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DIY RCC SMPS Circuits

Every non-battery-powered electronic device requires converting offline ac power to some dc voltage for powering electronics. Nowadays, efficient conversion of electrical power is becoming an important concern, and switching power supplies offers not only higher efficiency but also greater flexibility. This guide is designed to give an overview of switch-mode power supplies (SMPS), but focuses mainly on the working of the simplest of all switch-mode converters – the ringing choke converter (RCC).

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RCC: How Does It Work?

The ringing choke converter, also known as the self-oscillating flyback converter, is a popular circuit for cost-sensitive applications due to its simplicity and low component count. The RCC technique is now widely used in mobile phone battery chargers, electronic ballasts, dc/dc converters, and other applications with similar demands. It is called a ringing choke converter because the regenerative signal for oscillation comes from the ringing of the transformer choke. The basic circuit diagram of a self-oscillating flyback converter is shown below.

self-oscillating flyback converter

When T1 is ON as a result of the base current from the base winding, collector current flows. When the base current is insufficient and T1 is OFF, current flows on the secondary side. Since the converter is a self-exciting type, it performs this operation repeatedly. Note that a gap is placed in the transformer core to prevent magnetic saturation in this one-transistor-type simple converter.

The following is a detailed explanation of how the power supply circuit works, with reference to the above drawing. When the input voltage is applied, base current flows to the transistor through the starting resistor. When the transistor is turned on, the transformer’s primary winding is excited and the transistor is forward-biased by the voltage induced in the base winding. As a result, voltage induced in the base winding increases with the raising of the collector current and the transistor is turned on quickly. Now the transistor shifts to the active region, the collector voltage increases, the voltage of the primary and base windings drop, and the transistor is turned off. As a consequence, energy stored in the transformer is transferred to the load through the rectifier and filter components on the secondary winding. The circuit repeats this switching process with an operating frequency that varies with the input voltage and the state of the output load. The next figure depicts the typical operation waveform of each part of the ringing choke converter.

typical operation waveform

RCC: In Real Life

We started from the very basic theoretical circuit of a ringing choke converter. Now let’s examine the electronics of a real-world RCC SMPS. The circuit diagram shown below is an exact replica of a generic Chinese mobile phone travel charger, given here to help the analysis of some key components in the RCC circuitry.

rcc

When input power is fed to the circuit, a small startup current will flow through resistor R2 into the base of transistor Q1 to turn it partially on. Next, the primary winding (L1) and the auxiliary winding (L2) get the same polarity voltage. As the auxiliary winding voltage will generate current flow into the base of Q1, the base current increases, and Q1 will be fully on quickly. The current in primary winding also begins to increase linearly and reach a critical level. Beyond this critical level, according to transistor characteristic curves, a little primary current increase will force Q1 to leave its saturation region. As a result, the voltage on primary winding (also the auxiliary winding) begins to fall and the transistor quickly turns off. When the transistor is off, the secondary winding (L3) gets polarity reverse voltage, diode D8 becomes conducted, and energy stored in the primary winding transfers to the output load from the secondary winding. After all stored energy is released, this cycle starts again. Remaining components are used to enrich the overall functioning of the circuit. Capacitor C2 especially helps to keep the cycle frequency independent of the beta value of transistor Q1 (C2 reliably controls the cycle on time and frequency), zener diode ZD1 regulates the auxiliary winding voltage (drives the transistor with constant current), and diode D7 (including circling components) accelerates the on-off process of transistor Q1.

RCC SMPS: Do-It-Yourself Experiment

Inspired by the above design, I recently prepared a small SMPS based on RCC topology for some experimentation. In the prototype, I got an output around 250 mA at 9 V DC from an input of AC 230 V. I would like to share the initial design of my RCC SMPS here.

RCC SMPS

Needless to say, the transformer (flyback transformer/SMPS transformer) is the key component in an insulation type dc/dc converter. Since iron cores generate high losses (thermal losses) at high frequencies, they are not used in switch-mode converters. The small (~E19) ferrite-core transformer (X1) in the circuit has three windings — primary (L1), auxiliary/feedback (L2), and secondary (L3). The L1 winding consists of around 170 turns of 36 SWG; the L2 winding is made up of 13 turns of 36 SWG; and L3 has 13 turns of 27 SWG. A small E19-type ferrite core with its bobbin can also be used to make the transformer. Suitable layers of insulation should be added between the three windings.

(structure of the transformer)

(structure of the transformer)

homemade smps transformer

Note that this design idea is meant for educational and experimental purposes only. Frankly, figuring out how to work the ringing choke converter can be challenging. For this reason, be prepared to do some experimenting (cut-and-try approach) to get the results you want. I’ll need to test my prototype repeatedly to confirm many things before using it as an end-user product. Further design examples/notes may be added to this page over time if my experiments show some interesting/useful results!

WARNING!
HIGH-VOLTAGE, LETHAL POTENTIALS ARE PRESENT IN SMPS CIRCUITS

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