U.S. patent application number 09/873506 was filed with the patent office on 2001-11-15 for method for reducing losses during the commutation process.
This patent application is currently assigned to SIEMENS AG. Invention is credited to Baudelot, Eric, Bruckmann, Manfred, Mitlehner, Heinz, Weis, Benno.
Application Number | 20010040813 09/873506 |
Document ID | / |
Family ID | 7889915 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040813 |
Kind Code |
A1 |
Baudelot, Eric ; et
al. |
November 15, 2001 |
Method for reducing losses during the commutation process
Abstract
The invention relates to a method for reducing losses during the
commutation of a free-running, driven power converter valve (T2) of
an invertor phase (2) to a current-accepting power converter valve
(T1) of said invertor phase (2). According to the invention, the
current-accepting power converter valve (T1) is switched on at the
beginning of the commutation process and the free-running, driven
power converter valve (T2) is rapidly switched off as soon as the
value of its drain voltage (U.sub.D) is zero. The losses during the
commutation process can thus be significantly reduced in a simple
manner.
Inventors: |
Baudelot, Eric; (Weisendorf,
DE) ; Bruckmann, Manfred; (Nurnberg, DE) ;
Mitlehner, Heinz; (Uttenreuth, DE) ; Weis, Benno;
(Hemhofen, DE) |
Correspondence
Address: |
BAKER & BOTTS
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
|
Assignee: |
SIEMENS AG
|
Family ID: |
7889915 |
Appl. No.: |
09/873506 |
Filed: |
June 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09873506 |
Jun 4, 2001 |
|
|
|
PCT/DE99/03707 |
Nov 22, 1999 |
|
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Current U.S.
Class: |
363/40 |
Current CPC
Class: |
H03K 17/08148 20130101;
H02M 7/53803 20130101; H02M 1/0048 20210501; Y02B 70/10 20130101;
H03K 17/08142 20130101 |
Class at
Publication: |
363/40 |
International
Class: |
H02M 001/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 1998 |
DE |
198 55 900.3 |
Claims
We claim:
1. A method for reducing losses during the commutation of a
free-running, driven power converter valve of an invertor phase to
a current-accepting power converter valve of said invertor phase
wherein the current-accepting power converter valve is switched on
at the beginning of the commutation process, and wherein the
free-running, driven power converter valve is switched off as soon
as the value of its drain voltage (U.sub.D) is zero.
2. The method according to claim 1, wherein, at the beginning of
the commutation process, the free-running, driven power converter
valve has a gate voltage which is decreased until its drain voltage
is equal to a predetermined reference voltage.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a method for reducing losses during
the commutation of a free-running, driven power converter valve of
an invertor phase to a current-accepting power converter valve of
said invertor phase.
[0002] The publication entitled "Use of the MOSFET Channel Reverse
Conduction in an Invertor for Suppression of the Integral Diode
Recovery Current", printed in the Conference Report "The European
Power Electronics Association", 13. to 16.09.1993, in Brighton,
pages 431 to 436, discloses a method by which losses are reduced
during the commutation process. This known method is used in a
polyphase invertor having Metal-Oxide Semiconductor Field-Effect
Transistors (MOSFETs) as power converter valve.
[0003] MOSFETs are unipolar power semiconductors which are able to
carry current in both directions. Every MOSFET has a parasitic
bipolar freewheeling diode reverse-connected in parallel, said
diode generally being designated as inverse diode. This
freewheeling diode has properties which are not optimal for the
operation of the power converter valve, since it cannot be produced
as a separate chip in a separate process. It is an integral part of
the MOSFET. This inverse diode has a non-optimal on-state behavior
and non-optimized stored charge.
[0004] A known circuit of an invertor phase 2, which has a MOSFET
in each case as power converter valves T1 and T2, is illustrated in
more detail in FIG. 1. The antiparallel bipolar freewheeling diode
of the power converter valve T1 and T2 is designated by RD1 and
RD2, respectively. On the DC voltage side, this invertor phase 2 is
linked to a DC voltage source 4 across which a DC voltage U.sub.ZK
is dropped, the latter also being designated as intermediate
circuit voltage. The junction point 6 between the two power
converter valves T1 and T2 that are electrically connected in
series forms an AC connection to which a load can be connected. The
MOSFETs used are normally off MOSFETs, which are designated as
enhancement-mode MOSFETs. In n-channel enhancement-mode MOSFETs, a
drain current flows only when the gate-source voltage UGS exceeds a
predetermined positive value.
[0005] FIG. 2 illustrates a current/voltage characteristic of a
MOSFET which is disclosed in the "EPE" conference report mentioned
in the introduction. This current/voltage characteristic has
different characteristic curves running in the quadrants I and III.
That part of the characteristic curve in the quadrant I which is
designated by T.sub.C is used when the MOSFET is driven by means of
a gate-source voltage U.sub.GS=15 V. That part of the
characteristic curve in the quadrant III which is designated by
T.sub.RCC is used when the MOSFET is driven and a load current
I.sub.LOAD flows counter to the main direction through the MOSFET.
If the MOSFET is not driven (U.sub.GS=0 V), then the characteristic
curve in the quadrant III which is designated by T.sub.D is used.
In other words, the integral freewheeling diode RD of the MOSFET
carries the load current I.sub.LOAD.
[0006] In accordance with this characteristic, it can be seen that
the on-state losses of a MOSFET can be reduced if the MOSFET is
driven in free-running operation. As a result, the free-running
current is divided between the transistor and the integral
free-wheeling diode RD. This operation is characterized by the
characteristic curve T.sub.RCCD in the quadrant III.
[0007] During the commutation process from the power converter
valve T2, which is free-running and is driven, to the
current-accepting power converter valve T1 (FIG. 1), it is
necessary, in accordance with the publication "Commutation
Behaviour in DC/AC-Converters with Power MOSFET", printed in "PCI",
June 1986, pages 316 to 330, firstly for the power converter valve
T2 to be switched off before the power converter valve T1 is
allowed to be switched on. This is necessary in order to prevent a
short circuit as a result of the two power converter valves T1 and
T2 being switched on simultaneously. This means that, at the
instant of commutation, the integral freewheeling diode RD of the
free-running power converter valve T2 carries the load current
I.sub.LOAD and thus, on account of the stored charge, the
freewheeling diode RD causes switch-off losses.
[0008] The publication mentioned in the introduction specifies a
method whereby the load current I.sub.LOAD during the commutation
process from the free-running, driven power converter valve T2 to
the current-accepting power converter valve T1 is not carried by
the integral freewheeling diode RD2 of the power converter valve
T2. This known method is characterized in that the
current-accepting power converter valve T1 is driven so slowly that
only a minimal current overshoot occurs. The slow driving of the
current-accepting power converter valve T1 results in an increase
in the switch-on losses of said valve. The level of these switch-on
losses is dependent on the switch-on delay. The current overshoot
is comparable to a diode reverse current which additionally loads
the power converter valve T1. For this temporally extended driving,
overcurrent detection is required for each power converter valve T1
and T2 of an invertor phase 2. This current in the bridge path is
detected by means of voltage measurement on a leakage inductance.
To that end, on the one hand the value of the leakage inductance
must be known exactly and on the other hand a fast integrator must
be provided, at whose output the value of the current in the bridge
path is then present. Connected downstream of this integrator is a
peak value detector which, on the output side, is connected to an
overcurrent control device. This method reduces the amplitude of
the reverse recovery current and the switching losses of the
free-running, driven power converter valve during the commutation
process.
[0009] The invention is based on the object, then, of modifying the
known method in such a way that the abovementioned disadvantages no
longer occur.
[0010] This object is achieved according to the invention by virtue
of the fact that the current-accepting power converter valve is
switched on at the beginning of the commutation process, and that
the free-running, driven power converter valve is rapidly switched
off as soon as the value of its drain voltage is equal to a
zero.
[0011] The drain voltage of the free-running power converter valve
is required as measured value for this method. This measured value
is used during the known desaturation monitoring, which detects a
short-circuit current or an overcurrent. In other words, a further
measured-value detection device is not required in order to be able
to carry out the method according to the invention.
[0012] As a result of the driving of the current-accepting power
converter valve, the load current commutates from the free-running
power converter valve to the current-accepting power converter
valve. The value of the drain voltage of the free-running power
converter valve changes as a function of this current commutation.
At the beginning of commutation, the drain voltage has a negative
value of the order of magnitude of the saturation voltage of the
power converter valve. At the end of the load current commutation,
the entire intermediate circuit voltage is dropped as reverse
voltage across this power converter valve, since the
current-accepting power converter valve carries the load current.
From these two cut-off values of the drain voltage, it can be seen
that the profile of the drain voltage has a zero crossing during
the commutation process. It is exactly at this instant that the
load current completely commutates to the current-accepting power
converter valve. In order that the switch-off losses are minimized
as far as possible, the free-running power converter valve must be
switched off as rapidly as possible at this instant. Depending on
how rapidly this switch-off is effected, a parallel-path current
flows through the free-running power converter valve and through
the current-accepting power converter valve in addition to the load
current. In other words, the losses that occur cannot be
eliminated, but rather can only be reduced depending on how rapidly
the free-running power converter valve is switched off. This
reduction of the losses is significantly greater than the reduction
by means of the known method, since, in the case of the known
methods, on the one hand there are processing steps for the
measurement signal and on the other hand the overcurrent control
device can operate only when an overcurrent has already
occurred.
[0013] In an advantageous method, at the beginning of the
commutation process, the gate voltage of the free-running, driven
power converter valve is decreased until its drain voltage is equal
to a predetermined reference voltage. This additional method step
improves the identification of the voltage zero crossing of the
drain voltage since, irrespective of the value of the saturation
voltage of the free-running, driven power converter valve, the
initial value of the drain voltage at the beginning of the
commutation process always has the value of the reference voltage.
This becomes apparent particularly in the case of small load
currents.
[0014] For further explanation of the invention, reference is made
to the drawing which diagrammatically illustrates the method
according to the invention.
[0015] FIG. 1 shows a known invertor phase;
[0016] FIG. 2 shows a known current/voltage characteristic of the
power converter valve T2 of FIG. 1;
[0017] FIG. 3 illustrates, in a diagram against time t, the profile
of the gate-source voltage of the power converter valve T2 of FIG.
1 during the commutation process in accordance with the invention;
and
[0018] FIG. 4 illustrates, in a diagram against time t, the profile
of the associated drain voltage;
[0019] FIGS. 5, 6 illustrate the profiles of the drain-source
voltage and of the drain voltage in each case in a diagram against
time t in accordance with an advantageous embodiment of the method
according to the invention; and
[0020] FIG. 7 shows the circuit of a known hybrid power MOSFET.
[0021] The method according to the invention will now be explained
in more detail with reference to the diagrams of FIGS. 3 and 4 in
conjunction with the circuit arrangement according to FIG. 1:
[0022] At the instant t.sub.0, the power converter valve T1 is
turned off and the power converter valve T2 is driven. Assuming
that the load current I.sub.LOAD>0, the power converter valve T2
is free-running and carries the load current I.sub.LOAD counter to
its main direction. The integral freewheeling diode RD2
participates in the carrying of current, in accordance with the
diagram according to FIG. 2, as a function of the flowing drain
current I.sub.D. The power converter valve T1 is the power
converter valve which is intended to accept the load current ILOAD
during the commutation process. Therefore, this power converter
valve T1 is designated as the current-accepting power converter
valve. Si-MOSFETs which--as already mentioned in the
introduction--are numbered among the unipolar power semiconductors
which can carry current in both directions (drain-source,
source-drain) are provided as the power converter valves T1 and T2.
At the instant T.sub.1, the current-accepting power converter valve
T1 is driven, as a result of which it switches on. With this
driving of the current-accepting power converter valve T1, the
profile of the drain-source voltage U.sub.DS2 is monitored with
regard to a voltage zero crossing. This voltage zero crossing in
the case of the drain voltage U.sub.DS2 of the free-running power
converter valve T2 occurs at the instant t.sub.2. At this instant
t.sub.2, the free-running power converter valve T2 is switched off.
This switch-off should be performed as rapidly as possible. Once
the free-running power converter valve T2 is switched off, the
entire intermediate circuit voltage U.sub.ZK is dropped across this
power converter valve and the current-accepting power converter
valve T1 carries the entire load current I.sub.LOAD.
[0023] As can be gathered from the diagram in accordance with FIG.
4, the magnitude of a saturation voltage U.sub.DSsat is dropped
across the free-running, driven power converter valve T2. The value
of this saturation voltage U.sub.DSsat is dependent on the drain
current I.sub.D in the case of normally off MOSFETs. The smaller
the drain current I.sub.D, the smaller the value of the saturation
voltage U.sub.DSsat. However, the smaller the value of this
saturation voltage U.sub.DSsat, the more difficult it is to
identify the voltage zero crossing. If the voltage zero crossing is
not identified until after the instant t.sub.2, then a bridge
short-circuit is present and both power converter valves T1 and T2
can possibly be switched off owing to overcurrent. If the voltage
zero crossing is identified before the instant t.sub.2, then the
load current I.sub.LOAD flowing through the power converter valve
T2 commutates completely to the integral freewheeling diode RD2, as
a result of which switch-off losses are again caused on account of
the stored charge of said freewheeling diode RD2.
[0024] For these reasons mentioned, the method according to the
invention has been improved in such a way that, irrespective of the
drain current I.sub.D at the instant of the driving of the
current-accepting power converter valve T1, the drain-source
voltage U.sub.DS of the free-running power converter valve T2
assumes a predetermined value. The magnitude of this value,
designated as reference value U.sub.DSref, is greater than a
saturation voltage U.sub.DSsat, but less than the forward voltage
U.sub.DRD of the integral freewheeling diode RD2.
[0025] In order that, at the instant of the driving of the
current-accepting power converter valve T1, the drain-source
voltage U.sub.DS is equal to the reference voltage U.sub.DSref, at
the beginning of the commutation process, i.e. at the instant
t.sub.0 in accordance with FIG. 5, the gate-source voltage
U.sub.GS2 of the free-running power converter valve T2 is reduced
until its drain-source voltage U.sub.DS2 is equal to the reference
voltage U.sub.DSref. At the instant t.sub.1, in FIG. 6, the
magnitude of the drain-source voltage U.sub.DS2 of the free-running
power converter valve T2 has risen to the value of the reference
voltage U.sub.DSref. Since the value of the drain-source voltage
U.sub.DS2 of the free-running power converter valve T2 is equal to
the predetermined value of the reference voltage U.sub.DSref, the
current-accepting power converter valve T1 is switched on. With the
switching-on of the current-accepting power converter valve T1, the
profile of the drain voltage U.sub.DS2 is monitored with regard to
a voltage zero crossing. As soon as the drain voltage U.sub.DS2 has
reached the value zero (instant t.sub.2 in FIG. 6), the
free-running power converter valve T2 is switched off as rapidly as
possible. The invention's reduction of the gate-source voltage
U.sub.GS2 of the free-running power converter valve T2 causes the
drain-source voltage U.sub.DS2 thereof to be brought to a
predetermined value U.sub.DSref. Consequently, this method
according to the invention becomes independent of the flowing drain
current I.sub.D.
[0026] If MOSFETs made of silicon are used as power converter
valves T1 and T2, the threshold voltage U.sub.DRD of the integral
freewheeling diode RD1 and RD2 is approximately 0.7 V. However, if
MOSFETs made of silicon carbide are used, then the threshold
voltage U.sub.DRD of the integral freewheeling diode RD1 and RD2 is
approximately 2.8 V. This higher threshold voltage U.sub.DRD is
manifested because silicon carbide has a much greater energy gap
than silicon.
[0027] If the advantageous method according to the invention is
used for reducing losses during the commutation process in an
invertor phase 2 for whose power converter valves T1 and T2 MOSFETs
made of silicon carbide are provided in each case, this method is
simplified since the reference voltage U.sub.DSref can then be
chosen from a larger voltage range. It must be ensured that the
value of the reference voltage U.sub.DSref can never be equal to
the threshold voltage U.sub.DRD of the integral freewheeling diode
RD1 or RD2 of the power converter valve T1 or T2, respectively. If
the magnitude of the drain-source voltage U.sub.DS exceeds the
threshold voltage U.sub.DRD of the associated integral freewheeling
diode RD, the latter is turned on, as a result of which this
freewheeling diode RD again participates in the commutation process
with the disadvantages mentioned in the introduction.
[0028] The circuit of a known hybrid power MOSFET is illustrated in
more detail in FIG. 7. This hybrid power MOSFET is described in
detail in DE 196 10 135 C1. This hybrid power MOSFET endures high
reverse voltages, with the on-state losses being low, however. This
hybrid power MOSFET has a normally off n-channel MOSFET 8 and a
normally on n-channel junction FET. This junction FET 10 is also
referred to as Junction Field-Effect Transistor (JFET). The
normally off n-channel MOSFET 8 is made of silicon, whereas the
normally on n-channel JFET 10 is composed of silicon carbide. A
commercially available low-voltage power MOSFET may be provided as
Si-MOSFET 8. The Si-MOSFET has an integral freewheeling diode RD.
The Si-MOSFET 8 and the SiC-JFET 10 are electrically connected in
series, the gate of the SiC-JFET 10 being directly linked to the
source terminal S of the Si-MOSFET 8.
[0029] The Si-MOSFET endures a reverse voltage of 30 V, for
example. The SiC-JFET 10 connected in series with this is designed
for a much higher reverse voltage. The integral freewheeling diode
RD also has the low reverse voltage of the Si-MOSFET 8. A diode for
a low reverse voltage has a very thin silicon wafer, which results
in a very low stored charge. Owing to the virtually non-existent
stored charge, the switch-off losses of this integral freewheeling
diode RD of the Si-MOSFET 8 of the hybrid power MOSFET are minimal.
For this reason, when using this known hybrid power MOSFET as power
converter valve T1 and/or T2 of the invertor phase 2, precontrol of
the method according to the invention is no longer necessary. Since
the drain voltage U.sub.D of the Si-MOSFET 8 serves as control
voltage for the SiC-JFET 10, the latter is automatically switched
off as soon as a reverse voltage of 30 V, for example, is present
across the Si-MOSFET 8 of the hybrid power MOSFET.
[0030] If a hybrid power MOSFET in accordance with German patent
196 10 135 is in each case used as power converter valve T1 and T2
of the invertor phase 2, the method described in the "PCI"
publication in the introduction can be used, and, nevertheless,
high losses do not occur during the commutation process. This is
possible since the stored charge of the integral freewheeling diode
RD of the MOSFET 8 of the hybrid power MOSFET is minimal.
Participation of this freewheeling diode RD of the MOSFET 8 in the
commutation process is non-critical.
* * * * *