Application of low cost DC/DC converter 34063 (Fig.)

Linear regulator power supply is inefficient, so it is usually not suitable for large current or large difference between input and output voltage. Switched power supply has relatively high efficiency, and efficiency does not decrease with the increase of input voltage. Power supply usually does not need large radiators and is small in size, so it is an inevitable choice in many applications. Switching power supply can be divided into chopping type, converter type and charge pump type according to the mode of conversion, and soft switch and hard switch according to the mode of switch.

Chopping Switched Power Supply
Chopper switching power supply can be divided into three types according to its topology: Buck, Boost and Buck-boost. The step-down switch power circuit is usually shown in Fig. 1.
In Fig. 1, T is a switch, L1 is an energy storage inductance, C1 is a filter capacitor, and D1 is a continuous current diode. When the switch is on, the inductance is magnetized, the current in the inductance increases linearly, and the energy is converted to magnetic energy stored in the inductance. If the initial current of the inductance is iL0, the relation between the current flowing through the inductance and time t is:
ILt= iL1+ (Vi-Vo-Vs) t/L, Vs is the conduction voltage of T.
When T is off, L1 continues to flow through D1, thereby linearly reducing the current of the inductance. If the initial current of the inductance is iL1, then the current flowing through the inductance is related to time t:
ILt=iL1-(Vo+Vf)t/L, Vf is the positive saturated voltage of D1.

Figure 1Basic circuit of step-down switching power supply

Special application of 34063
Application of Extended Output Current
The peak current allowed for the switch of the DC/DC converter 34063 is 1.5A, beyond which 34063 may be damaged. Because the current passing through the switch tube is trapezoidal, there is a difference between the output average current and the peak current. If a larger inductance is used, the difference will be smaller so that the average output current can be larger. For example, the input voltage is 9V, the output voltage is 3.3V, using 220 μ The average output current of H reaches 900mA, and the peak current is 1200mA.

Simply relying on the switch inside 34063 to achieve a higher output current than 900mA is not impossible, but reliability will be affected. To achieve a larger output current, an additional switch must be used. Figures 2 and 3 are the voltage-reducing and boosting circuits of the external switch tube.

Figure 2Boost Darlington and Non-Darlington Connections

Figure 3Antihypertensive Darlington and non-Darlington connections

With a non-Darlington connection, the external triode can reach saturation. When the deep saturation is reached, the triode has a longer shut-off delay because the base area stores a considerable amount of charge, which prolongs the switch on time and affects the switch frequency. The Darlington connection will not be saturated, but when the switch is on, the pressure drop will be large, so the efficiency will be reduced. The anti-saturation drive technology can be used, as shown in Fig. 4. This drive circuit can keep the Vce of Q1 above 0.7V and make its conduction in a weak saturated state.

Figure 4Anti-saturation drive circuit

Three voltage outputs can be generated using a single 34063 chip, as shown in Fig. 5.

Figure 534063 Circuit with 3 Output Voltages

The peak output voltage of +VO can be up to 2 times V_ IN, -VO output voltage up to -V_ IN. It is important to note that the 3-way peak circuit must not exceed 1.5A, and the output power of the two additional power sources must be less than V_ IN. I. (1-D), where I is the primary output current and D is the duty cycle. In the case of these two small output currents, this circuit can very well reduce the cost of realizing boost and negative voltage power supply.

Circuit 34063 with shut-off function
34063 does not have shut-off function by itself, but it can use its over-current saturation function to increase a few devices to achieve the shut-off function, while also achieving delayed start.
Figure 6 shows a 34063 circuit with a turn-off function, with R4 taking 510_and R6 taking 3.9 K. When a high level is added to the control end, the output of 34063 becomes 0V without affecting the normal operation of its overcurrent protection function.
With a slight modification of this circuit, a 34063 circuit with delay start function can be obtained, as shown in Figure 7.
Take C11 for 1 μ F, R10 is 510_, and the startup delay of 200-500 MS can be reached (the delay time is related to the input voltage). The disadvantage of this circuit is that it cannot protect the peak current when it is overflowing, but only the average current.

Constant Current and Constant Voltage Charging Circuit
The constant voltage and current charging circuit is shown in Figure 8. It can be used to charge the accumulator. First, it charges with a constant current of 500mA. After charging to 13.8V, it becomes a constant voltage charging, and the charging current decreases gradually.

Limitations of 34063
Although the switch power supply composed of 34063 is cheap and widely used, its limitations are obvious. The main points are as follows:
(1) Low efficiency. For antihypertensive applications, the efficiency is generally only about 70%, and the efficiency is lower when the output voltage is low. This prevents it from being used in situations where power consumption is strictly required, such as when USB provides power.
(2) The duty cycle range is small, about 15%~80%, which limits its dynamic range. It is not applicable in some applications where the input voltage changes a lot.
(3) Due to the open-loop error amplification, the duty cycle can not be locked, which makes the selection of circuit parameters difficult. The inductance and capacitance have to be several times larger than the theoretical calculation values to achieve the expected results. Although 34063 has many drawbacks, it is also a good choice for manufacturers with limited profit margins.

Figure 634063 circuit with shutdown function

Figure 734063 Circuit with Delayed Start Function

Figure 8Constant voltage and current charging circuit

Frequency and ADSL performance of switching power supply
For ADSL, the upstream channel is between 30 and 100 kHz, and the downstream channel is between 100 kHz and 1.1 MHz. Long-line connection rate is often an important indicator of ADSL performance, but when the line is very long, the high-frequency channel in the downlink channel attenuates severely, so the signal-to-noise ratio in the low-frequency downlink band plays an important role in the long-line connection rate.
The output of the switching power supply contains the fundamental frequency of the switching frequency and the ripple component of its harmonics. Generally, the energy from the fundamental to the 10th harmonic is larger. If the switch frequency is 20 kHz, its harmonics are 40 kHz, 60 kHz, 80 kHz... In this way, there will be 10 interference frequency points in the downstream channel from 100 to 300 kHz. If the switching frequency is 100 kHz, the interference points will be reduced to 2. If the switching frequency is 1 MHz, the downlink channel will not be interfered, which can greatly improve the performance of the downlink channel.

Key Points for Device Selection
(1) Select a switch triode or power MOS tube only if the external switch is connected, and pay attention to the voltage and power consumption.
(2) If the switching frequency is very high, the inductance can be multi-wire wound side by side to reduce the effect of skin effect.
(3) A Schottky diode with short recovery time and low forward voltage is generally selected for the continuous current diode, but attention should be paid to the voltage withstanding. If the output voltage is small (a few volts at zero), MOS tube continuation must be used. Output filter capacitors generally use high frequency capacitors, which can reduce the output ripple and reduce the temperature rise of the capacitors. Upper arm resistance in sampling circuit and a 0.1-1 μ F capacitance, can improve the transient response.

Key points of PCB layout and wiring
There is a current loop for both on and off switches. Both loops are high frequency and high current loops, so the area of the two loops should be minimized in layout and wiring. The sampling voltage used for feedback should be derived from the output capacitance, and the heat loss of the chip or switch tube should be noted.