# SMPS input voltage

I find lots of SMPS, with a PWM current mode control IC in the input AC variation range of approx 1:3. From 85V AC ~ 265V AC at its input. Which parameter restricts this is it minimum 'Ton' possible? How to find practical maximum input range possible, say one want from input: 60V AC ~ 300V AC, and output: 5V DC. Please consider the PWM controller datasheet below.

Datasheet here: SG 2525A

• Maybe you should link a diagram showing the SG2525 used as an offline converter? Jun 4, 2014 at 18:10

The PWM range of the device you list is from 0% to 50% (theoretically) but I'm assuming, for the sake of this answer that it's from 3% to 45%. (this is a more guaranteed range)

For a decent load range of (say) 5 watt (basic internal switching losses) to 50 watts, the PWM has to "push" its duty cycle around a fair bit to maintain the output voltage at a constant level (even on a fixed input voltage of say 150 volts).

Given that the input voltage range of 3:1 reduces the duty cycle range of 45:3 to 45:9, it means that 5:1 of the PWM range is all that remains to regulate the output voltage against load power changes of maybe 10:1 (50 watts to 5 watts).

Without knowing the load range you have to cover it's impossible to say whether 60 Vac will be OK or not.

Another consideration is that the bridge diodes, capacitor and switching transistor have to be rated so much more powerfully at lower AC supplies because this type of switcher takes a constant power from the AC to provide the load power - you reduce voltage by 3:1 and current taken from the AC rises by 3:1 - if you want a 4:1 range then this has to be accommodated. Bear also in mind that the flyback transformer will also have to be made bigger to avoid saturating at the peak currents expected when operating from lower AC voltages.

I'd say it can be done but tread carefully.

• Actually , I selected SG2525 for the sake of question, and later found the limited duty cycle. Please consider SG3842 as it can go upto 96% . The load is about 50W 5v@10A or 12@4A. The tentative diagram added above.