# Boost Converter - Input follows Output

I have a boost converter.

$$\V_{in}\$$ = 7V

$$\V_{out}\$$ = 14V

$$\I_{load}\$$ = 0.35A

While testing, I observe that as long as Vin<14V, I observed switching at the inductor switch node.

But, as I increase my input voltage, Vin>14V, I dont observe the switching at the switch node.

How come this happens with the boost converter, when I want the output to be constant 14V irrespective of the input voltage?

Could someone please provide some suggestions to overcome this and why this happens?

Crudely said, a boost converter tries to get the input voltage up to the target output voltage and a buck converter tries to get the input voltage down to the target output voltage.
A boost converter therefore fails to get a voltage up/higher when the input voltage is already higher than the target output voltage.

The output of a boost converter is given by $$V_{out} = V_{in} \frac{ 1 }{ 1-D }$$ where D is the duty cycle for which applies 0 < D < 1. You can see this duty cycle at the mosfet's gate and/or at the switching node.
When the $$\V_{in} > V_{out} \$$, in order to obtain the target output voltage, according this equation, D < 0. If e.g. Vin = 28V and D could be -1, then Vout=14V. However, D cannot be smaller than 0. So, the mosfet stops switching (which is 0% duty cycle) and the inductor could be crudely approached as were it a wire.
And therefore, the ouput voltage follows the input voltage.

If you want a converter with an output (e.g. 14V) that lies within the range of the input voltage (e.g. 7V to 18V) you need another topology, e.g. a buck-boost converter.
Wiki about the types of SMPS

• Why does the Boost converter not maintain the output @14V, even if I designed it for 14V for the 7-18V input range? Can this output voltage be stable if I use Buck Boost converter? – Newbie Sep 27 at 5:37
• @Newbie please see updated answer. And yes, you can get a stable output voltage using a Buck Boost converter. – Huisman Sep 27 at 6:16
• Thanks @Huisman – Newbie Sep 27 at 7:00