The internal power supply of portable B-ultrasound system is complex. The power supply of external adapter and battery must be converted into the voltage required by the system through DC / DC conversion. In order to reduce the useless consumption of power supply and improve the battery efficiency, the power supply of system motherboard, B-ultrasound control board, LCD and keyboard is powered by switching power supply.Overall design of portable B-ultrasound power supply
Figure 1 is the overall design block diagram of portable B-ultrasound power supply. There are two kinds of power input voltage for portable B-ultrasound: one is the power adapter input, the voltage is 18V, and the other is the battery input, the voltage is 14.4V. It is required to realize the thermal switching between the two voltages, and the system operation will not be affected when switching the voltage, that is, it provides the function of external power supply and battery power supply without delay. It is required to output several voltages such as 12V, 5V, 3.3V and 48V. The specific indicators are 12V / 2.5A, - 12V / 0.5A, 5V / 4A, 3.3v/3a, 48V / 80mA and - 48V / 80mA. It has the function of single key on-off, that is, when there is no power, press the power key to turn on the power; When there is power, press the power key to send a shutdown signal to the control panel, and the upper computer can also shut down through software (i.e. support ATX shutdown command). The power output interface adopts standard computer ATX interface.
Figure 1 overall design block diagram of portable B-ultrasound power supplyDesign of power switching circuit
The power switching circuit of portable B-ultrasound is shown in Figure 2. When the external power adapter is connected, the voltage is input to AC 18V, which is divided by VD100 and vd101 diodes, and then applied to pin 3 (in-phase terminal) of n100a (lm193) voltage comparator through R100 and R107. The input voltage of the battery is 14.4V, which is divided by R101 and r108 and added to pin 2 (inverting end) of n100a (lm193) voltage comparator. Since the voltage of pin 3 is higher than that of pin 2, pin 1 of n100a (lm193) outputs high level to turn on triode V100, turn off V101, turn off FET v105 and turn off power_ The in terminal gets the 18V voltage of the external power adapter. When there is no external power adapter, or when the portable B-ultrasound machine cannot use the external power adapter due to sudden power failure of external AC, the voltage of pin 3 of n100a (lm193) is lower than pin 2, and pin 1 of n100a (lm193) outputs low level, so that the triode V100 is off, V101 is on, and the FET v105 is on. The battery voltage passes through the source and drain of the conducting FET v105, power_ The in terminal obtains the 14.4V voltage of the battery, which realizes the thermal switching between the two voltages. Vd102 and vd103 play an isolation role in the circuit, isolating the external power adapter and battery power supply.
Figure 2 power switching circuitDesign of single touch switch circuit
The circuit of portable B-ultrasound single touch switch is shown in Figure 3, which is powered by an external adapter or battery_ The in voltage is sent to the FET Q100 for startup, and the other is sent to one end of the touch button power-key2 through r104 and vd104. The other end of the tap button power-key2 is grounded. When the tap button power-key2 is pressed, the base of triode n106 is clamped at the low level, n106 is turned on, N102 is also turned on, FET Q100 is turned on, and power from external adapter or battery_ In voltage passes through the source and drain of Q100 to obtain power voltage to supply power to high and low voltage circuits, and the machine starts up. At the same time of startup, the power voltage passes through the 6V zener diode to turn on the triode n104, and the n104 collector is at the low level to maintain n106 on and maintain the startup state.
Fig. 3 single touch switch circuitWhen the portable B-ultrasound machine is in the power on state and the button power-key2 is pressed again, the optocoupler B101 is turned on, the optocoupler secondary is turned on, the four pins of the inverter D101 output a low level to the control panel, and the control panel sends a low-level shutdown command power-off1 to turn on the optocoupler B100, then turn on the triode n103, turn off n104, and the high level of the n104 collector turns off n106, Led to the end of FET Q100 and realized shutdown.When the upper computer sends a high-level shutdown command power-off, it will also turn on the optocoupler B100. The rest process is the same as the shutdown process of the control panel.
Design of low voltage power supply circuit
The portable B ultra-low voltage power supply circuit is shown in Figure 4, which is mainly composed of 6 tps5430 of TI company and 1 lm2576 of national semiconductor company. The six tps5430s provide two groups of 12V, 5V and 3.3V voltages, one of which is used to supply power to the main control board of the portable B-ultrasound machine, and the other is used to supply power to the computer in the portable B-ultrasound machine. The two groups are exactly the same, so we only give the power supply schematic diagram of one group. Lm2576 is responsible for generating - 12V voltage to the computer in portable B-ultrasound.
Figure 4 low voltage power supply circuit
Tps5430 has a wide voltage input range of 5.5 36V, continuous 3A current output capacity (peak value of 4A) and conversion efficiency of 95%. 8-pin small patch package, the back of the chip is a metal heat sink, which must be welded to the ground when using. When making PCB package, the heat sink is regarded as the ninth pin. There is no need to connect the heat dissipation device, and good results can be achieved by using the heat dissipation of the circuit board itself, which is especially suitable for the design and use of portable products. Pin 1 of tps5430 is the boot end, which requires an indirect pH of 0.01 between the boot and pin 8 Low ESR capacitance of F. 2. Pin 3 is the empty terminal, and pin 4 VSENSE is the feedback voltage terminal of the regulator, which is connected to the voltage dividing resistance terminal of the output voltage to obtain the feedback of the output voltage. Pin 5 ENA is the power on / off control terminal. When the voltage of this pin is lower than 0.5V, the chip will turn off the power conversion and reduce the power supply current to 18 A. The chip works normally when it is suspended. We do not control this pin, so it is suspended and not connected. Pin 6 is grounded, pin 7 Vin is the power supply terminal, connected to power . A high-quality, low ESR ceramic capacitor is indirectly connected between the power supply and the ground. The 8-pin pH is the source of the internal power FET, and the freewheeling diode and inductor are connected externally. The powerpad end of pin 9 is the heat dissipation metal sheet on the back of the chip, which must be connected to the ground (GND).
The output voltage value of tps5430 is determined by the partial voltage value of its 4 pins, and the output voltage Vout = (1.221 (R1 & Themes; 1.221) / r2) v. Wherein, R1 is the resistance above partial voltage and R2 is the resistance below partial voltage. For the design of tps5430, R1 can be taken as 10K , and R2 can be calculated according to the output voltage to be obtained. According to r206 (1.11k), r208 (3.07k) and R210 (5.36k) given in Figure 4, we can calculate that the output voltages are 12.2v, 5.2v and 3.5V respectively, which is slightly higher than the design value by 0.2V. The load is relatively heavy, and the voltage is just right when loaded.
N208 (lm2576-12) is a product of American National Semiconductor. Pin 1 is the power supply terminal and connected to power ; Pin 2 is the output end, which is externally connected with freewheeling diode and inductance; Pin 3 is the grounding terminal and pin 5 is the on / off control terminal of the power supply. Due to the output of negative voltage, pins 3 and 5 are not grounded, but connected to the - 12V power supply; Pin 4 is the voltage feedback terminal. We use lm2576-12 with fixed 12V output, so pin 4 is grounded and there is no need to connect the feedback resistor to divide the voltage.
Design of high voltage power supply circuitThe portable B ultra-high voltage power supply circuit is shown in Figure 5, using DC / DC converter. Ua3843 is a high-performance, fixed frequency and current mode controller specially used for DC / DC converter applications. It provides designers with a cost-effective solution using the least external components. It is divided into PWM control, cycle current limit, voltage control and so on.Figure 5 48V power supply circuit
1 PWM control
Power supplies voltage to pin 7 of N1 (ua3843) through resistor R34, and the sawtooth oscillator formed by external R36 and C33 of pin 4 of N1 and internal circuit starts to work. The PWM pulse is output by pin 6 of N1 to control the on time of MOSFET V9 and determine the output voltage. R37 is used to suppress parasitic oscillation and is usually connected in series near the MOSFET gate. The gate resistance r37 should not be too large, which directly affects the charge and discharge of the input capacitance of the MOSFET by the PWM drive signal, that is, the switching speed of the MOSFET. The induced electromotive force output by the 9th and 10th pins of the secondary of the switching transformer is rectified by vd14 and filtered by C35, L7 and C36 to form a 48V DC output voltage to supply power to the B-ultrasound probe. The induced electromotive force output by the 7th and 6th pins of the secondary of the switching transformer is rectified by vd13 and filtered by C39, L8 and C37 to form a - 48V DC output voltage to supply power to the B-ultrasound probe. Vd12, C34 and R38 form a spike pulse absorption circuit, which is used to suppress the spike pulse with extremely high amplitude generated by the drain of V9 at the moment when the switch turns from on to off. Its principle is: at the moment when V9 is cut off, the spike pulse generated by its drain forms a charging circuit through vd12 and C34. The charging current suppresses the spike pulse within a certain range to avoid V9 being impacted by the spike pulse. After C34 charging, C34 discharges through R38 to prepare for absorbing spike pulse again in the next cycle
2 cycle current limit
Pin 2 feed back is the feedback voltage input terminal. This pin is compared with the reference voltage (generally 2.5V) at the in-phase input terminal of the internal error amplifier to generate the control voltage and control the pulse width. This circuit grounds it and is controlled by pin 1 of the output terminal of the internal error amplifier. The cycle current limit signal of pin 3 determines the width of PWM pulse, that is, the output voltage. R47, r48, R14 and C32 externally connected to pin N1 (ua3843) form a cycle current limiting circuit. In each oscillation cycle, when the peak inductance current of winding 3 5 in the primary of switching power supply pulse transformer L6 reaches the set value, the PWM pulse will be turned off. The set value is determined by the voltage of pin 1 comp terminal of N1 (ua3843) (pin 1 comp is the output terminal of internal error amplifier), Usually, a feedback network is connected between this pin and 2 pins to determine the gain and frequency response of the error amplifier. The cycle current limit control process is as follows: the switch tube is turned on, the inductance current rises, and the sampling voltage V3 rises. When the voltage of pin 3 is greater than 1V, the internal current detection comparator turns over, the internal PWM latch resets, closes the PWM pulse, and is ready to enter the next cycle. In order to eliminate the sharp wave pulse interference of the current limiting circuit, a sharp wave filter circuit is composed of R14 and C32 to ensure that the cycle current limiting function is effective in each oscillation cycle. The current limiting sampling resistance of R47 and r48 determines the maximum output current of the whole switching converter. The maximum output current can be adjusted by changing its resistance.
3 voltage control
N1 (ua3843) is a current mode pulse width modulator with two closed-loop controls. The current sampling signal is sent to pin 3 of the in-phase input end of the current detection comparator to form a current closed-loop control, the error voltage is sent to pin 1 of the output end of the internal error amplifier, and its output is sent to the inverting input end of the current detection comparator as a comparison reference to form a voltage closed-loop control. It can be seen that the voltage closed loop and the current closed loop interact. Finally, both of them control the PWM latch through the current detection comparator, that is, control the width of the PWM pulse. The voltage closed-loop control circuit is composed of voltage reference N2 (tl431a), optocoupler B4 (tlp521) and resistors R31, vr7, R12, R32, etc. The current signal output by the optocoupler B4 (tlp521) is converted into a voltage signal and sent to pin 1 of the output terminal of the internal error amplifier. The internal reference of N2 (tl431a) error amplifier is 2.5V. The voltage closed-loop voltage stabilizing control process is as follows: the output voltage rises, the reference terminal VR of tl431a rises, the conduction of TL431 rises, the conduction of optocoupler B4 (tlp521) rises, the voltage of pin 1 drops, the turnover of internal current detection comparator is advanced, the reset of internal PWM latch is advanced, the PWM pulse becomes narrow and the output voltage becomes low, so as to stabilize the output voltage. High voltage output voltage Vout = (1 R31 / (R12 vr7)) VREF. By adjusting the value of potentiometer vr7, the output can be adjusted to 48V, and the adjustment range is 31 & times; 2.5=77.5V115&TImes; 2.5=32V
epilogueThis paper introduces the design of portable B-ultrasound power supply, including power switching