JPH0255574A - Switching converter - Google Patents

Abstract

PURPOSE:To reduce the loss of a transformer by increasing the switching frequency of a switching element through a switching frequency control circuit when the voltage of an input power source is low. CONSTITUTION:A switching frequency control circuit comprises a PWM control circuit 2 for a switching element Q1 and an oscillation circuit 1 for controlling the switching frequency. The PWM control circuit 2 performs ON/OFF control of the switching element Q1 so that the electromotive force of the secondary winding N2 of a transformer is rectified D1, C1 and outputted. The PWM control circuit 2 controls the ON duty ratio of the element Q1 from the output voltage from a resistor voltage dividing circuit R3-4 arranged between the output terminals so that the output voltage is kept constant. Furthermore, a resistor voltage dividing circuit R1-2 is provided and the oscillation frequency of the oscillation circuit 1 is varied by the divided voltage value. When the input source voltage is high, switching frequency of the element Q1 is lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION (AT Industrial Application Field) This invention relates to a switching converter used as a constant voltage power supply circuit.

(bl Conventional Technology) Conventionally, switching converters have adopted various configurations depending on the application, but generally they are separately excited type converters that control the switching elements using PWM as a DC power supply circuit with a large output capacity and a small output voltage fluctuation rate. A switching converter is used.

FIG. 5 shows a basic circuit diagram of the conventional switching converter. The primary winding N1 of the transformer as shown in the figure
A switching 1-run transistor Q1 is connected to the switching 1-run transistor Q1, and the PWM control circuit 2 is configured to control the gate of the switching 1-run transistor Q1. 1~Lance's 2 blow vA
A current diode D1 and a smoothing capacitor C1 are connected to N2 to supply a DC voltage to the load. A resistive voltage divider circuit consisting of resistors R3, R, and l is provided between these output terminals, and its output voltage is fed back to the PWM control circuit 2. The PWM control circuit 2 turns Ql on and off using the oscillation frequency of the oscillation circuit 1, and at this time controls the on-duty ratio of Ql so that the divided output voltage formed by R3 and R4 becomes a constant value.

(C) Problems to be Solved by the Invention Design conditions for a switching converter that performs PWM control on such switching transistors include input voltage, maximum and minimum values of output current, and switching frequency.

Generally, they are constructed so that maximum efficiency can be obtained at the rated input and rated load. However, even if the output current (fA - load current) is constant, if the input terminal changes, the efficiency as a switching converter will decrease. That is, if the load current is constant, the lower or higher the input voltage is than the rated input voltage, the less the maximum efficiency condition will be satisfied.

For this reason, conventional switching converters used under a wide range of input power supply voltage conditions have large losses under conditions other than the designed rated input voltage.

The purpose of this invention is to
The object of the present invention is to provide a switching converter that can obtain maximum efficiency depending on the input power supply voltage.

(di Means for Solving the Problems) This invention consists of a switching element that intermittents the current flowing through the primary wire of the transformer, a circuit that rectifies and smoothes the voltage generated in the secondary winding of the transformer, and a load. In a switching converter, the switching converter is equipped with a switching control circuit that detects a supply voltage and changes the on-duty ratio of the switching element so that the voltage is maintained at a constant value. The present invention is characterized in that a switching frequency control circuit is provided that lowers the switching frequency of the switching element when the input power supply voltage is high, and increases the switching frequency of the switching element when the input power supply voltage is low.

(e) Effect-i The loss of a switching converter is divided into the loss due to the transformer and the loss due to the switching element. Losses caused by transformers can be broadly divided into copper loss and iron loss. The switching frequency dependence of each loss will be shown below.

(1-1) 'The copper loss includes the copper loss of the primary winding and the secondary winding, and the total steel ff1Pc that summarizes these losses is expressed by the following formula.

Pc − (1/Ap+1/As) /) ・ j
l! (Np■■))2 ・ (Ra/Rd) ・
...(1) Ap: Conductor occupied cross-sectional area of the primary winding As: Conductor occupied cross-sectional area of the secondary winding ρ; Copper resistivity p: Average winding 1.4 Np: Number of primary windings ■p :Primary side effective current Ra/Rd: Ratio of AC resistance to DC resistance Among these coefficients, the one that depends on the switching frequency is Ra/Rd. If the switching frequency is 20KHz
A change of 1.02-118 at ~200KHz. Although there is a positive correlation between switching frequency and loss, the rate of change is extremely small. Therefore, copper loss hardly depends on the switching frequency.

(1-2) Iron loss Iron loss includes hysteresis loss and eddy current loss, which are expressed by the following equation.

Pi=Ph+Pe -K h -f-Bm" V e 10Kc・f2 ・8m2・4■e... (2) Here Ph: Hysteresis loss Pe: Eddy current loss Kh: Hysteresis loss constant Ke: Eddy current loss Constant ■e: Core effective volume Bm; Maximum magnetic flux density during operation.Bm is Bm-E-T/(AC-Np)...where E: Input voltage T: ON period of one cycle of switching element AC: Core midsection area.Here, for example, if the on-duty of the switching element is 30%, T is 'r=0.3/f (f switching frequency), so Bm=0.3E/ (Ac-Np- f) ... (41
By substituting this into equation (2), the iron loss Pi becomes P 1
=Kh (0,3E/ (Ac-Np)) 2°4・V
e ・f-”+Ke (0,3B/(Ac-Np)l
"Ve -f-"...(5) There is a negative correlation between the switching frequency and iron loss.

(1-3) Total transformer loss Therefore, the frequency dependence of the loss of the entire transformer is negative, and the higher the switching frequency, the lower the I/lance loss.

(2) Loss due to switching elements Loss due to switching elements occurs at the intersection of the current flowing when the switching element is on and the voltage applied when the switching element is off, so the loss increases as the switching frequency increases.

(3) Loss of the entire switching connector The loss of the entire switching connector is the sum of the I-lance loss and the switch-noting element loss, ignoring the loss consumed by other circuit components. Shown in Figure 3. In Figure 3, Ps is the switching element loss, pt is the transformer loss, and P is the total loss of the converter, which is the sum of both losses.The number 1 at the end is when the input power supply voltage is low, and 2 is when the input power supply voltage is high. are shown respectively. In this way, the loss of the entire switching conharc is the sum of frequency dependent or negative transformer loss and switching element loss having positive frequency dependence, so there is a switching frequency with the lowest loss. When the input power supply voltage is low, losses due to switching elements decrease overall, and transformer losses (
Eventually, copper loss) will increase overall. Conversely, when the input power supply voltage is high, switching element loss increases and transformer 146 loss (of which copper loss) decreases. For this reason, the third
f1 in the figure. ．． As shown by f2, the higher the input power supply voltage, the lower the switching frequency that provides the lowest loss.
Conversely, the lower the input power supply voltage, the higher the switching frequency at which the lowest loss can be obtained.

Basically, the switching control circuit of the present invention detects the load supply voltage by the switching control circuit, and changes the switching element to on-duty so that the load supply voltage is maintained at a constant value. I control you. This intermittents the current flowing through the switching element or the primary winding of the transformer, generates an electromotive force in the secondary winding of the transformer, and is rectified and smoothed by the rectifier and smoothing circuit to obtain a constant DC voltage. It will be done. Furthermore, the switching frequency control circuit lowers the switching frequency of the switching element when the input power supply voltage detected by the input power supply voltage detection circuit is high, and conversely increases the switching frequency of the switching element when the input power supply voltage is low. If the current is constant, the transformer loss will decrease when the input power supply voltage is high, but in this case, the switching frequency control means lowers the switching frequency, so the loss of the entire conharc is further reduced. On the other hand, when the input power supply voltage is low, the transformer loss increases, or by increasing the switching frequency of the switching frequency control means, the loss of the converter as a whole decreases.

FIG. 1 is a circuit diagram showing the configuration of the invention.

In FIG. 1, reference numeral 3 denotes a switching frequency control circuit, which is comprised of a circuit 2 for PWM control of the switching element Q1 and an oscillation circuit 1 for controlling the switching frequency. PWM control circuit 2 controls on/off of switching element Q1. As a result, the electromotive voltage generated in the secondary winding N2 of the transformer is rectified and smoothed by Dl and C1. Also, R3. is provided between the output terminals. The PWM control circuit 2 controls the on-duty ratio of Ql based on the output voltage of the resistor voltage divider circuit made up of R11 to keep the output voltage constant. Furthermore, a resistive voltage dividing circuit consisting of R1R2 is provided between the input power supply line and the ground, and the oscillation circuit 1 changes the oscillation frequency (switching frequency) depending on the voltage dividing value. As shown in FIG. 3, the oscillation circuit 1 oscillates at a switching frequency that minimizes converter loss depending on the input power supply voltage. In this way the switching converter losses are kept low over a wide range of input supply voltages.

(f) Embodiment FIG. 2 shows a circuit diagram of a switching converter which is an embodiment of the present invention. In the figure, reference numeral 3 denotes a control IC 14 constituting a switching control circuit, which is a voltage stabilizing circuit that supplies a constant voltage power to this IC. As shown in the figure, a switching element Q is connected to the primary winding N1 of the transformer T1.
1 is connected, and the control IC 3 controls on/off of Ql. A rectifying and smoothing circuit consisting of Di and CI is provided in the secondary winding N2 of the transformer, and R3. A resistive voltage divider circuit consisting of R4 is provided. IC3 changes the on-duty ratio of 01 so that this partial pressure value is constant. The transformer T2 is a transformer that steps down the input power supply voltage, and its output is rectified by the diode bridge DB2, smoothed by C3,
Further, the voltage is regulated by a constant voltage circuit 4 and is supplied as a power source for the IC 3. On the other hand, a diode bridge DBI and a smoothing capacitor C4 are connected to the input power source, and the rectified and smoothed output thereof is connected to one end of the primary winding N1 of the transformer T1. Also this diode bridge DB
A resistive voltage divider circuit consisting of R1 and R2 is connected between the output of I and the ground, and the input power supply voltage is detected by this.

An oscillation circuit O3C is configured in the control IC3, and the oscillation frequency is determined by the resistance between the reference voltage output terminal Vr and one input terminal of the OSC and the capacitance between the other input terminal of the OSC and ground. Vr and OSC as shown
A resistor R5 is connected between one input terminal of the
A circuit consisting of transistor Q2 and transistor Q2 is connected thereto. Further, the output of a voltage dividing circuit consisting of R1 and R2 is connected to the base of Ql. Therefore, when the input power supply voltage increases, the base potential of Ql increases, and the on-resistance of Ql increases. As a result, the resistance value between Vr and one of the input terminals of OSC increases, the oscillation time constant increases, and the oscillation frequency decreases. Conversely, when the input power supply voltage is low, the base potential of Ql decreases, so the on-resistance of Ql decreases. As a result, the resistance value between Vr and one input terminal of OSC decreases, and the oscillation frequency increases. The oscillation frequency at this time is determined, for example, as shown in FIG. 4, so that the loss of the switching converter is always the lowest regardless of changes in the input power supply voltage Vin. Therefore R1, R2 and R6
Set the value of

(g) Effects of the Invention As described above, according to the present invention, the loss of the switching converter related to changes in the input power supply voltage can always be kept at a minimum, so that highly efficient switching can be achieved in response to a wide range of input power supply voltages. A converter can be configured.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of the present invention, and FIG. 2 is a circuit diagram of a switching converter that is an embodiment of the present invention. FIG. 3 is a diagram showing the relationship of loss to the switching frequency of the switching converter, and FIG. 4 is a diagram showing the relationship of the loss of the switching converter to the switching frequency using the input power supply voltage as a parameter. Fifth
The figure is a basic circuit diagram of a conventional switching converter. Applicant: Zaiyo Seisakusho Co., Ltd.

Claims (1)

[Claims] (1) A switching element that switches on and off the current flowing in the primary winding of the transformer, a circuit that rectifies and smoothes the voltage generated in the secondary winding of the transformer, and a circuit that detects the load supply voltage and maintains the voltage at a constant value. In the switching converter, the switching converter is equipped with a switching control circuit that changes the on-duty ratio of the switching element so that the on-duty ratio of the switching element is maintained. A switching converter comprising a switching frequency control circuit that increases the switching frequency of the switching element when the input power supply voltage is low.