CN1812242A - Method and circuit for accurately controlling switch power supply output current - Google Patents

Abstract

This invention discloses a kind of method and circuit of precisely controlling to switch power supply to output current. This method uses switching power supply to detect and get the average current flowing from voltage transformer primary coil or inductance element during switching on the switch element. It also uses switching power supply to detect and get the time, which is either from switching off the switch element to the current of voltage transformer secondary coil or inductance attenuates to zero, or from switching off the switch element to the current of voltage transformer secondary coil or inductance does not attenuate to zero but the period of switch element switching off has finished; then, based on the detected times and average current, as well as their equivalent transformed signals, the said output current is controlled by switching power supply; in addition, it also provides a kind of circuit. It concludes: a current detector used for detecting the said average current; a demagnetizing time detector used for detecting the said time; a multiplier which gets average current, times or their equivalent transformed signals. The output of multiplier controls and outputs current.

Description

A kind of method of accurately controlling switch power supply output current and circuit

Technical field

The invention belongs to switch power technology, relate in particular to the accuracy control method and the circuit of switch power supply output current.

Background technology

In switch power technology, not only need control output voltage, also usually require the control output current, such as requiring output current constant.In flyback Switching Power Supply (flyback A.C.-D.C. converter) was used, modal method was to detect output current at resistance of transformer secondary output output series connection, controls the voltage at these series resistance two ends and just can control output current at present.Because control signal must feed back to elementary control chip,, reduced efficient so the necessary isolating device that increases between the primary and secondary though output current has obtained good control like this, has also significantly improved system cost simultaneously.

The method of another low-cost control output current is the input power of control input end at present, such as U.S. Pat 6721192.Because

Pin×Eff＝Pout

Vin×Iin×Eff＝Vfr×Iout………………………………..(1)

Vfr=Vout+Vfschottky wherein;

Vfschottky is the forward voltage drop of output rectifier diode;

Iin, Vin are primary coil average current input and voltage;

Eff is a power conversion efficiency, and Pout is a power output;

Iout, Vout are respectively output current, output voltage.

In a power-supply system, power conversion efficiency is geostationary.Therefore, need only size, just can control output current, promptly according to output voltage control input power

$\mathrm{Iout}=\frac{\mathrm{Pin}\×\mathrm{Eff}}{\mathrm{Vout}+\mathrm{Vfschottky}}\·\·\·\left(2\right)$

Fig. 1 is an accompanying drawing of U.S. Pat 6721192, uses a programmable input power limiter to obtain constant output current.This load limiter course of work is as follows: feedback voltage signal is controlled the maximum permissible value of primary current as the input of load limiter; If output voltage increases, then feedback voltage signal also increases, and load limiter will improve the maximum permissible value of primary current; Otherwise load limiter just reduces the maximum permissible value of primary current.The basic principle of this method is:

$\mathrm{Pind}=\frac{1}{2}\×L\×\mathrm{Ipea}{k}^2\×f\·\·\·\left(3\right)$

Wherein Pind is the energy of storing in the inductance;

L is the inductance value of primary coil;

Ipeak is the maximum of primary current;

F is a switching frequency.

Because equation 3 has only transformer according to DCM (interrupted current pattern) mode work just effectively, the working method that this just requires to use the Switching Power Supply of this method cannot enter CCM (continuous operation mode) has limited the use of Switching Power Supply greatly; In addition, input power and input maximum current are quadratic relationship, are not linear relationships, and this just requires to do a square conversion in Switching Power Supply inside, and this conversion will bring more loss of significance.

Summary of the invention

The invention provides a kind of method and circuit of accurately controlling switch power supply output current, it can realize the accurate control to switch power supply output current under the condition of elementary feedback.

In order to address the above problem, the invention provides following technical scheme: the method for accurately controlling switch power supply output current, described Switching Power Supply is made up of control circuit, transformer or inductance element that one or more switch elements, the described switch element of control open and close, and described Switching Power Supply detection also obtains the average current that the switch element open period flows through from transformer or inductance element; Described Switching Power Supply detects and obtains being closed to transformer secondary output coil current or inductive current from switch element and decays to zero or be closed to transformer secondary output coil current or inductive current from switch element and do not decay to zero but time of having finished switch element down periods; Described Switching Power Supply utilizes described detected time and average current and their equivalent transformation signal to control described output current.

A kind of circuit of accurately controlling switch power supply output current also is provided in addition, described Switching Power Supply is made up of control circuit, transformer or inductance element that one or more switch elements, the described switch element of control open and close, it at least also comprises: a current detector is used for the average current of sense switch element open period transformer or inductance; An erasing time detector, be used to detect from switch element be closed to transformer secondary output coil current or inductive current decay to zero or transformer secondary output coil current or inductive current do not decay to zero but time of having finished switch element down periods; A multiplier, an input of multiplier are the detected average current of described current detector or its equivalent transformation signal, and an input is detected time of erasing time detector or its equivalent transformation signal, the output control output current of multiplier.

Described current detector is average current detection device or peak current detector.

Described control circuit is the Pwm controller with pulse-skip function.

The present invention compared with prior art has following marked improvement: because the present invention utilizes the magnetic flux principle, and make equation $\mathrm{Iout}=\mathrm{Eff}\×\frac{\mathrm{Npri}}{N\sec }\×\frac{\mathrm{Tr}}{T}\×\mathrm{Iavg}\_\mathrm{on}$ Set up, wherein the efficient Eff of Switching Power Supply is considered as a constant, and Tr is the transformer secondary output coil erasing time, and Npri and Nsec are respectively the numbers of turn of primary coil and secondary coil, Iavg_on is a primary coil at the average current in energy storage stage, and T is the switch element period of oscillation.And the present invention utilizes Switching Power Supply to detect and obtain the average current that the switch element open period flows through from transformer or inductance element, and detect and obtain being closed to transformer secondary output coil current or inductive current and decay to zero or be closed to transformer secondary output coil current or inductive current from switch element and do not decay to zero but time of having finished switch element down periods, and utilize multiplier to control output current above-mentioned detected time and average current and their equivalent transformation signal from switch element.The present invention does not only need to increase the isolating device between the primary and secondary, and what output current and controlling formed between its average current is linear relationship, the loss of significance of having avoided repeatedly conversion to bring, thereby realized accurate control to switch power supply output current, and use the average current detection device simultaneously, it can make Switching Power Supply can both work under the pattern of DCM or CCM, avoid the embarrassment that under DCM or CCM, to work, and adopted Pwm controller, made integrated circuit reduce power consumption with pulse-skip function.

Description of drawings

Below in conjunction with the drawings and specific embodiments, the present invention is described in further detail.

The existing output current control of Fig. 1;

Fig. 2 flyback Switching Power Supply is used schematic diagram;

Fig. 3 is used for the circuit diagram of flyback Switching Power Supply control output current.

Specific embodiment

At first to realizing that principle of the present invention is described as follows.With the flyback Switching Power Supply is example, sees Fig. 2.Flyback Switching Power Supply (A.C.-D.C. converter) is made up of power switch, flyback transformer TM1 and controller.The information that makes full use of feedback voltage is starting point of the present invention, according to the magnetic flux principle for balance, has

$\frac{\mathrm{Vin}\×\mathrm{Ton}}{\mathrm{Npri}}=\frac{\mathrm{Vfr}\×\mathrm{Tr}}{N\sec }\·\·\·\left(4\right)$

Wherein Tr is the transformer secondary output coil erasing time, and under CCM, Tr is the time that switch element is closed; Under DCM, Tr is closed to secondary current from switch element to be reduced to the zero time; Npri and Nsec are respectively the numbers of turn of primary coil and secondary coil.

So

$\mathrm{Iout}=\mathrm{Eff}\×\frac{\mathrm{Vin}\×\mathrm{Iin}}{\mathrm{Vfr}}=\mathrm{Eff}\×\frac{\mathrm{Npri}}{N\sec }\×\frac{\mathrm{Iin}\×\mathrm{Tr}}{\mathrm{Ton}}\·\·\·\left(5\right)$

Notice

$\mathrm{Iin}=\mathrm{Iavg}\_\mathrm{on}\×\frac{\mathrm{Ton}}{T}\·\·\·\left(6\right)$

Wherein T is the switch element period of oscillation; Iavg_on is a primary coil at the average current in energy storage stage, particularly, and under DCM,

$\mathrm{Iavg}\_\mathrm{on}=\frac{\mathrm{Ipri}\_\mathrm{peak}}{2}\·\·\·\left(7\right)$

Under CCM

$\mathrm{Iavg}\_\mathrm{on}=\frac{\mathrm{Ipri}\_\min +\mathrm{Ipri}\_\max }{2}\·\·\·\left(8\right)$

So

$\mathrm{Iout}=\mathrm{Eff}\×\frac{\mathrm{Npri}}{N\sec }\×\frac{\mathrm{Tr}}{T}\×\mathrm{Iavg}\_\mathrm{on}\·\·\·\left(9\right)$

Experiment showed, that if the forward voltage drop of Schottky diode is counted in the effectiveness formula, the efficient Eff of Switching Power Supply can think a constant so.So, in equation 9, Tr can obtain by detecting FEEDBACK CONTROL voltage, and Iavg_on can obtain by the detection of primary coil current.As long as control primary current, just can control output current like this according to the size of secondary coil erasing time.

Fig. 3 is the circuit diagram that is used for A.C.-D.C. converter control output current.Its course of work is as follows: the erasing time detector detects transformer secondary output coil current attenuation to the zero time by detecting the anti-micro component that swashs voltage, otherwise the time T that the selector switch element is closed is as the input of multiplier as the input of multiplier the selective erase detected time of time detector to zero the time in the transformer secondary output coil current decays for MUX; The maximum and the minimum value of average current detection device detection of primary coil open period electric current under the control of control signal, and obtain average current according to equation 8, as another input of multiplier; The output signal of multiplier feeds back to the control end of transformer electric current, forms feedback control loop like this, thereby realizes controlling output current by the control primary current.

In like manner, according to the explanation of the above-mentioned principle of the present invention, under DCM, when adopting peak current detector also can realize the present invention, and be equally applicable to the situation that Switching Power Supply is isolated DC-DC converter.

Claims (9)

1, a kind of method of accurately controlling switch power supply output current, described Switching Power Supply is made up of control circuit, transformer or inductance element that one or more switch elements, the described switch element of control open and close, it is characterized in that:

Described Switching Power Supply detects and obtains the average current that the switch element open period flows through from transformer or inductance element;

Described Switching Power Supply detects and obtains being closed to transformer secondary output coil current or inductive current from switch element and decays to zero or be closed to transformer secondary output coil current or inductive current from switch element and do not decay to zero but time of having finished switch element down periods;

Described Switching Power Supply utilizes described detected time and average current and their equivalent transformation signal to control described output current.

2, the method for accurately controlling switch power supply output current as claimed in claim 1 is characterized in that: described control circuit is the Pwm controller with pulse-skip function.

3, as the method for the described accurately controlling switch power supply output current of claim 1-2, it is characterized in that: described Switching Power Supply is A.C.-D.C. converter or isolated DC-DC converter.

4, a kind of circuit that utilizes the method realization accurately controlling switch power supply output current of the described accurately controlling switch power supply output current of claim 1, described Switching Power Supply is made up of control circuit, transformer or inductance element that one or more switch elements, the described switch element of control open and close, it is characterized in that it at least also comprises:

A current detector is used for the average current of sense switch element open period transformer or inductance;

An erasing time detector, be used to detect from switch element be closed to transformer secondary output coil current or inductive current decay to zero or transformer secondary output coil current or inductive current do not decay to zero but time of having finished switch element down periods;

A multiplier, an input of multiplier are the detected average current of described current detector or its equivalent transformation signal, and an input is detected time of erasing time detector or its equivalent transformation signal, the output control output current of multiplier.

5, the circuit of accurately controlling switch power supply output current as claimed in claim 5 is characterized in that: described current detector is the average current detection device.

6, the circuit of accurately controlling switch power supply output current as claimed in claim 5 is characterized in that: described current detector is a peak current detector.

7, as the circuit of the described accurately controlling switch power supply output current of claim 4-6, it is characterized in that: described control circuit is the Pwm controller with pulse-skip function.

8, as the circuit of the described accurately controlling switch power supply output current of claim 4-6, it is characterized in that: described Switching Power Supply is A.C.-D.C. converter or isolated DC-DC converter.

9, the circuit of accurately controlling switch power supply output current as claimed in claim 7 is characterized in that: described Switching Power Supply is A.C.-D.C. converter or isolated DC-DC converter.