Wide-range Regulator delivers 12V, 3 a output from + to 100V source

Synchronous buck regulators offer high efficiency and is popular in applications in which available input voltages is 12 V or less. However, as input voltage approaches 100V, wide-range-regulator design becomes more difficult, and the choice of suitable ICs narrows considerably. This Design idea combines a Current-mode PWM ICS for Flyback-regulator circuits with a 100V gate-driver ICS to produce a rel atively high-performance Synchronous Buck regulator that can operate at inputs as high as 100V.

the circuit in figure 1 uses National Semiconductor ' s LM5020 current-mode PWM, IC1, to Dr Ive an LM5104 gate driver, IC2, forming a synchronous controller. The LM5020 contains an internal linear regulator this accepts input as high as 100V and can also deliver a output that CA N Supply drive the LM5104. To reduce power dissipation in high input voltages, after initial power application, diode D2, a 1N4148, supplies an 11.5V Bootstrap voltage to the remainder of the circuit. Transformer T1, a 100-to-1 current Transformer from Pulse Engineering, provides current feedback during MOSFET Q1 ' s On-tim E. q1 and Q2 are Siliconix Dpak devices, which has low gate-charge requirements and low on-resistances to Minim Ize total switching losses at the circuit ' s 200-khz operating-frequency. All capacitors is of ceramic-dielectric construction to withstand high temperature and to meet packaging-size constraints .

For sustained operation in high input voltage, maximum current, and elevated-temperature conditions, transistor q1 re Quires an adequate heat sink or cooling airflow to maintain its junction temperature below the 175°c maximum specification . Q1 has a low junction-to-case thermal resistance, and thus it case temperature must not exceed 160°c. L1, the model DO5010 unshielded ferrite-core inductor from Coilcraft, presents a small pc-board footprint and offers a hig H Saturation-current Rating but represents this design ' s dominant loss component. For applications with less critical space requirements, you can improve circuit efficiency by increasing L1 ' s inductance a nd size, thus reducing ripple and enabling use of a larger core and increased winding-wire gauge. Reducing the output voltage improves efficiency, but as output voltage drops below the circuit ' s 8V bootstrap voltage, IC 1 dissipates additional power and requires caution to avoid exceeding its ratings.  Figure 2 shows The circuit ' s measured efficiency versus output current for three input voltages.

One practical application for the circuit met a customer's requirement for A/DC converter that would operate from a 24V Source and deliver 12V output at currents as high as 3A. This routine-sounding specification also requires operation in a physically and electrically harsh environment in which th E packaged circuit resides on the engine block that reaches a temperature of 125°c, and ambient-air temperature reaches 100 °c. In addition, the power source comprises bi series-connected 12V batteries that provide a nominal voltage of 24V, which in Practice varies from 40V and includes inductively induced load-dump voltage spikes, that reach 100V.

Wide-range Regulator delivers 12V, 3 a output from + to 100V source