Buck converter with low duty cycle.

[eem2am]

I need to Provide 700mA to a LED (Cree XR-E).
I have to use a Buck converter as it’s the cheapest way.

The supply rail is pulsating at 100Hz from 27V to 38V.
(it’s the secondary of a mains operated flyback)


The low duty cycle is 3.5V/38V = 0.092
The high duty cycle is 24V/27V = 0.89

Also, I have to dim (by PWM or analog dimming) this current down to 65mA (+/-10%)
At times.
When the LED has 65mA in it, it has a Vf of just 2.8V
-This makes the duty cycle just 0.073

Also, from the same supply rail, I have to charge up two 1.2V Ni-MH batteries.
(200mA charge current)
Again I must use a buck converter.
These have a voltage of 1.8V when discharged.
-So the Buck duty cycle will be 1.8/38 at its minimum. (= 0.047)

Are these duty cycles too low to be realistic with a buck?

Surely the FET on time will be too small when we’re at 300KHz switching frequency.? (around 156ns at worst)

….

 

[dick_freebird]

You may be bound by "minimum on time" more than duty
per se. Quality of regulation may suffer or you may see the
output pulse start to "flicker" as you approach the minimum
on time (jitter, etc. become a significant fraction, the output
switching noise may not have settled out by next ramp
compare, etc.).

This is why you see some controllers fall back to a pulse
skipping mode (effectively downranging frequency) at very
light loads, as well as the simple switching-losses bonus.

 

[frankliner]

I agree with the comment above. Please look into the PFM controller 910.27 from Elmos. It may fit your requirements.

**broken link removed**

 

[eem2am]

frankliner.........i am trying to get pricing for that chip.

It sounds like just what i need and will handle both high and low buck conversion ratios due to PFM operation.

Though it didn't say what the minimum on time of the fet would be at low duty cycle.
We have been told never to let fet on time get below 800ns.

but i noted it is not priced on Digikey or Mouser

 

[frankliner]

The minimum on time is 500ns-1.5us.

Unfortunately there are no US distributor for Elmos yet. There are European and Asian distributors.

**broken link removed**

For North America you should fill out
**broken link removed**