Using Series-Connected Isolated μModule Converters to Increase Output Voltage and Current Range

We will use the UL60950 certified 2kV AC isolated μModule converters LTM8057 and LTM8058 to demonstrate this design approach, which can also be applied to LTM8046, LTM8047 and LTM8048. Suppose we want to get a 10V / 300mA output from a 20V input. By looking at the maximum output current curve given in Figure 1, we notice that relying on a single LTM8057 is not enough to meet the output current requirements in these cases.

Figure 1: Typical maximum output current vs. input voltage

However, once you notice that a single LTM8057 can provide 300mA at 5V output with a 20V input, the solution becomes immediately apparent. Since the output voltage is isolated from the input, the outputs of two LTM8057s set to 5V can be connected in series to achieve a 10V output at 300mA (Figure 2).

Figure 2: The outputs of two LTM8057 modules connected in series  support a 10V, 300mA output application from a 20V _IN.

The circuit in Figure 2 can also be used to increase the output voltage range when outputs above 12V are required. By adjusting the feedback resistors to provide a nominal output voltage of 7.5V, the combined output voltage is increased to 15V. The output current capability for 15V is the same as for the individual 7.5V modules (Figure 3).

The circuit shown in Figure 2 allows for a third option: using a common return to provide both positive and negative outputs. The return nodes of the two outputs are tied together in the middle of the output stack. With this approach, the circuit in Figure 2 will have 5V and –5V outputs. Each output can be different in size because the output voltage of each converter is determined independently.

Figure 3: The outputs of two LTM8057 modules are connected in series to provide a low output noise series converter  with over 160mA  at 15V _{OUT from 12V IN}

When using series output, the low output spectrum noise advantage of LTM8058 and its integrated LDO post regulator can still be maintained. Figure 4 shows the connection circuit schematic of two LTM8058s, where the output V _OUT2 of the LDO  is connected in series to provide 10V _OUT . Figures 5 and 6 show the output noise spectrum of LTM8058 under a 10V and 100mA load condition when the LDO outputs are connected in series (see the schematic shown in Figure 4) and the feedback outputs are connected in series, respectively.

Figure 4: Two LTM8058 modules  connected in series with V _{OUT2 to provide 10V OUT}

Figure 5: Noise spectrum of two LTM8058 LDO outputs connected in series (under a 100mA, 10V _OUT load)

Figure 6: Noise spectrum of two LTM8058 flyback outputs connected in series (under a 100mA, 10V _OUT load)

Linear Technology's isolated μModule converters provide a simple and compact solution for obtaining isolated power at a regulated output voltage. The LTM8057 and LTM8058 successfully demonstrate that the output capability of an isolated μModule converter can be increased by adding one or more isolated modules connected in series with the output, while still maintaining the original output noise characteristics.