JPS5857072B2 - switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching regulator that can obtain a stable output voltage through pulse width modulation and has an overcurrent protection function.

An example of a conventional switching regulator is shown in FIG. 1, and will be described below.

In the figure, reference numeral 1 denotes a transistor saturation blocking oscillator circuit, which includes a primary winding L1 and a feedback winding L3 of a transformer T, an oscillating transistor QI, a base resistor R1, a base capacitor C1, and a starting resistor R2.

Reference numeral 2 denotes an output rectifier circuit, which includes a secondary winding L2 of a transformer T, a diode D1, and a smoothing capacitor C2.

3 is a voltage comparison circuit, which includes a reference voltage element E1 and a voltage comparator A.
Consists of 1.

6A is a control circuit, which has a transistor Q2, and 6B
is an overcurrent protection circuit, which includes an overcurrent detection resistor R3 connected in series to the load RL, a reference voltage element E2, and a voltage comparator A3.
Consisting of

In the above configuration, the output voltage ■.

Stabilization is usually achieved by a means called a ringing choke method.

That is, when the transistor Q becomes conductive, an exciting current t1 flows through the primary winding L1 of the transformer T (where, 1: input power supply voltage, t: time, and inductance of the primary winding of the L1 transformer T). is flowing and trance T
Excitation energy is stored in.

At this time, the primary winding L1 and the secondary winding L2 of the transformer T are connected so that the phase is such that the diode D1 of the output rectifier circuit 2 is reverse biased.

Next, when the transistor Q1 is cut off, the excitation energy is applied to the output rectifier circuit 2 to produce an output voltage .

It is closed and taken out. However, RL is the load resistance, and tON is the transistor Q.

The conduction time tOFF is the cutoff time of the transistor Q□, and it is assumed that there is no forward voltage drop of the rectifier diode D1.

Therefore, if the conduction time tON in the above equation (1) is controlled, the output voltage becomes ■.

can be stabilized. Next, the operating voltage and current waveform of each part in the transistor saturation type blocking oscillator circuit 1, that is, the peak value icp of the base current ib1 and the collector current of the oscillation transistor Q1 and the collector current .

The relationship is shown in Figure 2.

The collector current peak value icp of transistor Q1 is 1c
p=hfe-ib (hfe two-current amplification factor), and in order to control the conduction time tON of the transistor Q1 when the transistor Q1 is conductive, that is, from the above equation (2), it is only necessary to add the base current ib. it is obvious.

Therefore, the control signal from the voltage comparison circuit 3 drives the transistor Q2, and the conduction time tON of the transistor Q1 is controlled by dividing the base current lb supplied to the transistor Q1 according to the control amount.

Furthermore, the transistor saturation blocking oscillation method controls the base current ib of the transistor Q0,
Its collector current i.

The collector current l of the transistor Q1 has the advantage that the saturation point of the transistor Q1 can be freely changed, thereby making it easy to control the pulse width.

Only the minimum base current 1b-□ of 0P necessary for saturation of hfe is given, and it is inevitable that the collector saturation voltage VcB(S) becomes large as shown in FIG.

In addition, the inversion trigger for cutting off transistor Q1 is
Since it is proportional to the change in collector current of 1°, generally when a switching transistor with a smooth collector current saturation point in the vo IC characteristics of the transistor t is used, the reversal trigger M = phantom is small and the reversal speed is t. As the switching speed becomes slower, switching loss increases and the collector saturation voltage also increases. As a result, the power conversion efficiency of the switching regulator decreases, and the operational reliability also deteriorates.

In particular, the transistor Q1 has an inductance component as a load.
Since the maximum collector current is flowing at the moment when the current transitions from conduction to cutoff, the transistor saturation blocking oscillator circuit 1 essentially has the disadvantage that switching loss increases.

Furthermore, the current amplification factor hfe of the transistor Q1 usually has large variations, and the current amplification factor hfe is (2).
As shown in the equation, since this is a factor that determines the conduction time tON, the circuit constant of the base current supply circuit is adjusted so that the optimum base current lb is supplied to the transistor Q1 according to the variation in the current amplification factor hfe. It also has the disadvantage that it requires adjustment, which is very troublesome.

Furthermore, the overcurrent protection circuit 6B connects an overcurrent detection resistor R3 in series with the load RL, detects the current flowing through the load RL with the overcurrent detection resistor R3, and uses the detected voltage and the reference voltage element E2. When the reference voltage is compared with the voltage comparator A3 and the control transistor Q2 is driven, the output current becomes ■.

- It becomes as shown in the following equation (3).

That is, as in the above equation (1), the transistor Q.

If the conduction time t□N of is controlled, the output current ■.

is 2 Constant current value ■.

-1 and is protected from overcurrent. 3. However, since the overcurrent protection circuit 6 is added independently to the switching regulator, the circuit configuration of the entire switching regulator becomes complicated.

Additionally, switching regulators generally have a high input voltage (■1) and an output voltage (■).

It is often used when the input current Ii is low, so the output current ■ is lower than the input current Ii.

becomes large, and in a method that detects an output current of 1° and provides overcurrent protection, the power loss P- in the overcurrent detection resistor R3 increases.
There was a drawback that IoxE2 was large and the power conversion efficiency of the switching regulator was reduced.

The present invention eliminates the above-mentioned drawbacks and provides a switching regulator that has excellent power conversion efficiency, can provide overcurrent protection without incorporating special overcurrent protection circuit components, and also has a soft start function. The purpose is to

That is, the present invention connects a first series circuit between the primary winding of the transformer, the collector-emitter of the oscillation transistor, and a current detection circuit that detects a current proportional to the current of the primary winding to a DC input power source. Further, a second series circuit including a parallel circuit of a capacitor and a resistor and a feedback winding of the transformer is connected between the base and emitter of the oscillation transistor,
a relaxation oscillation circuit comprising a first series circuit portion excluding a current detection circuit and a DC input power supply and a second series circuit;
a rectifier circuit that rectifies the AC output of the secondary winding of the transformer; a first voltage comparison circuit that compares the DC output voltage obtained from the rectifier circuit with a reference voltage and obtains an output corresponding to the deviation value; an error amplifier circuit that is driven by a first voltage comparison circuit and whose upper limit of the output voltage is controlled by a voltage proportional to the DC output voltage of the rectifier circuit; and an error amplifier circuit that compares each output of the current detection circuit and the error amplifier circuit. A trigger signal is sent to the base of the oscillation transistor of the relaxation oscillation circuit to change the relaxation oscillation circuit from an on-off state to an off state only when the output of the current detection circuit including the voltage comparison circuit 2 exceeds the output of the error amplifier circuit. A switching regulator is characterized in that it consists of a control circuit that applies voltage to the voltage.

An embodiment of the present invention is shown in FIG. 3 and will be described in detail below.

In the figure, reference numeral 11 denotes a blocking oscillation circuit which obtains a trigger signal to turn off from the outside, and the primary winding L1 of the transformer T,
It consists of a feedback winding L3, a base resistor R1, a base capacitor C1, a starting resistor R2, and a transistor Q1.

Reference numeral 12 denotes an output rectifier circuit, which includes a secondary winding L2 of a transformer T, a diode D1, and a smoothing capacitor C2.

Reference numeral 13 denotes a voltage comparison circuit, which includes a reference voltage element E1 and a voltage comparator A1.

14 is an error amplification circuit consisting of a resistor R4 and a transistor Q3.

Reference numeral 15 denotes a current detection circuit, which is composed of a current detection element such as a resistor R5 or a current transformer.

Reference numeral 16 denotes a control circuit that applies a trigger signal to turn off the blocking oscillation circuit 11, and is composed of a voltage comparator A2 and a transistor Q2.

In the above configuration, when an unstable DC voltage Vi is supplied from the input power source Ei, a stable desired DC voltage (2) is generated between the output terminals a and b.

can be obtained. FIG. 4 shows the voltage and current waveforms of each part of the circuit shown in FIG. 3, and the circuit operation will be described below.

In the figure, A is the base input current ib waveform of the transistor Q1, the collector current lc waveform of the transistor Q0, C is the output voltage vic waveform of the current detection circuit 15 and the output voltage V waveform of the error amplifier circuit 14, and 2 is the control This is the output current jpE waveform of the circuit 16.

First, a case will be described in which the circuit shown in FIG. 3 is in steady operation.

Now, when the transistor Q1 of the blocking oscillation circuit 11 starts conducting, a current i flows through the primary winding L1 of the transformer T.

is inflowed. At this time, an induced voltage is generated across the secondary winding L2 of the transformer T, but since the polarity of the diode D1 of the output rectifier circuit 12 is opposite to the induced voltage, the collector current i of the transistor Q1.

The excitation current Vi is mostly 1 and 4.

vinegar.ゎゎFigure 4 (, Show 1 Time t.

At the same time as the transistor Q1 starts conducting, the collector current i.

increases linearly.

At this time, the blocking oscillator circuit 11 applies a current n times the minimum base current i0F/hfe necessary for conduction of the transistor Q1 to the base, as shown in the base input current ib waveform shown in FIG. , transistor Q1 conducts and the collector current i due to sufficient base current, 7°, during the conduction period of transistor Q1.

The collector saturation voltage can be made sufficiently small without saturating the voltage.

Also, the output voltage ■.

The voltage V applied to the inverting input terminal of the voltage comparator A2 modulated by the signal from the error amplifier circuit 14 corresponding to the deviation value of the reference voltage E1 is the output voltage EV.

When the voltage increases, the voltage ■ applied to the inverting input terminal is controlled to decrease.

Furthermore, the collector current i output by the current detection circuit 15.

= ΣI The voltage V sc proportional to t is the voltage vi applied to the non-inverting input terminal of the voltage comparator A2.
It is applied as o.

In the control circuit 16, the voltage vic applied to the non-inverting input terminal of the voltage comparator A2 and the voltage ■ applied to the inverting input terminal are compared, and when they have the relationship vlo>V,
Transistor Q2 is conductive, and when the relationship vio<v exists, transistors Q2i-J are cut off.

Therefore, as mentioned above, the collector current l of transistor Q1.

increases linearly, so the voltage Vio proportional to the collector current l also increases linearly.

At time t1 shown in FIG. 4, the relationship between ■i6 and V is V
io) V, and as a result, the transistor Q2 becomes conductive, and a collector current iFE of the transistor Q2 flows as shown in FIG. 4B.

The collector current iFE acts to completely cancel out the base current ib flowing into the transistor Q1, and can quickly eliminate even the accumulated carriers remaining in the transistor Q1.

That is, when the output voltage vic of the current detection circuit 15 exceeds the output voltage V of the error amplifier circuit 14, the transistor Q2 becomes conductive and the transistor Q1 is rapidly cut off.

According to the base current ib waveform shown in FIG. , the collector loss can be reduced.

Also, the collector current i of the transistor Q1.

At the time when the peak value icp reaches a peak value corresponding to the stabilization of the output voltage, that is, at time t2 shown in FIG. If the transistor Q1 is forcibly cut off, the transistor Q
It is possible to further reduce the collector loss at the time of the first interruption, thereby increasing the power conversion efficiency of the switching regulator.

Furthermore, since the carrier accumulation time when the transistor Q1 is turned off can be significantly shortened, the maximum oscillation frequency can be increased, and thereby the pulse width modulation range of the switching regulator can be widened.

Therefore, the present invention can expand the load range to the light load side, and furthermore, the output voltage is reduced due to the frequency limit at light loads.

As a result, the output voltage can be prevented from abnormally rising.■.

stability can be improved.

Furthermore, the conduction time tON of the transistor Q1 in the oscillation state of the switching regulator according to the present invention is (
It is expressed by the following equation.

(However, Ll: primary winding inductance, R5: current detection resistor, Vi: input voltage, v: error amplifier circuit output)
Therefore, the conduction time tON of the transistor Q1 is independent of the current amplification factor hfe of the transistor Q1, etc., which has large variations, so that the base current supply circuit and the negative feedback circuit can be easily designed.

The conduction time tON is a function of the output V of the error amplifier circuit 14, and is accumulated in the transformer T by performing pulse width modulated oscillation in response to factors such as load fluctuation and input fluctuation in the new blocking oscillation circuit. ■ Output voltage by controlling excitation energy.

can be stabilized. Furthermore, the switching regulator of the present invention also has an overcurrent protection function as described below without incorporating any special parts for an overcurrent protection circuit.

As mentioned above, the present invention has an output current of ■.

Against the output voltage■.

In order to stabilize the reference voltage E, based on equation (4) above,
1 and output voltage ■.

Obtained according to the deviation value of. The conduction time tON of the oscillation circuit 11 is controlled by the output V of the error amplifier circuit.

However, the power supply to the error amplifier circuit 14 is the output voltage ■
.

Therefore, the operating range of the error amplifier circuit output V is the output voltage ■.

limited by. In other words, the error amplifier circuit output ■ and the output voltage ■.

Since the relationship is V≦■o, the output current is ■.

The conduction time tON, which increases as
), the increase is limited.

Furthermore, in the switching regulator of the present invention, (
5) After satisfying the formula, the output current ■.

When increasing the output voltage, that is, during overcurrent,
.

and output current■.

I will explain the relationship between In this case the output voltage■.

is the same as equation (1) above, and therefore the input power P.

is the following formula (6), output power P.

is expressed by the following equation (7). Generally, power conversion efficiency is 1
Assuming 00%, input power Pi and output power P.

are equal, so (6). From formula (7), transistor Q1
The ratio of the conduction time tON and the cutoff time tOFF is as follows.

Furthermore, from equations (5) and (8), the cut-off time tOFF of transistor Q1 at the time of overcurrent detection is tOFF= -R
6 ・・・・・・・・・(9)

That is, from equations (1), (5), and (9), toN, toF
If F is erased, the output voltage when overcurrent is detected is ■.

and negative L r The relationship between load resistance R0 is ■.

=vi, (----)...(1,0)R5L1 becomes.

Also an output type EV. and output current■. The relationship is ■o−■.

RL, and as a result, the output current ■. )”.

Therefore, uO), (If the load resistance RL is eliminated from equation 10, the output voltage becomes ■.

and output current■. The relationship is as shown in equation (■2) below.

becomes a hyperbolic function.

02) Equation holds true in the error amplifier circuit 1 as described above.
4 output V and output voltage■.

The relationship is V=V. In the case of , and in the case of V〈■o,
In other words, the output current■.

When there is no overcurrent, the output voltage ■
.

is controlled and stabilized by the reference voltage of the reference voltage element E1.

That is, in FIG. 5, the output voltage is ■. ≧・0, output current is ■.

If ≧O, the output voltage ■. and output current■.

The relationship is a combination of the above equations (1) and (12), and as a result, the so-called foldback characteristic of the overcurrent protection function as shown in FIG. 6 can be obtained.

Next is the output voltage ■ shown in Figure 6.

and output current■. Let's briefly explain the relationship.

Output current■.

is within the rated value, and the output V of the error amplifier circuit 14 is within an operating range proportional to the deviation value between the DC output voltage obtained from the output rectifier circuit 12 and the reference voltage of the reference voltage element E1 in the voltage comparator circuit 13. In the case of , the relationship between ■o and ■o becomes the P-Q line, and the output voltage is based on equation (1) above.

is stabilized.

On the other hand, the output current ■. is outside the rated value, that is, in an overcurrent state, and the DC output voltage (2) obtained between output terminals a and b is the supply source to the error amplifier circuit 14.

When the operating range is limited by , the VOIO relationship becomes a Q-0 curve, and the output current ①o is limited based on equation (12).

Therefore, the switching regulator of the present invention has the sixth
It is possible to obtain an overcurrent protection function, a so-called foldback characteristic, as shown in the P-Q-0 curve shown in the figure.

Furthermore, the operating point for overcurrent detection in the present invention is determined by the output (2) of the error amplifier circuit 14, and since the output V rises from zero at startup, the output current (2).

and the transformer drive current starts up while being suppressed.

That is, the present invention not only has the overcurrent protection function as described above, but also has a soft start function, and can sufficiently protect the transformer 1 driving transistor etc. from destruction that occurs during startup.

Although the embodiments described above have been described with respect to a self-excited switching regulator using a blocking oscillation circuit, the present invention operates in the same manner as described above even when applied to a separately-excited switching regulator.

According to the present invention, a switching regulator is provided that has excellent power conversion efficiency regardless of the oscillation method, can provide overcurrent protection without incorporating special overcurrent protection circuit components, and has a soft start function. can.

[Brief explanation of drawings]

Figure 1: A circuit diagram showing a conventional example; Figure 2: A diagram showing operating voltages and current waveforms of each part of the circuit in Figure 1; Figure 3; a circuit diagram showing an embodiment of the present invention; Figure 4: FIG. 3 is a diagram showing the operating voltage and current waveform of each part of the circuit, FIG. 5 is a diagram for explaining the present invention, and FIG. 6 is an output voltage of the present invention. and output current■. Characteristic diagram showing the relationship between. 11...Blocking oscillation circuit, 12...
...Output rectifier circuit, 13...Voltage comparison circuit, 1
4...Error amplification circuit, 15...Current detection circuit, 16...Control circuit.

Claims (1)

[Claims] 1 A first series circuit between the primary winding of the transformer, the collector-emitter of the oscillation transistor, and a current detection circuit that detects a current proportional to the current of the primary winding is connected to a DC input power supply, and a capacitor is A second series circuit consisting of a parallel circuit of a resistor and a feedback winding of the transformer is connected between the base and emitter of the oscillation transistor, and the first series circuit part excluding the current detection circuit and the DC input power supply A relaxation oscillator circuit consisting of two series circuits, a rectifier circuit that rectifies the AC output of the secondary winding of the transformer, and a DC output voltage obtained from the rectifier circuit and a reference voltage are compared and the deviation value is determined. a first voltage comparator circuit that obtains a corresponding output;
an error amplifier circuit driven by the voltage comparison circuit and whose upper limit of the output voltage is limited by a voltage proportional to the DC output voltage of the rectifier circuit; and an error amplifier circuit that compares each output of the current detection circuit and the error amplifier circuit. The voltage comparator circuit of No. 2 is called a voltage comparator circuit, and only when the output of the current detection circuit exceeds the output of the error amplifier circuit, a trigger signal is sent to the oscillation transistor of the relaxation oscillation circuit to change the on-off state of the relaxation oscillation circuit to the off state. A switching regulator comprising a control circuit that applies voltage to a base.