Which offline flyback controller chip has greatest resilience to overvoltage to internal FET drain?

[treez]

Hi,
We have made some offline Flybacks using the FSCQ1565 controller, for a group of students, who we want to teach about leakage inductance. The flyback transformer is very large and wont saturate even at the FSCQ1565 peak current level.
Anyway, we deliberately did them without Primary clamps so that we could demonstrate to the students that they will blow up. However, the FSCQ1565 has 800V leakage spikes on its drain and yet still runs fine !!! (its only rated to 650V). There are 5 members of the FSCQXX65 family, with a range of peak current limit levels.
Which one of the family will be most resilient to such drain overvoltages?
Is it the FSCQ1565?, because it has the greatest avalanche energy capability?, as per page 6 of the FSCQXX65 datasheet..

FSCQXX65 datasheet

https://www.onsemi.com/pdf/datasheet/fscq1565rt-d.pdf

 

[Easy peasy]

presumably - how wide are the clamped peaks ? < 100nS ?

--- Updated Apr 22, 2021 ---

800V x 0.1A x 100nS x 50kHz = 0.4 watt, so if the device is heatsunk - it may well handle this for quite a while, 8 uJ per hit ....

 

[treez]

Thanks, yes the peaks are about 100ns (the bit above 650V is about 100ns)......there is no clamp. (other than the fet avalanching (sorry, is that what you meant anyway?).
May i ask where did you get the "0.1A"?

 

[Easy peasy]

Guesstimate - it's not going to be the peak turn off current as a lot of energy will go into the capacitance - it is the width ( in time ) of the actual clampy bit that determines the avalanche power in the mosfet - if the overshoot was about to turn over near 800V ( i.e. at very light loads - very short on time of mosfet ) then the current in the clamp ( mosfet ) may be very low - worst case is of course full power ( max on time ).

One can judge the impact on the fet by measuring tab temp with and without a zener clamp - say 120V and good diode ... 600V <35nS Trr.

you can glue a fine thermo couple to the zener body too - just to see how hot it gets ( superglue )

 

[treez]

Thanks, are clamp-less flybacks quite common?
AN-4146 shows only offline flybacks without primary clamps...presumably the 650V internal mosfet gets overvoltaged , but it doesnt matter because the fet just avalanches and everythings fine?
As you say, we can look at the heat of the FET, with and without a clamp, but without a clamp the FSCQXX65 controller isnt getting particularly hot.
The FSCQXX65, with no clamp, is significantly cooler than the output diode which is on the same size heatsink.
It does have a 1nF , 1kV ceramic cap from drain to ground though.

AN4146

FAIRCHILD AN4146

dtsheet.com

FAIRCHILD AN4146

dtsheet.com

 

[Easy peasy]

You can design a QR flyback to be clamp-less - usually a bigger cap across the FET ( or Tx, or both ) - this guarantees a ring down to gnd - which gives losses less turn on - fast turn off gate drive gives very low loss turn off ( as the Vds rises more slowly ) - the slower rate of rise of volts across the mosfet ( and Tx ) helps the o/p diode turn on in the face of some leakage inductance - the turn off ringing overshoot can be quite high though - so 800 - 1kV fets often used - but the RFI is low and the eff is high - and less snubbing needed on the o/p diode - so win, win, win

The fet does take a bit of a hit at 1st turn on though - but usually larger fets are used anyway in flybacks - so not too much of an issue ....