JPH0223106Y2 - - Google Patents

Description

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

As shown in Figure 1, the switching regulator converts commercial AC power directly into DC using a rectifying and smoothing circuit 1, and generates a high frequency signal from an oscillating circuit 2, such as 20KHz to
The switching transistor 3 is switched with a 50KHz pulse, and a voltage stabilized by the ferrite transformer 4, which is the collector load of the switching transistor 3, is obtained from the secondary side. Such a switching regulator can replace the conventional low frequency transformer with a small and lightweight high frequency transformer 4,
This greatly contributes to making power supplies smaller and lighter.

In the switching regulator mentioned above,
In order to stabilize the secondary side, fluctuations in the rectified DC stabilizing voltage on the secondary side are detected by an error amplifier 5,
A feedback control system is provided which applies this output to a pulse width modulation circuit 6 to modulate the pulse width of the high frequency reference pulse from the pulse oscillation circuit 2 and apply it to the switching transistor 3.

On the other hand, in a system where a logic circuit, which is a microcomputer-applied product, and mechanical parts such as relays or motors coexist, there are two power supplies: a high-precision +5V power supply for the logic circuit and a rough-precision +12V (or +24V) power supply for the mechanical system. Grid power is required. To configure this power supply with a switching regulator, the first
As shown in the figure, it is economical to provide the above-mentioned feedback control system for stabilization only on the +5V power supply, and not to provide the feedback control system for stabilization on the +12V power supply.

However, in such a configuration, even if the +12V power supply has no load or a light load, the switching transistor 3 is switched with a constant pulse width if a certain load is applied to the +5V power supply. For this reason+
There was a risk that the voltage of the 12V power supply would rise rapidly and exceed the withstand voltage of the smoothing circuit capacitor or the allowable voltage of the load.

In order to eliminate this drawback, as shown in Figure 1, +
A leader resistor 7 was connected between 12V and the common potential as a dummy load. Therefore, an invalid current was allowed to flow through the leader resistor 7 until it reached a level at which a +12V power supply could be stably obtained. For this reason, if a current of _100mA is caused to flow through the leader resistor 7, then 12×
This resulted in a power loss of 0.1 = 1.2W, reducing the efficiency of the entire switching regulator.

However, depending on the application, a constant load current always continues to flow, so there may be no problem even if the +12V power supply suddenly increases during light loads.In such applications, the leader resistor 7 may be removed to avoid waste. It is desirable to reduce power consumption.

However, the problem in this case is that there is a risk that an overvoltage will occur in the +12V power supply if the load is not in the normal state, that is, if the load is accidentally dropped.

The present invention has been made in view of this point, and one embodiment of the present invention will be described in detail below with reference to FIG. 2. Since the circuit of the present invention is almost the same as that shown in FIG. 1, the same parts as in FIG. 1 are given the same figure numbers.

According to the present invention, a +5V voltage detection circuit 9 is connected to +5V via a Zener diode 10 and a resistor 11.
Connect to the 12V voltage end.

In this configuration, only +5V is normally detected by the voltage detection circuit 9 and inputted to the error amplifier 5 to stabilize the +5V voltage. However, if the +12V voltage increases for some reason and exceeds the protection voltage, the Zener diode 10 becomes conductive and inputs a voltage higher than +5V to the error amplifier 5. As a result, the pulse width is reduced by the pulse width modulation circuit 6 and +
Operates to lower the voltage by 5V, and accordingly +12V
The voltage can also be limited to a certain voltage level. In this case, the accuracy of the +5V voltage will be lost, but since this is not the normal usage condition, no inconvenience will occur.

As detailed above, according to the present invention, voltage abnormalities in the +12V system can be prevented using the +5V system feedback control system, so load protection can be achieved without using a leader resistor. In addition, reactive power in the leader resistor is eliminated, which is beneficial as it contributes to improving the efficiency of the switching regulator.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram for explaining a conventional example, and FIG. 2 is a circuit diagram for explaining the present invention. 1 is a rectifier and smoothing circuit, 2 is a pulse oscillation circuit, 3 is a switching transistor, 4 is a transformer, 5 is an error amplifier, 6 is a pulse width modulation circuit, 7 is a leader resistor, 9 is a voltage detection circuit, 10 is a zener The diode 11 is a resistor.

Claims (1)

[Scope of utility model registration request] It comprises a rectifying and smoothing circuit, a transformer, a switching element, and a control circuit connected to an AC power source, and applies a high frequency control pulse output from the control circuit to the switching element to form a high frequency AC voltage. In a switching regulator that obtains a plurality of stabilized power supplies on the secondary side by applying it to the primary side of the transformer, the pulse width of the high frequency control pulse is determined by detecting the voltage fluctuation of one output of the transformer. Stabilization is performed by modulation, and a Zener diode is connected between another output with a higher voltage than the first output and the first output, and when the other output becomes excessive, the Zener diode is made conductive and the first output is connected. A switching regulator having multiple outputs, characterized in that the output is reduced.