CN101453165A - Summit current operated electric voltage feed forward method in switch power source - Google Patents

Abstract

The invention discloses a voltage feedforward method for controlling peak current in a switch power, which comprises a main circuit and a control circuit of the switch power, wherein power components are connected in series at the primary side of the main circuit; and the control circuit mainly comprises a sample circuit, a comparator, a voltage control delay unit and a drive device. The method comprises: performing the voltage sampling and comparing with a threshold value of the comparator, and turning the comparator if the value is larger than the threshold value of the comparator; and triggering the voltage control delay unit by a turning signal of the comparator to generate a delay voltage signal to the drive device according to a specific arithmetic, to control the on-off of the power components to control the peak current in the switch power. The method improves the reliability of the whole system, simplifies the design of the system and reduces the requirements on the components.

Description

Voltage feedforward method in peak current control of switching power supply

Technical Field

The invention relates to a method for controlling peak current of a switching power supply, in particular to a voltage feedforward method in peak current control of the switching power supply.

Background

In a switching power supply system, a peak current limiting module is generally included. The introduction of the peak current limiting module strengthens the reliability of the system, reduces the requirements on components and improves the controllability of the system output.

The existing peak current limiting module works as follows: the system detects the system current in real time through the current detection device, compares the system current with a set value, and allows the current to continue to increase if the system current is smaller than the set value; if the system current is larger than the set value, a control signal is output to drive the power tube to be closed. As shown in fig. 1, during the on period of the power transistor 100, the inductor current gradually rises under the action of the input voltage, and the voltage Vcs on the sampling resistor Rcs rises. When the Vcs voltage rises to the threshold voltage V_thMeanwhile, the comparator 101 sends a control signal to the driving device 102, and the driving device 102 turns off the power transistor 100 according to the requirement of the control signal.

In the above scheme, the initial objective of the system design is to control the peak current according to the following equation: $\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}},$ the scheme ignores the influence of the system delay time on the peak current. During the turn-on period of the power transistor 100, the Vcs voltage gradually rises, and when the voltage rises to the threshold voltage V_thAt this time, the output of the comparator 101 changes from high to low, and the driver device 102 starts to pull the GATE voltage from high to low also after receiving the change of the input signal. In this process, the comparator 101 itself hasThe delay, and thus the driver device 102, is also delayed. During this delay period, the gate voltage of the power transistor 100 has not changed in time, the power transistor is still turned on, and the peak current continues to increase until the gate voltage of the power transistor 100 is pulled down and turned off.

Let the system delay time be T_DThen, then $\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}$

Where Vline is the input voltage; t is_DIs the system delay time; in the inductance of the primary coil of the Lpri transformer, the first part

Is a system control target value, the second part

Is the amount of peak current deviation from the control target due to system delay. When the input voltage variation range is very large, the deviation amount of the peak current caused by the system delay is also very large, taking an ac/dc converter as an example, a wide input voltage range is usually required, such as 85V to 265V ac, the input dc voltage variation range reaches 250V, and according to the circuit of fig. 1, the peak current will greatly vary under different input voltages. Therefore, it is necessary to introduce a voltage feed forward function to compensate for the effect of input voltage variations on the peak current.

In the system shown in FIG. 2, during system operation, the VDD voltage is held substantially constant and the current through resistor 206 is

Since Vline is much larger than VDD, the current can be approximated as

. The peak current is controlled within the controller by amplifying the current and superimposing this current with the existing threshold voltage to generate a new threshold voltage. The peak current thus becomes:

$\mathrm{Ipeak}=\frac{\mathrm{Vth}-k*\mathrm{Vline}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}$

from the above equation, to obtain equal peak currents at different input voltages, the external circuit must be adjusted so that

Is equal to

That is, when the external inductance lpi changes or the external current sampling resistor Rcs changes, the compensation coefficient k must be adjusted. In fig. 2, adjusting the compensation coefficient k is to adjust the starting resistor 206, which will affect the starting time and standby power consumption of the system. In addition, in the prior art, in order to realize voltage feedforward, one pin Vin is added, and the complexity and the cost of the system are inevitably increased.

Disclosure of Invention

The invention aims to provide a voltage feedforward method for controlling peak current in a switching power supply, which improves the reliability of a system and simplifies the design of the system while accurately limiting the peak current.

In order to solve the technical problem, the voltage feedforward method for controlling the peak current in the switching power supply comprises a main circuit of the switching power supply and a control circuit, wherein a power element is connected in series on the primary side of the main circuit, and the control circuit mainly comprises a sampling circuit, a comparator, a voltage-controlled delay unit and a driving device, and the method comprises the following steps:

the method comprises the following steps that firstly, a switching power supply is started, a main circuit power element flows through current, and the current flows through a sampling circuit to form a sampling voltage Vcs;

step two, with the increase of the current flowing through the power element of the main circuit, the sampling voltage Vcs is continuously increased, when the sampling voltage Vcs is greater than the threshold voltage of the comparator, the comparator outputs turnover, and a control signal is output to the voltage-controlled delay unit;

step three, after the voltage-controlled delay unit is triggered by the control signal, pressing T in the following formula_VCDGenerating a delay

$\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}+\frac{\mathrm{Vline}*T_{\mathrm{VCD}}}{\mathrm{Lpri}}$

Wherein,

$T_{\mathrm{VCD}}=\frac{\mathrm{Lpri}*I_{\mathrm{FIX}}}{\mathrm{Vline}}-T_{\mathrm{FIX}}$

I_FIXis a fixed current value T_FIXThe fixed delay time Rcs is the resistance of the sampling circuit

Vline is the input voltage; t is_DIs the system delay time; inductance of primary coil of Lpri transformer

Ipeak is the main circuit peak current;

and step four, outputting a control signal to the driving device by the voltage-controlled delay unit after the delay of the step three, and controlling the power element of the main circuit to be closed by the driving device so as to realize the control of the peak current in the switching power supply.

The invention adopts the technical scheme that the voltage-controlled delay unit is added in the control circuit of the switching power supply, and the voltage feedforward delay is realized according to a certain algorithm, so that the aim of controlling the peak current in the switching power supply is fulfilled, the reliability of the whole system is improved, the design of the system is simplified, and the requirements on components are reduced.

Drawings

The invention is described in further detail below with reference to the following figures and embodiments:

figure 1 is a schematic diagram of a prior art switching power supply peak current control,

figure 2 is a schematic diagram of another prior art switching power supply peak current control,

fig. 3 is a schematic diagram of the peak current control of the switching power supply of the present invention.

Detailed Description

As shown in fig. 3, the voltage feed-forward method in the peak current control of the switching power supply of the present invention includes a main circuit of the switching power supply and a control circuit, wherein a power element 300 is connected in series on the primary side of the main circuit, and the control circuit mainly includes a sampling circuit, a comparator 301, a voltage-controlled delay unit 302 and a driving device 303, and the method includes the following steps:

step one, the switching power supply is turned on, and the main circuit power element 300 flows current which flows through the sampling circuit to form a sampling voltage Vcs;

step two, as the current flowing through the main circuit power element 300 increases, the sampling voltage Vcs continuously rises, and when the sampling voltage Vcs is greater than the threshold voltage of the comparator 301, the comparator 301 outputs an inversion signal and outputs a control signal to the voltage-controlled delay unit 302;

step three, after the voltage-controlled delay unit 302 is triggered by the control signal, pressing T in the following formula_VCDGenerating a delay

$\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}+\frac{\mathrm{Vline}*T_{\mathrm{VCD}}}{\mathrm{Lpri}}$

Wherein,

$T_{\mathrm{VCD}}=\frac{\mathrm{Lpri}*I_{\mathrm{FIX}}}{\mathrm{Vline}}-T_{\mathrm{FIX}}$

I_FIXis a fixed current valueT_FIXThe fixed delay time Rcs is the resistance of the sampling circuit

Vline is the input voltage; t is_DIs the system delay time; inductance of primary coil of Lpri transformer

Ipeak is the main circuit peak current;

will T_VCDSubstituting the expression of (A) into the expression of Ipeak to obtain

$\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}+\frac{\mathrm{Vline}*T_{\mathrm{VCD}}}{\mathrm{Lpri}}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}-\frac{\mathrm{Vline}*T_{\mathrm{FIX}}}{\mathrm{Lpri}}+I_{\mathrm{FIX}}$ In the design, T is_FIXIs designed to be equal to T_DThen, then $\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+I_{\mathrm{FIX}}$ Will no longer be related to the input voltage Vline and the primary inductance Lpri, thus solving the problem that the peak current varies with the input voltage and at the same time the peak current also variesProblems that change as the primary inductance changes;

step four, the voltage-controlled delay unit 302 after the delay of step three outputs a control signal to the driving device 303, and the driving device 303 controls the main circuit power element 300 to be turned off, so as to control the peak current in the switching power supply.

Claims (1)

1. A voltage feedforward method in peak current control of a switching power supply comprises a main circuit of the switching power supply and a control circuit, wherein a power element is connected in series on the primary side of the main circuit, and the control circuit mainly comprises a sampling circuit, a comparator, a voltage-controlled delay unit and a driving device, and is characterized in that: the method comprises the following steps:

the method comprises the following steps that firstly, a switching power supply is started, a main circuit power element flows through current, and the current flows through a sampling circuit to form a sampling voltage Vcs;

step two, with the increase of the current flowing through the power element of the main circuit, the sampling voltage Vcs is continuously increased, when the sampling voltage Vcs is greater than the threshold voltage of the comparator, the comparator outputs turnover, and a control signal is output to the voltage-controlled delay unit;

step three, after the voltage-controlled delay unit is triggered by the control signal, pressing T in the following formula_VCDGenerating a delay

$\mathrm{Ipeak}=\frac{\mathrm{Vth}}{\mathrm{Rcs}}+\frac{\mathrm{Vline}*T_D}{\mathrm{Lpri}}+\frac{\mathrm{Vline}*T_{\mathrm{VCD}}}{\mathrm{Lpri}}$

Wherein, $T_{\mathrm{VCD}}=\frac{\mathrm{Lpri}*I_{\mathrm{FIX}}}{\mathrm{Vline}}-T_{\mathrm{FIX}}$

I_FIXis a fixed current value T_FIXThe fixed delay time Rcs is the resistance of the sampling circuit

Vline is the input voltage; t is_DIs the system delay time; the inductance Ipeak of the primary coil of the Lpri transformer is the main circuit peak current;

and step four, outputting a control signal to the driving device by the voltage-controlled delay unit after the delay of the step three, and controlling the power element of the main circuit to be closed by the driving device so as to realize the control of the peak current in the switching power supply.

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