When designs the non-isolation the voltage dropping manostat, the voltage dropping manostat usually is the first choice, only if ratio of VIN and the VOUT is bigger than 10, input voltages to be very high or two kind of situations has both at the same time. Occupied the spatial factor regarding the FET driver is possibly a question lowly, and will cause the current mode control loop to lose the control. Will be very low in VOUT, the output current to have several ampere hours, the efficiency will drop large scale to 60%~70%. The efficiency drop is causes by the translation loss, because upside in the switch winds through the full load current throughout. Shown in Figure 1 the electric circuit to look like a little looks like the voltage dropping manostat, and uses a voltage dropping controller, but in fact is one kind of voltage synchronization flyback circuit. Is higher than 85% in the efficiency, the input voltage scope is in the 36V~60V situation, it faces the application system when the output current is 2A requests the output voltage is 3.3V. This electric circuit in several kinds have appraised in the technology as if is most hopeful, because its efficiency and the cost surpass the voltage dropping manostat and the asynchronous flyback circuit.
Figure 1, this kind of synchronization flyback circuit has the very high efficiency as well as many kinds of input voltage/output ratio-voltage.
After the LM2743 controller starts, from lifts the coil from the MMBTA06 transistor and the 6.2V zener diode as well as from one to obtain the power. Its EN (start) the input end is provides UVL (low pressure cut-off) the comparator, uses for to prevent below 28V to start. The controller actuates a loss compared to the Short base diode also low synchronizer switch, and takes the current limiting examination resistance using the lower FET breakover resistance. Has a 250 kHz turn-on frequency in the pin 11 150kΩ resistors. The fly-back transformer which Corporation (www.pulseeng.com) designs by Pulse Engineering is a low cost part, its primary inductance is 50mH, turns compared to is 3:1, the size is 13 (long) ×15 (wide) ×11 (high) mm. 3:1 turning compared to prevents the primary switch to wind through the full load output current, thus causes the switching loss gradient profiling manostat to be small. Out-port’s small LC filter can cause a 10mF porcelain capacitor processing very big effective (rms) the ripple current, in addition, a low cost aluminum capacitor can also eliminate the ripple and the cushion load transient state.
Figure 2, shown in Figure 1 the electric circuit to have in the very wide output current scope is higher than 85% efficiencies.
Figure 2 showed shown in Figure 1 the electric circuit under three kind of input voltages and certain kind of output current metrical data. Left side three most place above curve expression efficiencies; Three low curve expressions according to the right side scale division measurement take W as unit’s total losses. In the coil loading situation, the VOUT ripple’s peak - peak value is not 6mV, when the output current is 4A rises 20mV. When the output current is 3.5A, the efficiency drops rapidly, this is the current limiting function creates. Is similar to any switching power supply, special flyback circuit such, the print circuit wafer layout is important. If uses four or the multi-layered print circuit wafer, the power source plane and the ground plane separate, the electronics grid actuates the segment to be short and to be wide, you may obtain the optimum performance. Although shown in Figure 1 the electric circuit to plan to apply in the 7W list output system, but this kind of synchronization flyback circuit suitably in bigger power range; So long as you increase the secondary, may expand easily it many kinds of outputs. Increases the out-port already may use the diode rectifier, may also use additional FET which the low grid-voltage driver actuates.
