Current Source Direct Base Drive for Transistor Power Switches

1. Field of the Invention

The present invention relates in general to power transistors operated in a switching mode, and more specifically, to a direct base drive circuit used to drive power transistors.

2. Description of the Prior Art

It has long been common practice to operate power transistors in a nonlinear mode as on-off switches. Commercial use for such transistor power switches has, for example, found successful application in power conversion systems.

Of key importance in the operation of transistor power switches in the base drive circuit, which drives the transistor into its "off" or "on" state. Base drive circuits, which are common in the prior art, are of two types: transformer coupling; and direct base drive, in which a transformless coupling is used.

The transformer coupling is limited to applications which only require operation over a limited or substantially fixed range of duty cycle. Duty cycle is defined as the fractional ratio of the time period during which the output power switch is "on", to the total time period for the repetitive on-off cycle. Since magnetic coupling devices require operation with alternations of equal volt-second wave form areas, a transformer coupled base drive circuit experiences saturation effects and consequently a collapse of drive signals at duty cycle extremes. This precludes application of transformer-coupled base drive circuits in, for example, "boost regulator" circuits which require a duty cycle operation of from zero to 80 percent.

High performance "transformerless" direct base drive circuits are known in the art and are not subject to duty cycle restrictions. They are, however, limited to applications which do not require high output power levels. Direct base drive circuits, which are known in the art, drive the power transistor from a normal voltage source. Such voltage source drive circuits produce unfavorably large "turn-on" switching losses at high output power levels, due to their inability to deliver and maintain a high level of base current with adequate fast rise time during the switch "on" event. The rate of rise of base current drive, which can be produced by a voltage source, is limited by circuit inductances and load current reaction resulting from the power transistor output current. This detrimental turn "on" switching loss is experienced for output power levels of 1 kilowatt or greater.

The object of this invention is to correct deficiencies in base drive capacity, experienced with the prior art, in an efficient and economic manner. To accomplish this a current source base drive circuit was sucessfully devised.

The novel feature of the present invention resides in the use of a constant current source which forces base drive current into the power transistor regardless of the above-described transient effects caused by unwanted base drive circuit stray inductances or fast rising load currents. Abrupt switching of the power transistor is accomplished by "steering" the current from the constant current source into the base of the power transistor for the "on" mode and through a reversed-biased voltage source for "off" mode.

Another novel feature is the modulation of the current source such that the current driven to the base of the power transistor is at an amplitude sufficient to keep the power transistor in saturation, but not to overdrive it. This results in increased efficiency and faster switching.

FIG. 1 is a schematic of a voltage source direct base drive which is representative of the prior art.

FIG. 2 is a diagrammatic representation of the base drive current "turn-on" response when a voltage source drive, known in the prior art, is used and low output power is required.

FIG. 3 is a diagrammatic representation of base drive current and collector-emitter voltage during "turn-on" response when the prior art voltage source drive is used and an output of approximately 1 kw is required.

FIG. 4 is a schematic of an equivalent circuit for a current source base driver.

FIG. 5 is a schematic of the constant current source base drive circuit.

FIG. 6 is a diagrammatic representation of base drive current when the constant current source base driver is used and low output power is required.

FIG. 7 is a diagrammatic representation of base drive current and collective-emitter voltage, during turn on response, when a constant current source base drive is used and output power of approximately 1 kw is required.

FIG. 8 is the schematic of the constant current source base drive circuit using a MOSFET for the "steering" function.

FIG. 9 is a schematic of a simplified current source direct base drive in which a single transistor provides both constant current and "steering" functions.

FIG. 10 is a schematic of a "programmed" current source base drive circuit.

A typical transformerless direct base drive circuit, known in the prior art, is shown in FIG. 1. The power transistor 10 is switched abruptly "on" by "turning-on" transistor 12, which acts as an "on-off" switch. With transistor 12 "on" base drive is supplied to the power transistor from the positive voltage source 14 through the limiting resister 16. The output transistor 10 is, thus, driven from a normal voltage source 14 with the rate of rise of the base drive current to the power transistor 10 bucked by circuit inductances and load reaction caused by the output current in the emitter ground power connection. The power transistor is switched "off" by using switching circuit 19 to simultaneously switch transistor 12 "off" and Transistor 18 "on" thereby abruptly reverse biasing the power transistor, switching it into the "off" condition.

The above-mentioned circuit inductances and load current reaction resists the rise of needed base current during turn-on, just the instant in time that base drive is needed. FIGS. 2 and 3 show the diagrammatic effect load current amplitude has on switching performance when the prior art voltage source drive is used. These figures show base drive current and collector-emitter voltage actually measured on the prior art drive circuit. Curve 20 shows base current versus time, with the condition of essentially zero power in the output collector-emitter circuit of the power transistor. As the load current increases, the rapid rise time, shown in curve 20, deteriorates. Curve 22 shows base current versus time when appreciable power, approximately 1 kw power, is permitted to flow in the output circuit. The effect of the rapid rise in output load current, as the power transistor attempts to turn "on", induces base-emitter voltages which buck the base drive current (curve 22; note that the rise of base current is arrested and, in fact, reduced back to nearly zero for a transient time interval. Curve 24 shows collector-emitter voltage when appreciable power flows in the output circuit (1 kw). It can be seen from curve 24 that collector-emitter voltage starts to fall rapidly and then actually rises again in response to the undesired base current perturbation before finally dropping to nearly zero as the output transistor finally attains the "on" conducting state. During the "turn-on" transient appreciable switching loss occurs, as a result of this "hesitation" in the collector-emitter voltage fall characteristic. This collector-emitter voltage "hesitation" is the direct result of the inability of the voltage source drive method used in the prior art, to deliver and maintain a high level of turn on drive during the "switch-on" event.

The invention remedies this collector-emitter voltage "hesitation" by using a constant current drive which forces current into the base of the power transistor regardless of the transient effects of fast rising load currents. FIG. 4 shows the equivalent circuit for this new direct base drive. The constant current source 26 forces base current into the power transistor 28 regardless of the above-discussed transient effects. Switching the power transistor 28 "off" is accomplished by "steering" the current from the constant current source 26 through a reverse-bias voltage source 32. This provides a path for the drive current during the "off" mode and also any reverse "junction-clearing" current which can naturally flow out of the power transistor base. In the equivalent circuit the "steering" is accomplished by switch 30.

A simplified schematic of the constant current source base drive is shown in FIG. 5. The PNP transistor 36 acts as a simple current source and supplies constant current. The constant current capabilities of the circuit are further enhanced by the inclusion of a base drive inductor 37. This inductor permits the "on" drive to be supplied from a low voltage positive source through a "slow" low-cost transistor operating as a constant current source. This inductor forces base current into the power transistor 38 despite base-emitter voltage reaction, thus, providing extremely high compliance. The NPN transistor 40 provides the functional equivalent of the switch 30 in FIG. 4, receiving square wave control signals 41 which drive it into "on" or "off" states. It thus steers the base drive current into the power transistor 38, for turn "on", or into the negative side of the low voltage power source 41 for reverse biasing and turn "off".

FIGS. 6 and 7 are diagrammatic representations of the constant current base driver's performance. For ease of comparison, FIGS. 6 and 7 are of the same scale as FIGS. 2 and 3, which represent turn "on" response of the prior art voltage source driver. Curve 44 shows base drive current turn "on" response for the current source drive. Comparison of curve 44 with the prior art response curve 20 shows that the current driver achieves superior response even at low output power, with three amps being obtained in about 300 nanoseconds as opposed to 1,200 nanoseconds for the prior art. The dramatic difference between the prior art drive method and the current source drive method can be seen when the power transistor is delivering high power. Curve 46 shows the base current drive response and curve 50 shows the collector-emitter voltage response during turn "on" when there is a high power condition. The current source driver forces base current to rise abruptly (see curve 46) despite reaction of the fast rise collector-emitter current which is evidenced by ringing in the base current. Comparing curve 46 with curve 22 shows that the current drive circuit, by forcing the base current, can achieve three amps current drive in 300 nanoseconds (its same time response as with low load) whereas, the prior art drive requires 1,600 nanoseconds to achieve the same three amp base current level. Curve 50 shows the collector-emitter voltage turn "on" response when there is a high power condition. The clean fall of the collector-emitter voltage (see curve 50) is evidence of much reduced switching losses. Curve 50 does not show the severe "hesitation" and resulting losses associated with the prior art voltage drive circuit (see curve 24). Thus, the constant current driver produces quick response and reduced switching losses not before possible with the prior art driver methods.

Once the current source concept has been made apparent further refinements of its embodiment are possible. One such alternative embodiment is shown in FIG. 8. In this circuit a MOSFET 52 is substituted for the steering transistor 40 (FIG. 5). Since MOSFET devices can switch about ten times faster than bipolar transistors, this modification would be useful if an even higher rate of rise of base drive current were required.

Another refinement is the combination of the constant current function of transistor 36 (see FIG. 5) and the current steering function of transistor 40 into a single transistor circuit. This simplified embodiment using one transistor, shown in FIG. 9, would have application when lower power is required and when the duty cycle range need only be from the zero percent to approximately 80 percent range. Power switching transistor 54 is biased "off" during the time interval when the driver transistor 56 is "on". During driver transistor's 56 "on" period it operates in its linear mode and sets the level of current in the base drive inductor 58 by "sensing" the voltage across resistor 60 and controlling this voltage drop to match the referenced potential established at the driving transistor 56 base to within one base-emitter forward biased junction potential. To abruptly switch power transistor 54 "on" the driver transistor 56 is switched "off". This forces the current in the base drive inductor 58 to flow to the base of the power transistor, effectively "swamping out" the much smaller, previously discussed, transient effects. Of course, continuous "on" base drive is no longer possible with this simplified circuit, since the energy storage in the base drive inductor has limits. When the driving transistor 56 is turned back "on" by the square wave control signals 62, the base current to the power transistor 54 is terminated and a low-impedance path for this current is provided through capacitors 68, 64, and through the driver transistor 56. During this interval the P-N junction drop across diode 70 provides additional reversed bias to assure fast switch off of the power transistor 54.

An additional novel feature of the current base drive is that it enables easy programming of the base current amplitude. By controlling the bias of the transistor current source through simple means, one can set the amplitude of the base drive current to be responsive to various quantities, such as, duty cycle or collector current. For example, FIG. 10 shows the simplified embodiment in which the base drive circuit is "programmed" to be linearly proportional to the duty cycle. Since it is known that the power transistor current drive requirements increase with duty cycle, it is advantageous to program drive current amplitude as a function of duty, providing a selected minimum drive at nearly zero duty and increasing smoothly as the duty is increased. This programming prevents overdriving of the power transistor, thereby increasing efficiency and switching speed. The programmed driver, shown in FIG. 10, has a steering circuit 72, a current source circuit 74, and a programming circuit 76, which controls the bias voltage to the transistor current source. Rectangular wave signals are generated in a control circuit 78, which senses the duty cycle, and are fed to the base of transistor 80. Transistor 80 acts as an "on-off" switch alternately connecting and disconnecting resistor 82 in parallel connection with resistor 84. The average resistance of the combination of resistor 82 and resistor 84 determines the voltage at the base of transistor 86, thereby, controlling the amplitude of the base drive current to the power transistor 88. Capacitor 90 filters the control signal at the base of transistor 86 so that the current output of transistor 86 is the average response to the currents being drawn through resistor 82 and resistor 84. At near zero duty, transistor 80 is "off" the majority of the time, hence, resistor 84 is selected to set the minimum base drive current level. At near unity duty, transistor 80 is switched "on" the majority of the time by the rectangular wave control signals 78, hence, the value of resistor 82 is selected so that, in parallel with resistor 84, current source transistor 86 is biased to provide the maximum base drive required. In this way a programmed base drive is obtained which adjusts base current amplitude to match the requirements of the power transistor.