Buck Converter Diode Current

[MOSFET KILLER]

Hello Forum,
I am building a very large buck converter to control power to my induction heater project. I am going to use three buck converters each 120 degrees out of phase of each other to handle the 10kW. Can anyone tell me what current would be expected through the diode if the output is 50A.

Alex Sears

 

[crutschow]

If you are referring to the switching diode that carries the inductor current when the series transistor switch is off, then it carries the same current as the output, or 50A in this case. The duty-cycle of this current depends upon the relative voltage difference between the input and output voltage.

 

[MOSFET KILLER]

Would a three phase buck converter share the total current across the three phases, even at low duty cycle or would all three phases need to be rated for full load current.

 

[Diver300]

Each diode would take 17 amps or so peak, but you would need to control the current in each of the phases so that they stayed the same. Otherwise component variations could make the phases take very different currents.

If you are handling 10 kW at 50 A, that is 200 V output. What is the input voltage?

 

[Claude Abraham]

The diode current in a buck converter is the (1 - duty factor) times the output current. For a 60% duty factor, Idiode is 20A when the output is 50A. But during a shorted output, the diode carries almost the full output current since the df is very small. The diode should be sized to withstand a short at maximum temp. Did this help?

 

[indulis]

At these power levels, a synchronous buck would probably be more appropriate.

 

[MrAl]

Hello,


To add a little to what Claude was saying...

Another point of interest is the peak diode current. Here are some estimates of the diode average current and the diode peak current:

Iavg=(1-DC)*Iout
Ipk=(2*Iout*L+Tp*Vout*(1-DC))/(2*L)

where
Iavg is the average diode current
DC is the duty cycle
Iout is the average output current to the load
Ipk is the peak diode current
L is the inductance of the inductor in Henries
Tp is the total switching period, Ton+Toff
Vout is the average output dc voltage

Note that if you had an error condition where the output resistance went very low, the regulator circuit would increase the duty cycle and this would increase the peak diode current to some fairly high level. The limit of the duty cycle is usually determined by the nature of the ic used as the controller. Most have some limit such as 0.95 and can not get to a full 1.00, which is nice for an overload but usually still isnt enough.
To guarantee proper operation for fault currents usually a second loop has to be used to sense current and cut back if needed, because the output voltage will try to go up to the set point voltage even with a short and this will cause a huge current in the switching transistor. Some controllers already have this built in in the form of peak inductor current sensing.

 

[meanu]

Hello Mosfet Killer

I would like to get in touch with you about your 10 kW Induction Furnace. I am collecting parts for the smae style of device myself. if you are not around anymore could some one else put me in touch with this person. Thankyou.

meanu