In driving a bipolar junction transistor (BJT), a dedicated transformer may supply current to a base of the BJT to turn-on and turn-off the BJT. Total base current for the BJT to turn-on and turn-off can be on a primary winding of the dedicated transformer and may be applied directly to the base of the BJT. Circuitry can be utilized to carefully control the total base current on the primary winding to reliably achieve a switching frequency of the BJT. Otherwise, the switching frequency may experience instability as current supplied from the dedicated transformer can vary.
Also in driving a BJT, a resonant tank may be utilized to provide current to the base of the BJT to turn-on and turn-off the BJT. The switching frequency of the BJT may be synched to a resonance frequency of the resonant tank. The resonance frequency may be dependent on load resistance and a bus voltage, which can vary. Thus, the switching frequency of the BJT may vary unless the load resistance and the bus voltage are made stable.
The present disclosure is directed to a switching circuit with a base discharge switch, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims.
The following description contains specific information pertaining to implementations in the present disclosure. The drawings in the present application and their accompanying detailed description are directed to merely exemplary implementations. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present application are generally not to scale, and are not intended to correspond to actual relative dimensions.
BJT Q1 includes base QB, emitter QE, and collector QC. In the present implementation, BJT Q1 is an NPN BJT, but in other implementations BJT Q1 is a PNP BJT. BJT Q1 is configured to switch power from a bus voltage (not shown in
The bus voltage can be connected to collector QC and a lower voltage (e.g. ground) can be coupled to emitter QE. By switching BJT Q1, switching circuit 100 can power a load that may be coupled to emitter QE.
Switching circuit 100 can have various topologies, which can vary from what is shown in
In switching circuit 100, base current supply 104 is configured to turn-on BJT Q1. More particularly, base current supply 104 is configured to provide current to base QB of BJT Q1 so as to turn-on BJT Q1. As shown in
Also in switching circuit 100, limiting resistor R1 is configured to limit current provided by base current supply 104. Blocking diode D1 is configured to block negative current from flowing into base current supply 104. A series configuration of limiting resistor R1 and blocking diode D1 are coupled between base current supply 104 and offset circuit 102.
Offset circuit 102 is optionally coupled between base QB of bipolar junction transistor Q1 and base current supply 104. For example,
Offset circuit 102 is configured to provide offset voltage VBO to base QB of BJT Q1 so as to turn-off BJT Q1. In doing so, base QB of BJT Q1 can be made to have a base voltage substantially lower than an emitter voltage of BJT Q1. For example, offset voltage VBO can impress a negative voltage on base QB of BJT Q1. By applying offset voltage VBO to base QB of BIT Q1, BJT Q1 can be made to turn-off rapidly. More particularly, BJT Q1 may other wise take longer to turn-off due to charge stored in base QB. By utilizing offset voltage VBO, BJT Q1 can be used in higher frequency applications than may otherwise be practical. For example, without offset voltage VBO, the switching frequency of BJT Q1 may be, for example, as much as approximately 100 kHz. With the offset voltage VBO, the switching frequency of BJT Q1 may be, for example, as much as approximately 300 kHz. However, offset circuit 102 and offset voltage VBO are not included in every implementation of the present disclosure.
Base discharge switch S1 is coupled across base QB of BJT Q1 and emitter QE of BIT Q1. Base discharge switch S1 can be many different types of switches. For example, base discharge switch S1 can include a transistor, such as an NPN or PNP transistor. Examples of base discharge switch S1 include a metal-oxide field-effect transistor (MOSFET) or a BJT. Base discharge switch S1 can be a low voltage switch and can thereby be provided at a low cost. Various implementations of base discharge switch S1 are not limited to using a single transistor as shown merely as an example in the drawings of the present application. Base discharge switch S1 can take many forms, and can include more than one transistor and might include other circuit elements as well, so as long as base discharge switch S1 can perform the switching functions discussed in the present application.
Base discharge switch S1 is configured to selectively draw current away from base QB of BJT Q1 so as to turn-off BJT Q1. This may be accomplished, for example, by turning-on base discharge switch S1 so as to provide a low resistance path for the current. The current may be drawn away from base QB of BJT Q1 through, for example, offset circuit 102. Base discharge switch S1 is further configured to selectively prevent base current supply 104 from providing current to base QB of BJT Q1. This may also be accomplished, for example, by turning-on base discharge switch S1 so as to provide a low resistance path for current from base current supply 104. Thus, control of BJT Q1 may not be as susceptible to variation in base current supply 104. As such, circuitry may not be required to carefully control switching of base current supply 104. Furthermore, switching of BJT Q1 can remain substantially stable and controllable even where load resistance and/or the bus voltage varies.
Pulse width modulator 106 can be utilized to control base discharge switch S1 using control signal VOFF. Thus, base discharge switch S1 can be controlled by a pulse width modulated signal corresponding to control signal VOFF in
Base discharge switch S1 is also configured to selectively apply offset voltage VBO to base QB of BJT Q1 so as to cause base QB of BJT Q1 to have a base voltage substantially lower than an emitter voltage of BJT Q1. This may be accomplished, for example, by turning-on base discharge switch S1. Furthermore, offset voltage VBO can be selectively applied to base QB of BJT Q1 so as to turn-off BJT Q1. By doing so, base discharge switch S1 can selectively draw current away from base QB of BJT Q1 more rapidly than may otherwise be possible. In this way, BJT Q1 can be made to turn-off rapidly.
Referring to
In switching circuit 200, base current supply 204 is an AC voltage source. More particularly, base current supply 204 includes a multi-winding transformer having primary winding T1:P and secondary winding T1:S (also referred to more generally as an inductor winding). Switching circuit 200 does not require a dedicated multi-winding transformer. Furthermore, the inductor winding may not be part of a multi-winding transformer, which can save on cost. Primary winding T1:P is configured to receive control signal VON. Control signal VON is configured to control turn-on of BJT Q1.
Also in switching circuit 200, offset circuit 202 includes a resistor-capacitor (RC) filter having resistor R2 and capacitor C1. Resistor R2 and capacitor C1 are coupled between base QB of BJT Q1 and base discharge switch S1. As such, base discharge switch S1 can draw current away from base QB through offset circuit 202. Furthermore, resistor R2 and capacitor C1 are coupled between base QB of BJT Q1 and base current supply 204. As such, base current supply 204 can power offset circuit 202 and can further provide current to base QB through offset circuit 202.
Referring to
At time t2, control signal VOFF goes high as shown by waveform 326 to turn-on base discharge switch S1. As such, base discharge switch S1 draws discharge current ID away from base QB of BJT Q1 so as to turn-off BJT Q1. As illustrated by waveform 324, base voltage VB is discharged at time t2 and BJT Q1 is thereby turned-off. Furthermore, in the present implementation, at time t2, base discharge switch S1 optionally applies offset voltage VBO to base QB of BJT Q1 so as to cause base QB of BJT Q1 transistor to have base voltage VB be substantially lower than an emitter voltage of BJT Q1. Thus, waveform 326 shows that offset voltage V80 impresses negative voltage VBN (e.g. −1 volts) on base QB of BJT Q1. As such, base QB of BJT Q1 is rapidly discharged in some implementations.
As control signal VON is low between times t2 and t3, base current supply 204 does not provide charge current ICH to base QB of BJT Q1. As such, base current supply 204 cannot turn-on BIT Q1 between times t2 and t3. If control signal VON were not carefully controlled, control signal VON could be high in waveform 320 at least partially between times t2 and t3. However, in
Thus, base discharge switch S1 controls turn-off of BJT Q1 and thereby can stably control switching of switching circuit 200. Furthermore, while it may be preferred that charge current is AC current in some implementations, charge current can also be DC current (e.g. substantially constant DC current) while maintaining stable switching of switching circuit 200. Also, by including base discharge switch S1, the duty cycle and/or switching frequency of BJT Q1 can be controlled more robustly than may otherwise be practical through control of control signal VOFF. For example, control signal VOFF may be generated based on various conditions, such as feedback from a load.
Thus, as described above with respect to
From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described above, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.
The present application claims the benefit of and priority to a pending provisional application entitled “Bipolar Junction Transistor Switching Control Circuit,” Ser. No. 61/674,654 filed on Jul, 23, 2012. The disclosure in this pending provisional application is hereby incorporated fully by reference into the present application.
Number | Date | Country | |
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61674654 | Jul 2012 | US |