The schematic circuit diagram for designing an HVDC power supply is needed for various applications such as Geiger counters, insect zappers, Nixie tubes, and sensors. These applications require high-voltage direct-current (HVDC) supplies. In the market, you can find various types of HVDC power supply designs, including voltage doubler or quadrupler, flyback converter, and boost converter.
Some of these designs have limited current-output capacity. However, by performing precise calculations using fundamental boost conversion equations, it’s possible to create an HVDC power supply design with both clean and high current capacity. Many component manufacturers provide application notes with useful formulas that are compatible with their components, derived from these fundamental equations. In this article, we’ll introduce a boost converter design using the MC34063 DC-DC converter. You can see the author’s prototype in Figure 1.
Boost converter basics
In a boost converter as depicted in Figure 2, energy is stored within the inductor (L1a) during the period when the transistor (T1a) is in the ‘on’ state (ton). Once the transistor is switched off (toff), this stored energy is transferred in series with the input voltage (Vin) to the output filter capacitor (Cout) and the connected load (RL). This particular arrangement enables the adjustment of the output voltage to a level higher than the input voltage.
The output voltage can be calculated as follows:
Circuit and working
The circuit diagram for the boost converter utilizing the MC34063 DC-DC converter can be observed in Figure 3. The MC34063 is a monolithic control circuit that encompasses all the essential active components necessary for the operation of DC-to-DC converters. This component signifies notable improvements in user-friendliness, featuring highly efficient and straightforward switching regulators. The adoption of switching regulators is gaining prominence over linear regulators due to the compact size and enhanced power efficiency demanded by modern equipment designs. Switching regulators enhance application versatility while concurrently lowering expenses.
The MC34063 was specifically engineered to cater to various converter applications, encompassing buck, boost, and voltage-inverter designs. It integrates a temperature-compensated reference voltage, oscillator, active peak-current limiting feature, output switch, and an output-voltage-sense comparator. These functionalities are encapsulated within either an 8-pin DIP or SOIC package.
The internal schematic of the MC34063, as outlined in the datasheet provided by Texas Instruments, is illustrated in Figure 4.
Its pin 5 (comparator inverting input) senses and sets the output voltage to a stable value for calculating feedback resistor values as shown in Fig. 5.
The internal voltage regulator generates a reference voltage of 1.25 volts for the internal comparator. Therefore, the external voltage divider, composed of R1a and R2a, should be configured to yield precisely 1.25 volts when the desired output voltage is attained. For instance, if an output voltage of approximately 501 volts is required, the resistor values for the voltage divider should be R2a=2.4 mega-ohms and R1a=6 kilo-ohms, respectively.
As depicted in the block diagram, the comparator’s output triggers and deactivates the SR latch. The oscillator, driven by the timing capacitor at pin 3, comprises current source and sink elements. These components charge and discharge the external timing capacitor between predetermined upper and lower thresholds. Typically, the charge and discharge currents amount to 35mA and 200mA, respectively, resulting in an approximate 6:1 ratio. Consequently, the ramp-up phase lasts six times longer than the ramp-down phase. The upper threshold corresponds to the internal reference voltage of 1.25V, while the lower threshold is approximately 0.75V.
The oscillator operates continuously at a rate regulated by the timing capacitor’s value. It also monitors peak current by sensing the voltage generated across a small-value, high-wattage sensing resistor connected to pin 7 due to inductor current. In the presented circuit (Figure 3), resistor R6, rated at 1.5 ohms and 2W, serves as the sensing resistor.
The block diagram illustrates that the output switch consists of an npn Darlington transistor. The collector is connected to pin 1, while the emitter is linked to pin 2. This configuration provides flexibility, allowing the MC34063 to be employed in buck, boost, or inverter configurations. The maximum collector-emitter saturation voltage at 1.5A (peak) is 1.3V, and the maximum peak current for the output switch is 1.5A. For higher peak output currents, an external transistor can be employed. The oscillating pulses drive the internal transistors, which can either deliver boost/buck conversion or drive an external power transistor with a higher rating to achieve a higher power capacity.
Certain circuit designs, particularly step-up and voltage-inverting configurations, necessitate a ton/(ton+toff) ratio exceeding 0.857. This can be achieved by incorporating a ratio extender circuit that employs a germanium diode and exhibits temperature sensitivity. The use of a negative-temperature-coefficient timing capacitor can help mitigate this sensitivity. In Figure 3, the extender circuit comprises transistor T2 (BC557), germanium diode D2 (1N34A), and timing capacitor C3. T2 primarily functions as a switch for charging and discharging capacitor C3, which is powered by pin 3 of the IC. Current limiting is imperative for all step-up and voltage-inverting designs employing the ratio extender circuit. This allows the inductor to reset between cycles of over-current during the initial power-up of the switcher. Regulation is controlled by the voltage feedback loop when the output filter capacitor reaches its nominal voltage.
In the main circuit, a wire connects the junction of resistors R1 and R2 to capacitors C1 and C2 for charge balancing in both output capacitors. The voltage divider consists solely of resistor R3, linked to pin 5 of the MC34063.