CN100386957C - Control circuit for carrying out current limitation on peak current controlled DC-DC transformer - Google Patents

Abstract

The present invention discloses a control circuit for carrying out constant current limitation on a peak current controlled DC-DC transformer. The present invention comprises a one-way switch, wherein the input end of the one-way switch is connected with the output end of a voltage error amplifier of the peak current controlled DC-DC transformer, and the output end of the one-way switch is connected with the midpoints of two divider resistors which are connected in series; the other end of one of the divider resistors is grounded, and the other end of the other divider resistor is connected with an emitting pole of a PNP triode; the emitting pole of a PNP triode is also connected with a power supply by one resistor, an electricity collecting pole is grounded, and the base pole introduces ramp signals by one resistor. The circuit can effectively counteract the influence of ramp compensation voltage on current limitation in different duty ratio so as to realize constant current limitation, and eliminates the phenomenon that current limitation values caused by ramp compensation changes following the changes of the duty ratio within the range of the whole duty ratio so as to make the current limitation values constant, reduce the selection of the current rating of a switching tube of the transformer, reduce the circuit cost and increase the working reliability of the circuit.

Description

Peak current control DC-DC converter is carried out the control circuit of constant current limliting

One, technical field

The present invention relates to a kind of control circuit of constant current limliting, relate in particular to a kind of control circuit that peak current control DC-DC converter is carried out the constant current limliting.

Two, background technology

In the existing peak current control DC-DC converter, often adopt current-limiting protection to guarantee the safe operation of circuit, adopt direct current signal to carry out current limliting usually; And, adopt slope compensation usually in order to guarantee the stability of system, and as shown in Figure 1, resistance R 3, R4 and R _sConstitute slope compensation circuit; resistance R 1, R2 and diode D1 constitute current-limiting protection circuit, and wherein the anode of voltage error amplifier output diode connects, and the negative electrode of diode is connected with the common port of resistance R 1, R2; resistance R 1 another termination DC power supply, R2 other end ground connection.When not adding slope compensation circuit; this current-limiting circuit can play a very good protection; yet after adopting slope compensation; when duty ratio changes; the actual cut-off current of converter changes, and no longer is the Constant Direct Current signal, in order to guarantee the safe operation of converter in whole duty cycle range; the switching tube electric current must be chosen for by norm the cut-off current under the limiting case, thereby the cost of whole system is improved greatly.

Three, summary of the invention

1, goal of the invention: the purpose of this invention is to provide a kind of control circuit that peak current control DC-DC converter is carried out the constant current limliting.

2, in order to reach above-mentioned goal of the invention, circuit of the present invention comprises: an input is controlled the single-way switch that the voltage error amplifier output in the DC-DC converter is connected with peak current, the conducting when input terminal voltage is higher than output end voltage of this single-way switch, the single-way switch output is connected with the mid point of two divider resistances that are in series, the other end ground connection of one of them divider resistance, the other end of another divider resistance connects the emitter of a PNP triode, the emitter of triode also connects power supply by a resistance, grounded collector, base stage is introduced ramp signal by a resistance.

Described single-way switch pipe can the root actual conditions select the triode in diode or the PWM chip to realize.

In this circuit, the partial pressure value of adjusting two divider resistances promptly can be adjusted the size of the value of current limliting signal.

As shown in Figure 2, principle of the present invention is signal identical with the slope compensation signal slope of stack on the basis of common direct current current-limiting signal, is applied to the current limliting end of converter.The base signal of triode is a voltage V identical with the slope compensation frequency _Ct(taking from the sawtooth waveforms that the convertor controls chip produces usually), then the emitter voltage of triode Q1 is base voltage V _CtThe constant DC voltage (being about 0.5V) that superposes, this voltage obtains the current limliting signal through divider resistance again, thereby the numerical value of current limliting signal is:

$V_{\lim }=(V_{\mathrm{ct}}+0.5)\&times;\frac{R2}{R1+\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}2}$

By choosing sampling resistor R in suitable R 1, R2 value and the converter _sValue (resistance is much smaller than R1, R2) and V _CtValue, obtain the voltage with the slope compensation same slope, make:

$V_{\lim }+0.5=V_C\frac{R4}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}+{\mathrm{ni}}_L{R}_S\frac{R3}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}$

V wherein _CBe the compensation ramp signal that applies,

Be the slope compensation magnitude of voltage,

Be sampling resistor voltage partial pressure value, i _LIt is the constant cut-off current of setting.

When converter operate as normal during at non-limited current state, this moment is by the output of the voltage error amplifier EA of peak current control DC-DC converter and the low saltus step that the inductive current sampled voltage decides PWM, promptly when the inductive current sampled voltage rises to the output of voltage error amplifier EA, saltus step takes place in latch output, and PWM exports step-down.

When peak current control DC-DC converter is operated in limited current state, the output of the voltage error amplifier of peak current control DC-DC converter converter reaches capacity, its output is greater than current limliting voltage Vlim, thereby make diode current flow, the output voltage of voltage error amplifier is pulled low to the voltage that differs a diode drop 0.5V with current limliting voltage Vlim by force, and this moment, the current limit increment was:

$\mathrm{\&Delta;}{i}_L=\frac{(V_{\lim }+0.5)-V_C\frac{R4}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}}{n{R}_S\frac{R3}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}}=\frac{[(V_{\mathrm{ct}}+0.5)\&times;\frac{R2}{R1+\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}2}+0.5]-V_C\frac{R4}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}}{n{R}_S\frac{R3}{\mathrm{<figure-callout\; id="3"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}3+R4}}$

Voltage $[(V_{\mathrm{ct}}+0.5)\&times;\frac{R2}{R1+\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="C20061003969500071.png"\; state="\{\{state\}\}">R</figure-callout>}2}+0.5]$ Have and slope compensation voltage Identical slope, and frequency is identical, so the time limit current increment Δ i _LWith the independent of duty cycle of circuit, be a constant.

Circuit of the present invention can be realized peak current control DC-DC converter is carried out the constant current limliting by the form that combines with UC384X series peak value current mode PWM chip, utilizes the respective element in the PWM chip to realize its function.

3, beneficial effect: advantage of the present invention is: (1) can effectively offset the influence to current limliting under different duty of slope compensation voltage, thereby be implemented in different duty constant current limliting, can in whole duty cycle range, eliminate slope compensation and bring cut-off current, make cut-off current constant with the phenomenon that change in duty cycle changes; (2) reduce choosing of converter switches tube current quota, reduced circuit cost, improved the reliability of circuit working.

Four, description of drawings

Fig. 1 is the control circuit figure of the existing constant current limliting that is unrealized;

Fig. 2 is the control circuit figure of realization constant current limliting of the present invention;

Fig. 3 is the specific implementation circuit that the constant current-limiting control circuit is applied to UC3846.

Five, embodiment

As shown in Figure 3, for combining with pwm chip, the present invention realizes peak current control DC-DC converter is carried out the control circuit of constant current limliting, comprise current sample and oblique wave compensation circuit 1, the UC3846 type pwm chip 2 in the peak current control DC-DC converter and peak current control DC-DC converter is carried out the control circuit 3 of constant current limliting, in current sample and oblique wave compensation circuit 1, the electric current sampling resistor R that flows through _sProduce sampled voltage, sampled voltage is through filter resistance electric capacity R7, C1 filtering, be applied to divider resistance R4 one end, ramp signal is connected by resistance R 9 on the base stage of NPN triode Q2, and ramp signal is from the CT end (8 pin) of UC3846, the collector electrode of Q2 connects the reference voltage terminal of UC3846 by resistance R 8, emitter meets another divider resistance R3, and the common port of R3 and R4 is access to the current sample positive input (4 pin) of UC3846, the current sample negative input of UC3846 (3 pin) ground connection; The current limliting signal end of UC3846 (1 pin) is the base stage of one of them triode, this signal end connects peak current control DC-DC converter is carried out in the control circuit 3 of constant current limliting two divider resistance R1 that are in series and the mid point of R2, the other end ground connection of divider resistance R2 wherein, the other end of divider resistance R1 connects the emitter of PNP triode Q2, the emitter of this triode Q2 connects the UC3846 power end by resistance R 5, the grounded collector of triode Q2, base stage is introduced ramp signal by resistance R 6 from the CT end of UC3846.

Claims (3)

1. control circuit that peak current control DC-DC converter is carried out the constant current limliting, it is characterized in that, this circuit comprises a single-way switch, and the input of this single-way switch is connected with the output of voltage error amplifier (EA) in the peak current control DC-DC converter; When the input terminal voltage of this single-way switch is higher than its output end voltage, this single-way switch conducting; The output of described single-way switch connects the mid point of two divider resistances that are in series, the other end ground connection of one of them divider resistance, the other end of another divider resistance connects the emitter of a PNP triode, the emitter of triode also connects power supply by a resistance, grounded collector, base stage is introduced ramp signal by a resistance, and described ramp signal is the voltage signal identical with the slope compensation frequency.

2. as claimed in claim 1 peak current control DC-DC converter is carried out the control circuit of constant current limliting, it is characterized in that single-way switch is a diode.

3. as claimed in claim 1 peak current control DC-DC converter is carried out the control circuit of constant current limliting, it is characterized in that single-way switch is the triode in the pwm chip.

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