Design of Floating Gate Power Supply of Half-Bridge Circuit Using ...

Design of Floating Gate Power Supply of Half-Bridge Circuit Using
Snubber Charge
Se-Kyo Chung, Jung-Gyu Lim, Hwi-Beom Shin *, and Hyun-Woo Lee **
* Department of Electrical and Electronics Engineering
Gyeongsang National University, Jinju, Gyeongnam 660-701, Korea
** Department of Electrical Engineering
Kyungnam University, Masan, Gyeongnam, Korea
Abstract
This paper deals with a bootstrapped power supply using a snubber energy recovery for a high-side gate driver of a
half-bridge inverter. The proposed circuit utilizes the snubber capacitor in the low-side power switch for charge
pumping to a bootstrap capacitor used for a high-side floating power supply. By employing the proposed circuit, a
simple floating power supply for a half-bridge inverter can be implemented without any magnetic components.
Moreover, the power dissipation in the RCD snubber can be reduced by the energy recovery from the snubber capacitor.
The operation principle and design method of the proposed circuit are presented. The simulation and experimental
results are also provided to show the validity of the proposed circuit.
Keywords: Gate driver, bootstrap circuit, snubber, energy recovery, half-bridge circuit
1 INTRODUCTION
A half-bridge topology is used as a basic building
block for power conversion circuits such as single- and
three-phase full-bridge converters. However, two
isolated power supplies are generally required to
control the gates of both high- and low-side power
semiconductor switches because the high-side switch
has a floating ground. This is a problem of significance
in miniaturizing power converter circuits. The bootstrap
and charge pump techniques have been considered as
possible solutions to overcome this problem [1]-[5].
A RCD snubber has been used to relieve a switching
stress of a power semiconductor device during a
turn-off transition. Its configuration is simple but the
power dissipation in the snubber resistor degrades
efficiency of the power converter. Non-dissipative
snubber circuits using energy regeneration have been
presented [6], [7]. However, these circuits need
additional magnetic components, such as transformers,
for energy regeneration from the snubber capacitor.
This paper deals with a bootstrapped power supply
using snubber energy recovery for the high-side gate
driver of a half-bridge inverter. In the proposed circuit,
the snubber capacitor in the low-side power
semiconductor switch is utilized for charge pumping to
a bootstrap capacitor as well as original snubbing action.
Thus, a simple floating gate power supply for a
Fig.1 Proposed circuit.
half-bridge inverter can be implemented without any
magnetic components. Moreover, the power dissipation
in the RCD snubber can be remarkably reduced by the
snubber energy recovery. The operation principle,
design method and characteristic analysis of the
proposed circuit are presented in this paper. The
simulation and experimental results are finally provided
to show the validity of the proposed circuit.
2 PROPOSED CIRCUIT
A. Circuit configuration
Fig. 1 shows a half-bridge inverter with the proposed
circuit which consists of the RCD snubber (R s , C s and
D s ), bootstrap circuit (C b and D b ), zener diode (D z ) and

initial charging circuit (D i and R i ). The MOSFETs are
considered for the power semiconductor switches (S 1
and S 2 ). The low-side gate driver is supplied from the
independent DC source (V DC ) with a common ground.
The details of the low-side circuit are omitted for
simplicity.
B. Operation of proposed circuit
The operation of the proposed circuit can be
explained using Fig. 2 which describes the operating
modes of the proposed circuit. Each operating modes
are explained as follows:
• Initial charging: If the bootstrap and snubber
capacitors are initially uncharged, it is required to
charge one of two capacitors for starting up. As shown
in Fig. 2(a), the low-side switch S 1 is first turned-on to
charge C b using the power supply V DC , where the
voltage across C b after initial charging is given as
V = V −V
−V
. (1)
cb, init DC Di S1(
on)
• Mode 1 (snubber capacitor charging): In this mode,
after S 1 is turned-off, S 2 is turned-on by using energy
stored in C b as shown in Fig. 2(b). The capacitor C s is
operated as a snubber during the turn-off transition and
charged by a surge and bus voltages. The stored charge
in C s is given as
Q = C v . (2)
cs
s
cs
As increasing the voltage across S 1 to V bus −V S2(on) , a
bootstrap action occurs. The voltage potential at the top
of C b (v cb + ) is highest in whole circuit. The current to
maintain the on-state of S 2 should be supplied from C b
during this mode.
• Mode 2 (charge pumping): As shown in Fig. 2(c), S 2
is turned-off and S 1 is turned-on in this mode. The
bootstrap capacitor C b is recharged by charge pumping
from C s through D b . The voltage v cb is limited by a
zener diode to V z . The charge stored in C b in this mode
is given as
Q = C V
(3)
cb
b
z
• Mode 3 (snubber capacitor reset): The energy
transfer from C s to C b is completed if the snubber
capacitor voltage is down to v
cs
= Vz
+ VDb
+ VS1 ,( on)
and
the diode D b is turned off. After that, the charge
remained in C s is dissipated in R s for the next snubbing
action.
In the operation of the proposed circuit, the initial
charging is needed once, only for starting up. Then, the
Modes 1, 2 and 3 are repeated. After the initial charging,
V DC is automatically disconnected from the proposed
circuit if the zener breakdown voltage is chosen to be
satisfied the condition given as
V > V −V
−V
. (4)
z
DC
Di
S1(
on)
(a) Initial charging
(b) Mode 1
(c) Mode 2
(d) Mode 3
Fig. 2 Operation of proposed circuit.

3 DESIGN OF PROPOSED CIRCUIT
A. Bootstrap and snubber capacitor values
The values of the bootstrap and snubber capacitors
can be calculated using the required charges and
leakage currents of the high-side circuit. The amount of
the charge required in C b to drive S 2 during Mode 1 is
represented as [1], [2]
I + I + I + I
qbs lgs lcb zk
∆ Qcb
= Qg
+ Qls
+
. (5)
f
The minimum value of C b can be determined using (5)
as
∆Qcb
Cb
≥ . (6)
∆v
cb
where ∆v cb is the desired ripple voltage in C b during
Mode 1 defined by ∆ v = V −V
. It is known from
cb
z
cb , min
(6) that choosing larger value of C b results in the lower
ripple voltage. The minimum voltage V cb,min should be
greater than a turn-off threshold voltage of S 2 .
Since the charge of C b is supplied from C s during
Mode 2, the amount of the charge required in C s can
represented as
Ilcs
∆ Qcs = ∆Qcb
+ . (7)
f
The minimum value of C s can also be determined using
(7) as
∆Qcs
Cs
≥ (8)
∆v
where
cs
cs
∆ v = V −V
−V
−V
−V
−V
. (9)
bus
S 2(
on)
Ds Ds z S1(
on)
The value of Cs should meet the snubber requirement
B. Diode ratings
Under the above design, the charge supplied from C s
to the high-side circuit is given as
∆ Q ' = C ∆v
≥ ∆Q
. (10)
cs
s
cs
Since the charge consumed in the high-side circuit
during Mode 1 is ∆ Q , the excess charge not stored in
cb
C b can be represented as
∆ Q = ∆Q
' −∆Q
. (11)
e
cs
cb
This charge is dissipated in D z and S 1 . The average
power loss by the dissipation of ∆Q
in D z and S 1 can
be calculated as
P = ∆Q
f ⋅V
(12)
z
P
e
z
cb
e
2
( ∆Qe
f ) ⋅ RDS1(
)
= ∆Q
f ⋅V
=
. (13)
S1 e S1(
on)
on
The average currents of the bootstrap and snubber
diodes D b and D s can be derived, respectively, as
I = ∆Q
' f
(14)
Db
cs
I = Q f . (15)
Ds cs
The peak current rating is important for the initial
charging diode D i , which is calculated as
I
Di,
peak
V −V
R + R
DC Di
= . (16)
i
Ds ( on)
The peak inverse voltages of D b , D s and D i can also be
derived, respectively, as
V
Db, PIV
= Vz
+ VDs
(17)
V = V −V
−V
. (18)
Ds, PIV bus S 2( on)
s1(
on)
Di, PIV
= Vbus
−VS
2( on)
+ Vz
−VDC
. (19)
V
C. Snubber resistor
In Mode 3, the remained charge in C s is dissipated in
R s and C s is then reset, where the reset time constant is
given as τ = C R . The power loss in R s s can be
calculated as
s
2
( V + V + V ) ⋅ f
1
P = C
(20)
Rs
s z Db S1(
on)
2
It is noted that the power loss in R s is independent to the
bus voltage.
4 SIMULATIONS AND EXPERIMENTS
A. Simulation and experimental conditions and device
parameters
The simulations and experiments were carried out to
show the validity of the proposed circuit. Fig. 3 shows a
test circuit used in both simulation and experiment. The
MOSFET IXFH58N20 and fast recovery diode DSEI8
by IXYS were used for the power switches (S 1 and S 2 )
and diodes (D i , D b and D s ), respectively. The IR2110 by
International Rectifiers was used as the gate drive for S 1
and S 2 . The device parameters used in the simulation
and experiment are summarized in Table 1.
Fig. 3 Test circuit.
The capacitor values can be calculated from (6) and
(8) as C b ≥93.3nF and C s ≥9.7nF for ∆v cb =3V. The
standard capacitor values with a margin were used in
the simulation and experiment as C b =100nF and
C s =22nF. Since metalized polyester capacitors with a
small equivalent series resistance (ESR) were used for
C b and C s , the leakage currents were neglected in this
calculation. However, if electrolytic capacitors with a
large ESR are used, the leakage currents should be
considered. Since the power dissipation in R s is
calculated as 0.03W, 10Ω/0.25W resistor was used.

Table 1 Experimental conditions and device parameters
Item Value
Experimental condition f 10kHz
s V bus 50V
MOSFET
Zener diode
Fast recovery diode
Gate driver
Q g
I lgs
R DS(on)
V z
I zk
t rr
225nC
100nA
40mΩ
15V
250uA
35ns
V Di , V Db , V Ds 1.0V
I qbs
Q ls
230uA
5nC
V gs1
0
V gs2
5V/div 0
V cs
10V/div
0
V cb
2.5V/div
0
V ds2
B. Simulation and experimental results
The simulation was carried out using the Pspice. Fig.
4 shows the simulation results for the test circuit as
shown in Fig. 3. The gate-source voltages of S 1 and S 2 ,
voltages across C s and C b , and drain-source voltage of
S 2 are shown from the top. It is shown in this figure that
the high-side power switch is well controlled by using
the proposed circuit.
10V/div
0
50us/div
Fig. 5 Experimental result for proposed circuit
(Duty=50%)
V gs1
15V
V gs2
0
V gs2
0
V gs1
V gs2
10V
5V
V gs1
5V/div
0V
V cs
60V
10V/div
V cs
40V
0
20V
0V
V cb
V cb
15V
10V
5V
2.5V/div
0
0V
V ds2
60V
10V/div
V ds2
40V
20V
0V
1.0ms 1.05ms 1.10ms 1.15ms 1.20ms 1.25ms 1.30ms
0
50us/div
Fig. 6 Experimental result for proposed circuit
(Duty=90%)
Fig. 4 Simulation result.
Figs. 5 through 7 show the experimental results for
the test circuit. Fig. 6 shows the experimental results
when a duty ratio of the high-side switching signal is
given as 50%. It is shown in this figure that the
proposed circuit well works as predicted in the
simulation result. The voltage decrease in C b is
dominant at the turn-on transition of S 2 . Thus, it can be
known that the gate charge Q g is the most important

20V/div
V cb
0
2.5V/div
0
V cb +
V cs
Fig. 7 Energy transfer from C s to C b .
0.5us/div
parameter in the design of the proposed circuit. It is
impossible for bootstrapped power supplies to achieve a
duty of 100% because the low-side switch must be
turned-on to charge the bootstrap capacitor for one
switching interval. Therefore, the operation under a
high duty ratio is important for this type of the power
supply. Fig. 6 shows the experimental results under a
duty ratio of 90%. Fig. 7 shows the energy transfer
from C s to C b during the turn-on transition of S 1 , where
vcb+ means the voltage between the top of C b and
common ground. It is shown in this figure that C b is
charged during Mode 2 and C s is reset during Mode 3.
5 CONCLUSIONS
A floating gate power supply of a half-bridge circuit,
using an energy recovery of a snubber capacitor, has
been proposed. The proposed circuit can be simply
implemented without any magnetic components.
Moreover, the power dissipation in the RCD snubber
can be reduced by the energy transfer to the high-side
supply. It is, therefore, expected that the proposed
circuit will be used for small-sized and low cost power
conversion circuits using a half-bridge topology.
ACKNOW LEDGEMENTS
This work was financially supported by MOCIE
through IERC program.
V s2(on) =on-state voltage drop of S 2
v cs =snubber capacitor voltage
V bus =DC bus voltage
V z =zener breakdown voltage
V Db =forward voltage drop of the bootstrap diode
V Ds =forward voltage drop of the snubber diode
Q g =gate charge of S 2
Q ls =level shifter charge of the gate driver
I qbs =gate driver bias current
I lgs =gate-source leakage current of S 2
I lcb =leakage current of the bootstrap capacitor
I zk =zener diode bias current
I lcs =leakage current of the snubber capacitor
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LIST OF SYMBOLS
V DC = voltage of the low-side DC source
V cb,init = initial charging voltage of the bootstrap
capacitor
V Di = forward voltage drop of the initial charging diode
V s1(on) =on-state voltage drop of S 1