Analyzing the current-controlled switching power supply solution

With the rapid development of power electronics technology, the relationship between power electronics equipment and people's work and life is becoming increasingly close, and electronic equipment cannot do without reliable power supply. In the 1980s, computer power supplies were fully realized as switching power supplies, and the power supply replacement of computers was completed first. In the 1990s, switching power supplies have successively entered various electronic and electrical equipment fields. Program-controlled switches, communications, electronic testing equipment power supplies, control equipment power supplies, etc. have widely used switching power supplies, which has promoted the rapid development of switching power supply technology. A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switch transistor on and off to maintain a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with a switching power supply and a linear power supply, the cost of both increases with the increase of output power, but the growth rate of the two is different. At a certain output power point, the cost of a linear power supply is higher than that of a switching power supply, which is the cost reversal point. With the development and innovation of power electronics technology, the switching power supply technology is constantly innovating, and this cost reversal point is increasingly moving towards the low output power end, which provides a wide range of development space for switching power supplies.

The voltage-controlled switching power supply will lose control of the switching current, which is not convenient for overcurrent protection, and has slow response and poor stability. In contrast, the current-controlled switching power supply is a voltage and current dual closed-loop control system that can overcome the disadvantages of current loss of control, and has reliable performance and simple circuits. Based on this, we designed a current-controlled switching power supply using the UC3842 chip. In order to improve the accuracy of the output voltage, the system does not adopt an offline structure, but a direct feedback structure. This system fully considers electromagnetic compatibility and safety in its design and can be widely used in industry, home appliances, audio-visual and lighting equipment.

Schematic diagram of current-controlled switching power supply

Current-mode control is developed to address the shortcomings of voltage-mode control. In addition to retaining the output voltage feedback control part of the voltage-mode control, a current feedback link is added. Its principle block diagram is shown in Figure 1.

Figure 1 Schematic diagram of a current-controlled switching power supply

The current-controlled switching power supply is a voltage and current double closed-loop control system, with the inner loop being the current control loop and the outer loop being the voltage control loop. When UO changes, UF changes, or I changes, US changes, and thus UO changes, achieving the purpose of output voltage stability.

Current mode control chip UC3842

UC3842 adopts fixed working frequency pulse width controllable modulation mode, with a total of 8 pins, and the functions of each pin are as follows: ① pin is the output end of the error amplifier, and the external resistor and capacitor components are used to improve the gain and frequency characteristics of the error amplifier; ② pin is the feedback voltage input end, and the voltage of this pin is compared with the 2.5V reference voltage of the error amplifier in-phase end to generate an error voltage, thereby controlling the pulse width; ③ pin is the current detection input end, when the detection voltage exceeds 1V, the pulse width is reduced to put the power supply in an intermittent working state; ④ pin is the timing end, and the operating frequency of the internal oscillator is determined by the external resistor and capacitor time constant, f=1.8/(RT×CT); ⑤ pin is the common ground end; ⑥ pin is the push-pull output end, the internal is a totem pole type, the rise and fall time is only 50ns, and the driving capacity is ±1A; ⑦ pin is the DC power supply end, with under-voltage and over-voltage locking functions, and the chip power consumption is 15mW; ⑧ pin is the 5V reference voltage output end, with a load capacity of 50mA.

Figure 2 UC3842 internal circuit

A brief introduction to the functions of each port of the UC3842, an 8-port dual in-line plastic package.

① Port COMP is the output of the internal error amplifier.

②Port VFB is the feedback voltage input terminal, which is compared with the +2.5V reference voltage at the non-inverting input terminal of the internal error amplifier to generate an error voltage to control the width of the pulse.

Port ③ ISENSE is the current sensing terminal. In the application circuit, a small-resistance sampling resistor is connected in series to the source of the MOSFET to convert the current of the pulse transformer into a voltage and send it to port ③ to control the width of the pulse.

④ Port RT/CT is the timing terminal. The oscillation frequency of the sawtooth oscillator is f=1.8/(RT·CT), and the current mode operating frequency can reach 500kHz.

⑤Port GND is grounded.

⑥ Port OUTPUT is the output port. This port is a totem pole output with a peak driving current of up to 1.0A.

⑦ Port VCC is the power supply. When the supply voltage is lower than 16V, UC3824 does not work, and the power consumption is less than 1mA. After the chip works, the input voltage can fluctuate between 10 and 30V, and the working current is about 15mA.

⑧Port VREF is the reference voltage output, which can output accurate +5V reference voltage and the current can reach 50mA.

UC3842 constitutes a current-controlled switching power supply

1 Circuit composition

The current-controlled switching power supply circuit composed of UC3842 is shown in Figure 3.

Figure 3 UC3842 constitutes a current-controlled switching power supply

2 Working Principle

220V AC first passes through the filter network to filter out various interferences. Resistor R1 is mainly used to eliminate the residual voltage at the moment of power failure, thermistor RT1 can limit the surge current, and varistor VDR protects the circuit from lightning strikes. When the positive terminal potential of C17 rises to ≥R16, port ⑦ gets the working voltage, the UC3842 circuit starts, the potential of port ⑥ rises, Q1 starts to conduct, and at the same time, the 5V voltage of port ⑧ is established through the internal circuit... C12 filter capacitor eliminates the spike pulse generated during switching, C11 is a noise elimination capacitor, R6 and C13 determine the oscillation frequency of the sawtooth oscillator, and R9 and C15 are used to determine the gain and frequency response of the error amplifier. C14 acts as a slope compensation, which can improve the reliability of the sampling voltage. After normal operation, the high-frequency voltage on coil N2 provides the working voltage for UC3842 through D2, R17, C18, and D3.

When the switch is turned on, the electric energy of the rectified voltage added to the primary winding of the switch transformer is converted into magnetic energy and stored in the switch transformer. After the switch is turned off, the energy is released to the load through the secondary winding. D7 and D8 are pulse rectifier diodes, C7 and R5 absorb the pulse current that appears at the moment of bypass startup, and L3, C8, C9, and C10 form a filter circuit. The output voltage can be described by the following formula.

UO=UI(TON/KTOFF)

Where UO is the output voltage, UI is the rectified voltage, K is the transformer ratio, TON is the on-time of Q1, and TOFF is the off-time of Q2.

From the above formula, we can know that the output voltage is proportional to the on-time of the switch tube and the input voltage, and inversely proportional to the transformer ratio and the off-time of the switch tube. C16, R12, and D5 are used to limit the gate voltage and current, thereby improving the switching speed of Q1, which is beneficial to improving electromagnetic compatibility. R13 is mainly used to prevent the gate of Q1 from being suspended. D1, R4, C5 and D6, R16, and C20 form a two-stage absorption circuit to absorb peak voltage and prevent Q1 from being damaged.

The voltage stabilizing circuits in the system are:

The voltage drop of the transistor is used to replace the voltage regulator resistor R in the voltage regulator diode circuit. When the change of UI or RL causes the output voltage UO to change, the change of UO will be reflected in the emitter junction voltage UBE of the transistor, causing the change of UCE, thereby adjusting UO to keep the output voltage basically stable. According to the role of the transistor, it is called an adjustment tube. Since the adjustment tube and the load are in series, Figure 15-2-1 is called a series voltage regulator circuit. It is mainly composed of a reference voltage, a comparison amplifier, a sampling circuit and an adjustment element. The comparison amplifier can be a single tube amplifier circuit, a differential amplifier circuit, or an integrated operational amplifier. The adjustment element can be a single power tube, a composite tube, or several power tubes in parallel. The sampling circuit takes out a part of the output voltage UO and compares it with the reference voltage VREF.

● Current feedback circuit. The source of Q1 is connected in series with the sampling resistor R15, which converts the current signal into a voltage signal and sends it to the non-inverting terminal of the current detection comparator inside UC3842. When Q1 is turned on and the current slope rises, the voltage of the sampling resistor R15 increases. Once the voltage of R15 is equal to the voltage of the inverting terminal of the current detection comparator, the internal trigger is reset and Q1 is turned off, which means that the duty cycle of the excitation pulse of port ⑥ is controlled by current to stabilize the output voltage. C19 is used to suppress the sharp pulse of the sampling current.

● Voltage feedback circuit. It is mainly composed of programmable precision voltage regulator TL431 and linear optocoupler PC817. The output voltage is divided by R21 and R22 to obtain the sampling voltage, which is sent to the reference port of programmable precision voltage regulator TL431. By changing the resistance values ​​of R21 and R22, the voltage regulation value of TL431 changes, and the output voltage of the switching power supply can be changed. When the output voltage increases, the voltage UKA across TL431 remains unchanged, the current at the control end of the optocoupler increases, and the voltage value at the feedback end of port ② increases accordingly. The voltage at the inverting end of the current detection comparator inside UC3842 becomes lower, the duty cycle of the pulse signal at the output port ⑥ becomes lower, the conduction time of the switch tube decreases, and the output voltage decreases; on the contrary, if the output voltage decreases, the output pulse duty cycle of UC3842 increases, the output voltage increases, and the voltage regulation is achieved. On the other hand, the power supply voltage at port ⑦ is generated by D2 rectification and C18 filtering, which reflects the change of the output voltage and plays a feedback role to stabilize the output voltage.

● The circuit has a feedforward adjustment function. When the load remains unchanged, the input voltage suddenly increases, and the inductive current of the switching transformer rises rapidly due to the increase in input voltage. Since the feedback signal and error signal have not changed, the current limiting effect occurs relatively quickly, so the pulse width becomes relatively narrow. Therefore, the change of the mains power has been compensated before affecting the output, which improves the response speed to the input voltage.

Figure 4 Slope compensation

When the system operates at a duty cycle greater than 50% or under continuous inductor current conditions, harmonic oscillation will occur, which is caused by the simultaneous operation of fixed frequency and peak current sampling. Figure 4A shows this phenomenon. If a disturbance is added to the control voltage, a small △I (dashed line in the figure) is generated, and the system will be unstable.

In order to make the system work reliably under the condition of duty cycle greater than 50% or continuous inductor current, the sawtooth voltage of port ④ is sent to port ③ through emitter follower Q2, thereby adding an artificial ramp synchronized with the pulse width modulation clock to the current sampling end, which can reduce the △I disturbance to zero in the subsequent cycle, as shown in Figure 4B. The slope of the compensation ramp must be equal to or slightly greater than m2/2 for the system to be stable.

The protection circuits designed in the system are:

● Output overvoltage protection circuit I. When the output voltage is high, the voltage at port ② exceeds 2.5V through the voltage feedback circuit, the internal trigger is reset and the external Q1 is cut off, thus achieving the purpose of output overvoltage protection.

● Output overvoltage protection circuit II. When the output voltage rises and is higher than the breakdown voltage of D9, the voltage stabilizing diode D9 breaks down, and the thyristor SCR is triggered to conduct, causing the negative terminal voltage of the photocoupler diode to drop to 0V, the photocoupler is saturated, the voltage at port ② is at the maximum value, and Q1 is always cut off, achieving the purpose of output overvoltage protection.

● Output overcurrent and overload protection circuit. When the circuit is overcurrent or overloaded, the output voltage decreases, and Q3, D4, and R8 form a secondary overcurrent and overload protection circuit. When the secondary is not overloaded, Q3 and D4 are turned off; when the secondary is overloaded, Q3 and D4 are turned on, the potential of port ④ decreases, and the sawtooth oscillator stops oscillating, achieving the purpose of overcurrent and overload protection.

● Q1 overcurrent protection circuit: When the power supply voltage is abnormal, the current in the switch circuit increases, and when the voltage on the sampling resistor R15 exceeds 1V, the internal trigger is reset and the external Q1 is cut off, effectively protecting Q1.

in conclusion

This system uses the current-controlled switching power supply designed by UC3842, which overcomes the disadvantages of poor voltage regulation and load regulation of the voltage-controlled switching power supply, and has reliable performance and simple circuit. This power supply is an ideal power supply for low-power switching power supplies of 20 to 80W.