Photocoupler-Free Isolated Flyback DC-DC ConverterUnique Adaptive On-Time Control Greatly Improves Transient Response of Isolated Power Supplies

－To begin with, please tell me about “adaptive on-time control”.

“Adaptive on-time control” is based on “on-time control” adopted in ROHM’s isolated power supply controller ICs. The BD7F series are isolated power supply controller ICs that have been the first to adopt this adaptive-type on-time control. In previous ICs for isolated power supply control, there remained room for improvement in output voltage fluctuations caused by instantaneous load current fluctuations, that is, in the load transient response.In addressing this problem, the BD7F series employs adaptive on-time control to successfully hold load transient fluctuations to 200 mV or less. This is a reduction by up to 65% compared with typical control methods. Please have a look at comparative data for these control methods.

－What is the meaning of “adaptive” here?

Adaptive on-time control employs three different types of operation to cope with load states.

When the load is constant and stabilized, general PWM control is executed, and the switching frequency is fixed at 400 kHz (Typ).

When the load fluctuates, on-time control is used, and the on-time for operation corresponds to load transients. At this time, the switching frequency fluctuates.

During light loading, PFM operation is used, and by lowering the switching frequency, self-consumption of power is suppressed, and high efficiency maintained.

As a result, high efficiency is realized over all loading regimes, to achieve fast response to load transients. Moreover, phase compensation components, which had been necessary when using conventional control methods, can be omitted.

－I see. Well, could you please discuss the other feature as well, the “output load compensation function”.

The “output load compensation function” is a function that compensates for output voltage fluctuations, that is, for errors, that occur due to fluctuations in the voltage V_F of the secondary-side output diode due to the load current. In applications where such errors pose a problem, voltage correction that is opposite to the V_Fcharacteristic of the secondary-side output diode resulting from load fluctuations can be input using this function, so that load regulationcan be improved.

－I’m sorry, I don’t quite follow.

When it was explained that the primary-side flyback voltage is used to stabilize the secondary-side output voltage, the following equation was presented.

Using this equation, we explained that the factors causing errors in V_OUT are V_Fand ESR. Due to fluctuations in the secondary-side transformer current I_S, which is the load current, errors due to ESR (the total secondary-side impedance: transformer winding resistance, board impedance, etc.) change in a simple manner, and V_Ffluctuations depending on the V_F-I_Fcharacteristic of the diode.

In order to compensate for this error, a function is provided to determine a compensation amount at the COMP pin, and to correct the feedback current that determines the output voltage. Put simply, the idea is that the errors occurring due to V_Fand ESR cancel each other, according to the load current.

The above graph visually represents compensation when using the output load compensation function. In order to use this function, it is only necessary to determine the component values of the resistor and capacitor that are connected to the COMP terminal to determine the compensation amount. Equations for calculating the component values and other details are provided in the data sheet.

－Viewing the graph, I am able to see that as the load increases, the error factors continue to cause errors, and how compensation works. And, since the BD7F series of ICs do not require a feedback path isolation circuit, and also incorporate MOSFETs, and so I guess fewer components are needed, and design is simplified. But, a transformer must be used, and so I guess that some would say it is rather harder to use.

To be honest, the need for a transformer can be somewhat troublesome. Of course, the data sheet provides the range for selection of the transformer; but when considering processes up to the actual physical transformer, it is quite a hassle to deal with specification creation, prototype fabrication, specification exchange, and the like.

It is with this in mind that we have made evaluation boards available, so that customers can first carry out evaluations. One representative product is the BD7F100HFN-EVK-001, an evaluation board for the BD7F100HFN-LB; this board can be purchased over the Internet from online distributors. An isolated power supply based on the BD7F100HFN-LB, with an input voltage of 24 V, an output voltage of 5 V, and an output current of 800 mA, can easily be evaluated using this product. This is a photo of the evaluation board.

Moreover, specific part numbers for other principal components, including the transformer, are given in application examples of the data sheet. The following is an excerpt from the data sheet for the BD7F100HFN-LB; where the transformer is concerned, transformers made by Sumida Corporation are recommended, and part numbers are already provided for widely-used voltages.

The following table is also excerpted from the data sheet. Data is also included for other applications, and the general specifications of recommended transformers are also indicated.

－With this kind of support, it should be possible to make decisions with confidence.

We have prepared a support system that is ready to answer any other questions our customers may have.

－Thank you very much for your very detailed explanations.