Base drive circuit for high-power switching transistor

Abstract: Disclosed is a base drive circuit (10-1, 10-2) that provides a base drive current particularly adapted to switch a switching power transistor (QA, QB) ON and OFF quickly. The base drive current includes a variable AC impedance device that responds to a control pulse to form a base drive having (a) an initial forward base drive peak that drives the switching power transistor toward deep saturation momentarily, followed by (b) an intermediate drive current of lower magnitude, sufficient to keep a high-power transistor at or near saturation, and (c) a high-amplitude reverse drive current for high-speed turn-off of the transistor.

Description: The present invention relates generally to high-frequency power conversion, and more particularly to a base drive circuit used to turn on and turn off high-power switching transistors. In the design of switch mode power supplies, the object is to produce an AC-type waveform which will then be transformed, rectified and filtered to produce a DC voltage. To generate the AC-type waveform, switch mode power supplies often utilize one, two, or more power switches in various configurations. Symmetrical configurations, those having an even number of power switches, generate AC waveforms by operating the switches so that when one-half of the switches are turned on the other one-half of them are off; one-half of the switches controls the application of a positive DC voltage to a load, while the other one-half controls the application of a negative DC voltage, with the result that by operating the switches in a push-pull manner an AC-type voltage is applied to the load. Bipolar transistors are frequently used as the power switches because of their fast switching speed and ability to handle high voltages and currents. In order to rapidly turn on and off conduction of these transistors, and thus be able to generate a high-frequency AC-type voltage, a base drive circuit couples control pulses to the respective bases of the power transistors to switch them into and out of their conductive states. Switching circuits of the type to which this present invention are directed typically include an input stage that couples a control signal, usually in the form of a pulse train, to the primary winding of an isolation transformer. The secondary winding is coupled to the base lead of the switching power transistor. The switching transistor drives a primary winding of a high-frequency power transformer. In order to achieve high efficiency, the power transistor is operated at relatively high frequency (in the kilohertz range), requiring the power transistor to be driven into its conducting and non-conducting states quickly. To hasten transition of the power transistor from its non-conducting to its conducting state (i.e., OFF to ON), the amplitude of the forward base drive current (IB1) must be relatively high. Unfortunately, if the base drive current is maintained at this amplitude for the time of the pulse the base drive circuit consumes more power than desired, and the design of the base drive circuit must take this fact into consideration. In addition, the continuous high amplitude drives the transistor deeply into saturation, causing the turn-off time to be extended because of the time required to sweep out stored charge from the transistor before it can turn OFF, and resulting in excessive power dissipation in the transistor. Further, in order to drive the switching transistor OFF in as short a transition time as possible, a reverse base drive current (IB2) having a negative-going amplitude can be provided. Unfortunately, to provide such negative-going IB2 signals, a negative power supply is often needed. An example of a known type of power transistor driving/switching circuit is disclosed in U.S patent 3,805,094. This circuit employs a driving transformer having a very high magnetising inductance, provided with an auxiliary winding, and an auxiliary switching transistor driven by the auxiliary winding for short-circuiting the base and emitter of the power switching transistor. The primary winding of the transformer has the terminals connected respectively to a positive voltage source and to the collector of a control transistor through a network comprising a resistor and a parallel connected capacitor. An object of this circuit is to reduce the time needed to turn the power transistor off. This known circuit uses the combination of a diode (7) and transistor (14) to draw current down. A resistor (11) and capacitor (12), in the base drive circuit in Figure 1, does not however perform the waveshaping functions but instead the resistor (11) and the capacitor (12) are used to protect the input transistor (10). Therefore although the object of this prior circuit is to switch a power transistor it still has at least some of the drawbacks outlined above. SUMMARY OF THE INVENTION According to the present invention, a wave-shaping circuit for producing, from a control pulse that alternately assumes a first voltage level and a second, lower voltage level, defining leading and trailing edges, a base drive current for a power switching transistor, including an input terminal for receiving the control pulse, a coupling transformer having a secondary winding coupled to a base lead of the power switching transistor and a primary winding connected with its second terminal to a virtual ground, the wave-shaping circuit having input circuit means comprising first resistance means coupled to receive and communicate the control pulse to a first terminal of the primary winding, and a second resistance means connected in series with a charge storage means, the second resistance means and the charge storage means being connected in parallel with the first resistance means, the input circuit means being responsive to the leading edge of the control pulse for producing a first current having a first amplitude portion, followed by a second amplitude portion of lesser magnitude, the first circuit means further comprising a diode means connected in parallel with the second resistance means and responsive to the trailing edge of the control pulse for producing a second current of opposite polarity to the first current having a third amplitude portion followed by a fourth amplitude portion of lesser magnitude. In the preferred embodiment of the invention, the circuit includes a first resistance that connects an input terminal, adapted to receive a control pulse, to a primary winding of an isolation transformer. The secondary winding of the isolation transformer is coupled to the base of the switching transistor. In parallel with the resistor is a series-connected capacitor and second resistor (of a value less than the first). This combination of components (the two resistors and the capacitor) forms that portion of the wave-shaping circuit that generates the initial and intermediate portions of forward base drive current applied to the switching transistor. Connected in parallel with and across, the second resistor is a high-speed diode configured to turn on when the control pulse terminates. When a control pulse is applied to the input terminal the invention responds to the initial edge of the pulse to induce an initially large current flow through the primary winding of the transformer. As the capacitor charges, the current decreases to a lower steady-state value, until the trailing edge of the control pulse occurs. The diode responds to the trailing edge of the control pulse to turn ON, to create a very low impedance across both resistances, in effect reconfiguring the wave-Shaping circuit in a manner that induces a high reverse current to flow through the primary winding. The reverse current terminates when all the stored charge is swept from the power transistor. The initial and final high current peaks are coupled to the base of the switching transistor, operating to provide a forward-base current (IB1) and a reverse-base current (IB2) that quickly turns the transistor on and off, respectively. At the same time, the initial current peak is followed by a base current of reduced amplitude, sufficient to keep the transistor at or near saturation. This reduced amplitude forward-base drive current reduces the power consumed by the base drive circuit of the power transistor. In addition, the base drive circuit incorporating the present invention is uniquely configured to operate from a single power source. In this respect, a second capacitor sets a DC voltage level that forms the AC ground for the circuit, deleting the need for a second power source. A number of advantages are achieved by the present invention. The invention allows, with a minimum of parts, a technique to produce an ideal IB1 and IB2 current drive ratio that can be optimized to the particular transistor and application. Additionally, the present invention provides a forward base drive current IB1 that is not a continuous DC current level. To the contrary, the forward base drive current of the present invention provides an initial high peak to turn on the switching transistor very quickly, and then drops to a lower DC level sufficient to keep the transistor at or near saturation. The low DC current level is easily adjustable (by the first resistance) to suit the forced beta requirements of the application, and to minimize the power dissipated by the switching transistor. Similarly, the reverse drive base current IB2 level is not a continuous DC level. The present invention provides a short, high reverse current peak to quickly sweep stored charge from the switching transistor and thereby turn it off very fast. This also provides a power dissipation saving because the reverse base drive current IB2 level is now only a brief pulse. Further still, the present invention provides the positive and negative-going peak signals with the use of a single-ended power source. It does not require two power sources, one negative and one positive. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description of the invention, which should be taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a switching circuit having a base drive circuit that includes the wave-shaping mechanism of the present invention; Fig. 2 is a waveform diagram of the input pulse voltage and the current generated by the present invention; Fig. 3 illustrates a switching circuit configured so that a single base drive circuit can drive multiple switching transistors; and Figure 4 illustrates a "unipolar" configuration of base drive circuit. DETAILED DESCRIPTION OF THE INVENTION Turning now to the Figures, and in particular Fig. 1, there is shown base drive circuits 10-1 and 10-2 are respectively coupled to supply base drive currents (via coupling transformers T1 and T2, respectively) IB1 and IB2 to switching power transistors QA and QB. The base drive currents IBA and IBB are formed in response to pulsed control signals P1 and P2 provided by a pulse generator 12. The power switching transistors QA and QB are operated substantially 180° out of phase, each alternately switching between conducting and nonconducting states to generate, in a push-pull manner, an AC-type voltage that is coupled through an output transformer T3. A DC supply 22 provides the DC voltage that is converted by the switching power transistors QA and QB to the AC-type voltage. As will be seen, the base drive currents IBA and IBB include three portions: An initial, fast-rising current peak (IB1) that functions to turn the switching power transistors QA, QB on quickly; a mid-portion current that supplies sufficient base drive to the switching power transistors QA and QB, ideally in proportion to the collector current if possible, to keep the transistors in or near saturation; and a fast-rising, negative current peak (IB2) that operates to sweep out any stored charge, minimizing the storage time and turn-off time. During the OFF time, the bases of the transistors QA and QB are clamped to their respective emitters through low impedances, such as resistors RA and RB, respectively. These low impedances minimize false turn-on due to noise and capacitively coupled currents induced in other parts of the circuit. The diodes DA and DB function to protect the switching power transistors QA and QB, respectively, from reverse voltage spikes appearing across the emitter-collector terminals. The base drive currents, IBA and IBB, are produced by the base drive circuits 10-1 and 10-2, respectively. More specifically, it is the function of the wave-shaping circuits of the present invention, designated by the numerals 14-1 and 14-2, to provide the fast-rising turn-on and turn-off current peaks. As can be seen in Fig. 1, the base drive circuit 10-1 that produces the base drive current IBA for the switching power transistor QA is identical to the base drive circuit 10-2 that produces the base drive current IBB for the switching power transistor QB. Accordingly, only the construction and operation of the base drive circuit 10-1 will be described, it being understood that the description of the construction and operation can be applied equally to the base drive circuit 10-2. As Fig. 1 illustrates, the base drive circuit 10-1 includes a current buffer comprising a PNP transistor Q1 and a NPN transistor Q2 connected in totem pole circuit fashion between a supply voltage V cc and ground. (The current buffer is used in high-power situations, and may not be needed for low-power application.) The base leads of the transistors Q1 and Q2 are connected together and to the pulse generator 12 to receive a control signal P1. The emitter leads of the transistors Q1 and Q2 are connected together and to an input terminal 16 that, in turn, connects to the wave-shaping circuit 14-1 of the present invention. The wave-shaping circuit 14-1, in turn, is coupled to one lead of a primary winding 18 of a coupling transformer T1. The second lead of the coupling transformer T1 is connected to ground through a parallel circuit consisting of a capacitor C3 and a resistor R5 that is shared with the base drive circuit 10-2. Through the use of capacitor C3 the base drive circuits are capable of being operated from a single supply voltage, V cc . The capacitor functions to (1) set an AC ground, as noted above, and (2) provide energy for both base drive circuits 10-1 and 10-2 without itself dissipating power. The secondary winding 20 of the transformer T1 has one lead connected to the base terminal of the power switching transistor QA, the other lead being connected to the emitter lead of said transistor. The wave-shaping circuit 14-1 includes a first resistance R1 that connects the input terminal 16 to the primary winding 18. Connected in parallel with the first resistance R1 are a capacitance C1 and a second resistance R2 connected in series circuit configuration with each other. A fast-switching diode D1 is connected in parallel across the second resistance R2, the diode D1 having its anode lead connected to the capacitance C1 and its cathode lead connected to the resistances R1 and R2, as well as the primary winding 18 of the coupling transformer T1. In operation, referring to Figs. 1 and 2 jointly, the control signal P1, a waveform having negative-going pulses, is applied to the base leads of the transistors Q1 and Q2. Typically, the control signal P1 has a pulse reoccurrence frequency (PRF) of approximately 25KHz, a pulse width (PW) of approximately 10 microseconds. As illustrated in Fig. 2, the control signal P2 that is applied to the base current drive circuit 10-2 is approximately 180° out of phase with the control signal P1. When the control signal P1 is HIGH the PNP transistor Q1 is OFF and the NPN transistor Q2 is ON connecting the supply voltage V cc to the input terminal 16. When the control signal P1 goes LOW, transistor Q2 turns OFF and transistor Q1 turns ON, pulling the input terminal 16 toward ground. This provides a discharge path for charged capacitor C3, creating a current to flow through the primary winding 18, the wave-shaping circuit 14-1, and through the transistor Q1 to ground. The value of that current flow will be determined primarily by the resistance R2 (actually by a resistance formed by the parallel configuration of resistances R1 and R2, but if resistance R2 is much less than resistor R1, resistance R2 will control) because initially the capacitor C1 will be a low impedance and transparent to current flow. This initial current flow is coupled by the transformer T1 to the base of the switching power transistor QA as the base drive current IB1, appearing thereat as the TURN-ON PEAK current illustrated in Fig. 2. While the control signal P1 is LOW, the capacitor C1 charges, and as it charges the current flow through the primary winding 18 will decrease exponentially until a lower value is reached, resulting in the mid-portion of the base drive current IB1 labeled DC LEVEL in Fig. 2. This DC LEVEL is determined primarily by the resistance R1 since the resistance R2 has been taken out of the picture by the (now) substantially charged capacitor C1. In effect, C1 acts as a variable AC impedance device. When the control signal P1 returns from the LOW to a HIGH the PNP transistor Q1 turns OFF, NPN transistor Q2 turns ON, and the input terminal 16 is pulled to the supply voltage V cc . Transistor Q2 couples a voltage to the anode of the diode D1 that is higher than that applied to the cathode, forward-biasing the diode D1 and turning it ON. This action effectively places a current short across the resistances R1 and R2. Current that had previously been flowing through the primary winding 18 of the transformer T1 from the wave-shaping circuit 14-1 reverses and begins recharging capacitance C3, limited only by the relatively negligible impedances presented by the diode D1, the capacitance C1, and the resistive impedance of the primary winding 18. Accordingly, at this time the base drive current IBA applied to the base of the switching power transistor QA experiences a rather large reverse current (IB2) turn-off peak, as illustrated in Fig. 2, that functions to sweep out stored charge at the junctions of the transistor QA, quickly turning it OFF. The base drive current IBB produced by the base current drive circuit 10-2 operates in a substantially identical manner in response to the control signal P2 which, as indicated above, is 180° out of phase with the control signal P1. Note that the power switching transistors QA and QB are driven ON by a high initial current, but are not driven deeply into and held in deep saturation. Rather, the TURN-ON PEAK of the base drive current IBA functions to provide a high amplitude drive current for a short time to drive the switching power transistor quickly into conduction. Thereafter, the base drive current returns to a lower amplitude (DC LEVEL) to inhibit the switching power transistor QA from dissipating excess power. In addition, the switching power transistor can be turned OFF much quicker because it is not deeply in saturation at turn-off time, assisted by the TURN-OFF PEAK that quickly sweeps out stored charge. The switching circuit of Fig. 1 illustrates use of the invention in a switching circuit that uses one base drive circuit 10-1 or 10-2 to operate one switching power transistor QA or QB, respectively. Fig. 3 illustrates a circuit configuration in which the coupling transformer T1 (Fig. 1) is replaced with a coupling transformer T1′ (Fig. 3). As Fig. 3 shows, the transformer T1′ has a single primary winding 18′ and multiple (two in Fig. 3) secondary windings 20′. Each secondary winding is coupled to a corresponding switching power transistor Q5, Q6. Operation is substantially the same as that described above with respect to Fig. 1. Fig. 1 illustrates what may be called a balance base drive circuit configuration; that is, the base drive circuits 10-1, 10-2 share the AC ground-setting capacitor C3. The alternate operating cycles of the base drive circuits (since operation of each base drive circuit 10-1, 10-2 is 180° out of phase of the other) function to establish an AC ground at voltage node A somewhere between V cc and ground. If, however, only one of the base drive circuits 10-1, 10-2 is used, compensation for the absence of the absent base drive circuit must be made; otherwise, the voltage node corresponding to voltage node A in Fig. 1 will tend to rise toward V cc . Such compensation is illustrated in Fig. 4, showing a base drive circuit 10-1′ in a "uni-polar" configuration. In order to ensure that the voltage node A establishes an AC ground with sufficient operating clearance between V cc and ground, there is added the parallel configuration of capacitor C3′ and resistor R5′ that connect the voltage node A′ to V cc The values of C3′ and R5′, as will be apparent to those skilled in the art, will establish the DC value that the voltage node A′ assumes during operation. It will be appreciated by those skilled in this art that the invention is capable of modification. For example, it is possible that the wave-shaping circuits 14-1 and 14-2 can be made a part of the base circuits of the power switching transistors QA and QB, respectively. This is believed less desirable, however, because circuit layout becomes much more difficult if the International Safety Standard (VDE0806)need be met. Having the wave-shaping circuits 14-1, 14-2 as shown in Fig. 1 makes it easier to meet creepage and clearance requirements. In summary, there is provided in accordance with one aspect of the invention a wave-shaping circuit to produce a base drive current from a control pulse signal (such as a square wave), comprising means to produce a fast-attack forward base current in response to a portion of the control signal (such a change in level of the square wave going in one direction), means to cause that current to be followed by an intermediate level forward base current, and means to produce a fast-attack reverse base current in response to a further portion of the control signal (such as an oppositely going change in the square wave).

Claims: A wave-shaping circuit for producing, from a control pulse (12) that alternately assumes a first voltage level and a second, lower voltage level, defining leading and trailing edges, a base drive current for a power switching transistor (QA), including an input terminal (16) for receiving the control pulse, a coupling transformer (T1) having a secondary winding (20) coupled to a base lead of the power switching transistor (QA) and a primary winding (18) connected with its second terminal to a virtual ground (A), the wave-shaping circuit having input circuit means (14-1) comprising first resistance means (R1) coupled to receive and communicate the control pulse to a first terminal of the primary winding, and a second resistance means (R2) connected in series with a charge storage means (C1), the second resistance means and the charge storage means being connected in parallel with the first resistance means, the input circuit means being responsive to the leading edge of the control pulse for producing a first current having a first amplitude portion, followed by a second amplitude portion of lesser magnitude, the first circuit means further comprising a diode means (D1) connected in parallel with the second resistance means and responsive to the trailing edge of the control pulse for producing a second current of opposite polarity to the first current having a third amplitude portion followed by a fourth amplitude portion of lesser magnitude. The wave-shaping circuit of claim 1, including a second charge storage means (C3) coupled to the second terminal of the primary winding and a third resistance means (R5) connected in parallel with the charge storage means coupling the primary winding to a ground potential. The wave-shaping circuit of claim 2, wherein the secondary winding (20) of the coupling transformer is coupled between a base lead and an emitter lead of the power switching transistor. The wave-shaping circuit of claim 1, wherein there is at least a second power transistor (QB) alternately switched by the wave-shaping circuit by the control pulse operating between first and second voltages, and the wave-shaping circuit including at least a pair of coupling transformers (T1,T2) each having at least one secondary winding (20) coupled to a base lead of a corresponding one of the power switching transistors (QA,QB), and a primary winding (18), charge storage means (C3) coupled to the second terminal of each primary winding, the input circuit means being coupled to the first terminal of the primary winding.

Inventors: PAUKER ARMANDO, PREUIT RICHARD B, PAUKER, ARMANDO, PREUIT, RICHARD B.
Applicants: TANDEM COMPUTERS INC, TANDEM COMPUTERS INCORPORATED
Application year: 1986
Main classification: H03K17
IPC codes: H03K17, H02M7