Indian Journal of Science and Technology, Vol 9(16), DOI: 10.17485/ijst/2016/v9i16/76672, April 2016
ISSN (Print) : 0974-6846
ISSN (Online) : 0974-5645
FPGA-based Digitally Controlled Isolated Full-Bridge
DC-DC Converter with Voltage Doubler (IFBVD)
J. Divya Navamani, K. Vijayakumar and A. Lavanya
EEE Department, SRM University, SRM Nagar, Potheri, Kattankulathur, Kancheepuram - 603203, Tamil Nadu, India;
divyateddy1@gmail.com, lavanya.a@ktr.srmuniv.ac.in, kvijay_srm@gmail.com
Abstract
An isolated full-bridge dc-dc converter with voltage doubler (IFBVD) is mainly used in higher power applications. This
paper presents the testing and implementation of a digitally controlled Random pulse width modulated isolated fullbridge DC-DC converters with voltage doubler (IFBVD). Different PWM techniques for full-bridge DC/DC converter are
investigated. Field Programmable Gate Array (FPGA) is used to obtain different PWM techniques. It is most commonly used
in designing PWM Generator for Power Converter Applications. A comprehensive comparison between the various PWM
techniques, including current stress, ripple and efficiency, is presented. Simulation is done using Matlab/Simulink. An
experimental setup has been provided to validate the theoretical and simulation results using FPGA control. Experimental
results of a 0.5kW, 50kHz prototype are presented.
Keywords: DC-DC Converter, FPGA, Phase Shift, Random PWM, Ripples
1. Introduction
Isolated dc-dc converters are the critical component
in power conditioning system of high voltage and high
power application. Figure 1 shows few use of isolated dcdc converter. The Transformer is commonly employed
in the converter for isolation and safety. The major
disadvantage of using the transformer in dc-dc converters
are high current stress, low efficiency, magnetic flux
saturation etc1,2. Unidirectional and Bidirectional
isolated dc-dc converters are introduced for automotive
applications3,5. Many multilevel converters are proposed
for many applications6. Different categories of isolated
dc-dc converters are flyback, half bridge, full bridge,
push-pull and forward converters. Most predominately
used an isolated converter for high voltage application
is full-bridge converter17,18. Several novel topologies are
introduced in modifying the full-bridge dc-dc converter.
Voltage input and current output full-bridge converter
is proposed and studied7. To improve the stability of
the converter, duty cycle exchanging control, individual
voltage and current loop are studied8. Recently multiport
* Author for correspondence
bidirectional full bridge dc-dc converters are introduced
for multiple energy interface10. Current fed full bridge dcdc converters are used specifically for fuel cell application
where low input current ripple is required11. Many ZVS
and ZCS techniques are introduced in the full bridge
topologies to improve the performance of the switch12.
Figure 1. Application isolated dc-dc converter.
Improvement in efficiency and reduction in harmonics
can be achieved by incorporating different pulse width
modulation technique. Commonly used PWM method
for full bridge dc-dc converter is phase shifted pulse width
modulation technique9. Random pulse width modulation
is initially used for inverter circuits. However, the random
modulation schemes in dc-dc converters are not widely
used so far. Different types of random pulse modulation
FPGA-based Digitally Controlled Isolated Full-Bridge DC-DC Converter with Voltage Doubler (IFBVD)
are random carrier frequency modulation with fixed
duty13, random carrier frequency modulation with
variable duty14 and random pulse position modulation15.
For our study we took random pulse width modulation16,
conventional PWM, Phase shifted PWM and compared.
This paper is organized as follows. Section I gives the
introduction about the full bridge dc-dc converter and
different pulse width modulation technique. Section II
briefs about the two isolated full-bridge topologies taken
for analysis. Section III gives efficiency analysis. Section
IV shows the comparison of current stresses. Section V
provides the hardware implementation, and Section VI
concludes the results.
2. Isolated Full Bridge DC-DC
Converter
An isolated full-bridge dc-dc converter is mainly used
in high voltage applications. Its primary function is
to boost or buck down the voltage. In hybrid vehicles,
these converters are required to boost up the low voltage
derived from the source such PV array, Fuel Cell, etc.
Isolation is obtained by the use of the transformer. The
predominant use of the transformer is to provide isolation,
and the other advantages are the incorporation of the
high-frequency transformer to reduce the transformer,
minimization of current and voltage stresses and when
a significant step-up or step-down conversion ratio is
needed1,2.
An unidirectional full bridge dc-dc converters are
mainly classified into two types:1. Isolated Full-Bridge
DC-DC Converter with Diode Rectifier (IFBDR) as
shown in Figure 2(a). Full-Bridge DC-DC Converter with
Voltage Doubler (IFBVD) as illustrated in Figure 2(b).
Table I gives the comparison between IFBDR and IFBVD.
Figure 2(b). Isolated Full Bridge DC-DC Conveter
with Diode Rectifier (IFBDR).
Table 1. Comparison of IFBDR and IFVD
Sl. Parameters
IFBDR
No
1
voltage gain
n
2
Number of switches 4 power switches,4 diodes
3
4
Number of passive
components
voltage rating
IFBVD
1 capacitor
n/(1-D)
4 power
swiches, 2
diodes
2 capacitor
Higher Voltage
Rating
Lower Voltage Rating
3. Efficiency of IFBVD
The converter efficiency is simulated and calculated as,
æ P ö
÷(1)
h = 100ççç1 - loss ÷
÷
÷
ç P ø
÷
è
input
Ploss= Psw+Pcond+Pdiode+PTR(2)
é æ 1ö
ù
æ 1ö
÷
÷
ççI + ÷
ê1 ççI - ÷
ú (3)
V
V
1
÷
÷
ç 2 ÷ in .t + çç
2 ÷ in .t ú. f
Psw = 4 ê
÷
r
f ú sw
ê çç
çç 2 ÷
÷
n
n ú
2
÷
÷
ê2 çç 2 ÷
÷
÷
ç
÷
÷
ç
çè
ê
ú
ø
ø
ëè
û
2
Pcond = 4.Ron .I
2
rms
æ 2- D ö
(4)
÷
÷
= 4.Ron ççI .
÷
÷
ç
2 ø
è
2
2
Pdiode = 4(VFO .I rms
+ Rdiode .I rms
) 5)
I rms =
1 1 - D (6)
n
2
2
2
æ1
ö PTR = RPRI (I 1 - D ) + Rsec çç 1 - D ÷
÷
÷
èn
ø
Figure 2(a). Isolated Full Bridge DC-DC Conveter
with Voltage doubler (IFBVR).
2
Vol 9 (16) | April 2016 | www.indjst.org
(7)
where, Ploss= Converter Total Loss.
Indian Journal of Science and Technology
J. Divya Navamani, K. Vijayakumar and A. Lavanya
Psw = Switching losses.
Pcond = Conduction loss of switch.
Pdiode = Diode Losses (Voltage doubler).
PTR = Transformer Losses.
tr = Rise time of the switch= 100ns.
tf = Fall time of the switch = 100ns.
Using equation 2,3,4,5 and 7 the losses of the converters
are calculated for different PWM technique and the
efficiency is determined. Figure 3 shows the comparison
of efficiencies of Conventional PWM, Traditional phase
shift and random PWM technique. The effectiveness of
the random PWM technique is quite small compared to
other techniques due to significant increase of switching
count.
to conventional PWM. Table 2 and 3 gives comparison
of efficiency and current stress of IFBVD with different
PWM technique.
Figure 4. Comparison of current stress of IFBVD with
different PWM technique.
Table 2. Efficiency comparison
Sl.no
1
2
3
IFBVD
CONVENTIONAL PWM
PHASE SHIFT TECHNIQUE
RANDOM PWM
Efficiency
92.15%
97.62%
84.62
Table 3. Current Stress comparison
Figure 3. Comparison of efficiency of IFBVD with
different PWM technique.
4. Current Stress Analysis of
IFBVD
Current Stress for conventional PWM technique=
nV2
(2D - 1 + k )
4fs L
(8)
where, k =
1 - 2D
1- D
Current stress for random PWM technique==
nV2
{k(1 - D1 ) + (2D1 + 2D2 - 1)}
4fs L
(9)
Equation 8 gives the current stress for conventional
PWM technique. Equation 9 gives the current stress
for Random PWM technique. Figure 4 shows that the
current stress is slightly less in Random PWM compared
Vol 9 (16) | April 2016 | www.indjst.org
Sl. no IFBVD
1
CONVENTIONAL PWM
2
RANDOM PWM
Current stress
1.38
1.2
5. Hardware Implementation
5.1 Digital Control Techniques for DC-DC
Converters:
Many digital control techniques are implemented and
analyzed for non-isolated and isolated dc-dc converter4.
Recently, it attracts many researchers to use those
techniques for pulse width modulation. Last two decades,
microcontroller occupies a definite place in generating a
pulse for the dc-dc converter due to their simplicity and
cost effectiveness. However, the flexibility and reliability
of FPGA attract the power electronic engineers to use
the device for controlling the converter and also for
random pulse generation. An open and closed loop of
a dc-dc converter is performed with the help of FPGA.
Other digital controllers used as the compensator for dcdc converters are DSP, ASIC4 and dspace. To study the
stability of the converter, many control system techniques
Indian Journal of Science and Technology
3
FPGA-based Digitally Controlled Isolated Full-Bridge DC-DC Converter with Voltage Doubler (IFBVD)
are used, and the compensator is obtained from the
derived transfer function. It can be implemented through
the above-mentioned methods. In this work, we compared
the different PWM techniques using PIC and FPGA.
The circuit diagram to interface driver circuit with PIC
16F877A is shown in Figure 5. Table 4 gives the simulation
and prototype data. Figure 5(a) shows the hardware
circuit for generating pulse with PIC microcontroller. In
Figure 6, a voltage of 15 V DC is fed to the FBVD through
a step down transformer and a bridge rectifier. The
converter circuit consists of four power MOSFETS. The
function of the converter is to convert DC voltage into
AC voltage. The AC voltage is fed to a step up transformer
of turns ratio 1:2.The output voltage from the transformer
will be 30V AC and which is further given to the voltage
doubling circuit. The voltage increasing circuit consists of
two diodes and two capacitors in parallel which doubles
and rectifies the 30 V AC into 70 V DC across the load of
5.6-kilo ohm.
Table 4. Hardware Parameters
Parameter
R = 5.6k
TR = 1:2
C = 0.1
fs= 50kHz
D = 0.5
VIN= 15 V
Vout= 60-90 V
Description
R - load resistance
TR – Turns Ratio
C- output capacitance
fs- switching frequency
D - duty ratio
VIN- input voltage
Vout - output voltage
Figure 5. Circuit diagram to interface driver circuit
with PIC 16F877A.
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Vol 9 (16) | April 2016 | www.indjst.org
Figure 5(a). Pulse generating circuit using PIC
microcontroller.
Figure 6. Hardware circuit of FBVD using PIC
Microcontroller for Conventional PWM Technique.
Figure 7 shows an FPGA kit used to generate random
PWM pulses to the switches of isolated full-bridge DCDC converter with voltage doubler (IFBVD). Figure 8
and 9 show the corresponding hardware result. Harmonic
analysis of different random PWM scheme for dc-dc
converter is given by Yash Shrivastava. With a relatively
low degree of random nature, the RPPM system slightly
reduces the discrete harmonics when compared with
the deterministic PWM scheme, but creates low-level
continuous noise19. Figure 10-13 shows that the simulation
results are validated with hardware results. Figure 14 and
15 shows the transformer primary and secondary voltage.
Figure 7. FPGA Kit-Spartan-3A.
Indian Journal of Science and Technology
J. Divya Navamani, K. Vijayakumar and A. Lavanya
Figure 8. Conventional PWM pulses-Hardware result.
Figure 9. Output across the load with Random l
PWM Technique-Hardware result.
Figure 10. Output across the load with Random
PWM Technique-Simulated result.
Figure 12. Random PWM pulses using FPGA.
Figure 13. Output across the load with Conventional
PWM Technique-Hardware result.
Figure 14. Transformer primary voltage.
Figure 11. Random PWM pulses-Simulation result.
Figure 15. Transformer secondary voltage.
Vol 9 (16) | April 2016 | www.indjst.org
Indian Journal of Science and Technology
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FPGA-based Digitally Controlled Isolated Full-Bridge DC-DC Converter with Voltage Doubler (IFBVD)
Comparative study on different PWM technique
with isolated full-bridge DC-DC converter with voltage
doubler are analyzed and tabulated (Table 5). Ripple
comparison is shown in Figure 16 and 17. Hardware results
are obtained to verify the performance of the converter.
Figure 18 and 19 shows the ripples obtained from the
converter in random and conventional pulse width
modulation respectively. Ripples are reduced in random
PWM compared to conventional PWM technique.
Table 5. Comparison between different PWM
Techniques
Parameters
Conventional
PWM
Technique
Efficiency
High
Ripples
Very High
Current Stress High
Switching
Low
Loss
Conduction
Low
Loss
Traditional
Phase Shift
Technique
Very High
High
Low
Very Low
Random
PWM
Technique
Low
Low
Low
High
Very Low
High
Figure 18. Ripples in Random PWM techniqueHardware results.
Figure 19. Ripples in Conventional PWM techniqueHardware results.
6. Conclusion
Figure 16. Ripple Analysis With Different PWM
Techniques In Isolated Full Bridge Voltage Doubler
(IFBVD)-Simulation Results.
Figure 17. Ripple Analysis with Different PWM
Techniques in Isolated Full Bridge Voltage Doubler
(IFBVD)--Hardware Results.
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Vol 9 (16) | April 2016 | www.indjst.org
This paper gives a comparative investigation in the level
of harmonics, ripples of three pulse width modulation
schemes that are applied to isolated full bridge dcdc converters operating in Continuous Conduction
Mode (CCM). As expected, higher harmonic spectral
lines in the input current were reduced and; therefore,
the negative influence on the source is reduced as well.
Experimental and simulation results are almost identical,
which confirms the simulation model of the converter.
Efficiency results show there is significant efficiency drop
of the random PWM technique due to the large increase of
switching count. After a thorough analysis of FBVD with
different PWM techniques, it has been concluded that the
random PWM method is best suited for application in
converters where minimum EMI interference is preferred.
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