JPS5827215A - Overload protecting device for multi-output switching power source - Google Patents

Abstract

PURPOSE:To improve response characteristics, by detecting a forward current by a current transformer installed in the secondary winding of a conversion transformer and controlling on-time of a switching transistor so as to make the peak value of momentary current constant. CONSTITUTION:A DC power source 1 is supplied to a conversion transistor 3 through an NPN switching transistor 2, and at the same time, smoothing chokes 6, 11 and current transformers 4, 9 that detect a forward current are installed in each of the secondary winding of a conversion transformer 3, and two constant voltage V1, V2 are outputted. Conversion circuits 8, 13 consisting only of resistance are connected to both ends of the secondary windings of current transformers 4, 9 and the momentary peak value of the forward current is detected for each cycle. The On-time of the switching transistor 2 is controlled by a controlling circuit 14 to make the momentary peak value constant. By this way, the number of parts can be reduced and response characteristics can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention controls the ratio of on-time and off-time of a switching transistor from a DC power source, and obtains a plurality of desired types of DC voltages from a plurality of secondary winding outputs provided in a conversion transformer. The present invention relates to an overload protection device that protects each output circuit from overload in a forward type multi-output switching power supply.

FIG. 1 shows a forward-type two-output switching power supply using a generally well-known NPN switching transistor 2, in which forward smoothing chokes 6 and 11 are connected to each of the secondary windings of a conversion transformer 3. A circuit configuration is shown in which current transformers 4 and 9 are provided to detect current, and overload protection is provided for each output circuit.

In FIG. 1, 1 is a DC power supply, 5 and 10 are secondary side high-speed rectifier diodes, 7 and 12 are Vi smoothing capacitors, and 8 and 13 are conversion circuits that obtain predetermined signals from the current transformers 4 and 9, respectively. , its output is connected to a control circuit 14 that controls the switching transistor 2. When the output signal level of the conversion circuit 8 or 13 exceeds a preset reference value, it is assumed that the load of the (1) or (2) output is overloaded, and the control circuit 14 operates to provide overload protection for each output. The control circuit 14 normally detects the output voltage (2) and performs a constant voltage control operation so that the output voltage remains constant. The other output voltage v1 is not directly controlled. Other operations of the phase 1 diagram are well known and will not be described here.

In the conventional overload protection device, as shown in FIG. 2 as the conversion circuit 8 or 13 in FIG. 1, a resistor 15 is connected across the secondary winding of the current transformer 4 to convert it into a voltage signal. The signal is rectified and smoothed into a DC voltage by a voltage doubler rectifier circuit 16 consisting of a diode 16a and a capacitor 16b. A resistor 17r/i is used to set an appropriate discharge time constant. The reason for using the voltage doubler rectifier circuit 16 is that the current waveform detected by the current transformer 4 or 9 is as shown in FIG. This is because the output current (1) (shown in FIG. 3) would not be detected in this case.

The DC signal detected and converted as described above is sent to the control circuit 14.
The overload protection function is performed by connecting the

1. If the signal level detected from the converter is not sufficiently large, it cannot be rectified by the voltage doubler rectifier circuit 16.

Therefore, the loss of the resistor 15 increases and the number of turns of the current transformer 4 or 9 increases.

2. Since diodes are used in the conversion circuit 8 and 13, there are problems with detection accuracy including temperature characteristics.

When the number of output circuits on the 32nd order side increases, the number of converter circuits is 8 or 1.
3 has a large number of parts, which is a problem.

4. Due to rectification and smoothing, the response characteristics of overload protection are limited.

The overload protection device for a multi-output switching power supply of the present invention includes:
This method solves the above-mentioned conventional problems, and its operation will be explained next. In the present invention, as the conversion circuit 8 or 13 in FIG. 1, a current transformer 4tf? as shown in FIG. : is an extremely simple structure in which only a resistor 15 is connected to both ends of the secondary winding 9. The control circuit 14 includes a switching transistor that detects the instantaneous peak value of the output signal of the conversion circuit 13 every cycle and controls the switching transistor so that the peak value (11p) (shown in FIG. 3) is constant. However, it is possible to control such a peak value by adding a hold circuit outside the commonly used control circuit integrated circuit. Since it is well known, its circuit configuration will not be discussed here.

As explained earlier, the conversion circuit 8 still has 13 resistors 15
The voltage signal across both ends of is as shown in FIG. 3, and although the peak value (11p) of the signal and the output current (min) do not match, they have the following relationship.

i = Kxllx(1-A) p Here, K: proportionality constant A: t1/12 Therefore, if the control circuit 14 controls the on-time t1 of the switching transistor 2 so that 11p is constant, the output current This results in a "fold-back" voltage/current characteristic as shown in Figure 5. In Fig. 3, a shows 11p and 11 when the on-time t1 of the switching transistor 2 is long, and b shows 11p and 11 when the on-time 11' is small. At the same 11p, the output current It illustrates that things change. However, the value of the output current 11 when the output circuit is short-circuited, that is, when the output voltage v1 in FIG. Since the term can be ignored, there is no significant difference from that detected by the voltage doubler rectifier circuit 16 as in the conventional example. 6th
In the figure, characteristic a is the conventional one, and characteristic a is the one of the present invention.

Most of the overload protection for multi-output switching power supplies is short-circuit protection, and the protection of the present invention can be said to be sufficient.

As explained above, the overload protection device for a multi-output switching power supply according to the present invention can be extremely configured by combining a peak value control type control circuit and a smoothing choke forward current detection/conversion circuit using a current transformer. It is possible to provide a protection device suitable for a multi-output switching power supply that is simple and has good response characteristics.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram of a generally well-known forward type two-output switching power supply, a detection/conversion circuit diagram of a conventional overload protection device for a 2Nd multi-output switching power supply, and Fig. 4 is a circuit diagram of a multi-output switching power supply according to the present invention. The detection/conversion circuit diagrams of the overload protection device for the output switching power supply, FIGS. 3 and 5, show the detection signal waveform and output voltage/current characteristics to explain its operation. 1...DC power supply, 2...Switching transistor, 3...Conversion transformer, 4.9.
...Current transformer, 5.10 ...Secondary high-speed rectifier diode, 6.11 ... Smoothing choke, 7,12 ... Smoothing capacitor, 8,13
... Conversion circuit, 14 ... Control circuit, 1
5...Resistance. Name of agent: Patent attorney Toshio Nakao and 1 other person 2nd
Figure 3 (b) 77- [Figure 4

Claims (1)

[Claims] The DC power supply controls the time ratio between the on time and off time of the switching transistor, and the configuration is configured to obtain multiple desired types of DC voltage from the multiple secondary winding outputs provided in the conversion transformer. A current transformer is installed in the secondary winding of the conversion transformer corresponding to the output so as to detect the forward current of the smoothing choke, and the peak value of the instantaneous current detected by each of the current transformers is constant. 2. An overload protection device for a multi-output switching power supply, comprising an overload protection circuit that can individually protect output overloads of the multi-output switching power supply by controlling the on-time of the switching transistors.