SMPS bench supply - keeps on popping output rectifiers

[oldway]

Rated for 20V at 5A. However, with original hardware, it can output up to 30A without completely breaking.

You are a dreamer 

You buy a low cost SM bench power supply and you think you can "upgrade" it and increase it's output current four of five time higher !

That's not possible....you should have to make a new power supply and not only substitute a rectifier and 2 Mosfets. 

[richnormand]

Okay so... I'm trying to understand. Is this right?

The newer MOSFET has a lower Rds-on, therefore more current flows through the transformer. Therefore there is a much higher current on the output rectifier - so much so that it fails.

I'd always assumed that the transformer winding would limit the primary side current.

I would try to go back with replacements that are all original, as initially designed, and get the PSU running well.
Only then would I try modifications, one by one, to understand the interactions.

Considering this is a bench PSU, getting another unit for the times when a higher current is needed (like fleaBay or such)with the proper rating might be easier and safer overall.

[Prehistoricman]

Look at Vds vs gate voltage and see how much each device is putting out. For IRFP460 it's way above it's rated 30A (close to 100A).

I scanned the datasheets and couldn't see a Vds vs gate voltage. Why would there be 100A of current?

Just know that your power circuit was designed around IRF840 so a higher powered mosfet needs
a different gate voltage(less) and  different component values because of device capacitance but I'm not qualified to comment.

I guess the question that sums up what I'm failing to understand is: why is one MOSFET's fully turned-on state different to another's?

I would try to go back with replacements that are all original, as initially designed, and get the PSU running well.
Only then would I try modifications, one by one, to understand the interactions.

Considering this is a bench PSU, getting another unit for the times when a higher current is needed (like fleaBay or such)with the proper rating might be easier and safer overall.

I will definitely use the original MOSFETs that came with the supply. It'd be nice to have a different output rectifier though and I still don't know why the first replacement failed.

[singapol]

Look at Vds vs gate voltage and see how much each device is putting out. For IRFP460 it's way above it's rated 30A (close to 100A).

I scanned the datasheets and couldn't see a Vds vs gate voltage. Why would there be 100A of current?

Just know that your power circuit was designed around IRF840 so a higher powered mosfet needs
a different gate voltage(less) and  different component values because of device capacitance but I'm not qualified to comment.

I guess the question that sums up what I'm failing to understand is: why is one MOSFET's fully turned-on state different to another's?




Considering this is a bench PSU, getting another unit for the times when a higher current is needed (like fleaBay or such)with the proper rating might be easier and safer overall.

I will definitely use the original MOSFETs that came with the supply. It'd be nice to have a different output rectifier though and I still don't know why the first replacement failed.

Device characteristics...
See pg.3 for IRF840 Fig.2  and for IRFP460 pg.3 Fig.1 @ 25 degree Celcius.
Example..trace gate voltage of 4.5V ( bottom line) at 30V Vds (3rd. line of horizontal) follow vertical line up to meet 4.5V curve then horizontal line to meet vertical drain current for respective device.
@25C

IRF840= about 14A  and IRFP460 = 40A (compare fig.3 of 460 for high 150C and 25C low temperatures- higher when hot same for 840) Everything scales up for higher gate voltage and higher Vds. Note :10 exponential 1 or 10 to the power 1 = 100.

IRFP460 has higher gate capacitance so switches slower with PWM (pulse width modulation) signal at the gate so more heating at higher current  for device until...poof!

Edit:

The first time I broke the supply, I replaced its original MBR2045CT (45V, 10A per leg) output rectifier with a TO-247 packaged rectifier from a PC supply. It didn't last 20 seconds before popping the fuse and the mains-side MOSFETs.

More recently, I installed new MOSFETs and a STPS61H100CW (100V, 30A per leg) output rectifier. This lasted about 10 minutes and broke in exactly the same manner. It was under about 100W of load, which is what the unit is rated to output.

Am I doing something wrong here? Do I need to purchase a specific kind of output rectifier for this job?

First you substituted higher current mosfet and used diode from PC so diode has aged..Like what Blue skull said all the ringing ( oscillation) heats up the devices. The number 1 enemy of semiconductor is heat!
which leads to melting of bonding wires and if sufficient surge current the silicon metalisation of the transistor literally melts or explodes.

[Prehistoricman]

You have replaced its output diode with another part that has 3 times junction capacitance, which leads to more ringing on primary side. When the ringing is out of the snubber's capability, the main FET has to take avalanche breakdown. If the device is not avalanche reliable rated, you have a problem.

Datasheets from Diodes.com say that the original output rectifier had 650 pF of capacitance, whereas the 3045PT has 500pF of capacitance. Also, the IRF840 datasheet says "100% avalanche tested".

I was still using the original MOSFETs at that point.

[anoxios]

Hello there,

I have a psu, bk precision 1550, i bought it broken from ebay, b1n60fh - dead, irf3205 - dead, s8550 - dead and two resistors, I replaced all dead parts, have voltage on the screen, can change the voltage, but no output and the on/off button won't respond when is pressed. My question, can somebody help me with some schematics, I saw this model have the same board with Tenma 72-8345, Manson NSP 3630, Voltcraft LSP 1363.

[perieanuo]

Okay so... I'm trying to understand. Is this right?

The newer MOSFET has a lower Rds-on, therefore more current flows through the transformer. Therefore there is a much higher current on the output rectifier - so much so that it fails.

I'd always assumed that the transformer winding would limit the primary side current.

nooo, lower rdson means lower switchig losses and colder mosfet. it'c commutation not linear, you must read smps theory