What is the chance of UK or EU mains going much below 190VAC, for long periods?

[treez]

Hi,
An external contractor has designed a 120W offline flyback SMPS for 190-270VAC input. It uses the FSCQ1565RT controller.
This flyback has no way of shutting down should the mains go below 190VAC....it will just increase its input current as much as is necessary to feed the load.

The FSCQ1565RT has an in-built primary current limit of 8A.....but for 120W load from 190VAC input, the peak primary current is only some 5A.
What is the chances of the EU and UK mains ever going below 190VAC for long periods?
Is there any situation whereby the EU or UK mains could go down to eg, 100VAC for long periods?

 

[scopeprobe]

Are you sure your understanding the FSCQ1565RT operation correctly it looks like it monitors the peak current so as you go down in Volts the frequency will slow as it will take longer for the peak current to get to its peak vaiue which limits your Joules throughput. As your inductance is chosen at low line, as you go below this the power thoughput of your converter will drop as the frequency of operation slows this will naturally drop the output power capability so your current perhaps won't just keep getting bigger.

Figure 32 of their datasheet shows the current measurement control of the PWM

 

[albbg]

As far as I know the energy suppliers should be compliant with the EN50160 standard, at least in EU. I think you should refer to that standard or that in use in UK.

 

[treez]

Are you sure your understanding the FSCQ1565RT operation correctly it looks like it monitors the peak current so as you go down in Volts the frequency will slow as it will take longer for the peak current to get to its peak vaiue which limits your Joules throughput. As your inductance is chosen at low line, as you go below this the power thoughput of your converter will drop as the frequency of operation slows this will naturally drop the output power capability so your current perhaps won't just keep getting bigger.

Thanks, yes thats usually the way with QR controllers, but FSCQ1565RT has a minimum switching frequency of 20kHz.....and so it goes into CCM.

--- Updated Mar 29, 2021 ---

As far as I know the energy suppliers should be compliant with the EN50160 standard, at least in EU. I think you should refer to that standard or that in use in UK.

Our power network in UK is entirely run by French and German co's, so i guess we are same as EU.
I dont think a standard comes into it though.....we are speaking of fault situations where anything goes.....the mains just ends up being what it is......no matter whether or not the power network operator has got all the boxes ticked for standards.

 

[scopeprobe]

Thanks, yes thats usually the way with QR controllers, but FSCQ1565RT has a minimum switching frequency of 20kHz.....and so it goes into CCM.

--- Updated Mar 29, 2021 ---

Our power network in UK is entirely run by French and German co's, so i guess we are same as EU.
I dont think a standard comes into it though.....we are speaking of fault situations where anything goes.....the mains just ends up being what it is......no matter whether or not the power network operator has got all the boxes ticked for standards.

mmmm. Ok. I've done boundary/transition mode control of flybacks and how i describe is how they work, i thought resonant converters wern't that much different, perhaps they are.

I still would have thought if you starve the supply of input volts it would go into DCM rather than CCM though, my reasoning being if your inductance is fixed & your max peak current is fixed then as you reduce the input volts it will take longer to get to the peak current thus lowering the frequency. Your duty cycle is limited to 50% to achieve the volt second balance so your inductor won't be able to supply full load.

V = L di/dt ,
Delivered Watts = ((L * (Ipeak * Ipeak))/2) * Frequency

 

[betwixt]

It happens occasionally here when overhead power cables fail and one of the three phases is lost. However, the voltage drop is largely due to the network not being able to provide enough current so it is unlikely anything trying to draw more would succeed.

Brian.

 

[FvM]

What's the objective of your question? Your device isn't required to operate regularly at the discussed undervoltage. But it should be still safe, not catch fire or get damaged in all cases of grid brown-out. A correctly dimensioned fuse should achieve this. Undervoltage lockout could be an additonal means.

 

[Easy peasy]

As FvM says, paraphrasing, it really doesn't matter how long the mains dips to low values, any power supply working off the mains line, should, either inherently, or by design, limit currents under these conditions, by turning off if necessary, thus protecting itself and allowing it to operate another day. Fuse protection is an alternative, provided that nuisance tripping is avoided and the fuse can be easily replaced - these days some poly-fuses allow self-resetting protection - but the mains side power electronics would have to handle the over-power, over-current until the poly-fuse does in fact go to a high resistance state ...