# Burst mode in synchronous buck converter with current mode control?

The meaning of Burst Mode

It says burst mode happens when the resistance of the load is light and the system stops switching the MOSFETs. However, I just cannot understand how the light load decreases the load current and therefore the inductor current, as mentioned in the link.

As I know $$\Vload=Rload*Iload\$$ and $$\Vload=D*Vin\$$. So if we have a low load and highest input voltage, this implies that the duty cycle is small and the current is going high - which means the inductor current must be high as well and never go to DCM!

What I am missing?

• ... and the current is going high... this is where I can not follow your reasoning. Why should the current go high with small duty cycle? – Stefan Wyss Nov 17 '18 at 20:31
• as the we have light load, this means the output current must increase to compensate the the decrease of resistance!! – Yaakov Nov 17 '18 at 20:47
• Light load does not mean decrease of resistance. It means increase of resistance and this decrease of current. – Stefan Wyss Nov 17 '18 at 20:55
• well normally it does but if this is a current regulator, not a voltage regulator (eg. a LED driver) a light load is a low voltage – Jasen Nov 17 '18 at 22:08

However I just cannot understand how the light load decreases the load current

Light load is defined as low load current. High load resistance is just one way that may happen.

and therefore the inductor current, as mentioned in the link.

The inductor current is controlled by the converter circuitry; the low inductor current is not a direct effect of the light load; it is an effect of the controller reducing the duty cycle.

As I know Vload=Rload∗Iload and Vload=D∗Vin. So if we have low load and highest input voltage, this implies that duty cycle is small and the current is going high

Again, Rload is not necessarily an accurate model. Low load means that the load current is low, by whatever means that may happen. When the load current is low, the load voltage goes up, and the controller reduces the duty cycle.

What I am missing?

I'm not sure, but I suspect you're not taking the behavior of the controller into account.

• thanks for the clarification. however, I am still don't understand how can the load current is low and the load voltage goes up!. it is like for me the load voltage as well goes down – Yaakov Nov 17 '18 at 21:21
• The voltage is being driven by the converter; the converter, like most sources, increases its voltage as the load current goes down. So if the load, for whatever reason, starts demanding less current, then for any given duty cycle the converter voltage will just naturally rise. – TimWescott Nov 17 '18 at 22:32
• Ok I understand here. But how can the inductor current start increasing while the converter is in idle mode!? When the converter goes to the idle mode, both switches are turned off which means the inductance is not fed with current and must decrease in my understanding! – Yaakov Nov 18 '18 at 10:13
• In burst mode, in between bursts, both switches end up off, and the inductor current is zero. The output voltage is allowed to drop slightly, and then the controller turns on for a short burst. This causes inductor current to flow and injects some charge into the output, raising it slightly. – TimWescott Nov 18 '18 at 16:17