MC34063 flyback power supply

MC34063 is a monolithic bipolar linear integrated circuit that contains a temperature compensated bandgap reference source, a duty cycle controlled oscillator, driver and high current output switch tube, and can output a 1.5A switching power supply. It uses only a few external components to create switching boost converters, buck converters, and power inverters. Although the conversion efficiency is relatively low, because it is a veteran IC and the price is very low, it is still widely used in some low-cost circuits.

Recently, the author needed a DC boost circuit for work. The input power supply is a battery from 10V to 14.4V, the output voltage is 100V DC, and the load current is 10mA. At first, we designed the boost converter application circuit according to MC34063, but after many tests, the output voltage never met the requirements. Moreover, there is a restriction on the application of MC34063, that is, the sum of the absolute values ​​of the input and output voltages cannot exceed 40V, otherwise the IC cannot work safely and stably. To be clear about this restriction, it is necessary to limit the back electromotive force voltage value of the inductor in the circuit. Otherwise, the switching tube inside the IC may be broken down.

To complete the given tasks, other devices and circuit forms can also be used, but the circuits are relatively complex. After analysis and weighing, we decided to still use the MC34063 IC as the core, but replaced the inductor in the circuit with a step-up transformer, relied on the step-up transformer to increase the voltage, and then connected a rectifier circuit to the secondary of the transformer to convert the AC pulse into It supplies the load for the DC voltage output, and at the same time, it is divided by the resistors R4 and R3 and then fed back to the ⑤ pin of IC1 (the reverse input terminal of the comparator, used to stabilize the output voltage).

This approach has three major advantages. First, the boost circuit is simple and stable, and the manufacturing process is good, making it suitable for mass production. Second, it avoids the restriction that the sum of the absolute values ​​of the output voltage cannot exceed 40V, which ensures the safe use of the device. Third, the circuit cost is low. Except for a low-cost IC and a wire-wound small high-frequency transformer, there are no other active components and adjustable components in the circuit (see the attached diagram for the circuit principle). When designing, we choose the operating frequency of the circuit to be around 100kHz, so that the size of the step-up transformer can be made smaller. In order to improve the load regulation rate of the output voltage, a fixed load can be connected to the output end.

After actual testing, the voltage regulation rate and load regulation rate of this 100V boost circuit are better than 0.2%.