. Figure 6.6 confirms the ZVS operation of the

switches over the entire conversion range. From Figure 6.7 it is observed that

there is no overlapping between voltage and current waveforms. Hence the

turn off loss is nearly zero. Simulation waveforms of FB secondary resonance

converter are shown in Figure 6.8. It is also observed that resonance is

established in the secondary circuit without additional inductor that is done

with the help of leakage inductance. DC output voltage and output current are

shown in Figure 6.9.

Figure 6.8 Simulation wave forms of FB with secondary resonance

135

Figure 6.9 (a) DC output voltage and (b) Output current

Though the rectifier ringing and overshoot can be controlled by

using fast recovery diodes, the interaction of the transformer leakage

inductance with the reverse recovery process of the rectifier diode leads to

overshoot in output voltage and current. Because the interaction leads to loss

of duty cycle on the secondary side of the transformer the converter loses

ZVS for wide variation in line and load condition. In order to overcome the

above drawbacks a half bridge secondary resonance converter with C filter is

proposed.

In FB secondary resonance converter both ZVS and ZCS are

achieved, ZVS range is extended, high power density since two numbers of

diodes are reduced when compared to PSRC, no need of output inductor since

transformers secondary leakage inductance is used for resonance. This will

minimize the inductance requirement and reduce the losses. The effective

duty ratio is not reduced, hence the conduction loss due to circuating energy is

low.Though conduction losses are less the converter suffers from voltage and

current spikes at the output. To overcome these draw backs a half bridge

secondary resonance with C filter is proposed.

136

6.5.2 Half Bridge Secondary Resonance Converter With C Filter

The full bridge inverter shown in the previous section is replaced

by a half bridge inverter. The output voltage is controlled by phase shifted

method. The leakage inductance of the transformer is used for resonance

hence no need for external resonant inductor. The reverse recovery currents of

the diodes are reduced hence the voltage stresses of output diodes are clamped

to the output voltage. The half bridge secondary resonance with C filter is

presented in Figure 6.10.Two MOSFET switches are replaced by capacitors.

Therefore MOSFET count and driver count are reduced by two.

Figure 6.10 Simulink model of half bridge secondary resonance

converter with C filter

DC input voltage and current are shown in Figure 611. Simulation

waveforms of FB secondary resonance converter are shown in Figure 6.12. It

is observed that resonance is established in the secondary circuit with the help

of leakage inductance. From Figure 6.13 it is observed that the switches are

turned on softly. Therefore switching loss is almost zero. DC output voltage

and output current are shown in Figure 6.14.

137


Figure 6.12 Simulation wave forms of HB secondary resonance with C

filter

138




139

Figure 6.15 (a) Output ripple voltage and (b) Output current

From Figure 6.13 it is observed that there is no overlapping between current and voltage waveforms hence switching losses are reduced. From Figure 6.14 it is observed that there is no overshoot in the output voltage and is free from voltage and current spikes and from Figure 6.15 it is found that there are ripples in the output voltage.

The peak to peak ripple voltage is found to be 0.2V. The ripples in the output will increase heat in the load. Hence in order to reduce the ripples a half bridge secondary resonance with pi filter is proposed.

6.5.3 Proposed Half Bridge Secondary Resonance Converter With Pi Filter

In order to overcome the drawbacks of Full bridge secondary resonance converter and Half bridge secondary resonance converter with C filter, a Half bridge secondary resonance converter with pi filter is proposed. Figure 6.16 shows the simulink model of the proposed half bridge secondary resonance converter with pi filter. The capacitor filter in the output is replaced by the pi filter. The pi filter is proposed in the output to reduce the ripple. Llkg is increased either by loosely coupled windings or by increasing the number of turns.

140

Figure 6.16 Simulink model of proposed half bridge secondary

resonance converter with pi filter

Ripple voltage Vr,

fCIV d

r 2

Ripple factor r,

LRCLCr

21324

1

Vr - Ripple voltage

r - ripple factor

RL - Load resistance

C1 = C2

C1, C2 - Capacitances of the pi filter

- Reactance

L - Inductance of the pi filter

141

The simulation is done with the following specifications.

Table 6.2 Simulation parameters of HB with secondary resonance

Llkg 15.7 µH Cs1 6.8 pF

Lm 350 µH Cs2 6.8 pF

Ci 1000 µF Cr 1.1 µF

C0 470 µF R0 4

Cb2 200 µF Cb3 200 µF

Lo 1 µH DC input voltage 48V

The capacitor filter in the output is replaced by pi filter. Therefore

the MOSFET count and driver count are reduced by two. DC input voltage

and current are shown in Figure 6.17. It is observed that resonance is

established in the secondary circuit with the help of leakage inductance. From

Figure 6.18 it is observed that the switches are turned on softly. Simulation

waveforms of FB secondary resonance converter with pi filter are shown in

Figure 6.19


142



Figure 6.19 Simulation wave forms of proposed HB secondary

resonance with pi filter

143


Figure 6.21 (a) Output ripple voltage and (b) Output current

Figure 6.18.shows the driving signal, drain source voltage Vds3 and

the current flowing through drain source Ids3 of switches T2 and B1. It can be

observed Ids3 is negative before the arrival of the driving signal, which

assures Vds3 decreases to zero before the switch turning on and achieves ZVS.

From Figure 6.19 it is observed that there are no transients in primary voltage;

hence the switches are not subjected to voltage stress. It is also observed that

resonance is established in the secondary circuit without additional inductor

that is done with the help of leakage inductance. DC output voltage and

144

output current are shown in Figure 6.20. The proposed half bridge secondary

resonance with pi filter has no overshoot in the output voltage and the peak to

peak ripple voltage is reduced to 0.05V as shown in Figure 6.21. The output

current becomes smoother by adding pi filter.

Table 6.3 Performance of the HB secondary resonance with pi filter

for changes in load

% of

load

Output

voltage(V)

Output

current(A)

Output

power(w)

Input

power(w)

Efficiency

(%)

35.5 12.28 0.41 5.03 5.59 89.96

50 12.27 0.49 6.01 6.67 90.17

62.5 12.25 0.61 7.47 8.22 90.89

75 12.23 0.82 10.03 10.98 91.32

87.5 12.23 1.22 14.92 16.29 91.6

100 12.22 2.98 36.42 39.59 92

Table 6.4 Performance of the HB secondary resonance with pi filter for

changes in input voltages

Input

voltage

(V)

Input

current

(A)

Input

power

(w)

Output

voltage

(V)

Output

current

(A)

Output

power

(w)

Efficiency

(%)

40 0.66 26.4 12.1 1.99 24.08 91.21

44 0.82 36.08 12.15 2.73 33.19 92

48 0.82 39.4 12.22 2.98 36.42 92.43

52 0.94 48.88 12.4 3.6 44.64 91.32

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6.5.4 Comparison of open loop FB Secondary Resonance system with

closed loop system for step change in input voltage

The simulink model of open loop FB Secondary Resonance system

is shown in Figure 6.22. A step change in voltage is applied at the input. The

DC input voltage, output current, and output voltage with input step change

are shown in Figure 6.23. When input voltage is increased at 0.4s to a value of

60V, the output voltage also increases and settles at a new value of 16.5V.

Figure 6.22 Open loop system with input step change

Figure 6.23 Results of open loop system with step change in input

(a)Input voltage (b)Output current (c) Output voltage

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The simulink model of closed loop FB Secondary Resonancesystem is shown in Figure 6.24. In order to maintain the required output voltage level, closed loop control is used. The instantaneous voltage signal is taken from the output and given to a comparator. Other input to the comparator is the set voltage of 12V.Output of comparator is the error signal which is given to the PI controller. The output of PI controller is given to the two comparators whose outputs are quasi waves. In order to generate control pulses, the output of PI controller is compared with a triangular reference wave. The generated pulses are used as control signals for the gates of MOSFET T3 as shown in Figure 6.26. The DC input voltage, output current, and output voltage with input step change is shown in Figure 6.25.

Figure 6.24 Closed loop system with input step changes

Figure 6.25 Results of closed loop system with step change in input (a) Input voltage (b) Output current (c) Output voltage

147

The step change is applied at 0.4 seconds for open loop system as

shown in Figure 6.22. From Figure 6.23 it is observed that the open loop

system has steady state error. For the closed loop system shown in Figure

6.24, when the input is increased to 60V at 0.4s the control circuit takes

proper action and the output voltage is maintained at 12 V as shown in Figure

6.25. Set voltage is taken as 12V.The closed loop system reduces the steady

state error. The settling time is 0.75s.

Table 6.5 Parameters of PI controller

Proportional gain(Kp) 0.1

Integral gain(Ki) 30

Output limits [1 e6 – 1e6]

Sample time 50 e-6

Figure 6.26 (a) Output of PI controller (b) Triangular wave (c) Driving

pulse

148

6.5.5 Comparison of Open Loop FB Secondary Resonance System

with Closed Loop System for Output Load Regulation

The simulink model of open loop FB Secondary Resonance system

without output load regulation is shown in Figure 6.27. Input voltage is 48V

DC. A breaker is connected in parallel with the load. Load resistance is 4 .

The breaker is opened at initial state and it is closed at 0.4s. DC output

voltage is shown in Figure 6.28 where the output voltage is increased at 0.4s

due to change in the load.

The simulink model of closed loop FB Secondary Resonance

system for output load regulation is shown in Figure 6.29. Input voltage is

48V DC. Set voltage is 12V DC. In order to maintain the required output

voltage level, closed loop control is used. The instantaneous output voltage

signal is given to a comparator. Other input to the comparator is the set

voltage .Output of comparator is the error signal which is given to the PI

controller. The output of PI controller is given to the two comparators whose

outputs are PWM waves. They are used as control signals for the gate of

MOSFET T3.

Figure 6.27 Open loop system without output load regulation

149

Figure 6.28 DC output voltage with step change in load

Figure 6.29 Closed loop system with output load regulation

Figure 6.30 DC output voltage with output load regulation

150

The breaker is opened at initial state and it is closed at 0.4s.When

the breaker is closed, the output voltage increases to a value of 14V, due to

the action of closed loop system it settles to a value of 12V at 0.67s as shown

in Figure 6.30.

6.6 EXPERIMENTAL RESULTS

The DC-DC converter was built and tested for open loop half

bridge decondary resonance converter with pi filter at 48 V DC. The hardware

layout of half bridge secondary resonance converter with pi filter is shown in

Figure 6.31. The circuit parameters are as follows:

Table 6.6 Experimental parameters of HB with secondary resonance

Llkg 15.7 µH Cs1 6.8 pF

Lm 350 µH Cs2 6.8 pF

Ci 1000 µF Cr 1.1 µF

C0 470 µF R0 4

Cb2 200 µF Cb3 200 µF

The pulses are generated using PIC microcontroller. IR21110 driver

IC is used for isolation and amplification of triggering pulses. IRF840

MOSFET switch is used. Experimental waveform of driving pulses of

switches T2 and B2 is shown in Figure 6.32. The voltage across the primary is

shown in Figure 6.33. The voltage across the secondary is shown in Figure

6.34. Load voltage wave form is shown in Figure 6.35.

151

Figure 6.31 Hardware layout of Half bridge converter with secondary

resonance

Figure 6.32 (a) Driving pulses of B2 (b) Driving pulses of T2

152

Figure 6.33 Primary side voltage of the transformer

Figure 6.34 Secondary side voltage of the transformer

153

Figure 6.35 Load voltage waveform

From the open loop experimental result of half bridge secondary

resonance converter with pi filter the output voltage is 12V as shown in

Figure 6.35 and the output voltage from simulation result is found to be 12V,

hence the experimental results closely agree with simulation results.

6.7 SUMMARY

The conventional FBZVS uses a large leakage inductor to achieve

ZVS. The large leakage inductor increases the circuating energy, thereby

increasing the conduction losses and reducing the effective duty ratio.

Conversely in FB with secondary resonance the transformer leakage

inductance is used to achieve the resonance. This will minimize the

inductance requirement and will reduce the losses. Since the effective duty

ratio is not reduced, the conduction loss from the circuating energy is low.

154

Soft switched Full Bridge DC to DC Converter with Secondary

Resonance is analysed, simulated, tested and results are presented. The

experimental results are similar to the simulation results. Even though the

converter has many advantages, like minimum number of devices, soft

switching of the switches and no output inductor, it suffers from voltage and

current spikes at the output. To overcome these draw backs a half bridge

secondary resonance with C filter is proposed. It is observed that there is no

overshoot in the output voltage but peak to peak ripple voltage is found to be

0.2V. The ripples in the output will increase heat in the load. Hence to reduce

the ripples a half bridge secondary resonance with pi filter is proposed. The

half bridge secondary resonance with pi filter has no overshoot in the output

voltage and the peak to peak ripple voltage is reduced to 0.05V. The output

current becomes smoother by adding pi filter.

The analysis and design consideration of a HB secondary resonance

with pi filter is proposed. The open loop experimental results coincide with

the simulation results. The efficiency under full load is 92 %. The converter is

adequate for low power applications since it has minimum number of devices

and soft switching ability. The absence of an external resonant inductor makes

the converter cost optimal.

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