Issues with buck converter circuit - Page 1
Forum section: electronics / beginners

author: tyguy2
author_rank: Unknown
timestamp: August 11, 2021, 05:14:46 pm
content: Hello all!I've been designing a project recently that needs to run a heater, a lot of LED's, and some fans via a Raspberry Pi. I'm using a 24V power supply to run the LED's, so I decided to use 2 buck converters running off the 24V supply to create the voltages required for the heater and fans. I also added an overcurrent monitor to the heater's power supply just in case its fan dies and the current draw of the heater begins rise to dangerous levels. Each device is connected to an N Channel MOSFET, which allows the RPI to turn them on and off. I've attached the schematics below.I just built the circuit, and I have run into an issue I can't seem to solve. The power supplies turn on to their correct voltages (I measure 12.5V and 9.6V, but that's within plus minus 5%), and the fans run fine at 12V. This issue is when I try to turn on the heater, which should draw about 4.5A. When the MOSFET is turned on, both buck converters shut down. The 12V supply sits at about 3V after this, and the 9.2V supply sits at close to 0V.I thought the over current monitor was causing this, but its ALERT pin never goes to 0V (indicating an over current fault). I also know my power supply can supply 24V at up to 10A, so I know it's not a power supply issue.Any ideas as to what I've done wrong?Just a quick update:I replaced the 10k resistors on the PGOOD pins with 100k resistors, seems like I misread the datasheet. This change has fixed the issue with both buck converters shutting off when the heater is turned on, and the 9.2V buck converter now recovers to 9.2V when the heater load is removed, but the buck converter still drops to 0V if the heater is turned on and drawing current.

author: ace1903
author_rank: Regular Contributor
timestamp: August 12, 2021, 02:11:22 pm
content: From schematic is not clear if buck converters are properly powered, VCC pin is not connected to any net.Check that, maybe it works partially because it draws its power from other pin.

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: August 12, 2021, 02:46:57 pm
content: According to your schematics, it seems that you’re using a separate ic to detect over currents. Have you checked if the ics that you’re using for implementing the buck converters have internal protections? That could be caused by an internal short circuit or over current protection

author: tyguy2
author_rank: Unknown
timestamp: August 12, 2021, 07:56:24 pm
content: Quote from: ace1903 on August 12, 2021, 02:11:22 pmThe 24V net goes to a 24V 10A power supply, it definitely has powerQuote from: Just_another_Dave on August 12, 2021, 02:46:57 pmThe internal protections of these IC's don't kick in until ~15A of current draw, much lower than the 7.5A cut off I set with the separate IC. I disabled that chip just to be sure, though, and no change to the device's operation

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: August 13, 2021, 07:46:02 am
content: Quote from: tyguy2 on August 12, 2021, 07:56:24 pmIf it continues failing with the external protection circuit disabled, the problem needs to be caused by the converter IC. The most probable cause is that one of the internal protections is being activated. However, as it can’t be the over current one, maybe it is some type of short circuit or under voltage protection, but it could even be the temperature one.Another possibility could be that the control loop of one of them could be becoming unstable at that operating point. That could happen if you have used the equations provided by the manufacturer in the datasheet, but, if you simulated the circuit or measured it’s stability with a VNA, it won’t probably be the causeTaking that into account, I would measure the input and output voltage of the ICs and their temperature to check if those protections are the cause. Then, I would connect the load that is causing the problem before starting them, to avoid causing a step in the demanded current as that might be triggering the short circuit protection (the demanded current will increase slowly due to soft start). Finally, if none of those protections are the cause of the problem, I’ll check the circuit stability if you haven’t