High voltage full bridge circuit and method for operating the same

A circuit apparatus for providing an AC voltage to a load. The apparatus may include: a first, second, third and fourth switch, wherein the first switch is connected between a first node and a first terminal of the second switch and the fourth switch is connected between a second node and a first terminal of the fourth switch; a voltage source, wherein the voltage source is electrically connected between the first node and the second node; a resistive divider connected across the voltage source; a first resistor connected across the first switch a second resistor connected across the third switch; first connecting means for configurably connecting a second terminal of the second switch to the second node; second connecting means for configurably connecting a second terminal of the third switch to the first node; and third connecting means for configurably connecting the second terminal of the second switch in series with the second terminal of the third switch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the generation of AC voltage from either an AC or DC source. Such AC voltages may be used in induction and dielectric heating, ultrasonic cleaning and welding, audio, and battery powered AC mains replacement (“AC inverter”), or other uses. The AC output may also feed a load such as an AC to DC converter so that the final delivered output is DC.

2. Discussion of the Background

Previous output circuits have performed the function of generating an AC voltage. One implementation of such an output circuit, which uses two switching devices, is shown inFIG. 1. In this circuit100, switching devices Q1and Q2are alternately turned on, neither being on at the same time. DC is blocked by C1and AC is coupled to load RL, which could be a transformer, resonant circuit, piezo element or other device or circuit which requires large AC drive levels. Note that when either device Q1or Q2is turned on, the entire V1voltage is applied to the device which is off.

Another popular implementation of an output circuit is shown inFIG. 2. Here, switching devices Q1and Q4are turned on at the same time, alternating with Q2and Q3. Blocking capacitor C1may or may not be used in a given implementation. Again, when a switching device is off, it can be seen that the full voltage V1may appear across it. That is, for example, when Q1is on and Q2is off, Q2will have essentially all of V1across it, and must have a voltage rating so as to not fail in this condition.

Switching devices typically may be FET's, IGBT's, or other devices. Whatever technology is chosen for implementation, the devices used may have a maximum voltage specification. When the DC supply available is of high voltage, it may be difficult, expensive or impossible to select or obtain devices with an appropriate voltage specification to operate directly from this source and meet other system requirements. Sometimes, additional circuitry is required to reduce the voltage the switching devices will be exposed to, adding to cost and weight and reducing reliability.

There are also circumstances within integrated circuits where higher voltage devices may not be feasible. This could relate to linear circuits such as operational amplifiers and to switching circuits, such as drivers, or other circuitry.

Additionally, transient voltages or surges may be expected on some sources of applied DC, and if these voltages are directly applied across the switching devices then these devices would need a higher voltage rating for reliable operation.

SUMMARY OF THE INVENTION

The present invention aims to overcome at least some of the disadvantages of the prior art.

In one aspect the invention provides an output circuit having two complementary electrical switching circuits that are connected so that an applied DC voltage is split equally between the two. None of the switching devices of the circuit are exposed to more than one half of the applied DC voltage. The output of the system is taken between the outputs of the two switching circuits. This output may be blocked from the flow of DC current with a capacitor. Relatively low current resistors control the voltage split initially, and are needed primarily to overcome leakage and other minor asymmetries. The voltage split is inherently maintained in circuit operation regardless of duty cycle, allowing non-square (duty cycle other than 50%) and modulated waveforms. A minimum of four switching devices accomplishes this task. Additional switching devices may be added in parallel with those described herein to increase current handling or power dissipation. Additionally, a linear device may be substituted for each switching device, in which case analog signals may be amplified.

In another aspect, the invention provides a method for operating an output circuit that comprises a first, second, third and fourth switch, wherein the switches are connected in series such that the second switch is connected between the first switch and the third switch and the third switch is connected between the second switch and the fourth switch. The method, in some embodiments, includes operating the switches so that the output circuit provides an AC voltage to a load by first enabling the first and fourth switches at the same time, and then enabling the second and third switches at the same time. This cycle may then repeat. There may be a brief interval at the switching transitions to insure that devices that had been on have time to turn off. In some embodiments, the method may also include regulating the output of the circuit through, for example, cycle dropping or pulse width modulation, so as to maintain a controlled average value of lesser output.

In another aspect, the invention provides a method of prebiasing a blocking capacitor with ½ the input voltage (V1) when all switches have been off for an indefinite period. For this purpose a first resistor is connected across the first switch and a second resistor is connected across the third switch. This prevents larger current surges through C1than would otherwise occur.

The above and other features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3illustrates an output circuit300according to some embodiments of the invention (FET's are used for illustration, however IGBT's or other devices may also be used). Circuitry controlling switching devices Q1-Q4has been omitted for clarity as has rectifier diodes typically built into Q1-Q4, which may also be supplemented with external diodes.

Input voltage V1supplies power to the circuit300, and may be a battery or may be derived from an AC or another DC source. As shown inFIG. 3, V1is connected in parallel with switches Q1-Q4such that V1is electrically connected between the first switch and the fourth switch. This input voltage is split into two voltages (which may be equal) by the resistive divider399, which may be composed of a resistor R2connected in series with a resistor R3(as shown, resistive divider and V1form an electrical loop398). One half of this split voltage is applied to a filter capacitor C2, and then to the half bridge power stage302consisting of switches Q1and Q2. Similarly, the other half of the split voltage is applied to a filter capacitor C3and to the half bridge power stage304consisting of switches Q3and Q4. Typically, R2=R3and C2=C3.

The output AC voltage is taken between the outputs of the two half bridge stages. One point of connection for the output is between Q1and Q2, while the other point of connection is between Q3and Q4. Blocking capacitor C1, if used, may have one terminal connected to output node397and the other connected to load (RL) and ensures that there is no DC path through load resistor RL which could cause the applied voltage split to significantly deviate from its nominal value if the switching devices were unmatched, improperly controlled, or for any other reason.

Load resistor RL is representative of the output device or circuitry to follow, and could also consist, for example, of a transformer, piezoelectric transducer or resonant circuit. If a resonant circuit is chosen for the load, it may use the blocking capacitor C1as a part of the resonant circuit. The repetition rate may either be the same as the frequency of resonance, or offset from it.

In operation, control circuitry, which is not shown, is used to turn on Q1and Q4at the same time, while Q2and Q3are off for one half of a complete cycle. During this time, current is delivered from V1and C2and C3to the output, charging C1through load resistance RL. During the other half of the cycle, Q2and Q3are turned on and Q1and Q4are turned off. At this time, C1is discharged through load resistor RL. Control circuitry may impose a dead time during which all switching devices are off briefly.

During the time when Q2is on and Q1is off, Q1will have across it only the voltage applied to that half bridge, or ½ V1. Similarly, When Q1is on and Q2is off, Q2will only have ½ V1across it. This relationship also exists between Q3and Q4.

Switching devices may be put in parallel with those shown, for the purpose of allowing larger currents to be controlled, or for improved power dissipation limits. Similarly, rectifier diodes may be put in parallel with the switching devices to handle currents larger than the ratings of those diodes built into the switches.

Linear devices may be substituted for each switching device, maintaining the improved device operating voltage margin and allowing analog signals to be amplified.

As discussed above, an AC voltage is created by: (1) placing switching devices Q1and Q4in a conducting state while devices Q2and Q3are in a non-conducting state, which configuration causes C1to charge through load RL and then (2) placing devices Q1and Q4to a non-conducting state while devices Q2and Q3are enabled to conduct, which configuration will cause C1to discharge through load RL. In this fashion, RL has an alternating current forced through it, and may represent any load which can benefit from AC power.

Switching Q1through Q4in the above manner does not vary the voltage split defined by resistors R2and R3, and so these resistors may be large in value and waste little energy, and the switching devices are not exposed to the full potential of V1. Thus, the switching devices may be less expensive than otherwise would be the case, or a higher voltage for V1may be used than would otherwise be practical.

When the circuit is used to supply power to a load, it is often the case that the amount of power delivered must be regulated. One way of regulating the output is to briefly stop supplying an AC voltage to the load when too much output has been measured, and then resume output shortly thereafter so as to maintain the desired average value. During such a reduction, all switches could remain open.

Referring now toFIG. 4,FIG. 4is a schematic of an output circuit400according to another embodiment. Circuit400is similar to circuit300, however, circuit400includes a resistor R4connected across Q1and a resistor R4connected across R3. Resistors R4and R5function to pre-bias blocking capacitor C1with ½ the input voltage (V1) when all switches have been off for a period of time.

Referring now toFIG. 5, is a schematic of a configurable output circuit500according to an embodiment. Circuit500is similar to circuit400, however, circuit500includes a first connecting means501, a second connecting means502, and a third connecting means503. The connecting means501-503may be a switch, a pair of jumper connectors for receiving a jumper or other connecting means that, preferably, can handle a large current. First connecting means501is used to connect a terminal531of switch Q2with node510, second connecting means502is used to connect a terminal532of switch Q3with node511, and third connecting means503is used to connect terminal531of switch Q2with terminal532of switch Q3.

The connecting means501-503allow for operation at 220 VAC (190-260 VAC) or at 440 VAC. More specifically, when: (1) connecting means501is used to connect terminal531of switch Q2with node510(e.g., a jumper is connected between a first jumper connection and second jumper connection of the connecting means, wherein the first jumper connector is electrically connected to node510and the other jumper connector is electrically connected to terminal531); (2) connecting means502is used to connect terminal533of switch Q3with node511; and (3) connecting means503is not used to connect terminal531with terminal532(e.g., no jumper is present), the circuit500can operate at 220 VAC. Similarly, when: (1) connecting means501is not used to connect terminal531of switch Q2with node510; (2) connecting means502is not used to connect terminal533of switch Q3with node511; and (3) connecting means503is used to connect terminal531in series with terminal532, the circuit operates at twice the voltage and one-half the current.

The current when operation is at 220 VAC is twice the current when operation is at 440 VAC. The circuit500therefore provides the same power output at either voltage range. This is because the switches (e.g., MOSFETS) are configured in series for high voltage operation or in parallel for low voltage (at 2× the current). That is, for example, when a jumper is used to connect terminal531in series with terminal532, then the switches Q1-Q4are in series, but when jumpers are used to connect terminal501with node510and terminal502with node511, respectively, then switches Q1and Q2are in parallel with switches Q3and Q4.

Realizing this ability allowed one to build an versatile power structure that requires, for example, nothing more than a simple jumper change to make the circuit operate either from 220 VAC (as most often available in the US) or 400-520 VAC as is available throughout the world.

Additionally, while the process described above and illustrated in the drawings is shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and/or some steps may be performed simultaneously.