JPH04299063A - Switching power supply - Google Patents

Abstract

PURPOSE:To operate a voltage detecting circuit stably, to prevent oscillation of a switching power supply circuit and to suppress heating. CONSTITUTION:Voltage rise on load side is detected through a voltage detecting circuit 6 and a switching transistor driving circuit 9 tries to turn a switching transistor Tr1 OFF thus decreasing the base current IB1 thereof. A flip-flop operating means 12 detects decrease of the base current IB1 to actuate the switching transistor driving circuit 9 thus interrupting the switching transistor Tr1 quickly.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply. Specifically, the present invention relates to a switching power supply device whose operation is stabilized.

0002

2. Description of the Related Art FIG. 4 shows a circuit diagram of a conventional switching power supply device. A diode bridge D1 is connected between the power input terminals 1 and 2, and the emitter of a switching transistor Tr1 for power control is connected to the positive end of the diode bridge D1. A flywheel diode D2 is connected between this collector and the negative end of the diode bridge D1, and a smoothing capacitor C and a load 4 are connected via a choke coil L.
Connect in parallel. The choke coil L and the smoothing capacitor C form a smoothing circuit 5 that smoothes the voltage applied to the load 4.
It consists of A series body of a Zener diode D3 and a resistor R5 is connected in parallel between both ends of the smoothing capacitor C.
A resistor R6 is connected between one end of the resistor R5 and the negative terminal of the diode bridge D1. Further, a transistor Tr4 is connected to the anode end A of the Zener diode D3 via a resistor R4.
Connect the base of. The transistor Tr4 is a voltage detection transistor, and its collector is connected to the base of the transistor Tr5. Transistor Tr5
is a control transistor for controlling the base current of the switching transistor Tr1. Resistors R7 and R8 are placed between the base and emitter of the transistor Tr4.
are connected in series, and a transistor Tr6 is connected to the midpoint of
Connect the collector. It is assumed that the base of the transistor Tr6 is connected to the collector of the transistor Tr4 via a resistor R9. And transistors Tr4, Tr
6, resistors R4 to R9, and Zener diode D3 constitute a voltage detection circuit 6 that detects the voltage of the load. Resistance R
7, R8, R9 and the transistor Tr6 are a hysteresis circuit that changes the threshold value of the voltage detection transistor Tr4 depending on its operating state. In addition, the transistor Tr5
A series connection body of diodes D4 and D5 is connected between the base and the ground terminal of the transistor Tr5 in order to fix the base potential of the transistor Tr5 (=collector potential of the transistor Tr4). Transistor Tr5 and diodes D4, D5
constitutes a switching transistor drive circuit 7 that drives the switching transistor Tr1. Furthermore, on the emitter side of the switching transistor Tr1,
A constant current circuit 8 is connected. In the constant current circuit 8, series connections of a transistor Tr7, a resistor R10, a resistor R11, and a transistor Tr8 are connected in parallel between the positive end of the diode bridge D1 and the base of the transistor Tr5. The base of the transistor Tr7 is connected to the midpoint between the resistor R11 and the collector of the transistor Tr8, and the base of the transistor Tr8 is connected to the midpoint between the emitter of the transistor Tr7 and the resistor R10.

Next, the operation of the above conventional example will be briefly explained with reference to the waveform diagram of FIG. First, input terminal 1,
When the power source 3 is connected to the AC power source 2, the AC power source is rectified by the diode bridge D1, and a voltage is applied to the base of the transistor Tr5 and the base of the transistor Tr6 via the constant current circuit 8. Therefore, as shown in FIGS. 5A and 5B, the transistor Tr5 becomes conductive at time t1, the switching transistor Tr1 also becomes conductive, and a voltage is applied to the capacitor C and the load 4 via the choke coil L. Since the voltage applied to the load 4 is detected by the voltage detection circuit 6, when the load voltage increases, the voltage at the anode end A of the Zener diode D3 increases. FIG. 5C is a waveform diagram showing the voltage at the anode end A of the Zener diode D3 as viewed from the negative end (earth end) B of the diode bridge D1. At time t2 when the potential of the anode end A reaches a predetermined threshold value (threshold value when the transistor Tr6 is conductive) Von, as shown in FIG. 5(d), the transistor T
r4 becomes conductive, the base potential of transistor Tr5 decreases, and transistor Tr5 is turned off. Therefore, since no base current flows through the switching transistor Tr1, the transistor Tr1 also turns off at time t3 after a charge accumulation time determined by the capacitance between the base and emitter of the transistor Tr1. Then, the back electromotive force of the choke coil L supplies current to the load 4 and the smoothing capacitor C via the resistor R6 and the diode D2. At this time, since the transistor Tr6 is off, the threshold value of the anode end A of the Zener diode D3 is lowered to Voff. Then, due to the power consumption of load 4, transistor T
The base voltage of r4 decreases, and as shown in FIG.
When the potential at the point reaches the threshold Voff level, the transistor Tr4 is turned off, and the transistor Tr5 and the switching transistor Tr1 are turned on again to supply current to the load 4. At the same time, the transistor Tr6 is turned on, raising the threshold level of the anode end A of the Zener diode D3 to Von.

[0004] By repeating such operations, a substantially constant voltage is supplied to the load 4.

[0005]

[Problems to be Solved by the Invention] As described above, in the conventional switching power supply device, the load voltage is detected by the voltage detection circuit, and the voltage between the points A and B (the voltage between A and B) that is sensitive to the load voltage is detected. ) The switching transistor drive circuit 7 is operated according to VAB to control the switching transistor Tr1 on and off, but in such a conventional example, the operation of the voltage detection circuit 6 may become unstable. There was a risk that the switching power supply would oscillate.

Next, the operation of the conventional switching power supply device will be analyzed and the reason for this will be explained. Figure 6 (a) and (b) are
FIGS. 6(c) and 6(d) show changes in the collector current IC4 and collector voltage VC4 of the transistor Tr4 with respect to the voltage VAB between AB, and FIGS.
FIG. 6(E) shows the change in the collector current IC5 of the transistor Tr5 with respect to the voltage VAB between AB, and FIGS. It shows changes in collector current IC1. Also, Figure 6 (a) to (e)
For reference, the portion indicated by a broken line in FIG. 2 shows the operation when feedback by the transistor Tr6 is not applied.

Now, the transistors Tr5, Tr6 and the switching transistor Tr1 are on, and the transistor T
Considering the case where the AB voltage VAB increases starting from the state where r4 is off, and assuming that the junction voltage of each diode or each transistor is VF, the base-emitter voltage of the transistor Tr4 is R7・VAB/(R4+R7
) Therefore, when VAB≧(R4+R7)·VF/R7, the collector current IC4 of the transistor Tr4 starts to flow (point a in FIG. 6(a)). As a result, first, the currents in diodes D4 and D5 decrease. Diode D4,
When the current of D5 becomes 0, the collector voltage VC4 of the transistor Tr4 starts to decrease (point b in FIG. 6(b)). At this time, the base current IB6 of the transistor Tr6, the collector current IC5 of the transistor Tr5, and the base current IB1 of the switching transistor Tr1 also begin to decrease ((c), (e), and (f) in FIG. 6).

Furthermore, when the AB voltage VAB increases, the collector current IC of the switching transistor Tr1 increases.
1 begins to decrease. When the collector current IC1 starts to decrease (point c in Figure 6 (G), the voltage between AB at this time is VAB1
shall be. ), coil L, smoothing capacitor C, resistor R6,
Through the Zener diode D3, the AB voltage VA is delayed.
Since the voltage between AB and VAB decreases, the operation becomes unstable and transistors Tr1, Tr4, Tr5
is in an amplified state, making it easier to oscillate.

Furthermore, when the AB voltage VAB is increased, the collector voltage VC4 of the transistor Tr4 continues to decrease, and eventually the collector current IC6 of the transistor Tr6 starts to decrease (point d1 in FIG. 6(d). (The voltage is set to VAB2.) When the collector current IC6 starts to decrease, the decrease in the collector current IC6 is caused by the transistor T
r4, the collector voltage VC4 of the transistor Tr4 further decreases, and the collector voltage V
C4 becomes OV (Fig. 6 (b)), and transistor Tr6
The collector current IC6 also suddenly becomes 0V (Fig. 6 (d)).
d2 point). At the same time, the transistor Tr4 is turned on, and the transistors Tr6, Tr5, and Tr1 are turned off, and then the current in the coil L decreases and the AB voltage VAB decreases.

[0010] The above AB voltage VAB1 is expressed by the following equation 1.

[0011]

[Math 1]

The above VAB2 is determined by the following number 2.

0
013]

[Math 2]

It is determined by [0014].

As explained above, in the conventional switching power supply device, the switching transistor Tr1
Even when it is time to turn off the base current, the feedback circuit that turns off the base current does not operate immediately, and the operation of the voltage detection circuit may become unstable.

Furthermore, although the voltage detection circuit is composed of a Schmitt trigger circuit (comparator circuit), the conditions for determining its hysteresis strongly depend on the amplification factor hfe of the transistor, so depending on the design conditions, VAB1<
VAB2 may occur. In that case, the voltage detection circuit will not operate stably as a comparator, but will operate more like an amplifier, and the entire power supply may start to oscillate at a higher frequency than the designed value in the region between VAB1 and VAB2. there were.

As a result, the efficiency of the switching power supply device deteriorates and heat generation increases.

The present invention has been made in view of the drawbacks of the conventional example described above, and its object is to stably apply hysteresis in the operation of a voltage detection circuit.

[0019]

[Means for Solving the Problems] A switching power supply device of the present invention includes a switching transistor for intermittent DC power input, a smoothing circuit connected to an output terminal of the switching transistor, and a smoothed output voltage of the smoothing circuit. a switching transistor drive circuit that controls a base current of the switching transistor based on a voltage detection output from the voltage detection circuit; and a switching transistor that detects the base current of the switching transistor and is in an on state. The present invention is characterized in that it has a means for detecting when the base current of the transistor starts to decrease and rapidly cutting off the switching transistor by means of a switching transistor drive circuit.

[0020]

[Operation] For example, when the voltage on the load side increases, the voltage detection circuit detects this and the switching transistor drive circuit reduces the base current of the switching transistor in an attempt to turn off the switching transistor. When the base current starts to decrease, the decrease in the base current is detected by the fast-acting means, and the fast-acting means operates the switching transistor drive circuit to abruptly turn off the switching transistor.

[0021] In this way, when the timing to turn off the switching transistor comes, the switching transistor is forcibly turned off quickly, which eliminates unstable conditions in the switching power supply and stabilizes the operation of the voltage detection circuit. Can be done. Therefore, oscillation of the switching power supply device can also be prevented. Furthermore, as a result of faster off-operation of the switching transistor, heat generation from the switching transistor can be suppressed.

[0022]

Embodiment FIG. 2 is a circuit diagram of a switching power supply device according to an embodiment of the present invention. In this figure, the same parts as in the conventional example described above are given the same reference numerals. terminals 10, 11
As in the conventional example, an AC power source rectified by a diode bridge is connected to the . Terminal 10 is connected to the emitter of switching transistor Tr1 for power control. A resistor R1 is connected between the emitter and the base. On the other hand, a flywheel diode D2 is connected between the collector of the switching transistor Tr1 and the terminal 11, and a smoothing capacitor C and a load 4 are connected in parallel via a choke coil L. The choke coil L and the smoothing capacitor C constitute a smoothing circuit 5 that smoothes the voltage applied to the load 4. A series body of a Zener diode D3 and a resistor R5 is connected in parallel between both ends of the smoothing capacitor C, and a resistor R6 is connected between one end of the resistor R5 and the negative end of the diode bridge D1. Furthermore, the base of a transistor Tr4 is connected to the anode end A of the Zener diode D3 via a resistor R4. transistor T
r4 is a voltage detection transistor whose collector is connected to the base of the transistor Tr5 and whose emitter is connected to ground. Transistor Tr5
is a control transistor for controlling the base current of the switching transistor Tr1, and its collector is connected to the base of the switching transistor Tr1. A series connection body of resistors R7 and R8 is connected between the base and emitter of the transistor Tr4, and the collector of the transistor Tr6 is connected to the midpoint thereof. The base of the transistor Tr6 is connected to the transistor T through a resistor R9.
It shall be connected to the emitter of r5. The transistors Tr4 and Tr6, the resistors R4 to R9, and the Zener diode D3 constitute a voltage detection circuit 6 that detects the voltage of the load. Further, the resistors R7, R8, and R9 and the transistor Tr6 are a hysteresis circuit that changes the threshold value of the voltage detection transistor Tr4 depending on its operating state. Resistors R13 and R12 are connected in series between the emitter of the transistor Tr5 and the ground. Here, the value of resistor R13 is k times the value of resistor R12 (
R13=kR12), and the value of k is smaller than 1, for example k=0.1 to 0.3 (preferably k=0.2). A transistor Tr9 is connected between the base of the transistor Tr5 and the ground terminal in order to fix the emitter potential of the transistor Tr5, and the base of the transistor Tr9 is connected to the midpoint between the resistors R13 and R12. Transistors Tr5 and Tr9 and resistors R12 and R13 constitute a switching transistor drive circuit 9 that drives switching transistor Tr1. The flip-flop operation circuit 12 for quickly turning off the switching transistor Tr1 includes a transistor Tr4.
, Tr6, Tr9 and resistors R7, R8, R9, R12,
It is composed of R13. The flip-flop operating circuit 12 monitors the base current of the switching transistor Tr1 by detecting the emitter potential of the transistor Tr5, and the base current IB1 of the switching transistor Tr1 is, for example, 80% (this flip-flop operating point is set at the resistor R13). (can be adjusted by the ratio k of R12 and R12), the switching transistor drive circuit 9 is quickly operated as described later to rapidly reduce the base current IB1 of the switching transistor Tr1 to 0, thereby quickly cutting off the base current IB1 of the switching transistor Tr1. Further, a constant current circuit 8 is connected to the emitter side of the switching transistor Tr1. In the constant current circuit 8, the series connections of the transistor Tr7, the resistor R10, and the resistor R11 and the transistor Tr8 are connected in parallel between the terminal 10 and the base of the transistor Tr5. The base of the transistor Tr7 is located at the midpoint of the collector of the resistor R11 and the transistor Tr8.
The base of r8 is connected to the emitter of transistor Tr7 and resistor R.
It is connected to the midpoint between 10 and 10. FIG. 1 is a functional block diagram showing some of the functions of the circuit diagram of FIG. 2 as blocks.

Next, the operation of the above switching power supply device will be explained. 3(a) and 3(b) show changes in the collector current IC4 and collector voltage VC4 of the transistor Tr4 with respect to the AB voltage VAB, and FIG. 3(C) shows the emitter voltage V of the transistor Tr5 with respect to the AB voltage VAB.
3(d) and (e) show the change in E5, and FIG. 3(d) and (e) show the voltage between AB
FIG. 3(f) shows changes in the base current IB6 and collector current IC6 of the transistor Tr6 with respect to AB.
FIG. 3(G) shows a change in the collector current IC5 of the transistor Tr5 with respect to the voltage VAB between AB.
It shows the change in the base current IB1 of the transistor Tr1 with respect to AB. Note that the portions indicated by broken lines in FIGS. 3A to 3G show the operation when feedback by the transistor Tr6 is not applied.

Now, transistors Tr5, Tr6, Tr
9 and the switching transistor Tr1 is on and the transistor Tr4 is off, and the case where the AB voltage VAB increases is considered. If the junction voltage of each diode or each transistor is VF, then the resistance R
Since the potential at the midpoint between R12 and R13 is VF, the emitter voltage of transistor Tr5 is: VE5=(1+k)VF=VF+ΔV However, ΔV=kVF (Figure 3 (c)), and the collector voltage of transistor Tr4 (= The base voltage of transistor Tr5) is VC4
=VE5+VF=2VF+ΔV (FIG. 3(b)), and the base current of the transistor Tr6 is IB6=(VE5−VF)/R9=ΔV/R9 (FIG. 3(d)).

[0025] From this, the AB voltage VAB increases, and V
When AB≧(R4+R7)・VF/R7, first the collector current IC of transistor Tr4
4 begins to flow (point e in Figure 3 (a)). As a result, the collector current IC9 of the transistor Tr9 decreases. When the voltage VAB between AB further increases and the collector current IC9 of the transistor Tr9 becomes 0, turning off the transistor Tr9, the collector voltage VC4 of the transistor Tr4
starts to decrease (point f in Figure 3 (b)). At this time, the emitter voltage VE5 of the transistor Tr5 also begins to decrease at the same time (point g in FIG. 3(c)), and the base current IB6 of the transistor Tr6 also decreases (point h in FIG. 3(d)).

At this time, emitter voltage VE5=VF+Δ
V=(1+k)VF, and k<1 (for example, k=0.
1 to 0.3), the emitter voltage VE5 is a small amount Δ
By simply decreasing V=kVF, the base current IB6 and collector current IC6 of the transistor Tr6 each become 0 (points p and q in FIGS. 3(d) and (e)). Furthermore, the decrease in the collector current IC6 suddenly flows into the base of the transistor Tr4, and the collector voltage V of the transistor Tr4
C4 is lowered (point r in FIG. 3(b)), and as a result, the emitter voltage VE5 of the transistor Tr5 is suddenly lowered (see FIG. 3(b)).
c) point s).

Due to this positive feedback, the transistor T
r6 is turned off, transistor Tr4 is turned on, and transistor Tr5 is turned off, and the collector current IC5 of transistor Tr5 becomes 0, and the switching transistor Tr
1 suddenly turns off (Fig. 3 (f) and (g)). The collector current (control current) IC5 just before this positive feedback starts is:
The collector current IC5 is about a little smaller (for example, 80%) than the collector current IC5 when the transistor Tr6 is on,
At this time, since the switching transistor Tr1 is stably turned on, unstable oscillation does not occur. In other words, in the switching power supply device of the present invention, while the switching transistor Tr1 is stably turned on, the transistors Tr4 and Tr5 operate as flip-flops to abruptly cut off the base current IB1 of the switching transistor Tr1 to turn it off, causing oscillation. does not occur. In addition,
Point t in FIG. 3(g) is the point at which the switching transistor TR1 becomes unstable when transistors Tr4, Tr5, and Tr6 are not allowed to perform flip-flop operations, and transistors Tr4 and Tr5 are not allowed to perform flip-flop operations. By doing so, the switching transistor Tr1 is turned off before reaching point t.

[0028]

[Effects of the Invention] According to the present invention, when the timing to turn off the switching transistor comes, the switching transistor is forcibly turned off quickly, so it is possible to eliminate unstable conditions in the switching power supply device, and to prevent malfunctions in the voltage detection circuit. A stable operating area can be eliminated. Therefore, oscillation of the switching power supply device can also be prevented. Furthermore, as a result of faster off-operation of the switching transistor, heat generation from the switching transistor can be suppressed.

[Brief explanation of drawings]

FIG. 1 is a functional block diagram of a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a specific circuit diagram of the embodiment same as above.

FIG. 3 is a characteristic diagram showing the operation of each transistor same as the above.

FIG. 4 is a circuit diagram showing an example of a conventional switching power supply device.

FIG. 5 is a waveform diagram showing waveforms of various parts in the conventional example.

FIG. 6 is a characteristic diagram showing in detail the operation of each transistor same as the above.

[Explanation of symbols]

Tr1 Switching transistor 5 Smoothing circuit 6 Voltage detection circuit 9 Switching transistor drive circuit 12 Flip-flop operation circuit

Claims (1)

[Claims] 1. A switching transistor for intermittent DC power input, a smoothing circuit connected to an output end of the switching transistor, a voltage detection circuit for detecting a smoothed output voltage of the smoothing circuit, and the voltage detection circuit. a switching transistor drive circuit that controls the base current of the switching transistor based on a voltage detection output from the switching transistor; and a switching transistor drive circuit that detects the base current of the switching transistor and detects when the base current of the switching transistor in the on state starts to decrease. 1. A switching power supply device comprising: quick-cut operation means for abruptly turning off a switching transistor by a switching transistor drive circuit.