SMPS meant to drop out when pump heavily loaded.

[Flyback]

Hello,
We are doing an offline flyback SMPS to drive a pump motor.
It doesn’t matter if the pump doesn’t go round at full speed when most heavily loaded, so we will just let the smps drop out in such cases, and deliver what power it can to the motor.
The motor draws its power in pulses of ~25ms, spaced about 400ms apart.
So the smps will just drop out slightly for the ~25ms high power pulses. (when the customer has the maximum load (max tubing length) attached.
Obviously when lightly loaded the pump wont drop out at all (by drop out I mean its vout drop away slightly).
The motor is a dc motor.
However, we do not want to have subharmonic oscillation in the smps when it drops out, so we need a pwm controller which has a max duty cycle of 50%.

So to summarize, we need a current mode pwm control chip which has....
1…max duty cycle of 50%
2…Overvoltage protection (ie internal comparator for sensing the auxiliary voltage)
3….under voltage lockout (so it doesn’t keep working at stupid low mains.)
4….oscillator that can be set to 85khz or thereabouts.
Does anyone know of such a controller? it seems a rarity.
UCC28C45 does it, but doesn’t have the OVP or the UVLO.

 

[kubeek]

what´s the problem with adding two comparators to the UCC28C45?

 

[Flyback]

It ends up being more than just the two comparators........you need a supply for them because the vref of ucc28c45 drops away even if only lightly loaded.
you also need the divider resistors, filter and decoupling caps etc etc.