I designed a dc dc converter with the LT8390 in 12V out 47V 4.5 Amper, is part of a UPS,to switch on the main power supply and dc-dc converter I used an ideal diode LTC4355 and 2 mosfets IRF3710,
I have the following problem (occurs for load currents above 1.5 - 2 amperes)
is it a latch-up? How can I solve the problem?
Thanks in advance
Hi SPAXIO, I don't think it is latch-up. Otherwise, the LTC4355 will be destroyed with Amps of current through it. Please try or check the following to isolate the problem:
- MOSFETs are not installed backwards or any MOSFET or LTC4355 is not already damaged/shorted.
- Test the supplies independently with load current, especially your LT8390 circuit.
- OR the supplies with power Schottky diodes instead of LTC4355 (for a short period of time to limit dissipation).
If the input supply collapses, the LTC4355 cannot generate the gate drive needed to turn on the MOSFET channel and current will flow through the body diode, warming up and possibly damaging the MOSFET.
For further questions specific to your application, please submit a Technical Support Request: https://form.analog.com/form_pages/support/integrated/techsupport.aspx
Can you please provide waveforms for the Gate Pin voltages and Vout during this occurrence ?
as recommended by Psachdev (whom I thank a lot) I tried to isolate the various stages of the UPS,
after some tests I think the problems are not of the ideal LTC4355 diode but of the DC-DC converter LT8390.
I removed the 48 VDC Main fuse, and I notice this:
If there is a load with the circuit off at power-up the dc-dc fails to start correctly, vice versa if you connect the load after lt8390 has started there are no problems.
In particular, I note that in the presence of loads at power up the DC-DC converter reduce output voltage from 47 VDC set :
for load under 0.2 Ampere dc-dc converter star-up regularly
load = 0.3 A the output voltage is reduced at 40 V
load = 0.6 A the output voltage is reduced at 32.5 V
load = 2 A the output voltage is reduced at 13.2 V
load = 4 A the output voltage is reduced at 16-18 V (instable)
in this abnormal condition if the load is disconnected and reconnected DC-DC back to work.
At this point I think of connect the load with a P mosfet drive to pin LOADTG, and/or use pin PGOOD.
Any suggestions to avoid the external mosfet, and use the ideal diode LTC4355 already present on the circuit ?
I add that as the battery voltage drops, the LT8390 tends to deliver less current,
example at this time the battery is at 10.2 volts, if the load exceeds 0.5 amps the output voltage out DC-DC drops to 12-13 volts. I think that there is probably some problem on the PCB, which I hastily did
I attach the schematic
The problem that you are experiencing might be due to output inrush conditions.
Please used article linked below as reference:
Please check if the PSU is reaching max duty when output is dropping at lower input voltages. If it is indeed reaching max duty, the PSU might have unaccounted losses during design stage. I would suggest checking the component losses and as you said PCB loss.