JPS631028B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a switching regulator having a transformer.

In this type of switching regulator, as shown in FIG. 1, an input voltage E _i is applied to a series circuit of a primary winding n ₁ of a transformer 1 and a transistor 2 which is a switching element. When transistor 2 is turned on, primary current i ₁ flows, and when transistor 2 is turned off, primary current i ₁ flows.
As a result, the energy stored in the iron core of the transformer 1 is released to the secondary windings n ₂ and n ₃ as secondary currents i ₂ and i ₃ . The secondary current i ₂ on the side of the secondary winding n ₂ is rectified and smoothed, and is applied to the load RL as an output voltage Eo.
Also, the secondary current i ₃ on the side of the secondary winding n ₃ is the rectifier smoothing circuit 4
The feedback voltage E _f related to Eo is rectified and smoothed by
It is applied to the non-inverting input terminal (+) of the error amplifier 5 via resistors R ₁ and R ₂ . The error amplifier 5 amplifies the deviation between the reference voltage Es applied to the inverting input terminal (-) via the resistor R ₃ and the feedback voltage E _f , and the output Ea is used as the comparison voltage of the triangular wave from the oscillator 7 at the comparator 6. Compared to Er. Based on the comparison result, drive circuit 8 turns transistor 2 on or off. In this way, transistor 2 becomes E _f =Es
Repeat on and off until the output voltage Eo
is controlled to a constant value. In a switching regulator having such a configuration, if a disconnection occurs in the secondary winding _n3 of the transformer 1 that constitutes the feedback circuit, or if a failure occurs in the rectifier and smoothing circuit 4 and the resistors _R1 and _R2 , Since transistor 2 is mostly turned on, an overvoltage occurs on the output side. The value is more than twice the normal value, and especially when the input voltage range is wide, such as 24V to 100V, the value becomes large, so countermeasures are required. Further, since the output current becomes excessive due to a load short-circuit accident, etc., countermeasures are also required. Generally, the output voltage and output current are monitored, and when an abnormality occurs, the transistor 2
is turned off to provide overvoltage and overcurrent protection. However, as shown in Figure 1, when the feedback voltage E _f is obtained from a separate winding from the output voltage Eo to provide input/output isolation, the overvoltage protection circuit and overcurrent protection circuit must also be of the isolated type, resulting in a complicated configuration. I was getting used to it. Furthermore, if a large number of secondary windings on the output side are provided to form a multi-output type, overvoltage detection means and overcurrent detection means will be required for each output, making the configuration even more complicated.

By the way, in a switching regulator having a transformer, when the transistor 2 is switched from on to off and a secondary current flows, a flyback voltage is induced on the primary side. At this time, a spike voltage is generated due to the leakage inductance of the transformer, so as shown in the figure, the primary winding of the transformer 1 n ₁
A snubber circuit 9 consisting of a diode D ₁ , a resistor R ₅ and a capacitor C ₁ is generally added in parallel to. This snubber circuit 9 stores an average value E _fl of the flyback voltage. There is a relationship between the average value E _fl of the flyback voltage and the output voltage Eo as E _fl ≈kEo, where the winding ratio of the transformer 1 is k (=n ₁ /n ₂ ).

Therefore, in the present invention, the average value of the flyback voltage is monitored and the value is limited, and the primary current of the transformer is monitored and when the value exceeds the limit value,
By switching the limit value of the flyback voltage to a low level, a switching regulator that can provide overvoltage and overcurrent protection with a simple configuration has been realized.

FIG. 2 is a connection diagram showing one embodiment of the switching regulator of the present invention, in which the same parts as in FIG. 1 are given the same reference numerals. The difference between FIG. 2 and the conventional example shown in FIG. 1 is that a second error amplifier 10 is provided, and the sum of the average value E _fl of the flyback voltage and the input voltage E _i is connected to the non-inverting input terminal (+) of the resistor. _R6 , _R7
The input voltage is divided and applied to the inverting input terminal (-).
E _i and reference voltage Es are applied through resistors R ₈ , R ₉ and R ₁₀ respectively, and the output Eb is applied through resistor R ₁₁
A point that produces an output voltage Eb related to the deviation between the limit value E _l (=R ₉ /R ₈ Es) and E _fl by feeding back through the output voltage Eb and the first error amplifier. A selection circuit 11 is provided to select the larger output Ea of the transformer 5 and provide it to the comparator 6, and a resistor _R12 is provided on the emitter side of the transistor 2 to detect the primary current _i1 of the transformer 1. The voltage drop Ec of _R12 and the reference voltage Es are applied to the base via resistors _R13 and _R14 , respectively, the collector is connected to the inverting input terminal (-) of the second error amplifier 10, and the emitter is connected to the reference point. The point is that a connected transistor _Q1 is provided. Note that the selection circuit 11 may be constructed using a diode, or may have various configurations as necessary, such as by connecting the emitters of transistors in the output stages of the first and second error amplifiers in common. . And also during normal times Ea
＞Eb, set the value of the limiting voltage E _l to the normal value.
It is selected to be about 1.4 times that of E _fl .

Therefore, in the present invention, the first
The error amplifier output Ea of the second error amplifier output Eb
The output voltage Eo is controlled on the first error amplifier 5 side as in the conventional case. By the way, an abnormality occurred in the feedback circuit on the side of the first error amplifier 5, and the output voltage Eo
When E fl becomes large, the flyback voltage increases, so Eb > Ea, and the second error amplifier 10 operates to control the on/off of transistor 2 so that E _fl = E _l , thereby limiting the flyback voltage. . As a result, the output voltage Eo is also limited to about 1.4 times the normal value, effectively preventing overvoltage from occurring on the output side.

On the other hand, the primary current i ₁ increases due to a load short-circuit accident, etc., and the voltage drop Ec across the resistor R ₁₂ becomes a limit determined by the base-emitter voltage V _BE of the transistor Q, the reference voltage Es, and the values of the resistors R ₁₃ and R ₁₄ . When the value (R ₁₃ +R ₁₄ /R ₁₄ V _BE −R ₁₃ /R ₁₄ Es) is exceeded, transistor Q ₁ turns on. As a result, the inverting input terminal (-) of the second error amplifier 10, that is, the limit value _El of the flyback voltage, is forced to a low level, and the second error amplifier 10 operates, turning off the transistor 2 and reducing the primary current i. Stops the increase in ₁ and performs overcurrent protection. At this time, a capacitor _C2 is provided in the feedback circuit of the second error amplifier 10 to adjust the response time so that the transistor 2 does not repeatedly turn on and off several times during one cycle and generate heat. Also, when the power is turned on, transistor _Q1 is activated and starts operating from a short on time.
A soft start is possible because the on time is lengthened every cycle to transition to a normal state.

In addition, as shown in FIG. 3, the feedback voltage E _f is connected to the inverting input terminal (-) of the second error amplifier 10 through a resistor _{R15.
If the flyback voltage is given through} Can be limited to a value. Also, first and second error amplifiers 5, 10, comparator 6, oscillator 7, and drive circuit 8
When using the feedback voltage E _f as the operating voltage of a control circuit consisting _of
It is sufficient to provide such that voltage is not applied from the starting circuit 12 to the inputs of the error amplifiers 5 and 10 when the feedback circuit is abnormal. In addition, in FIG. 4, the starting circuit 1
A series regulator consisting of a transistor Q ₂ , a resistor R ₁₆ , and a Zener diode ZD is shown as 2. At startup (when the power is turned on), the control circuit has a voltage between the Zener voltage Vz of ZD and the base-emitter of Q ₂ . A voltage determined by the voltage difference is applied, and _when the steady state is reached, the regulated voltage E _f (=
Es) can be added. If Es is chosen to be larger than the Zener voltage, Q ₂ will be turned off in steady state, which has the advantage that the power consumed by the control circuit will hardly change even if the input voltage E _i changes significantly.

Note that the feedback of the first and second error amplifiers may be performed from the output side of the selection circuit 11.

As explained above, in the present invention, the flyback voltage is monitored and its value is limited in case of an abnormality, and the primary current of the transformer is monitored and when the value exceeds the limit value, the flyback voltage limit value is forcibly set. Since the switching is made to the low level, a switching regulator capable of overvoltage and overcurrent protection can be obtained with a simple configuration.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing an example of a conventional switching regulator, Fig. 2 is a connection diagram showing one embodiment of the present invention, and Figs. 3 and 4 are connection diagrams showing another embodiment of the present invention. It is. 1...Transformer, 2...Switching element,
3, 4... Rectifier smoothing circuit, 5, 10... Error amplifier, 6... Comparator, 7... Oscillator, 8... Drive circuit, 9... Snubber circuit, 11... Selection circuit, 12
...Start circuit.

Claims (1)

[Claims] 1. In a switching regulator having a transformer, a first error amplifier that produces an output related to the deviation between the feedback voltage corresponding to the output voltage and the reference voltage, and the average value and limit of the flypack voltage generated in the primary winding of the transformer. a second error amplifier producing an output related to the deviation from the value; means for selecting one of the first error amplifier output and the second error amplifier output; and comparing the selected output with a comparison voltage. and means for controlling on/off of the switching element based on the result, and when the average value of the flyback voltage exceeds the limit value, the selection means selects the second error amplifier output to protect the output side from overvoltage. At the same time, when the primary current of the transformer exceeds the limit value, the limit value of the flyback voltage is switched to a low level, and the second error amplifier is forcibly selected to provide overcurrent protection.・Regulator.