Method for reducing losses during the commutation process

Details Method for reducing losses during the commutation process Method for reducing losses during the commutation process

2001-06-04

2002-08-13

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S056050

Reexamination Certificate

active

06434019

ABSTRACT:

FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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 (“EPE”), 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.
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 an 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.
FIG. 1
illustrates a known circuit of an invertor phase
2
, which has a MOSFET in each case as power converter valves T
1
and T
2
. The antiparallel bipolar freewheeling diode of the power converter valve T
1
and T
2
is designated by RD
1
and RD
2
, respectively. On the DC voltage side, this invertor phase
2
is linked to a DC voltage source
4
across which a DC voltage U
ZK
is dropped, and which is also designated as intermediate circuit voltage. The junction point
6
between the two power converter valves T
1
and T
2
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.
FIG. 2
illustrates a current/voltage characteristic of a MOSFET which is disclosed in the “EPE” conference report. 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
c
is used when the MOSFET is driven by means of a gate-source voltage U
GS
=15 V. That part of the characteristic curve in the quadrant III which is designated by T
RCC
is used when the MOSFET is driven and a load current I
LOAD
flows counter to the main direction through the MOSFET. If the MOSFET is not driven (U
GS
=0 V), then the characteristic curve in the quadrant III which is designated by T
D
is used. In other words, the integral freewheeling diode RD of the MOSFET carries the load current I
LOAD
.
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
RCCD
in the quadrant III.
During the commutation process from the power converter valve T
2
, which is free-running and is driven, to the current-accepting power converter valve T
1
(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, for the power converter valve T
2
to be switched off before the power converter valve T
1
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 T
1
and T
2
being switched on simultaneously. This means that, at the instant of commutation, the integral freewheeling diode RD of the free-running power converter valve T
2
carries the load current ILOAD and thus, on account of the stored charge, the freewheeling diode RD causes switch-off losses.
The “EPE” publication specifies a method whereby the load current I
LOAD
during the commutation process from the free-running, driven power converter valve T
2
to the current-accepting power converter valve T
1
is not carried by the integral freewheeling diode RD
2
of the power converter valve T
2
. This known method is characterized in that the current-accepting power converter valve T
1
is driven so slowly that only a minimal current overshoot occurs. The slow driving of the current-accepting power converter valve T
1
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 T
1
. For this temporally extended driving, overcurrent detection is required for each power converter valve T
1
and T
2
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 the output of which 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.
SUMMARY OF THE INVENTION
The present invention is based on the object of modifying the known method in such a way that the above mentioned disadvantages no longer occur. This object is achieved according to the present 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 zero.
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.
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 on

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