Electrical – SMPS Input Circuit – Formulas resolving diodes, capacitors, voltage, etc

Two devices placed in parallel, regardless of whether they are rectifiers, transistors, resistors, etc. in any configuration, even with different devices will operate with the same voltage. In the case of two rectifiers, the total current passed will be distributed across the rectifiers according to their V-I curves such that both have the same forward voltage.

With two similar rectifiers this means that they will have approximately the same current, but with, say, a 1N4007 and a HER307, with a current of 1A the former will pass 800mA and the latter 200mA, with a V_f of approximately 0.9V.

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.