The invention relates to switching voltage converters in halfbridge topology such as switching power supplies, DC/DC converters or class xe2x80x9cDxe2x80x9d amplifiers (amplifiers operating in switching mode). The purpose of switching voltage converters such as primary-switched power supplies or DC/DC converters is to transform voltages and currents with potential separation. The centrepiece in the design of such converters or transformers is a high-frequency transformer which transmits the energy with potential separation.
Primary-switched power supplies and DC/DC converters are manufactured worldwide in very large numbers. They find application in industrial electronics, household appliances, entertainment electronics, as well as in PCs and telecommunication devices.
The following characteristics are of particular importance for switching power supplies:
high efficiency and resulting low heat dissipation
compact design
low weight
good quality of output voltage
favorable price
high reliability
the smallest possible number of components.
A number of converter principles are used in the prior art. We can generally differentiate between two basic types: forward converters and flyback converters. These are described by examples by Tietz/Schenk, chapter 18 (ISBN 3-540-19475-4, 9th edition). Flyback converters are regarded as advantageous in the prior art, sincexe2x80x94in addition to their high efficiencyxe2x80x94they also provide good quality output voltage (e.g. in case of DC voltage sources a low amount of residual pulsation, or ripple).
To realize a compact arrangements with low weight, a flyback converter is chosen which can transmit as much power as possible using a given high-frequency transformer. This makes it practical to transmit the required power through the smallest possible core. This not only has cost advantages but also allows for a more compact switching arrangement.
Push-pull converters meet these requirements. Due to their working principle, they canxe2x80x94under equal conditionsxe2x80x94transport about twice the power through a given magnet core than single-ended converters. The maximum power that can be transmitted through a magnet core also depends on the coil space utilized by the transformer. Optimal coil space utilization is ensured by converters in half bridge or full bridge topology, since they need only a primary coil which carries a current in all phases of magnetic flow formation.
To control the output voltage of a halfbridge converter, a control circuit uses pulse width modulator (PWM), for example, in which the duty ratio is adjusted so that it responds to a given control variable to maintain the output voltage or current at a desired setpoint. It is also necessary to synchronize the duty ratio of both switches of a half bridge arrangement to generate an AC voltage without a DC component at the output of the half bridge.
In the prior art, the two switches of the half bridge arrangement are realized using two n-channel transistors. The control circuit of the pulse width modulator generates two signals in such a way that, to prevent short circuits, only one of the two switches is closed at any given time. These are called mutually exclusive, also called xe2x80x9cone-hotxe2x80x9d, control signals. In a large number of applications, the supply voltage or intermediate circuit voltage is higher than the operating voltage of the pulse width modulator.
According to prior art, a pulse width modulator is realized using a voltage source which generates a delta voltage or sawtooth voltage. The instantaneous amplitude of the delta or sawtooth voltage is compared with a control voltage by a comparator. A D flip-flop controls two AND gates to generate alternating ON signal for drivers which control the two switches of the half bridge arrangement. (Tietze/Schenk, chapter 18.7.5).
The advantage of the half bridge converter topography is that in contrast with other topologies, a small HF transformer can be used. However, a disadvantage is the technical complexity in controlling the switches of the half bridge.
The prior art requires generating two separate signals, one each for the switch in TOP position and the switch in BOT position. These two signals must be mutually exclusive and they must have exactly the same pulse lengths.
Since a large number of applications have a supply voltage or an intermediate circuit voltage which is higher than the operating voltage of the pulse width modulator, and since the reference potential of the pulse width modulator is at the reference potential of the BOT switch, it is also necessary to provide an expensive level shifter for controlling the TOP switch.
This increases the expense for technology even further, since a level shifter is also required to adjust the level of the control signal for the TOP switch of the half bridge arrangement. Such level shifters impose not only with additional design requirements, but also entail additional financial costs.
There are integrated components on the market for generating the pulse width modulated signals. However, when the supply voltage or intermediate circuit voltage is higher than the operating voltage of the pulse width modulator control circuit, control of the power switch in the TOP position still requires considerable technical complexity.
Since it involves such high technical complexity, this half bridge topology is applied only to special converters requiring this technology or to high-power converters.
Included in the prior art is DE 33 11 662, in which a circuit arrangement is intended for use in an electromagnetic flowmeter, and JP 632 537 20. These two circuit arrangements do not achieve the objects of the present invention, since they introduce a half bridge arrangement with a constant locking time.
The reason for this constant locking time is the type of control of the TOP and the BOT switches by a driver.
It is an object of the present invention to provide a circuit which substantially reduces the technical complexly in controlling the half bridge in switching voltage converters with variable locking in half bridge topology.
It is a further object of the invention to provide a circuit which avoids the use of level shifters, which are sometimes necessary in prior art when the supply voltage or intermediate circuit voltage is higher than the operating voltage of the pulse width modulator control circuit.
In contrast to the prior art, the solution according to the invention uses a p-channel transistor, such as a bipolar transistor, MOS transistor or an IGBT (insulated gate bipolar transistor). This is the equivalent of a so-called source design or emitter/follower design Thus, the final step in this case is not designed as a half bridge as in the prior art, but in accordance with the invention as a so-called complementary source follower step.
The potential of the output signal of the half bridge is chosen as the reference potential for controlling the pulse width modulator. Generally this has no detrimental effect, since the control variable in a large number of applications is transferred with separate potential from the secondary side to the primary side or power supply side. Thus, with the solution according to the invention, the need for a level shifter for level adjustment is eliminated.
According to an embodiment of the invention, there is provided a switching voltage converter in half bridge topology which operates in switching mode with variable locking, consisting of two power switches and one control circuit. The TOP switch is realized by one or more n-channel transistors. The BOT switch is realized by one or more p-channel transistors. The outpwm of the half bridge is selected as a reference potential, which is not constant and varies during operation, for the control circuit and supply voltage Ubtop, Ubbot of the drivers of the TOP and BOT switch.
According to an embodiment of the invention, there is provided a switching voltage converter in half bridge topology with variable locking, comprising: first and second power switches, a control circuit, a TOP switch which includes at least one n-channel transistor, a BOT switch which includes at least one p-channel transistor, and an output Upwm of the half bridge is applied as a reference potential which is not constant and which varies during operation for the control circuit and supply voltage Ubtop, Ubbot of the drivers of the TOP and BOT switches.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.
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