AN2626
Application note
MOSFET body diode recovery mechanism in a phase-shifted
ZVS full bridge DC/DC converter
Introduction
The ZVS exploits the parasitic circuit elements to guarantee zero voltage across the
switching device before turn on, eliminating hence any power losses due to the
simultaneous overlap of switch current and voltage at each transition [1].
In order to allow the ZVS condition, the intrinsic body diode of the MOSFET has to conduct;
in no or low load operation the extremely low reverse voltage, could be not sufficient to
guarantee the reverse recovery charge sweep out before turning off the MOSFET. Hence,
the body diode could be stressed by high dv/dt that latching the parasitic internal bipolar
transistor brings the MOSFET to the failure.
In the market of power applications like telecom power supply, main frame computer-server,
welding and steel cutting, the demand of power density is growing each year. Increasing
power density means reducing component counts, power losses, heat-sink and reactive
component size. The alternative to the hard switched full bridge, typical topology for these
applications, was the phase-shifted zero voltage switching (ZVS) full bridge. This ZVS
technique guarantees zero voltage across the switching device before turn on, eliminating
hence any power losses due to the simultaneous overlap of switch current and voltage at
each transition.
By this switching technique also at high frequencies, the switching losses are low; hence it
allows the reduction of the components reactive size only. Obviously, by having lower losses
lower heat-sink size is allowed. Furthermore, by avoiding the hard-switching condition the
EMI/RFI noise is reduced.
The zero voltage condition occurs by the intrinsic MOSFET body diode conduction; an
extremely low reverse voltage, occurring at no or low load operation, which could be not
sufficient to guarantee the reverse recovery charge sweep out before turning off the
MOSFET. In this condition high dv/dt values could turn on the intrinsic bipolar and destroy
the MOSFET.
The deep studies of these failure mechanisms have led STMicroelectronics to design new
technology in order to develop MOSFETs really suitable for high power phase-shifted ZVS
applications. In this technical note we will investigate the possible triggering on of the
internal parasitic bipolar.
September 2007
Rev 1
1/13
www.st.com
Contents
AN2626
Contents
1
2
3
4
5
Topology description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
MOSFET body diode recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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AN2626
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Phase-shifted ZVS full bridge circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Switching sequence in a P-S ZVS FB converter DC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Typical waveforms in a P-S ZVS FB converter DC/DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Current flow into the parallel body diode-channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . 7
Current flow into the parallel body diode-channel MOSFET. . . . . . . . . . . . . . . . . . . . . . . . . 8
Current flow into the channel MOSFET (first quadrant) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Current flow due to the reverse recovery of the body diode . . . . . . . . . . . . . . . . . . . . . . . . 9
Comparison between standard and fast diode technology . . . . . . . . . . . . . . . . . . . . . . . . . 10
A typical leading leg MOSFET waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
A typical lagging leg MOSFET waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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Topology description
AN2626
1
Topology description
The basic circuit of the phase-shifted converter is composed by four switches; two for each
"leg".
The switches, labeled Q1 through Q4 in the
Figure 1
are shunted by their intrinsic body
diode (D1 through D4) and intrinsic output capacitance (C1 through C4), shown separately
in order to clarify their role in the global functioning.
Figure 1.
Phase-shifted ZVS full bridge circuit
Figure 2.
Switching sequence in a P-S ZVS FB converter DC/DC
V
gate
Q
1
Q
2
Q
3
Q
4
t
4/13
AN2626
●
Topology description
Transition t
0
-t
1
From the
Figure 3,
at t
0
Q1 and Q3 are in on state and on the primary side there is an
energy equal to:
Equation 1
E
t
0
–
t
1
1
-
=
E
mag
–
E
output
–
reflected
+ --
(
L
leak
+
L
res
) ⋅ (
I
mag
(
t
0
)
+
NI
out
(
t
0
) )
2
Instantly, at t
0
the switch Q1 is turned off and the resonant transition begins. The primary
current continues to flow, with a like-linear shape (since the current is forced by the output
inductor) using the charge stored in the switch output capacitance C2. Simultaneously, the
primary current will charge the output capacitance C1 of Q1 from essentially 0 V to the
supply voltage V
dd
, and will discharge the output capacitance C2 from V
dd
to zero. The
transition finishes when the Q2 source voltage exceeds the Q2 drain voltage sufficiently to
directly bias the internal body diode D2.
●
Freewheeling t
1
-t
2
Now D3 is directly biased and the output inductor freewheels. The voltage across the switch
Q2 is equal to the drop on its internal body diode D2 hence the ZVS condition is verified.
The switch Q3 is turned on and the current, flowing through the primary side, now is shared
between the body diode D2 and the channel of MOSFET Q2. During the freewheeling state
both output rectifier diodes are forward biased, and hence the reflected voltage to the
primary is null.
Figure 3.
Typical waveforms in a P-S ZVS FB converter DC/DC
V
A
V
B
V
A -
V
B
I
PI
t
0
t
2
t
4
t
1
t
3
●
t
5
t
Transition t
2
-t
4
The switch Q3 is turned off, the energy available to complete the transition is:
Equation 2
E
t
2
–
t
4
1
-
= --
(
L
leak
+
L
res
) ⋅ (
I
mag
(
t
2
)
+
NI
out
(
t
2
) )
2
It is much lower than it was in the lead leg, since the magnetizing and output inductance do
not contribute [4]. For this reason it is easier to miss the ZVS condition. If the energy stored
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