DC-DC converter maximum current rating

Well the suitable way would be to have the PSU connected to the old DVD player innards so that its normal operational load could be running. You would then simply insert a current meter (DMM in current measure mode) in series with one output at a time and monitor the current consumption into the normal load. This will give a good picture of that that power supply was nomimally expected to deliver.

If the "old DVD player" is junked out and not available to be used for a nominal load then you have a harder job on your hands. One method to determine capability of the supply would be to put the thing on the test bench and connect resistor loads to the outputs. You want to utilize high wattage resistors for this so that they do not get hot and cause a change in the reistance of these load resistors. For this type of test you will need to simultaneously load all the outputs that you would like to use on the PSU. The idea is to start with relatively light loads and then interatively increase the loads to more and more current while monitoring the output voltage across each resistor and periodicaly checking the temperature of critical components on the PSU board. As you get to heavier loads it also pays to check each output with an oscilloscope to check each output for amount of noise and ripple.

When you reach load sizes that start to cause the PSU to get hotter than a nominal working value, increased ripple beyond more than 1 or 2% of the voltage levels of the output, or the nominal output voltage sags more than 4 to 5% then you know you are reaching the design limits of the power supply. From this check the current flow into each load resistor and you will have a close idea of the supply capacity.

For sustained usage of the supply it is recommemded to back off on the limits determined above. Also make sure to use the PSU in no worse enclosue space than it was in when in the DVD player. For exwmple if the original application had a fan blowing air over the PSU then make sure to provide as good or better environment in the re-use application.

When working with a boost regulator, the first thing you want to know is the critical inductance and critical current. Critical current defines the boundary between continuous conduction (CCM) and discontinuous conduction (DCM) in the inductor. Circuit dynamics are very different between the two modes of operation, and you want to be in one or the other mode.

Critical current for the boost is approximately:

\$i_{\text{crit}}\$ = \$\frac{V_o T_s}{16 L_{\text{crit}}}\$

In this case with L = 1.8uH, \$V_o\$ = 10.5V, \$T_s\$ = 1uSec; \$i_{\text{crit}}\$ would be about 0.37 Amps. Normally the load current is 0.2 Amps, but pulses to 1.2 Amps. That's bad.

During the pulse the regulator goes from DCM to CCM adding a pole the the modulator response.

• If the regulator is compensated for DCM, the move to CCM will make it unstable and it will oscillate.
• If the regulator is compensated for CCM, operation in DCM will likely be stable, but the transient response will be very poor.

To keep the regulator in CCM mode at a current of 0.15 Amps an inductor of about 4.7uH would be needed.

Another thing to keep in mind is that switching regulators have negative input impedance. This means that if the impedance of the source voltage is equal or greater than the input impedance of the regulator, the system will oscillate until it runs out of regulation range. In this case with about 15W of input power from 6.5V, the input source impedance needs to be less than about 1.4 Ohms everywhere below the loop crossover frequency (including any LC resonances). Looking at the input voltage variations in the pictures, it's not clear that the supply source is up to that.