METHOD AND DEVICE FOR LOWERING THE IMPEDANCE OF A TRANSISTOR

Abstract

A method and circuit for lowering impedance of a transistor bridge having two pairs of cooperating transistors, comprising receiving a pair of DC input signals which enable activation of one of the pairs of transistors, the activation providing a path for the DC input signals through the two activated transistors, the pair of DC input signals having voltages differing from each other by a first amount; and applying a second pair of DC signals each to a different gate of the two activated transistors, the second pair of DC signals having voltages differing from each other by a second amount that is greater than the first amount, wherein as a result of the second amount being greater than the first amount, impedances of the two activated transistors are lower as compared to if the pair of DC input signals were used in substitution for the second pair of signals.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 11/993,701, now allowed, filed on Jun. 22, 2010 as a National Entry Application of PCT application Serial No PCT/CA2006/001059 filed on Jun. 23, 2006 and published in English under PCT Article 21(2), which itself claims benefit of U.S. provisional application Ser. No. 60/693,447, filed on Jun. 24, 2005. All documents above are incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The disclosure relates to conversion circuits and, in particular to power source polarity converters.

BACKGROUND OF THE INVENTION

A four-diode rectifier bridge is commonly used in converting an AC input voltage to a DC output voltage. This type of bridge can also be used in translating a DC input of arbitrary polarity into a DC output of known polarity; however a consequence of using the four-diode rectifier bridge is a forward voltage drop of two diodes when current is flowing. This consequence means less than ideal efficiency in power supply applications.

Accordingly, it would be advantageous to improve DC power source polarity converters.

SUMMARY OF THE INVENTION

According to one example embodiment, there is a method for lowering impedance of a transistor bridge having first and second pairs of cooperating transistors. The method includes the step of receiving a pair of DC input signals which enable activation of one of the cooperating pairs of transistors. The activation provides a path for the DC input signals through the two activated transistors. The pair of DC input signals have voltages differing from each other by a first amount. The method also includes the step of applying a second pair of DC signals each to a different gate of the two activated transistors. The second pair of DC signals have voltages differing from each other by a second amount that is greater than the first amount. As a result of the second amount being greater than the first amount, impedances of the two activated transistors are lower as compared to if the pair of DC input signals were instead used in substitution for the second pair of DC signals.

In one aspect of the above-mentioned example embodiment, the second pair of DC signals are boosted voltage signals.

According to another example embodiment, there is a circuit for lowering impedance of a transistor bridge having first and second pairs of cooperating transistors. The transistor bridge receives a pair of DC input signals which enable activation of one of the cooperating pairs of transistors. The activation provides a path for the DC input signals through the two activated transistors. The pair of DC input signals have voltages differing from each other by a first amount. The circuit includes means for generating a second pair of DC signals having voltages differing from each other by a second amount that is greater than the first amount. The circuit also includes means for applying the second pair of DC signals each to a different gate of the two activated transistors. As a result of the second amount being greater than the first amount, impedances of the two activated transistors are lower as compared to if the pair of DC input signals were instead used in substitution for the second pair of DC signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanying drawings:

FIG. 1 is a block diagram of a power source polarity converter, a power source and a load in accordance with at least some example embodiments;

FIG. 2A is a schematic diagram of a rectification circuit in accordance with an example embodiment;

FIG. 2B is a schematic diagram of a control circuit in accordance with an example embodiment;

FIG. 3 is a schematic diagram of a charge conditioning circuit in accordance with at least one example embodiment;

FIG. 4 is a schematic diagram of a monostable multivibrator in accordance with an example embodiment; and

FIG. 5 is a graph of the relationship between R_DS and −V_GS in a typical P-channel MOSFET.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring now to FIG. 1, a power source polarity converter in accordance with at least some example embodiments will be described. The converter, generally referred to using the reference numeral 10, comprises a rectification circuit 12 interconnected with a charge conditioning circuit 14. A DC power source 16 having positive and negative poles provides DC power to the converter 10 which is conditioned by the rectification circuit 12 and the charge conditioning circuit 14 to provide a DC output for driving a load 18. In the illustrated configuration, the load 18 is connected between a circuit output node 20 of the converter 10 and the circuit ground. It will be understood that the load 18 receives the DC output delivered by the converter 10 by way of the node 20.

Still referring to FIG. 1, the rectification circuit 12 conditions DC power provided by the DC power source 16 between the converter inputs 22, 24 such that the polarity of the voltage which appears between the inversion circuit outputs 26, 28 has the same polarity, regardless of the polarity of the DC power source 16 between the converter inputs 22, 24. The voltage which appears between the inversion circuit outputs 26, 28 (illustratively labelled Vout+ and Vout−) are in turn input into the charge conditioning circuit 14. The charge conditioning circuit 14 provides an output voltage across the charge conditioning circuit outputs 30, 32, illustratively labelled Vpol+ and Vpol−. Examples of conditioning circuits as in 14 include DC-to-DC converters such as charge pumps, buck and boost converters, etc. Additionally, the voltage between the charge conditioning circuit outputs 30, 32 is fed back to the rectification circuit 12 via the polarity inversion circuit conditioning inputs 34, 36.

Referring now to FIG. 2A, the rectification circuit 12 comprises a pair of P-Channel MOSFETs as in 38, 40 and a pair of N-Channel MOSFETs as in 42, 44. The illustrated circuit also includes a control circuit 46 which illustratively receives Vpol+ and Vpol− on the polarity inversion circuit conditioning inputs 34, 36; however one skilled in the art will appreciate that in some alternative examples DC voltage signals similar to Vpol+ and Vpol− might be generated within the control circuit 46 by a self-contained DC source (for example, a battery and, as necessary, complementary control circuit, both not shown, for generating the requisite signals). In operation, the control circuit 46 selectively activates either the FETs 40 and 42 or the FETs 38 and 44 as later explained in this disclosure.

Referring to FIG. 2B in addition to FIG. 2A, in some examples, the control circuit 46 will include the components illustrated in FIG. 2B. Four (4) level sensing transistors 48, 50, 52 and 54 are electrically connected to the converter inputs 22, 24 for “sensing” voltage of the DC signals found on those inputs. In particular, the transistor 48 is connected to the input 22 via conductor 56, the transistor 50 is connected to the input 22 via conductor 56, the transistor 52 is connected to the input 24 via conductor 58, and the transistor 54 is connected to the input 24 via the conductor 58. Also, the gates of the illustrated transistors 48 and 50 are connected to the input 24 via the conductor 58, and the gates of the illustrated transistors 52 and 54 are connected to the input 22 via the conductor 56. In at least one example, the level sensing transistors 48, 50, 52 and 54 are PMOS transistors.

The illustrated control circuit 46 also includes four (4) switching transistors 60, 62, 64 and 66 (in at least one example, the switching transistors 60, 62, 64 and 66 are NMOS transistors). The illustrated control circuit 46 also includes eight (8) resistive elements R 68. In at least one example, the resistive elements as in 60 each have the same nominal value such as 1 M Ω, for instance. The power MOSFETs 38, 40, 42 and 44, level sensing transistors 48, 50, 52 and 54 switching transistors 60, 62, 64 and 66, and resistive elements as in 68 are interconnected by conductors such as, for example conductive traces on a PC Board (PCB) or the like, on which the various elements have been mounted.

Referring now to FIG. 3, the illustrated charge conditioning circuit 14 comprises a switch circuit 70 (alternatively referred to in this disclosure as charge directing circuitry) which supplies a switched voltage to first and second charge transfer capacitors 72, 74 and a storage capacitor 76 interconnected by diodes as in 78₁, 78₂ and 78₃. Beginning with a more general explanation of function, a number of capacitors are, in accordance with at least some examples of the charge conditioning circuit 14, in communication with charge directing circuitry (such as, for example, a monostable multivibrator). In configuration for voltage boosting, these capacitors have their charging regulated by the charge directing circuitry. As understood by those skilled in the art, the implementation details for this voltage boosting by way of a suitable capacitor arrangement will vary; however it is instructive to mention some implementation details of the illustrated example embodiment.

With respect to the circuit illustrated in FIG. 3, this example circuit provides for a trebling of the input voltage and as a result a voltage between the charge conditioning circuit outputs 30, 32 will be approximately three times the voltage between the inversion circuit outputs 26, 28. As will be appreciated by persons of ordinary skill in the art, charge conditioning circuits which double, quadruple or provide other multiples of the voltage input to the inversion circuit outputs 26, 28 at the conditioning circuit outputs 30, 32 may also be provided for.

Referring now to FIG. 4 in addition to FIG. 3, the illustrated switch circuit 70 is comprised of first and second PNP type transistors 84, 86, first and second collector resistors 88, 90, first and second biasing resistors 92, 94 and first and second capacitors 96, 98. The elements 84 through 98 of the switch circuit 70 form a monostable multivibrator. As known in the art, the transistors within the monostable multivibrator circuit alternate between conducting and non-conducting states, wherein one transistor is in a conducting state while the other is in the non-conducting state. When the first transistor 84 is conducting, second transistor 86 is not conducting and the first diode 78₁ is forward biased. As a result the first charge transfer capacitor 72 is charged to the same voltage as that which is found between the inversion circuit outputs 26, 28. When the second transistor 86 is forward biased and conducting, first transistor 84 is reversed biased (and therefore not conducting). At the same time, the first diode 78₁ is reversed biased and the second diode 78₂ is forward biased. As result the second charge transfer capacitor is charged to the same voltage as that which is found between the inversion circuit outputs 26, 28 plus the voltage across the first charge transfer capacitor 72. When the first transistor 84 is once again forward biased and the second transistor 86 reversed biased, the second diode 78₂ is reversed biased and the third diode 78₃ forward biased. As a result, the storage capacitor 76 is charged to the voltage found between the inversion circuit outputs 26, 28 plus the voltage across the second charge transfer capacitor 72, which gives rise to a boosted voltage across the positive and negative charge conditioning circuit outputs 30, 32.

Still with reference to FIG. 3, in order to invert the output voltage to provide a negative output of equal magnitude, there is provided an inverter circuit 100 which inverts the positive output found on the positive conditioning circuit output 30, this negative voltage being available on the negative conditioning circuit output 32. Thus in the illustrated embodiment, generation of the DC signal on the circuit output 32 is carried out by inverting the DC signal on the circuit output 30 (after this latter signal has itself been generated of course).

Alternatively, such voltage multiplying circuits, multivibrators, or portions thereof are also available as integrated circuits.

Referring back to FIGS. 2A and 2B, in steady state operation, provision of a positive or negative voltage between the converter inputs 22, 24 causes a positive voltage to appear between the inversion circuit outputs 26, 28. This output voltage is boosted by the charge conditioning circuit 14 (FIG. 1) such that the voltage between the charge conditioning circuit outputs 30, 32 is greater than the voltage between the converter inputs 22, 24.

The MOSFETs 38, 40, 42 and 44 are the principle transistors, and act both as diodes and switches between the converter inputs 22, 24 and the inversion circuit outputs 26, 28. When they are activated, the MOSFETs 38, 40, 42 and 44 are in saturation and therefore acting as variable resistances. Conversely (as will be appreciated by one skilled in the art) when they are non-activated, any of the MOSFETs 38, 40, 42 and 44 will present such high resistance as to essentially behave like an open circuit.

Each of the MOSFETs 38, 40, 42 and 44 includes a diode body between source and drain. A positive DC voltage applied between the converter inputs 22, 24 causes a current to flow through the MOSFET 38 from the source 102 via the diode 104 to the drain 106. Similarly, the current flows through the transistor 44 from the source 108 via the diode 110 to the drain 112. (In the context of the illustrated rectification circuit 12, a positive DC voltage applied between the converter inputs 22, 24 enables activation of the FETs 38 and 44, whereas a negative DC voltage applied between the converter inputs 22, 24 does not enable activation.)

With current flowing through the FETs 38 and 44, a similar voltage to the initial voltage appears between the inversion circuit outputs 26, 28 which is boosted by the charge conditioning circuit 14 (FIG. 1). The boosted voltage is provided back to the polarity conversion circuit via the polarity inversion circuit conditioning inputs 34, 36.

The illustrated rectification circuit 12 includes the control circuit 46 that is electrically connected via conductors 114, 116, 118 and 120 to the gates of the FETs 42, 38, 44 and 40 respectively. As will be explained in more detail below, in the illustrative embodiment disclosed in the figures, the control circuit 46 operates to make the boosted voltage signals on the polarity inversion circuit conditioning inputs 34, 36 available to those of the FETs 38, 40, 42 and 44 that happen to be the activated pair.

Within the control circuit 46, the level sensing transistors 48, 50, 52 and 54 each selectively enable a respective one of the switching transistors 60, 62, 64 and 66 depending on polarity of the power source applied between the converter inputs 22, 24. This in turn allows boosted voltage signals provided via the polarity inversion circuit conditioning inputs 34, 36 to be selectively applied to the gates of the MOSFETs 38, 40, 42 and 44.

For example, assuming that the DC power source applied between the converter inputs 22, 24 has a positive polarisation and the charge conditioning circuit 14 provides for a voltage between the polarity inversion circuit conditioning inputs 34, 36 which is three (3) times the voltage between the inversion circuit outputs 26, 28, a voltage will be provided on polarity inversion circuit conditioning input 36. A potential difference equal to the voltage at the converter inputs 22, 24 will appear between the source and gate of the level sensing transistor 50, thereby turning the level sensing transistor 50 on and causing a voltage drop across the resistive element 68₁. This in turn causes a voltage drop between gate and drain of the switching transistor 62 thereby causing the voltage provided on polarity inversion circuit conditioning input 36 to be available at the gate of the MOSFET 38, thereby increasing the potential difference between gate and source of the MOSFET 38.

With an increase in potential difference between source and gate of the MOSFET 38, the resistance in the drain of the MOSFET 38 drops, causing a similar drop in the potential difference between source and drain for the same current. Similarly, a potential difference equal to the voltage at the converter inputs 22, 24 will appear between the gate and source of the level sensing transistor 52, thereby turning the level sensing transistor 52 on and causing a voltage drop across the resistive element 68₂. This in turn cases a voltage drop between drain and gate of the switching transistor 64 causing the voltage provided on polarity inversion circuit conditioning input 34 to be available at the gate of the MOSFET 44, thereby increasing the potential difference between gate and source of the MOSFET 44.

With an increase in potential difference between gate and source of the MOSFET 44, the resistance in the drain of the MOSFET 44 drops, causing a similar drop in the potential difference between source and drain for the same current. With the positive and negative poles of the power source 16 attached to the converter inputs 22, 24 so that the power source 16 is oriented for circuit behaviour as described above, only the cooperating pair of MOSFETs 38 and 44 are enabled (i.e. activated). In other words, the cooperating pair of MOSFETs 42 and 40 are non-activated when the other pair of FETs in the bridge are activated.

Given the symmetry of the circuit, as will now be apparent to a person of ordinary skill in the art, when the DC power source 16 placed between the converter inputs 22, 24 is inverted (e.g. the attachment of the converter inputs 22 and 24 to the positive and negative poles of the power source 16 is switched around) the cooperating pair of MOSFETs 42 and 40 will be enabled (i.e. activated) and the cooperating pair of MOSFETs 38 and 44 disabled (i.e. non-activated) thereby inverting the input.

Referring to FIG. 1, in a particular embodiment, and with appropriate selection of the components used for its manufacture, alternatively the rectification circuit 12 can be used alone without the charge conditioning circuit 14 as a minimal impedance universal circuit protector for protecting electronic circuits or other loads from what would otherwise be an accidental reversal of the DC power source 16.

Referring to FIG. 5, the resistance R_DS between drain and source in a typical P-channel MOSFET varies with the voltage V_GS applied between gate and source. As will be evident from the graph to a person of skill in the art, as the voltage difference between the gate and source (V_GS) becomes a larger negative value, the MOSFET moves into a region of low impedance operation giving rise to a corresponding decrease in the resistance R_DS. It follows that provided the gate-to-source voltage difference is sufficiently large, the voltage V_DS across drain and source, and therefore loss of power which would otherwise be experienced in the circuit, can be reduced to negligible amounts. A similar phenomenon arises in an N-channel MOSFET. Putting the above described relationship in the context of an example applicable to this disclosure rather than applying the DC signals from the power source 16 each to a different gate of the appropriate FET, the boosted voltage signals generated by the charge circuit 14 are instead applied so that (as graphically shown in FIG. 5) −V_GS increases and R_DS decreases, reducing the impedances of the activated FETs is possible.

Referring now to FIGS. 1 and 2, provided the DC power source 16 is able to provide a sufficient voltage difference between converter inputs 22, 24, complete saturation of the Source-Gate junction of (depending on polarity of the DC power source 16) the MOSFETs 38, 44 or 40, 42 may be obtained and the MOSFETs 38, 44 or 40, 42 will be placed in a low impedance operation mode. The positive inversion circuit output 26 is fed back to the positive polarity inversion circuit conditioning input 34 (i.e. Vout+ is fed back to Vpol+) which assists in maintaining the low impedance saturation mode without a need for a boosted voltage. Similarly, the negative inversion circuit output 28 is fed back to the negative polarity inversion circuit conditioning input 36 (i.e. Vout− is fed back to Vpol−). As a result, and given the low internal impedance of the saturated MOSFETs, the potential difference between the inversion circuit outputs 26, 28 is virtually identical to the potential difference between converter inputs 22, 24, regardless of the polarity of the DC power source 16 polarity connection for as long as DC power source 16.

It is to be understood that the invention is not limited in its application to the details of construction and parts illustrated in the accompanying drawings and described hereinabove. Example embodiments are capable of being practised in various ways. It is also to be understood that the phraseology or terminology used herein is for the purpose of description and not limitation. It will further be understood that example embodiments described hereinabove can be modified, without departing from the spirit, scope and nature of the subject invention as defined in the appended claims.

Claims

1. A method for lowering impedance of a transistor bridge having first and second pairs of cooperating transistors, the method comprising the steps of:

receiving a pair of DC input signals which enable activation of one of the pairs of transistors, said activation providing a path for the DC input signals through the two activated transistors, said pair of DC input signals having voltages differing from each other by a first amount; and

applying a second pair of DC signals each to a different gate of the two activated transistors, said second pair of DC signals having voltages differing from each other by a second amount that is greater than said first amount,

wherein as a result of said second amount being greater than said first amount, impedances of the two activated transistors are lower as compared to if said pair of DC input signals were instead used in substitution for said second pair of DC signals.

2. The method as claimed in claim 1, wherein the transistor bridge is a Field Effect Transistor (FET) bridge and said transistors are FETs.

3. The method as claimed in claim 1, further comprising the step of generating said second pair of DC signals, the generating step occurring before the applying step.

4. The method as claimed in claim 3, wherein the generating step includes generating, after a first signal of said second pair of DC signals has already been generated, the other signal of said second pair of DC signals by inverting said first signal.

5. The method as claimed in claim 1, wherein said second amount is between two and four times said first amount.

6. A circuit for lowering impedance of a transistor bridge having first and second cooperating pairs of transistors, the bridge receiving a pair of DC input signals which enable activation of one of the cooperating pairs of transistors, the activation providing a path for the DC input signals through the two activated transistors, the pair of DC input signals having voltages differing from each other by a first amount, and the circuit comprising:

means for generating a second pair of DC signals having voltages differing from each other by a second amount that is greater than said first amount; and

means for applying said second pair of DC signals each to a different gate of the two activated transistors,

wherein as a result of said second amount being greater than said first amount, impedances of the two activated transistors are lower as compared to if said pair of DC input signals were instead used in substitution for said second pair of DC signals.

7. The circuit as claimed in claim 6, wherein said transistor bridge is a Field Effect Transistor (FET) bridge and said transistors are FETs.

8. The circuit as claimed in claim 6, wherein said generating means comprises a charge conditioning circuit.

9. The circuit as claimed in claim 8, wherein said charge conditioning circuit includes a monostable multivibrator and a number of capacitors configured for voltage boosting and in communication with said monostable multivibrator, said monostable multivibrator for regulating charging of said capacitors.

10. The circuit as claimed in claim 9, wherein said charge conditioning circuit includes an inverter, said inverter including an input and an output, and when one of said second pair of DC signals is received at said inverter input the other of said second pair of DC signals is outputted at said inverter output.

11. The circuit as claimed in claim 6, wherein said applying means includes a number of level sensing transistors and a number of switching transistors.

12. The circuit as claimed in claim 11, wherein said level sensing transistors are P-channel transistors and said switching transistors are N-channel transistors.

13. The circuit as claimed in claim 6, wherein said second amount is between two and four times said first amount.

14. The circuit as claimed in claim 6, wherein said second pair of DC signals are substantially of equal and opposite magnitude.