JP3416715B2 - Switching power supply circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit for switching DC input and transmitting energy to a secondary side. 2. Description of the Related Art A conventional switching power supply having a soft start circuit will be described with reference to FIG. An AC power supply 1 is connected to a bridge rectifier circuit 3 via a power switch 2.
It is connected to the. A first smoothing capacitor 4 is connected to the output of the bridge rectifier circuit 3 in parallel. The first smoothing capacitor 4 includes a series circuit of a starting resistor 18 and a second smoothing capacitor 19,
The secondary winding and the source / drain of the switching element 5 are connected in parallel. The connection between the starting resistor 18 and the second smoothing capacitor 19 is connected to the power input of the control circuit 13, and the control signal output of this control circuit is connected to the gate of the switching element 5 via the gate resistor 6. . Output-side rectifying / smoothing circuits 21 and 22 are connected in parallel between both ends of the secondary winding of the converter transformer 20. The load circuit 2 is connected between both output terminals of the output-side rectifying / smoothing circuit.
An isolation amplifier 24 that detects the third and secondary output voltages and transmits this information to the primary control circuit is connected. The control circuit 13 includes an error amplifier 7, a triangular wave oscillator 8, a first PWM comparator 9, a second PWM comparator 10, an AND circuit 11, and a driver circuit 12.
It consists of. The reference voltage source 14 is provided on the positive input side of the error amplifier 7 and the isolation amplifier 24 is provided on the negative input side.
The output of the error amplifier 7 is connected to the negative input of the first PWM comparator 9 and the triangular wave oscillator 8 is connected to the positive input.
The other terminal of the parallel circuit of the third capacitor 17 and the resistor 15, one of which is connected to the GND of the control circuit, the other of which is connected to the triangular wave oscillator 8 on the positive input side of the second PWM comparator 10 and the negative input side. Resistor 1 with one connected to the control circuit power supply
6, the other terminals of the two comparators 9 and 10 are input to an AND circuit 11,
The output of the AND circuit 11 is connected to the input of the driver circuit 12. In the switching power supply having the above configuration, when the power switch 2 is turned on, the input of the AC power supply 1 is supplied to a bridge rectifier circuit 3 where the input is subjected to bridge rectification and then smoothed by a first smoothing capacitor 4. Starting resistance 1
The connection point voltage between the second smoothing capacitor 8 and the second smoothing capacitor 19 rises together with the terminal voltage of the first smoothing capacitor 4, and the control circuit 13 starts operating at the start-up start voltage. The operation of the circuit shown in FIG. 7 will be described with reference to a time chart shown in FIG. Even if the voltage of the first smoothing capacitor 4 rises and the voltage at the connection point between the starting resistor 18 and the second smoothing capacitor 19 reaches the starting voltage of the control circuit 13 and the triangular wave oscillator 8 starts operating, the second PWM Comparator 1
The negative input side voltage of 0 indicates that the terminal voltage of the second smoothing capacitor 19 is Vc, the capacitance of the third capacitor 17 is C,
5, 16, R ₁ and the resistance values, R _2, when the time is for t, to increase in ^{Vc (l-e -t / C} (R1 + R2)), the third
The on-period gradually increases until the charge of the capacitor 17 is completed. As a result, even if the secondary side output voltage is low, the ON period is limited, so that the switching element 5
Can perform a soft-start operation in which the peak current also gradually increases. The operation of the circuit shown in FIG. 7 is apparent from the time chart shown in FIG. 8, and a detailed description thereof will be omitted. However, such a method is effective in a forward converter or a separately-excited flyback converter in which the output voltage is stabilized by changing the duty, but the duty is constant. Therefore, it cannot be used for a self-excited flyback converter or the like that controls the output voltage according to the length of the on-time. Therefore, in the above-described method, the current flowing through the switching element is limited regardless of the method of the converter and the response of the control circuit, and the destruction of the switching element due to saturation of the switching transformer cannot be prevented. According to the present invention, a pulsating current after DC or AC smoothing is input and a switching transformer having a primary winding and a secondary winding, and a primary of the switching transformer is provided. A switching element having a negative terminal connected to a winding, a control circuit for driving and controlling the switching element, and a rectifier circuit connected to a secondary winding of the switching transformer; Limiting the gate drive current of the switching element from the circuit to limit the current of the switching element irrespective of the response of the control circuit and the operating state of the circuit prevents the destruction of the switching element due to saturation of the switching transformer. Has solved the problem. [0008] A switching power supply circuit according to a first aspect of the present invention receives a pulsating current after DC or AC smoothing, and has a primary winding and a secondary winding. A switching element having a negative terminal connected to a primary winding of the switching transformer, a control circuit for driving and controlling the switching element, and a rectifier circuit connected to a secondary winding of the switching transformer. In the switching power supply circuit, the drive current of the switching element from the control circuit is changed over time.
It has a function of soft-starting based on a configuration for increasing the switching element, and prevents the switching element from being destroyed due to overcurrent by limiting the current of the switching element regardless of the response of the control circuit and the operation state of the circuit. . First Embodiment A first embodiment of the present invention will be described below with reference to FIGS. 1 to 4. In FIG. 1, 1 is an AC power supply, 2 is an AC switch, 3 is a bridge rectifier, and 4 is a smoothing capacitor. 5 is a switching transformer , 6 is a switching transistor composed of a MOSFET,
7 is a gate resistor, 8 is a control circuit, 9 is a starting resistor, 10 is a power supply smoothing capacitor of the control circuit 8, 11 is a current detection resistor, 12 is a rectifier diode of a secondary output, 13 is a smoothing capacitor, and 14 is a secondary capacitor. This is a load circuit that operates with side rectification and smoothed output voltage. Reference numeral 15 denotes a transistor, 16 denotes a resistor, 17 denotes a transistor, and 18 denotes a resistor. 19 is a diode for pulling out the gate charge of the switching transistors 6. 20 is a capacitor, 21 is a resistor. Here, from FIG. 2, the switching transients scan
And six other is the gate-source voltage V _GS is the gate charge amount Q
g has a characteristic that increases almost in proportion to g.
It has a characteristic that the drain current ID increases in proportion to the gate-source voltage _VGS . In the switching power supply circuit having the above configuration, when the AC switch 2 is turned on and electric charge is being accumulated in the capacitor 20 by the current through the starting resistor 9, the terminal voltage of the capacitor 20 is changed to Vcc, Assuming that the capacitance of the transistor ₂₀ is C ₂₀ and the resistance of the resistor 21 is R ₂₁ , the emitter voltage Ve ₁₅ of the transistor 15 is equal to the time t.
With the passage of time, The equation is changed by the following equation. In this case, since the transistor 15 forms a current mirror circuit together with the transistor 17, the maximum value of the gate drive current IG flowing from the control circuit 8 to the switching transistor 6 is equal to the emitter current Ie _{17 of the} transistor _17.
Thus, the gate drive current IG of the switching transistor 6 is determined by setting the emitter voltage of the transistor 17 to V
e ₁₇ , assuming that the voltage between the base and the emitter of the transistor 17 is Vbe ₁₇ , and the resistance values of the resistors 18 and 7 are R ₁₈ and R ₇ , IG = (Ve ₁₇ −Vbe ₁₇ ) / (R ₁₈ + R ₇ ) Become. FIG. 4 shows these as time charts. In FIG. 4, the emitter voltage Ve ₁₇ and the emitter current Ie ₁₇ of the transistor ₁₇ and the control circuit 8
, The gate drive current IG of the switching transistor 6 and the drain current ID of the switching transistor 6 are shown. As is evident from Figure 4, with decreasing emitter voltage Ve ₁₇ of the transistor 17, but the emitter current Ie ₁₇ of the transistor 17 is to continue to increase, the output of the control circuit 8 constant pulse width at this time Output, the gate drive current IG
Gradually increases, and accordingly, the drain current ID gradually increases. As described above, by limiting the gate drive current IG so as to gradually increase, the gate charge amount of the switching transistor 6 per one pulse with respect to the output pulse of the control circuit 8 is reduced, and the switching transistor 6 drains itself. The current ID can be limited. By gradually increasing the gate drive current IG with the passage of time, the drain current ID increases. As a result, the soft start operation can be realized regardless of the converter system and the operation characteristics of the control circuit. Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIGS. Here, the same reference numerals are given to portions corresponding to the first embodiment. The second embodiment is different from the first embodiment in that the capacitor 20 and the resistor 2
1, the resistor 18 and the diode 19 are omitted, a capacitor 22 is connected between the resistor 16 connected to the emitter of the transistor 15 and GND, and a resistor 23 is connected between the emitter of the transistor 17 and GND. That is. When the AC switch 2 is turned on, the capacitor 2
When the charge of the capacitor 2 is in the process of being accumulated, assuming that the capacitance of the capacitor 22 is C ₂₂ and the resistance value of the resistor 16 is R ₁₆ , the emitter voltage Ve ₁₅ of the transistor ₁₅ becomes: ## EQU1 ## Therefore, assuming that the resistance value of the resistor 16 is R ₁₆ and the resistance value of the resistor 23 is R 23, the gate drive current IG of the switching transistor 6 is IG = Vcc / R ₁₆ − (Vcc−Ve ₁₅ ) / R ₂₃ . FIG. 6 shows these as a time chart. As is apparent from FIG. 6, the emitter voltage Ve ₁₅ of the transistor 15 gradually increases, the emitter current Ie ₁₅ of the transistor 15 gradually decreases, and the output pulse width of the control circuit 8 is constant. Gate drive current I
G gradually increases, and the drain current ID gradually increases. As described above, the gate drive current IG is bypassed to reduce the gate charge amount of the switching transistor 6 per pulse in the output pulse of the control circuit 8, thereby limiting the drain current ID by the switching transistor 6 itself. Can be.
Then, the gate drive current IG gradually increases with time.
Increases, the drain current ID increases, and as a result, a soft start operation can be realized irrespective of the type of converter and the operating characteristics of the control circuit. Here, in each of the embodiments, the switching transistor 6 is used as a switching element. However, it goes without saying that a similar effect can be obtained in the case of a bipolar transistor. As described above, according to the switching power supply circuit of the present invention, by using the configuration in which the drive current of the switching element is increased with the passage of time, the response of the converter system and the control circuit can be improved. Irrespective of this, it is possible to limit the current flowing to the switching element and prevent the switching element from being destroyed due to saturation of the switching transformer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a switching power supply circuit according to Embodiment 1 of the present invention. FIG. 2 is a characteristic diagram of a gate-source voltage-gate charge amount characteristic of a switching transistor. FIG. Gate. Source voltage characteristic diagram [FIG. 4] Time chart at startup of circuit of FIG. 1 [FIG. 5] Circuit diagram of switching power supply circuit according to Embodiment 2 of the present invention [FIG. 6] Time chart at startup of circuit of FIG. FIG. 7 is a circuit diagram of a conventional switching power supply circuit. FIG. 8 is a time chart at the time of startup of the circuit of FIG. 7 [Description of symbols] 1 AC power supply 2 AC switch 3 Bridge rectifier 4 Smoothing capacitor 5 Switching transformer 6 Switching transistor 7 Gate Resistor 8 Control circuit 9 Start-up resistor 10 Smoothing capacitor 11 Current detection resistor 12 Rectifier diode 13 Smoothing capacitor 14 Load circuit 15 Transistor 16 Resistor 17 Transistor 18 Resistor 19 Diode 20 Capacitor 21 Resistance Vcc Power supply voltage of control circuit

Claims (1)

(1) A pulsating current after DC or AC smoothing is input, and a switching transformer having a primary winding and a secondary winding and a primary winding of the switching transformer are provided. A switching element having an end connected thereto, a control circuit for driving and controlling the switching element, and a rectifier circuit connected to a secondary winding of the switching transformer; Increase the gate drive current of the switching element over time
A switching power supply circuit having a function of performing a soft start based on a configuration to be added .