Supply voltage dependent power source switching - Page 1
Forum section: electronics / projects

author: Krotow
author_rank: Regular Contributor
timestamp: January 15, 2025, 03:11:43 pm
content: What would be the best way to switch between direct power or power through step-down converter when supply voltage reach certain voltage?The problem: DC motor controller with HIP4081A H-bridge driver exists. The driver IC require power supply voltage between 9-15V. Controller power supply can be 12V, 24V or 36V. HIP4081A obviously works from 12V but last two voltages need some DC step-down converter to prevent frying the driver. Switching the converter on and off according to the supply voltage isn't a problem. Particularly TPS560430X step-down converter nearby has enable pin and with proper resistor divider (like 100 kOhm / 6.2 kOhm) to VIN turns on for 24/36V just fine. Remaining issue is separating power supply line from step-down converter output if input voltage is too high.The question - how to properly do that to avoid high voltage passing by for a brief moment to prevent driver IC damage. Maybe P-MOSFET with a gate connected to DC converter output and a diode between DC converter output and MOSFET drain to prevent high voltage feeding back into DC converter output? Or maybe there is something better for supply voltage switching?

author: ajb
author_rank: Super Contributor
timestamp: January 15, 2025, 04:15:30 pm
content: The simplest solution may be to use a buck-boost converter, and have it power the control circuitry at all times. Or if the control circuitry can safely run at, say, 11V, you might be able to just use a buck converter. Or buck the input down to 5V or 3.3V or whatever to power your main logic, and then boost back up to 12V for the bridge driver. Lots of options, depending on your overall power needs.Quote from: Krotow on January 15, 2025, 03:11:43 pmYou would need to have the bypass transistor off by default -- just connecting the gate to the output of the DCDC converter won't work as you say. You'll need to keep both the DCDC and bypass off until the supply voltage stabilizes and you can reliably measure it to determine whether to use the DCDC or not. A voltage supervisor would be ideal for handling the turn-on delay, but a comparator with an RC delay could work. The problem with using the UVLO built into the DCDC is that you don't have access to the *output* of the internal comparator, so it's hard to guarantee that the bypass transistor is on when the DCDC is off.

author: Krotow
author_rank: Regular Contributor
timestamp: January 15, 2025, 05:52:23 pm
content: Actually just recalled a meaning "100% duty cycle" in memory. And the fact that it was related to DC converters. Googled for 100% duty cycle DC converters and found some. Damn how EXPENSIVE they areHowever found two two exact matches for TPS560430X eq. they can work up to 36V 0.6A and has already OK price. MP2457GJ-Z and NCV890430MW50TXG. Seems these will be better choice than dancing around transistors.UPDATE: ...and even better match - L7986. Seems the right thing, because used in PLCs from beginning of small buck converter era with great success. Still in production and cheap.