DC/DC - Fixed Input

Low-power converters have a higher requirement for ripple & noise which need further filtering. 

There are the following measures: 

(1) add a capacitor at the output terminal (the function of the capacitor is to further lower ripple & noise with a relatively low frequency); 

2) add a PI filter at the output terminal by filtering methods of capacitor + DM inductor + capacitor  OR capacitor + CM inductor + capacitor (this method lowers ripple & noise at the same time. If the frequency of the noise is high, it is recommended to use the pattern of capacitor + CM inductor + capacitor);

 (3) customers can choose a power module whose output voltage is one grade higher and add an LDO to reduce ripple & noise (for instance, customers practically need a 3.3V output power module but there are demanding requirements for ripple & noise. Please recommend customer to choose a 5V output power module and add an LDO to lower it to 3.3V, and the LDO can lower ripple & noise).

The starting current of a module is generally about 1.4-1.6 times that of the Iin (max). The increase of capacitive load of the output terminal will also increase the starting current value of the module. It is recommended to select the forestage power supply with higher power. For the minimum input current in the unloading condition, please check the ‘No-load current’ section in the datasheet of the corresponding module.

In practical application, the capacitive load exceeds the maximum capacitive load of operating module. In addition, when the output capacitance is extremely large, large starting current is required at the moment of start-up, resulting in the failure of starting the module.

It is recommended to decrease the connected capacitance of the output terminal or add a buffer circuit at the output terminal to improve the capacity of the module to withstand capacitive load

The module may be damaged when it starts up at a voltage that is lower than that of input voltage range. Because there is no under-voltage protective circuits for the wide input voltage converters. 

The starting voltage should be determined on the basis of the threshold voltage of MOSFET and soft starting circuit, and the starting voltage of the MOSFET is 2V or more lower than that of the minimum input voltage of the module.

In this circumstance, lower voltage input requires higher input current so as to supply equal power; the power module is susceptible to damage in a higher input current condition.

(1) It should be ensured that the actual input voltage is in the rated range specified in the datasheet; 

(2) it is recommended to add an input under-voltage protective circuit on the front-terminal of the module in accordance with the Wide Input Voltage DC/DC Converter Application Guide.

There is high reflected ripple during the operation of the relay. 

It is recommended to implement some filtering measures or add a Y capacitor between the input and output grounds.