High Voltage to low voltage DC/DC converter

[CataM]

Hello everyone,

I need to design a DC to DC converter with the following specs:

Input voltage = 311 V (mains rectified)
Output Voltage ~ 62 V
Power = 1 kW
Switching frequency = to be decided

First of all, I can not use a transformer to step down the voltage beforehand because of size and money constraints.
Isolation for this stage is not necessary because the next stage of the application will have isolation.

The first converter that comes to mind is the Buck converter, however, I do not know why but seems like nobody uses a Buck converter for such a large voltage to drop to.
The only difficulty I see here is that I would need around 20% of duty cycle (assuming ideal Buck).
With Duty Cycle = 20 %, means that t_ON=0.2·T (0.2 times the period). Depending on the switching frequency I use, the "on time" could not be reached due to the influence of the turn on/turn off times of the transistors.

However, for switching freq = 20 kHz (seems reasonably well due to high power involvement and higher than audible range), t_ON=0.2*50 us= 10 us.

10 us of on time could not be influenced with nano second transistors.

My questions is: Why is not the Buck converter used for high voltage drop downs as the above one ? Am I overlooking something ?

Thank you for your time !

 

[schmitt trigger]

"My questions is: Why is not the Buck converter used for high voltage drop downs as the above one ? Am I overlooking something ?"

Yes. How to drive the high side switch correctly.
For this you would likely require a gate transformer. Driving a Mosfet with a gate transformer requires many design considerations, therefore people shy away from them.
There is an old Unitrode app note, now Texas Instruments, which provides detailed design information.

 

[sac1991]

Why can't we go with bootstrap capacitor for high side switching. With careful design it can operate very reliably at fixed frequency and a range of duty cycle.

But as of original question

1. A step down from 311V - 62V calls for isolation. Consider the failure of your buck converter power switch(MOSFET), whole input voltage will appear across low voltage bus(62V), destroying all the load as well as Point of Load converters.

2. As you have said 311V is main rectified. For 1kW design Power Factor correction is mandatory.

3. As per IEEE, special care should be taken for Offline DC DC converters, because surges as high as 3kV and 2kA can occur (Not exact values) depending upon your converter distance from Main line.

My suggestion is go with a PFC and a Isolated DC DC topology.

 

[CataM]

There is an old Unitrode app note, now Texas Instruments, which provides detailed design information.

Can you share it ? Thanks.