The design of a DC stabilized power supply that implements 7.4 V input and +12 V output involves DC boost. Since the output of the MC34063 chip is not very stable [1], it cannot directly output +12V voltage through it. In order to achieve this requirement, the LM7812 chip was introduced in the design, as long as the voltage input requirements are met, a +12 V DC voltage can be stably output. That is, the MC34063ADG chip is only used as a transition to convert the original +7.4 V voltage to +15 V after boosting to provide the required input voltage for the LM7812 chip, thereby realizing the design of a +12 V DC regulated power supply. Figure 1 shows the basic ideas of stabilized power supply.

The design input voltage of the nuclear instrument power supply system is a 7.4 V DC battery voltage. After simple filtering, the DC power supply control circuit is provided with a PWM control signal to the boost converter circuit to control the on and off of the switch tube, which is used to adjust the DC voltage. Duty cycle, finally get a stable or adjustable DC output voltage. By sampling, comparing and amplifying the output voltage, the pulse width is adjusted, and finally the purpose of outputting a stable DC voltage is achieved by filtering. Figure 2 shows the overall design block diagram.

According to the above introduction about MC34063ADG chip and the basic principle design of the second section boost circuit, a 15 V boost converter circuit can be obtained [2], whose output voltage is adjustable from 1.25 to 40 V. The output voltage sampling loop resistance is determined by R7 and R6. The output voltage can be changed by adjusting the resistance of R7. The calculation formula of the output voltage is V_out = 1.25 V (1 + R7/R6), we take R7 = 11 K'Ω, R6 = 1 K'Ω here, calculate The output is 15 V, the LM7812 input terminal can work normally in the voltage range of 14.5 ~ 35 V, 15 V meets the requirements, so the LM7812 output terminal can output +12 V [3].

From the above related discussion, the following circuit diagram of the 12 V boost power converter is obtained, as shown in Figure 3.

1) Sampling loop resistance R7 and R6: U_O (output voltage): Its voltage regulation value is determined by R7 and R6, and its calculation formula is:

U O = 1.25 ( 1 + R 7 R 6 ) (1)

Sampling loop resistors R1 and R2 must shunt the output loop. In order to reduce power consumption, the shunt current value should be less than 0.001A to not affect the performance of the system. therefore:

R 7 + R 6 ≤ U O 0.001 = 15 000 (2)

Calculated by the actual circuit:

R 7 = 11 k ,   R 6 = 1 k (3)

2) T_on and T_off values:

T o n T o f f = ( U o + U f − U i ) U i − U s a t = ( 15 + 1.2 − 7.4 / 7.4 − 1 ) = 1.375 (4)

Among them: U_o (output voltage), Uf is the forward voltage drop of the rectifier diode can take 1.2 V, UI (input voltage), Us is the saturation voltage drop of the switch tube can be taken 1.0 V, T_on is the switch tube conduction time, T_off is Switch off time.

T o n + T o f f = 1 f min = 5 × 10 − 5 s (5)

3) The remaining relevant data, such as the current limiting resistor Rsc, load peak current Ipk, timing capacitor C_T, inductance L, filter capacitor C_o, etc., are calculated by the actual circuit:

I p k = 4.75 A R s e = 0.0695 Ω C T = 1.2 n F L = 37.8 u H C o = 13.4 m F (6)

According to the relevant performance of the MC34063ADG chip and the basic principles of the circuits in the second section, the principle diagram of the boost converter circuit is designed in conjunction with the LM7812. For the boost circuit, the parameters such as inductance, sampling loop resistance, filter capacitor C_o, timing capacitor C_T, switch-on time T_on, switch-off time T_off and other parameters need to be calculated and simulated according to the calculated parameter values.

Based on Multisim simulation, the input voltage is 7.4 V, the timing capacitor is 1.2 nf, the filter capacitor is 13.4 mf, the current limiting resistor is 0.0695 ohms, the output voltage is measured with a multimeter 1 (XMM1), the output current is measured with a multimeter 2 (XMM2), and the oscilloscope 1 (XSC1) Measure the AC waveform generated at the input and output to analyze the ripple.

The simulation circuit is as follows (Figure 4).

Insert the voltage and current probe, the output voltage is 12.0 V, the effect is obvious and relatively stable, the output current is 1A;

According to the output voltage and input voltage, calculate the boost power supply adjustment rate η = | 12 / 12 − 1 | × 1 00 % = 0 . (Because the simulation error is small and the voltage probe sensitivity is low);

Output voltage ripple is 170 uV, good stability;

Insert the voltage and current probe, the output voltage is 12.0 V, the effect is obvious and relatively stable, the output current is 1.2 mA; According to whether the two voltages of the load are connected, the boost load adjustment rate η = | 12 / 12 − 1 | × 1 00 % = 0 is calculated. (Because the simulation error is small and the voltage probe sensitivity is low).

In Multisim, the circuit is simulated in the experimental design schematic diagram, mainly to study the output voltage, output current, voltage regulation rate, load regulation rate, ripple and other parameters of each circuit under normal circuit operation. The simulation results show that the voltage and load adjustment rate of the circuit meet the requirements, of which the voltage adjustment rate is less than 1.0%, the circuit output voltage is 12.0 V, and the voltage adjustment rate is 0, showing excellent high-precision voltage characteristics; measuring ripple, The output voltage ripple of the circuit is less than 5 mV, showing excellent voltage regulation characteristics.