DC-DC converter

Abstract: A DC-DC converter includes a transformer (5), a capaci­ tor (6) connected in parallel with a primary winding of the trans­ former, a switching transistor (4) connected between one end of the primary winding of the transformer and one end (Vcc) of a DC power source, and a blocking oscillating circuit (3) for driving the switching transistor (4). The blocking oscillating cir­ cuit (3) is directly driven by the DC power source and contains circuits (17, 18; 13c, 19, 20) for properly setting a pulse width and a pulse stop period of output pulses from the blocking os­ cillating circuit.

Description: The present invention relates to a DC-DC converter including a blocking oscillating circuit and a voltage resonance type switching circuit. A DC-DC converter of the type in which a first DC voltage is converted into a voltage with a rectangular wave by means of a switching circuit, the rectangular wave voltage is smoothed through a transformer, and the smoothed voltage is supplied to a load, as a second DC voltage, has been known. This type of DC-DC converter, because of meritorious features of small size and light weight, has widely been used as DC power sources for terminal equipment of computers and general communication equipment. This is especially true for a small capacity DC-DC converter with an output of 200 W or less of the single end type in which a switching element for turning on and off a first DC voltage is inserted between one end of the transformer and a first DC voltage source, because of its low cost and simple construction. For further detail of the single end type DC-DC converter, reference is made to IEEE Power Electronic Conference, 1977, pp 160, "A New Optimum Topology Switching DC-DC converter". A basic switching scheme in the DC-DC converter is categorized into a buck power stage system, a boost power stage system, a buck-boost power stage system, and a boost-buck power stage system. Common to those systems is a conversion of the DC power source voltage having a voltage with a rectangular wave. The common feature increases a switching loss of the switching element making up the switching circuit to necessitate a radiator plate for radiating heat. A self-exciting type circuit with omission of an oscillating circuit for driving the switching circuit, employed in the switching system, hinders the increase of the switching speed by a feedback action. A voltage resonance type switching system as a modification of the buck-boost power stage system as mentioned above, allows the improvement of a switching efficiency by shaping "L" the switching characteristic of the switching element. The "L shaping" of the switching characteristic means that in the graph in which a current of the switching element is plotted on an ordinate and a voltage between terminals of the switching element is plotted on an abscissa the switching element is operated to form the voltage-current characteristic of "L" Shape. This approach indeed succeeds in increasing the switching efficiency, but needs an external drive circuit and an auxiliary power source for driving the switching element, resulting in the-complexity of the construction of a DC-DC converter. In this respect, there has been a demand for driving the switching element by the output of an oscillating circuit driven by the first DC voltage mentioned before. The attempt, however, has been unsuccessful in that the mere use of an oscillating circuit provides an unstable or abnormal oscillation output due to a counter action of the switching circuit. FR-A-2 298 898 discloses a DC-DC converter having a transformer with a leakage inductance and an exciting inductance in the primary side. A switching element is connected between the primary side of the transformer and an input DC power source, the switching element periodically interrupting the voltage of the DC power source. A blocking oscillator is energised from the DC power source and controls the switching element; the blocking oscillator includes stabilizing means. Accordingly, an object of the present invention is to provide a DC-DC converter which ensures a stable DC-DC conversion, in which, in a DC-DC converter having a switching element inserted between a DC power source and an input terminal of a transformer, for example, a blocking oscillator with an output stabilizing means directly driven by the DC power source voltage is used as an oscillator for driving a switching circuit of the voltage resonance type. According to the present invention, there is provided a DC-DC converter comprising: a transformer with a leakage inductance and an exciting inductance in the primary side; a capacitor connected in parallel with said primary side of said transformer; switching means including a switching element connected between said primary side of said transformer and an input DC power source, said switching means periodically interrupting a voltage of said DC power source; means for rectifying and smoothing an output signal from said secondary side of said transformer and supplying the rectified and smoothed output signal to a load; and a blocking oscillating circuit with output stabilizing means which directly receives a voltage of said DC power source and controls said switching means by the oscillating output; characterised in that said blocking oscillating circuit comprises, an auxiliary transformer including a primary winding connected in series to a switching drive transistor and excited by the voltage of said input DC power supply source, a first secondary winding for applying a feedback signal to the base electrode of said switching transistor, a second secondary winding for applying a control signal to the control inputs of the switching element for intermittently interrupting said switching means, and a third secondary winding; a first setting means connected to said first secondary winding for setting a pulse width of the oscillating output pulse from said second secondary winding; a second setting means connected to said third secondary winding for setting a pulse stop period from said second secondary winding; and electrostatic shielding means for electrostatically shielding said second secondary winding from the remaining windings of said transformer. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram of a prior blocking oscillating circuit; Figs. 2A to 2E and 3 are waveforms useful in explaining an operation of the circuit shown in Fig. 1; Fig. 4 is a circuit diagram showing an embodiment of a DC-DC converter according to the present invention; Fig. 5 shows a circuit diagram of the blocking oscillating circuit shown in Fig. 4; Figs. 6A to 6C are waveforms useful in explaining the operation of the blocking oscillating circuit shown in Fig. 5; Figs. 7A to 7C show waveforms useful in explaining the operation of the DC - DC converter shown in Fig. 4; and Figs. 8 is a circuit diagram of a modification of the blocking oscillation circuit shown in Fig. 5. Fig. 4 shows an embodiment of DC - DC converter according to the present invention. An AC source of 100 V. for example, is rectified by a full-wave rectifying circuit 1, and a rectified output is smoothed by a smoothing capacitor 2, thereby to obtain an input DC voltage Vcc. A blocking oscillating circuit 3 is supplied with the input DC voltage Vcc and a voltage G of a ground level as its inputs. A transformer 5 for power transmission includes a primary coil 5a and a secondary coil 5b with polarities indicated by black dots. The primary coil 5a is connected in parallel with a resonance capacitor 6. A collector-emitter path of a power transistor 4 is connected between the power source voltage Vcc and one end of the primary coil 5a. A damper diode 7 polarized as shown is connected in parallel to the collector-emitter path. An output terminal B of the blocking oscillating circuit 3 is coupled with the base of the transistor 4, and another output terminal E is connected to the emitter of the transistor 4. The power source voltage Vcc-G (referred often to as Vcc) is converted into a voltage with a rectangular wave by the transistor 4 controlled by the outputs B and E of the oscillating circuit 3, and the rectangular wave is taken out after being voltage-converted by the primary coil 5a and the secondary coil 5b magnetically coupled with each other. The secondary coil 5b is connected in series to a choke coil 8 and a rectifying diode 9, and the voltage-converted rectangular wave is supplied with a predetermined load 11 after being smoothed by the smoothing capacitor 10. In the DC-DC converter shown in Fig. 4, an output characteristic of the blocking oscillating circuit 3 must be extremely stable. The blocking oscillating circuit used in the present invention has a novel output stabilizing means. The output stabilizing means includes a means which is either for keeping both an output pulse width and a pulse stop period constant or for freely setting the output pulse width and the pulse stop period. The blocking oscillating circuit 3 can be modified variously. For clearer understanding the differences of the arrangement, function and operation of those modifications from a prior blocking oscillating circuit, the prior blocking oscillating circuit will be described referring to Figs. 1, 2A to 2E and 3. In Fig. 1, a series circuit of a resistor R and a capacitor C and another series circuit of a primary coil TA of a transformer T and a switching transistor TR are connected between terminals of a power source E. A base of the transistor TR is connected to a junction point of the resistor R and the capacitor C via a secondary coil TB. A diode D with polarities as shown is connected between the terminals of the primary coil TA. An oscillation output is taken out from the tertiary coil TC. Fig. 2 shows a waveform of a collector voltage of the transistor TR, Fig. 2B shows a waveform of a charge/discharge characteristic of the capacitor C, Fig. 2C illustrates a waveform of a collector current of the transistor TR, Fig. 2D shows a waveform of a base current of the transistor TR, and Fig. 2E illustrates a waveform of an output from the tertiary coil TC. Fig. 3 shows the detail of a charge/discharge characteristic of the capacitor C. During the output pulse stop period T s , the capacitor C is charged negative and the transistor TR is in the cut-off condition. During this period T s , the charge in the capacitor C is discharged to the power source E through the resistor R, and the potential across the capacitor C rises to become positive, a base current flows through the transistor TR, so that the transistor TR is turned ON. More particular, because of the flow of the collector current, a positive feedback is active, and the base current increases, with the result that the transistor TR is rapidly saturated. During a period T p (a pulse period) that a voltage of the power source E is fully applied to the primary coil TA, the base current gradually decreases with a gradual increase of an exciting current for an inductance of the primary coil TA. At an instant that a value obtained by multiplying the base current by a current amplification factor of the transistor TR becomes equal to the collector current, the transistor TR is rapidly cut off. At the cut-off time, the exciting current residual in the primary coil TA is absorbed by the diode D. The cut-off condition is kept until the capacitor C charged during the OFF period of the transistor TR is discharged. That is, the pulse stop period T s is set up. In the blocking oscillating circuit shown in Fig. 1, an internal resistance of the transistor TR is very small at the time of its saturation. Then, it is deemed that the transformer T is shorted by the transistor TR. Therefore, the output impedance of the blocking oscillating circuit is low and therefore the oscillating circuit is suitable for a pulse power source. However, when thw voltage of the power source E is high or the circuit is operated with a high frequency output, the rise and fall characteristics of the output pulse become poorer, and the stop period T s becomes shorter than a designed value. The reason for this follows. When transistor TR shifts from the ON state to the OFF state, the current flowing through the secondary winding TB is inverted. Nevertheless, the transistor TR is incompletely cut off. Therefore, the capacitor C is temporarily shorted through the transistor TR. As the result of the instantaneeous shortcircuiting, the charge in the capacitor C is partially lost as shown in Fig. 3. Also, there is a case that the collector current and the base current vary with temperature and hence the oscillation frequency is not stabilized. Therefore, it is difficult to directly apply the prior blocking oscillation circuit shown in Fig. 1 to the DC-DC converter shown in Fig. 4 without modifying. An embodiment of the blocking oscillating circuit 3 used in a DC-DC converter according to the present invention is illustrated in Fig. 5. Since the blocking oscillating circuit 3 has a small output impedance, as well known, it is convenient in use when the switching element 4 is a bipolar transistor. In Fig. 5, a collector-emitter path of an npn switching drive transistor 14 is connected between the input terminals Vcc and G in series with a resistor 12 and a primary winding 13a of a drive transformer 13 . A series circuit of a resistor 15 and a capacitor 16 is connected between the input terminals Vcc and G, as a time constant circuit to determine an off period of the transistor 14. The not grounded terminal of the capacitor 16 is connected to the base of the transistor 14 through a secondary winding 13b of the drive transformer 13 . A base current is designated by i b . The polarities of the primary winding 13a and the first secondary winding 13b of the transformer 13 are set so as to be opposite to each other, as indicated by the black dots. In other words, the polarities are set so that when an excitation current of the primary winding 13a is generated backward, it is fed back to the base of the transistor 14. A series circuit of a resistor 17 and a diode 18 with the polarity as shown is connected in parallel between the terminals of the capacitor 16. The pulse width of the oscillating circuit 3 is adjusted by means of this series circuit to determine the discharge characteristic of the capacitor 16. Further, a resistor 20 and a capacitor 21 are connected in parallel with the series circuit of a third secondary winding 13c of the transformer 13 and a diode 19 with the polarity as shown, thereby to determine a pulse stop period of the output signal of the oscillator 3. Further, a second secondary winding 13d to lead out an output pulse of the oscillating circuit 3 is provided in the transformer 13. An output B of the second secondary winding 13d is applied to the base of the transistor 4 and an output E thereof is also applied to an emitter of the transistor 4. A resistor 22 is connected between the outputs B and E. A parallel circuit of a resistor 24 and a diode 23 with the polarity as shown is provided to prevent the backward current from one terminal of the second secondary winding to the output terminal B is inserted between both the terminals. A shield plate 25 designated by a dotted line is set between the second secondary winding 13d and the other windings to prevent a voltage induced by a voltage change, as a reaction to the switching operation of the transistor 4, from sneaking into the winding 13a, 13b and 13c except the second secondary winding 13d. The potential of the electrostatic shield plate 25 is kept at one level G of the input DC voltage, thereby to prevent a capacity coupling of the second secondary winding 13d with the remaining windings 13a, 13b and 13c. Accordingly, the induced voltage arising from a voltage change of the switching circuit including the transistor 4 is prevented from reaching to the blocking oscillating circuit 3. An operation condition of the DC-DC converter shown in Fig. 4 is set up as given below. It is assumed that the switching circuit including the transistor 4 executes a switching operation of the voltage resonance type with a sub-class E mode. More specifically, a magnetic energy stored in an inductance of the primary winding 5a of the transformer 5 during the conduction of transistor 4 is charged into a capacitor 6 for the resonance, during the off-period of the transistor 4. As a result, an output voltage of the transistor 4 changes like a sine wave. A voltage between the collector and the emitter of the transistor 4 is set at zero through the action of the damper diode 7 after the half period of the sine wave. The transistor 4 is driven so as to conduct again during the period that the collector-emitter voltage is zero. Specifically, a conduction period and a nonconduction period of the transistor 4 are set by an inductance of the primary winding 5a of the transformer 5 , a choke coil 8 including a leakage inductance and a capacitance of the resonance capacitor 6, for supplying the electric power to the load 11 at the input voltage Vcc. Even if a feedback winding (not shown) is provided in the transformer 5 , and the output from the feedback winding is fed back to the base of the transistor 4, it is impossible to satisfy the operating condition. On the other hand, the DC-DC converter according to the present invention can execute the switching operation of the voltage resonance type by driving the transistor 4. The conduction period of the transistor 4 is substantially determined by an inductance of the primary winding 13a of the blocking oscillating circuit 3, the current amplification factor β of the transistor 14, and the resistance of the resistor 17 for setting the pulse width. The off-period of the transistor 4, i.e. a nonconduction period, is substantially determined by the time constant given by the resistance of the resistor 15 and the capacitance of the capacitor 16, and the resistance of the resistor 20 for setting the off-period of the transistor 4. With this circuit arrangement, only the pulse width of the output signal from the blocking oscillating circuit 3 may be changed by properly selecting a set value of the resistor 17 for setting the pulse width, without any change of the basic operating condition in the oscillating circuit 3. In other words, this does not affect the determination of the off-period of the transistor 4. Additionally, only the off-period of the transistor 14 can independently be set by the set value of the resistor 20 for setting the off-period of the transistor 14, without any effect upon the pulse width setting condition of the output pulses of the transistor 14. With the arrangement to independently set the pulse width of the output pulses from the oscillating circuit 3 and the pulse stop period, the operating condition of the apparatus shown in Fig. 4 can be satisfied by a simple circuit construction. How the output voltage waveform of the transistor 4 in the DC-DC converter shown in Figs. 4 and 5 affects the oscillating condition of the blocking oscillating circuit 3, will be discussed referring to waveforms shown in Figs. 6A to 6C, and Figs. 7A to 7C. Waveforms in the respective portions of the blocking oscillator 3 are illustrated in Figs. 6A to 6C. Waveforms in the respective portions of the switching circuit containing the transistor 4 are shown in Figs. 7A to 7C. In Fig. 6A, a time length 0-T ON indicates an on-period of the transistor 14; a time length T ON -T1 an off-period of the transistor 14; i c a collector current of the transistor 14; i cp a peak value of the collector current i c . In Fig. 6B, i b designates a base current of the transistor 14, and i bo indicates a base current for determining a current amplification factor β=i cp /i bo of the transistor 14. In Fig. 6C vc designates a terminal voltage across the capacitor 16, Vs=n b ×Vcc (n b designates a turn ratio of the primary winding 13a of the transformer 13 to the first secondary winding 13b), V R =n Vconst (n indicates a turn ratio of the tertiary winding 13c of the transformer 13 to the secondary winding 13b, and Vconst stands for a terminal voltage across the resistor 20), V BE designates a minimum base voltage required for the conduction of the transistor 14. In Figs. 6A to 6C, a waveform indicated by a broken line is the one produced due to a voltage induced in the capacitor 16 during the off-period of the transistor 4. In Fig. 7A, i c stands for a collector current of the transistor 4, 30 a waveform of the current i c when the load 11 has a rated value, lp1 a peak value of the waveform 30, 31 a waveform of the current i c when the load 11 is light, Ip2 a peak value of the waveform 31. In Fig. 7B, vc designates an output voltage of the transistor 4, 32 a waveform of the voltage vc at the rated load, vcmax1 its peak value of the waveform 32, 33 a waveform of the voltage vc at a light load, vcmax2 its peak value of the waveform 33, Vcc an input voltage level of the oscillating circuit 3. In Fig. 7C, iLP represents a waveform of an excitation current flowing through the primary winding 5a of the transformer 5 at the rated load, 34 a waveform of the current iLP at the rated load, and I LP1 the peak value of the waveform 34, 35 a waveform of the current iLP at a light load, I LP2 the peak value of the waveform 35. When the switching circuit containing the transistor 4 operates in a stationary mode, the transistor 4 turns off for the period of T ON -T1 (Fig. 7A). The waveform of the output voltage of the transistor 4 during the off-period is substantially sinusoidal, as indicated by the waveform 32. The sinusoidal voltage 32 is induced into the capacitor 16 by way of a capacitance between the quartic winding 13d and the secondary winding 13b of the transformer 13 in the blocking oscillating circuit 3. As the result of the voltage induction, the polarity or the amplitude of the voltage across the capacitor 16 changes, as seen from curves 36 and 37 designated by a broken line in Fig. 6C. A problem encountered here is that the voltage across the capacitor 16 has a spike (see a curve 37) at t=T2 (Fig. 6C) and the level of the spike exceeds the base voltage level V BE necessary for the transistor conduction between the base and emitter of the transistor 14. When the spike exceeds the level V BE , the transistor 14 conducts at time t=T2 (Fig.6A) before t=T1 by the positive feedback action of the oscillating circuit 3. As a result, the oscillating period of the oscillating circuit 3 becomes short. The conduction of the transistor 14 at time t=T2 has a serious effect on the operation of the DC-DC converter. Under this condition, during the off-period of the transistor 4 (T ON -T1 in Fig. 7A) during this time duration that a voltage is applied to the collector of the transistor 4, the transistor 4 starts the next ON operation. The result is that an excessive spike current flows into the transistor 4, resulting in an abrupt increase of the switching loss of the transistor 4. It is noted that the DC-DC converter of the present invention uses an electrostatic shielding plate 25 to reduce the capacitance among the windings of the transformer 13 and therefore the capacitive coupling among the windings is prevented. Accordingly, no voltage is induced in the capacitor 16 when the transistor 4 is turned on and off. Then, the voltage vc across the transistor 14 never exceeds the voltage V BE during the period T ON -T1, as indicated by the curve 37 in Fig. 6C. Thus, the oscillating circuit 3 operates stably. In addition to the stable operation, the pulse width of the output signal from the oscillating circuit 3 and the pulse stop period between the adjacent pulses can independently be set, as mentioned above. Therefore, the oscillating circuit 3 may be oscillated, while the input DC voltage Vcc is applied to the oscillating circuit 3, and the switching circuit including the transistor 4 can be driven without any auxiliary power source. In the DC-DC converter shown in Figs. 4 and 5, even if the load 11 changes, the oscillating period of the oscillating circuit 3 automatically changes to allow the switching waveform of the voltage resonance type by the transistor 4 to be invariable. In the voltage resonance type switching circuit, when the load 11 gradually changes from the rated load to a light load, the switching period of the transistor 4 gradually becomes long if the conduction duration of the switching transistor 4 is fixed. For example, when the waveform 30 of the collector current i c of the transistor 4 at the rated load is changed to the wave form 31 at the light load, as shown in Fig. 7A, the nonconduction period of the transistor 4 changes from the period T ON -T1 to T ON -T2. The half period of the output voltage VC (Fig. 7B) in the off-period of the transistor 4 also changes T ON -τ1 to T ON -τ2; that is τ2>τ1. When the load 11 becomes lighter and the half period of the wave form 33 becomes long, T ON -τ2 exceeds T ON -T1 (Fig. 7A). In this case, when the operation period of the transistor 4 is left equal to the period at the rated load, a large spike current flows into the transistor 4 at time t=T1, thereby possibly to destroy the transistor 4. For example, in the blocking oscillating circuit 3 according to the present invention, the oscillating period changes in accordance with a variation of the load of the second secondary winding 13d. The change of the load has a direction that when the load of the second secondary winding 13d is light, the oscillation stop period (T ON -T1 in Fig. 7A) becomes long and hence the drive period of the transistor 4 becomes long. Therefore, according to the DC-DC converter shown in Fig. 4, the transistor 4 can be stably driven, irrespective of the variation of the load 11. With respect to the control of the output from the blocking oscillating circuit 3, the resistor 17 for pulse width setting and the pulse stop period setting resistor 20 in Fig. 5 may be substituted by a variable impedance element, respectively. Further, in Fig. 4, the voltage transformer 5 , as a means for transmitting electric power to the load 11, may also be replaced by a choke coil. The power transmission means including a voltage transformer or a choke coil may be expressed by an ideal transformer including a leakage inductance and an exciting inductance in the primary winding. Further, the connecting positions of the transistor 4 and the transformer 5 may be replaced with each other. Another arrangement shown in Fig.8 may be used in place of the blocking oscillating circuit 3 shown in Fig. 4. In Fig. 8 like reference numerals are used to designate like portions in Fig. 5, and the explanation of those portions will be omitted. A bidirectional Zener diode 41 is connected between the base of the transistor 14 and the secondary winding 13b of the transformer 13, through a resistor 42. A capacitor 43 is connected between the base of the transistor 14 and the secondary winding 13b. A transitor 44 has a collector-emitter path connected between the base and emitter of the transistor 14. A resistor 45 is connected between the junction point of the first secondary winding 13b with the capacitor 43 and the base of the transistor 44. Another capacitor 46 is connected between the base and emitter of the transistor 44. The oscillating circuit in the present embodiment operates in substantially the same manner as the blocking oscillating circuit shown in Fig. 5. In each of the blocking oscillating circuits shown in Figs. 5, and 8, the pulse width and the pulse stop period of the output from the oscillating circuit can be individually set and the shield plate 25 is used.

Claims: A DC-DC converter comprising: a transformer (5) with a leakage inductance and an exciting inductance in the primary side; a capacitor (6) connected in parallel with said primary side of said transformer; switching means including a switching element (4) connected between said primary side of said transformer and an input DC power source (Vcc, G), said switching means periodically interrupting a voltage of said DC power source; means for rectifying (9) and smoothing (8, 10) an output signal from said secondary side (5b) of said transformer (5) and supplying the rectified and smoothed output signal to a load (11) and a blocking oscillating circuit (3) with output stabilizing means which directly receives a voltage of said DC power source and controls said switching means by the oscillating output; characterised in that said blocking oscillating circuit (3) comprises, an auxiliary transformer (13) including a primary winding (13a) connected in series to a switching drive transistor (14) and excited by the voltage of said input DC power supply source (Vcc), a first secondary winding (13b) for applying a feedback signal to the base electrode of said switching transistor (14), a second secondary winding (13d) for applying a control signal to the control inputs of the switching element (4) for intermittently interrupting said switching means, and a third secondary winding (13c); a first setting means connected to said first secondary winding (13b) for setting a pulse width of the oscillating output pulse from said second secondary winding (13d); a second setting means connected to said third secondary winding (13c) for setting a pulse stop period from said second secondary winding (13d); and electrostatic shielding means (25) for electrostatically shielding said second secondary winding (13d) from the remaining windings of said transformer. A DC-DC converter according to claim 1, characterised in that: said first setting means includes a time constant circuit formed of a series circuit of a resistor (15) and a capacitor (16), said series circuit being connected between output terminals (Vcc, G) of said input DC power source; a series circuit of a diode (18) and a setting resistor (17) connected across said capacitor of said time constant circuit; the base electrode of said switching drive transistor (14) being connected to the connection point of said setting resistor and said capacitor through said first secondary winding (13b). A DC-DC converter according to claim 1, characterised in that: said second setting means includes a setting resistor (20) connected across said third secondary winding (13c) through rectifying means (19) and a smoothing capacitor (21) connected across said setting resistor. A DC-DC converter according to claim 1, characterised in that said switching drive transistor (14) is a bipolar transistor.

Inventors: MASUDA YOUICHI, ONODERA TOSHIHIRO, MASUDA, YOUICHI, ONODERA, TOSHIHIRO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1981
Main classification: H02M3
IPC codes: H02M3