7 segment display flickering when using switching power supply
You may be having some noise problems in your circuit due to the use of a switching power supply. Switchers are notorious for injecting noise spikes into the PWR and GND rails and can be coupling noise into other nearby conductors. Only with the most careful design and layout of the circuit board along with the proper selection of bypass and filtering components can keep the noise contained and under control. 

You provided few details of how your circuit is configured so it is hard to provide specific suggestions of how to correct the noise problem in your specific case. Even with that said I suspect that with two 4-digit 7-segment displays you may be using an MCU with some external logic to multiplex the display. Or you are using a long shift register approach to drive all the display segments with a typical periodic display update. In any case the flickering you are seeing is maybe being caused by the switcher noise disrupting the currently stored data driving the display until the next MCU display update comes along. 

It is also possible that the design of the switcher is not up to par for supplying the amount of surge current required by the display. If this is the case there will be high ripple on the supply voltage which is then modulating the intensity of the display and causing the flicker that you see.

Come back with a photo of your circuit setup, a schematic and maybe we can offer more specific suggestions.