Photocoupler-Free Isolated Flyback DC-DC ConverterMethod for Regulating the Secondary by Controlling the Primary

－Just looking at the circuit diagram, it looks like an unregulated isolated power supply; is the output really regulated?

Of course. Allow me to explain how the output of the BD7F series is regulated. The explanation will be easier using graphics, which I have prepared.

The circuit diagram on the left side of the graphic shows the simplified circuit block for the DB7F series device, an external transformer, and a secondary-side rectifying circuit. On the right side are waveforms of the voltage (Vx) and current (Ix) at each node.

How to perform regulation without feedback from the secondary side? The flyback voltage V_SW on the primary side contains V_out as a component. Then V_SW is monitored and operation is performed to indirectly detect V_out for regulation control.

In order to explain in a little more detail, I’ll ask you to look at the waveforms and the equation within the graphic. V_sw is the primary-side flyback voltage, and is generated, when the internal switch (MOSFET) turns off, at the point at which one end of the transformer primary-side windings is connected to the SW pin of this IC. This V_SW is the result of the product of the “transformer windings ratio (primary-side Np/secondary-side Ns)” and the “sum of V_out and the output rectifying diode voltage V_f”, to which V_in is added. Put simply, the voltage is the result of adding V_in to the voltage (vout+V_f) multiplied by the windings ratio of the transformer. But rather than an explanation, the waveforms and equation are easier to understand. One thing must be noted though: this equation has been simplified to make it easier to grasp. The voltage appearing on the secondary side is represented at (V_out+V_f), but strictly speaking, it is necessary to add a voltage resulting from the total secondary-side impedance (the windings resistance and ESR), multiplied by I_S. This is an error factor, and in actual design the term must be included in calculations.

－I see that the primary-side flyback voltage is used. I understand that it is used in some kind of operation, but if it’s not too complicated, could you explain just how this works?

The basic principle is not all that difficult. But, it is easier to understand using a circuit diagram and an equation, and so let’s have a look at another diagram and another equation. What’s important here are the two places that are circled in the circuit diagram. Details are indicated in the product data sheet, and the equation from the data sheet is not the simplified version we presented above, but includes all the terms for the secondary-side voltage, and so for consistency, from here on we will use the data sheet equation. The symbol notation (lowercase, subscripts) are somewhat different, but the meanings are the same.

The following is the basic equation. V_SW is the flyback voltage at the SW pin. Here “I_S×ESR” is added to the previous simplified equation.

V_SW is converted into the current I_RFB by the resistor R_FB. The voltage V_FB at the FB pin is roughly equal to the voltage V_IN due to the differential circuit with V_IN. This operation should be thought of and remembered as like that of the differential circuit of an op-amp, based on loop control. I_RFB is represented by the following equation.

V_FB≒V_IN, and therefore it is clear that the voltage obtained by subtracting V_IN (V_FB) from V_SW, divided by R_FB, gives us I_RFB. Here, a term with V_OUT appears in the equation. From here we shall determine V_OUT.

Because this current I_RFB flows through the resistor R_REF, the voltage V_REF at the REF pin is as given by the following equation.

We have already expanded I_RFB, and as is only to be expected, retracing gives us V_REF＝ R_REF × I_RFB.

Here, let’s look at the circuit diagram. The REF pin voltage is input to the non-inverting pin of the comparator within the IC and is compared with the reference voltage of 0.78 V at the inverting pin. For control ICs in general, this would be an error amplifier, but in this IC the control method uses the comparator, and so the action is that of a comparator. Similarly, to an error amplifier, the voltage at the REF pin (the comparator inverting input) is equal to the reference voltage (0.78 V). Hence the output voltage V_OUT and the voltage V_REF at the REF pin are related as follows.

In other words, the output voltage V_OUT is determined by the transformer windings ratio and the ratio of the resistances R_FB and R_REF. V_F and I_S×ESR are factors resulting in output voltage errors.

－I see. I now understand the meaning of using loop control of the primary-side flyback voltage to regulate the output voltage indirectly. During design, then, the output voltage should be set based on this equation?

That is not a problem. The data sheet indicates 3.9 kΩ as the standard value for R_REF, and V_REF is 0.78 V (typ), and so the output voltage can be set using these values. Moreover, application circuit examples and recommended transformers are provided. By all means, please have a look at the data sheet.

－You have clarified the reasons why a photocoupler or auxiliary windings are unnecessary. Next, I would like to ask about the functions and features of the DB7F series of ICs.

(to be continued）