A design method of RCC power transformer based on MOS tube

RCC circuits are divided into two types according to different power tubes. One is made of triodes, and the other is made of MOS tubes. The circuits are slightly different, but the principles are not much different. We know that the triode is a current-controlled current source, that is, if its base current is Ib, then its electrode current is this IB value multiplied by an amplification factor. MOS is a voltage-controlled current source, that is, the maximum collector current allowed to flow is determined by the voltage value of the GS pole. Correspondingly, the RCC circuit made of triodes is to control the maximum collector current, that is, the primary peak current, by controlling its base current to adjust the output energy size, that is, adjust the output voltage, while the MOS tube controls its primary peak current by adjusting the voltage between the GS poles.

Please see the figure above, which is a typical RCC circuit made of MOS tube. Below I will analyze the working process of this circuit based on my own understanding. 1. Startup. When the power is turned on, the high voltage passes through RST, passes through the GS pole of MOS, and then passes through RS to inject base current. Because there is a junction capacitance between the GS poles of MOS, the GS pole voltage increases, GS is turned on, and the upper side of RS will generate a voltage to the ground. This voltage passes through RF and injects current into the base of Q1. Because MOS is conducting, the same-name end of NS2 induces a positive voltage. This voltage passes through RL2, D2, RZCD, CZCD, and then to the Q1 electrode. Because RS has injected base current into Q1, Q1 is turned on.

2. Pull down the VG voltage and turn off the MOS. When the MOS is turned off, the voltage is reversed, and the voltage at the same terminal of NS2 is pulled to 0, which is the ground voltage. Since the upper end of the RCD is the ground voltage, the voltage at the pole of Q1 is negative at this time, and the junction capacitance of the GS pole of the MOS is quickly discharged. This accelerates the closure of the MOS. At the same time, the reverse energy is transmitted to the load through NS1, so the secondary establishes the output voltage, and the secondary control circuit also starts to work. When the transformer has discharged the stored energy, the voltage at both ends of NS2 disappears, CO2 has stored energy, and there will be a voltage at its upper end. This voltage passes through the NS2 winding, RZCD, CZCD, and Q1 collector, causing the voltage on Q1 to rise, that is, another voltage is added to GS. Then it starts to oscillate again.

3. The above is the startup process of the RCC circuit. Let's talk about its voltage stabilization process. Under a certain input voltage and a certain output load, its optocoupler current should be a constant value. The upper end of the phototransistor is a constant voltage maintained by capacitor CO2. This voltage injects current into the base of Q1 through the phototransistor, RA. The base current of Q1 determines the current flowing through its electrode. If the input voltage remains unchanged, when MOS is turned on, the voltage value at the upper end of RCD (that is, the same-name end of NS2 -) is VIN.NS2/NP+C02. As long as the input voltage value remains unchanged, the voltage value at this point when turned on is the same and will not change. The voltage at the upper end of Q1 is determined by the current flowing through Q1. Its voltage is equal to the voltage at the upper end of RCD, minus the voltage drop of RL2, RCD, D2, RZCD, and CZCD. When the load on the secondary side becomes lighter, the current flowing through the optocoupler increases, that is, the injected base current increases, the electrode current increases, and the voltage drop of the above four components also increases, so the voltage of Q1 decreases, so the peak current of the primary side increases, reducing the energy input and achieving voltage stability. When the primary input voltage increases, the voltage at the same-name end of NS2 increases. At this time, if the optocoupler current remains unchanged, then Q1 The voltage will rise, the energy will increase, the output voltage will rise, and the optocoupler current will increase, thus forming a series of automatic adjustments. This will adjust the primary peak current and keep the output voltage stable.

Through the above analysis, it is not difficult to see the difference between the RCC circuit and the flyback circuit. I summarize it as follows.

1. The frequency of the RCC circuit is variable, while the frequency of the flyback circuit is fixed. When the load becomes heavier, the frequency of the RCC circuit becomes smaller and the cycle becomes longer.

2. The RCC circuit always works in the critical conduction mode, and there will be no continuous mode of flyback current, that is, its primary current is always a triangular waveform, and there will be no trapezoidal wave, that is, the waveform of its primary current is as follows:

3. The way the RCC circuit regulates the voltage input is by controlling the peak current of the primary side, rather than the duty cycle, which is determined by the primary input voltage and the output voltage. Well, knowing the above principles, we can design this RCC power transformer.

To design an RCC transformer, you must first know 1. Input voltage, for example, wide voltage 90V to 264V AC. 2. Output specification, for example 12V1A. 3. Cross-sectional area of ​​the selected magnetic core. Here I chose EF20 magnetic core, with an area of ​​30 square millimeters. With the above conditions, according to the above circuit, I will design this RCC circuit transformer.

1. According to the input conditions, determine the lowest input DC voltage. Since the lowest input AC voltage is 90V, after rectification and filtering, and considering its voltage fluctuation, I can still take the lowest input DC voltage VIN as 90V.

2. According to the type of switch tube and other conditions, select the lowest frequency (i.e. the maximum cycle) at low voltage and full load, you may choose the longest on time, and set the duty cycle yourself. This step is very important. Here, I choose the maximum cycle of this circuit to be 17US, the on time to be 8US, and the off time to be 9US.

3. Calculate the primary peak current. First, estimate an efficiency, then derive the input power from the output power and the estimated efficiency, and then the input average current. For example, if the output of this model is 12W and the estimated efficiency is 0.8, then the input power is 15W, and the input average current is 15/90, which is 0.16A. Then calculate the peak current based on the duty cycle. The formula is IP=IAVG/D(1-0.5), and IP and IAVG are the peak current and the average current respectively. Here the average current is 0.16A, and D is 0.47, so the peak current is 0.69A. Based on this value, the RS value can be set. For a general transistor, VBE is about 0.6V, so RS=0.6/IP, which is about 0.86R in this case. In practice, a value slightly smaller than this resistor can be selected, and this resistor value limits the maximum output power. Combining the above two points, the detailed diagram is shown below.

In fact, the key to setting an RCC transformer is the setting of the primary current waveform. This current waveform can be observed with an oscilloscope by clamping the high voltage end of the oscilloscope to the upper end of RS. The formula for calculating the primary peak current is based on the primary average current.