JPH049033B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in the output control circuit of a ringing choke converter (RCC). Figure 5 shows a conventional basic circuit diagram. When the input power supply voltage Ei is applied, the main switching transistor _Q1 is turned on through the starting resistor _R1 , and this causes a voltage induced in the return winding NB of the output transformer T. A constant base current flows through the main transistor Q ₁ due to the base resistor R 2 and the base resistor R ₂ . In this case, the voltage induced in the secondary winding N ₂ is blocked by the rectifier diode D ₁ ,
No current flows through the smoothing capacitor _C2 and the load RL. At this time, approximately the input power supply voltage Ei is applied to the primary winding N ₁ of the transformer T, and the current in the primary winding N ₁ , that is, the collector current of the transistor Q ₁ increases linearly, and energy is transferred to the primary winding N ₁ . can be stored. However, since the base current is constant, the collector current of transistor Q ₁ cannot increase any further when it reaches a certain value, and after that, the primary winding N ₁
The voltage induced in the base current decreases and therefore the base current also decreases. Due to this feedback regeneration effect, the transistor _Q1 is rapidly turned off. At the same time, a voltage is induced in the secondary winding _N2 of the transformer T in the direction of making the diode _D1 conductive, and the energy stored in the primary winding _N1 is supplied to the capacitor _C2 and the load RL through the diode _D1 . Ru. Eventually, the transistor Q ₁ is turned on again and the diode D ₁ is cut off, but current is continued to be supplied to the load RL due to the discharge of the capacitor C ₂ , and thereafter the operation as described above is repeated. However, although such conventional circuits have the voltage conversion function described above, they do not have an output control function. Therefore, the present invention provides an inexpensive converter that efficiently provides an output voltage control function and an overcurrent protection function. This will be explained below with reference to the drawings. 1 and 2 are a circuit diagram of an embodiment of the present invention and operation waveform diagrams of each part thereof, and the same reference numerals as in the conventional example indicate equivalent parts. In the figure, A and B are a voltage detection section and a control circuit section that constitute the main parts of the present invention. First, the voltage detection section A consists of a voltage detector IC1 containing a reference voltage, a photocoupler light emitting section PD that outputs this, and resistors R ₄ and R ₅ that divide the output voltage E ₀ . In the control circuit section B, Q ₂ is a control transistor connected between the base and emitter of the main transistor Q ₁ , R ₃ and C ₄ are resistors and capacitors forming a time constant circuit, and are connected to both ends of the feedback winding NB. The connection point a between the resistor R ₃ and the capacitor C ₄ is connected to the base of the control transistor Q ₂ . Next, PT is the photocoupler light receiving section connected between one end of the feedback winding NB and the connection point a, thereby adjusting the charging time constant of the capacitor _C4 . Note that _C3 is a capacitor for improving the off-characteristics of the main transistor _Q1 , and _D2 is a diode for preventing the base starting current from sneaking around. The basic operation of this circuit is almost the same as that of the conventional example, and will therefore be omitted. Next, the operation of the control circuit section B will be explained with reference to FIG.

<Voltage control operation> First, in Fig. 2, a is the main transistor Q ₁
b is the voltage waveform diagram of the feedback (base) winding, c is the voltage waveform diagram of the capacitor C ₄ (point a), photocoupler light receiving part PT and resistor
In the time constant circuit formed by the current flowing through _R3 and the capacitor _C4 , the potential at point a is the control transistor.
When the voltage of Q ₂ (VBE) is exceeded, the transistor Q ₂
turns on and absorbs the base current of main transistor _Q1 , turning it off. (Figure 2 a) Furthermore,
Ignoring the loss in each part of the converter, the output voltage
The relationship between the on-time TON of E ₀ and Q ₁ can be approximated as shown in equation (1).

E ₀ = Ei (KiEi/I ₀ TON−K ₂ ) …(1) Here, Ei is the input voltage, I ₀ is the output current, K ₁ , K ₂
is a constant. By detecting the output voltage and varying the current of the photocoupler transistor to control TON, the output voltage E ₀ can be kept constant using equation (1).

<Overcurrent protection operation> When the output voltage is controlled to be constant as described above, the on time (TON) of the main transistor _Q1 is reduced by increasing the load current or decreasing the input voltage (Ei). increases and the frequency decreases. Therefore, when the input voltage (Ei) is the lowest and the maximum load current (I ₀ MAX), the above-mentioned on-time (TON) becomes the maximum (TONMAX). This is expressed below.

TONMAX=K ₁ I ₀ MAX/Ei (E ₀ /Ei+K ₂ )...(2) Now, if we set the current of the photocoupler light receiving part PT through voltage detection part A to be zero, the output current will be higher than this. , the charging time constant of capacitor C ₄ is maximum, and the main transistor Q ₁ at this time
If the on time (TON) of is TOC, then TOC>TONMAX...(3). During this TOC time, no matter how much the load current is increased, the TON width will not increase any further. If these relationships are expressed in terms of output voltage and current, the output characteristics will be as shown in Figure 3.The load current will increase up to the output current IOC corresponding to TOC, but the TON width will not increase even if the load is increased beyond this point. This results in a fold-back drooping characteristic. Note that the voltage EB generated in the base winding in Figure 1 (Figure 2 b) is the voltage generated in the forward direction.
If the voltage generated in the reverse direction of EBF is EBN, the reverse voltage EBN is proportional to the output voltage E ₀ , so if E ₀ is a constant voltage, EBN is also constant, but the forward voltage EBF is proportional to Ei. Therefore, the higher Ei is, the shorter the charging time of capacitor _C4 becomes, and the lower the TOC becomes. Therefore, by optimally designing EB, IOC can minimize changes due to input voltage.

FIG. 4 is a main circuit diagram showing another embodiment of the present invention, and is an example in which the base winding NB and the control winding NC are separated. In FIG. 1, the voltage EB generated in the base winding NB is also the base current source of the switching transistor _Q1 , so providing the control winding NC has the advantage that any voltage can be obtained.

As is clear from the above description, the present invention can be applied to switching power supply devices by adding a simple circuit to efficiently control the output voltage and prevent the output current from becoming excessive. The effect is extremely large.

[Brief explanation of drawings]

Fig. 1 is an implementation circuit diagram of the present invention, Fig. 2 is a voltage waveform at each point for explaining the present invention, Fig. 3 is the output characteristic of the implementation unit of the present invention, and Fig. 4 is another example of the present invention. Embodiment (principal part) circuit diagram, FIG. 5 is a conventional circuit diagram. In the diagram, C ₁ is the input capacitor, Q ₁
is the main transistor, T is the output transformer, N ₁ ,
N ₂ , NB, and NC are its primary winding, secondary winding,
Feedback (base) winding and control winding. D ₁ is a rectifier diode, C ₂ is a smoothing capacitor, D ₂ is a reverse blocking diode, C ₃ is a bias capacitor,
R ₁ is a starting resistor, R ₂ is a base resistor, A is a voltage detection section, R ₄ and R ₅ are voltage dividing resistors, and VD is for voltage detection.
IC and PD are photocouplers (light emitting part), B is a control circuit part, PT is a photocoupler (light receiving part), _R3 is an overcurrent setting resistor, _C4 is a control capacitor, and _Q2 is a control transistor.

Claims (1)

[Claims] 1. A leakage transformer having a primary winding, a secondary winding, a feedback winding, and, if necessary, a control winding, a collector is connected to the primary winding, and a base and an emitter are connected to the feedback winding. In a ringing chain converter in which a rectifying diode is connected to the main transistor for switching and the secondary winding, a control transistor is connected between the base and emitter of the main transistor, and the feedback winding or the control winding A circuit that provides a time constant circuit consisting of a resistor and a capacitor between them, connects the connection point of the resistor and capacitor to the base of the control transistor, and changes the charging time constant of the capacitor according to the detected output voltage value. A ringing chain yoke converter characterized by connecting. 2. The ringing chain converter according to claim 1, characterized in that the feedback circuit includes a photocoupler that supplies a current proportional to the output voltage of the output voltage detection section.