In the previous article, matters to be considered when raising the switching frequency to reduce the application size were discussed. In this article, cases in which the input voltage is increased, including parts of a circuit in which losses increase as well as matters for consideration and possible countermeasures, are explained.
Important Matters when Studying High Input Voltage Applications
As input power supplies to DC/DC converters, the 12 V buses of industrial equipment and the like maintain nearly constant voltage, but in the case of the voltages of batteries for automobiles and the like, equipment must be designed assuming voltages that vary over a considerable range, including transient fluctuations, deviating from the nominal 12 V.
In this article, we examine efficiency for cases in which the input voltage, which in applications we have studied up till now has been 12 V, may increase up to a maximum 60 V.
We calculate the actual losses when VIN is 12 V and 60 V.
PSWH(12VIN)＝0.5×12V×2A×(20 nsec＋20 nsec)×1MHz＝0.48W
PSWH(60VIN)＝0.5×60V×2A×(20 nsec＋20 nsec)×1MHz＝2.4W
VIN rises by a factor of 5, and so the calculated switching loss likewise undergoes a fivefold increase. Below we indicate the change in overall losses with changes in the input voltage. In essence, the increase in switching loss dominates.
Examination and Countermeasures
In order to expand the input voltage range to 12 V to 60 V, a number of specifications must be revised, beginning with the voltage rating (rated voltage) of MOSFETs initially selected for use when the input voltage VIN is 12 V. The parameters that require reexamination are summarized below.
Up to this point we have considered only losses, but in order to accommodate higher input voltages, there is another matter that requires study. It is not the main subject of this article, but is important for practical reasons and so is here mentioned.
A maximum voltage VIN of 60 V is stepped down to a VOUT of 5 V, but because the step-down ratio is limited by the minimum on-time that is one control parameter of a power supply IC, the step-down ratio and minimum on-time must be studied. The step-down ratio is 60:5, and so a simple calculation for a switching frequency of 1 MHz indicates that a power supply IC that can be controlled to 1/12 of a 1 μs period, that is, with a minimum on-time of 83.3 ns or less, is necessary. However, in actuality there are not many power supply ICs with a minimum on-time of 83.3 ns or less. Taking ROHM as an example, the DB9V100MUF power supply IC can be used, but in many cases the minimum on-time requirement cannot be met and the switching frequency must be lowered. Lowering the switching frequency in turn necessitates a review not only of losses, but of the values of all related components. However, in equipment for automotive applications, switching frequencies of 2 MHz and above are basically required, and so the option of lowering the switching frequency is not available.
Thus when studying high-voltage applications, the two issues of step-down ratios and loss increases must be examined.
・When the input voltage rises, the increase in switching loss dominates.
・Because the switching loss increases, MOSFET voltage ratings and allowable power dissipation must be reexamined.
・In addition, MOSFETs with faster tr and tf values and with lower on-resistance and Qg values should be considered.
・In power supply specifications, the switching frequency should be lowered if possible. If fSW is halved, the loss is also reduced by half.
・In the case of an IC with an internal switching transistor, use of the IC itself must be reconsidered.