Wide Range off-line switcher

[Pierre de Vos]

Hi,

I need an off-line switcher with a very wide input range. Typically
50Vac - 555Vac. I have seen a motorola appnote describing an offline
switcher that approaches this range, but from what I can remember that
one had a 6:1 input ratio. The one I need is 10:1.

Anyone have any ideas?

Regards
Pierre

 

[legg]

On 7 May 2004 05:32:15 -0700, pierre.devos@webmail.co.za (Pierre de
Vos) wrote:

Hi,

I need an off-line switcher with a very wide input range. Typically
50Vac - 555Vac. I have seen a motorola appnote describing an offline
switcher that approaches this range, but from what I can remember that
one had a 6:1 input ratio. The one I need is 10:1.

Anyone have any ideas?

At what power level?

RL

 

[Terry Given]

"Pierre de Vos" <pierre.devos@webmail.co.za> wrote in message
news:b9ca3aa.0405070432.329fece6@posting.google.com...

Hi,

I need an off-line switcher with a very wide input range. Typically
50Vac - 555Vac. I have seen a motorola appnote describing an offline
switcher that approaches this range, but from what I can remember that
one had a 6:1 input ratio. The one I need is 10:1.

Anyone have any ideas?

Regards
Pierre

The motorola app note was a low power flyback converter (< 50W IIRR). Marty
Brown's cute trick was to organise the frequency to vary with input
voltage - IIRC to get constant volt-seconds, ie Fsmps = k*Vin

One way to get a wide input range is to run your smps at low frequency (say
30kHz) BUT use a fast switch/controller, so you can have very narrow duty
cycles - the duty cycle limitation is what causes "squegging," as its sets a
lower bound on the energy quanta delivered to the load; for lighter loads
the voltage rises, forcing the error amp to set D=0 until Vout sags....A
known minimum load can be helpful in this regard.

Keith Billings' book has a suggestion whereby he transitions to fixed on
time, variable off time at very light loads.

Like R.Legg says, how many watts?!?