In practical applications, the design of the booster circuit is often involved. For a large power output, such as a DC/DC boost circuit of 70 W or more, it is difficult to achieve high power boost due to the limitation of the internal switching transistor of the dedicated boost chip. The conversion, and the price of the chip is expensive, and is greatly limited in practical applications. Considering that the Boost boost structure has a large choice of external switch tubes, a DC/DC boost circuit with high power output can be designed by selecting a suitable control chip.
UC3S42 is a current-type pulse width modulation power supply chip, which is low in price and widely used in power circuit design of electronic information equipment. It is often used as a control circuit for isolated flyback switching power supply. According to the functional characteristics of UC3842, combined with Boost topology, completely It can be designed as a current-mode controlled step-up DC/DC circuit with few external components, flexible control, low cost, and easy output power of more than 100W. It has functions that are difficult to implement with other dedicated chips.
1 UC3842 chip featuresUC3842 operates at 16~30V and operates at approximately 15mA. There is a frequency-programmable oscillator in the chip; a totem-type output structure capable of source and sink large currents, especially suitable for MoSFET driving; a fixed temperature compensated reference voltage and high gain error amplifier, current sensor; with lock The logic of the memory function and the PWM comparator that provides pulse-by-pulse current limit control with a maximum duty cycle of up to 100%. In addition, it has internal protection functions such as hysteresis undervoltage lockout and controllable output dead time.
The DC/DC boost circuit designed by UC3842 is a current type control. The circuit directly controls the peak current of the inductor with an error signal, and then indirectly controls the PWM pulse width. The main features of this current mode control circuit are:
1) The change of the input voltage causes the change of the inductor current ramp, the inductor current is automatically adjusted without the error amplifier output change, and the transient voltage adjustment rate is improved;
2) The current type control detects the inductor current and the switch current, and compares the output of the error amplifier on a pulse-by-pulse basis to control the PWM pulse width. Since the inductor current changes with the error signal, it is easier to set the control loop. Improved linear adjustment rate;
3) Simplified the current limiting circuit, while ensuring the reliability of the power supply, the current limit makes the inductor and the switching tube work more effectively;
4) The current-type control circuit needs to compensate the slope of the inductor current, because the average inductor current is the factor that determines the output size. Under different duty cycles, the peak inductor current cannot change with the average inductor current. Corresponding, especially the duty cycle, 50% instability, there is an error between the peak current and the average current that is difficult to correct. Even if the duty ratio is "50%, high-frequency harmonic oscillation may occur, so slope compensation is required. The peak inductor current is consistent with the average inductor current variation, but the slope compensation technique with synchronization and distortion is difficult to implement.
2 Boost circuit structure and characteristics analysis2.1 Boost circuit structure controlled by UC3842
The Boost topology and circuit controlled by UC3842 are shown in Figure 1 and Figure 2, respectively.
In Fig. 2, the input voltage Vi=16~20V is supplied to the chip and is supplied to the boost converter. The switch tube is opened and closed at a frequency cycle set by UC3842, so that the inductor L stores energy and releases energy. When the switch is turned on, the inductor is charged at Vi/L and the energy is stored in L. When the switch is turned off, L generates a reverse induced voltage, and the stored electric energy is discharged to the output capacitor C2 at a speed of (Vo-Vi)/L through the diode D. The output voltage is controlled by how much energy is transferred, and the amount of energy transferred is controlled by the peak value of the inductor current.
The entire voltage regulation process is controlled by two closed loops, ie
The closed-loop 1 output voltage is sampled and fed back to the error amplifier for error voltage comparison with the internal 2.5V reference voltage. The error amplifier controls the change in output voltage due to load changes.
The closed loop 2 Rs is the current detecting resistor between the source and the common terminal of the switching tube. The voltage generated by the current flowing through the inductor L during the conducting period of the switching tube is sent to the noninverting input terminal of the PwM comparator, and is compared with the error voltage. The pulse width of the pulse is modulated to maintain a stable output voltage. The error signal actually controls the peak inductor current.
2.2 Boost boost structure characteristics analysis
The Boost boost circuit can operate in current interrupt mode of operation (DCM) and current continuous mode of operation (CCM). The CCM working mode is suitable for high-power output circuits. Considering that the load reaches 10% or more, the inductor current needs to be kept in a continuous state. Therefore, the CCM operating mode is used for characteristic analysis.
The basic waveform of the Boost topology boost circuit is shown in Figure 3.
When ton, the switch S is in the on state, the diode D is in the off state, the current flowing through the inductor L and the switch tube is gradually increased, and the voltage across the inductor L is Vi, taking into account the conduction of the drain of the switch S to the common terminal. The voltage drop Vs is Vi-Vs. When ton, the current increase portion ΔILon passing through L satisfies the formula (1).
Where: Vs is the sum of the voltage drop when the switch is turned on and the voltage drop across the current sampling resistor Rs, about 0.6~0.9V.
When toff, the switch S is turned off, the diode D is in the on state, the energy stored in the inductor L is supplied to the output, and the current flowing through the inductor L and the diode D is in a decreasing state, and the forward voltage of the diode D is Vf, toff At the time, the voltage across the inductor L is Vo+Vf-Vi, and the reduced portion of the current ΔILoff satisfies the equation (2).
Where: Vf is the forward voltage drop of the rectifier diode, the fast recovery diode is about 0.8V, and the Schottky diode is about 0.5V.
In the steady state of the circuit, that is, from the continuous current to the maximum output, â–³ILon=â–³ILoFf, which can be obtained by equations (1) and (2).
If the inductor loss is ignored, the inductor input power is equal to the output power, ie
The average current of the inductor obtained by equations (4) and (5)
At the same time, the inductor current ripple is obtained by equation (1).
Where: f is the switching frequency.
In order to ensure continuous current, the inductor current should meet
Considering equations (6), (7), and (8), the inductance value that satisfies the continuous current can be obtained.
In addition, the Boost boost circuit structure shows that the switching current peak value Is(max)=diode current peak Id(max)=inductor current peak ILP,
The circuit diagram of the prototype is shown in Figure 2. It is a step-up DC/DC converter based on UC3842 control. The technical specifications of the circuit are: input Vi=18V, output Vo=40V, Io=2A, frequency f≈49 kHz, output ripple noise 1%.
According to the requirements of technical indicators, combined with the qualitative analysis of Boost circuit structure, the design of the prototype circuit and the selection of key parameters of Figure 2 are specifically described.
3.1 Energy storage inductance L
The maximum duty cycle is determined based on the input voltage and the output voltage. Obtained by formula (4)
When the maximum load is output, at least the circuit should work in CCM mode, that is, formula (9) must be satisfied.
At the same time, considering the continuous current above 10% of the rated load, the actual design can assume that the inductor ripple current is 20%~30% of the average current when the circuit is rated. The increase of â–³IL can reduce the inductance L, but Do not increase the output ripple voltage and increase the output capacitor C2, taking 30% as the balance point, ie
L can use an inductor with an inductance of 140~200μH and no saturation through 5A or more. The design of the inductor includes magnetic core material, size, model selection, winding number calculation, wire diameter selection and so on. When the circuit works, it is important to avoid saturation of the inductor and excessive temperature rise. The choice of core and wire diameter has a great influence on the performance of the inductor and the temperature rise. The magnetic core with good material is a toroidal iron powder core, which has strong peak current capability and low EMI. The use of a wire with a large wire diameter to wound the inductor can effectively reduce the temperature rise of the inductor.
3.2 Output voltage sampling resistor R1, R2
Because pin 2 of UC3842 is the inverting input terminal of the error amplifier, the positive input terminal of the chip is the reference 2.5v. It can be seen that the output voltage Vo=2.5(1+R1/R2), and the sampling resistors R1 and R2 can be determined according to the output voltage. value.
Due to the function of the energy storage inductor, a large peak current is formed when the switch tube is turned on and off, and a spike is generated on the detecting resistor Rs. To prevent the malfunction of the UC3842, the Rs sampling point is added to the foot 3 of the UC3842. R, C filter circuit, R, C time constant is approximately equal to the duration of the current spike.
3.3 Switch tube S
The current peak of the switch tube is obtained by equation (10)
Iv(max)=ILP=5.11A
The withstand voltage of the switch tube is obtained by equation (11)
Vds(off)=Vo+Vf=40+0.8=40.8V
According to the 20% margin, the switch tube of 6A/50V or above can be selected. In order to make the temperature rise lower, the MOS switch tube with smaller Rds should be selected. It is considered that the on-state resistance Rds will increase with the increase of the PN junction temperature T1.
Figure 4 is a diagram showing the switching voltage waveform and current transient waveform of the measured switch tube.
3.4 Output Diode D and Output Capacitor C2
The output diode D in the boost circuit must withstand a reverse voltage equal to the output voltage value and conduct the maximum current required by the load. The peak current Id(max) of the diode=ILP=5.11A. This circuit can select a fast recovery diode of 6A/50V or higher. If the Schottky diode with reduced forward voltage is used, the efficiency of the whole circuit will be improved.
The selection of the output capacitor C2 depends on the output ripple voltage requirement. The ripple voltage is related to the equivalent series resistance ESR of the capacitor. The allowable ripple current of the capacitor is greater than the ripple current in the circuit.
The ESR of the capacitor "ΔVo/△IL=40x1%/1.33=O.3Ω.
In addition, in order to meet the requirements of the relative value of the output ripple voltage, the filter capacitance should be satisfied.
According to the calculated ESR value and capacity value, the capacitor is selected. Since the ESR value increases at a low temperature, the capacitance should be selected according to the ESR at a low temperature. Therefore, an electrolytic capacitor having a frequency characteristic of 560 μF/50 V or more can be satisfied.
3.5 External compensation network
The compensation pin Rf, Cf is externally connected between the output pin l of the UC3842 error amplifier and the inverting input pin 2. The values ​​of Rf and Cf depend on the UC3842 loop voltage gain, rated output current, and output capacitance. The loop closed-loop gain and frequency response can be changed by changing the values ​​of Rf and Cf. In order to get the best compensation of the loop, the stability of the loop can be tested and the transient response of the output voltage Vo during Io pulsation can be measured.
Figure 5 shows the difference between the dynamic response control waveforms when Cf selects 0.01 μF and 470 pF. The amplitude of the overshoot and the reset time are different.
3.6 Slope compensation
In the practical circuit, to increase the slope compensation network, there are generally two methods. One is to connect the compensation network Rx and Cx from the ramp pin 4 to the inverting input pin 2 of the error amplifier, so that the error amplifier output is ramped, and then with Rs. The voltage induced on the comparison. The second is to connect the compensation network Rx and Cx from the ramp pin 4 to the current sensing pin 3, and increase the slope of the ramp on the induced voltage of Rs, and then compare it with the smoothed error voltage to prevent harmonic oscillation and avoid The UC3842 is unstable and improves the noise characteristics of the current-mode control switching voltage. This article uses Method Two.
3.7 Protection circuit
When the voltage of pin 3 of UC3842 rises above 1V or the voltage of pin 1 falls below 1V, the PWM comparator can output a high level, causing the PWM latch to reset. According to the UC3842 shutdown feature, it is easy to set overvoltage protection and overcurrent protection in the circuit. The peak current induced on Rs in this circuit forms a pulse-by-pulse current limiting circuit. When the pin 3 reaches 1V, the current limiting phenomenon occurs. Therefore, the inductive magnetic component and the power switch tube in the whole circuit do not have to be designed with a large margin. , can ensure the stability of the voltage regulator circuit and reduce costs.
4 ConclusionAccording to the above principle and calculation design, input 18V, output 40V 80W step-up DC/DC circuit, the whole circuit is easy to debug, stable in operation, high in reliability, and the efficiency is over 80%, especially low cost, which has been applied in practical equipment. . In addition, according to the specific circuit index requirements, the circuit can be flexibly controlled and changed to design other application circuits.
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