Buck converter protection and enable/Disable circuit
From my earlier post Controllable Enable/Disable Buck converter circuit, I have updated the design and built it on a breadboard. It works as intended.
Circuit Operation -
- Default: When the battery is plugged in, the buck is OFF.
- Turn ON: Pressing SW1 pulls the SW1 line low, which turns Q3 and drives EN high -> the buck turns ON. The Q1 will keep the EN high when user releases the Switch and no need to wait for the controller to turn ON and take over control.
- Turn OFF (firmware): The microcontroller uses a NOT + AND gate combination to drive Q1 for shutdown. When asserted, Q1 pulls EN low and disables the buck. The NOT/AND gating is intended to make accidental shutdown during MCU resets unlikely (requires a specific two-input state, not just a single GPIO glitch).
Updated Schematic -
Design notes -
- Battery: Li-ion, 48 V nominal, 60 V max, board load ≤ 1 A
- R24 = 1MΩ and R25 = 1MΩ
- Measured Standby current(buck OFF, Q1 ON)
IOFF = (VIN/R25) + (VIN - 0.7)/R24. At 48 V (R24 = R25 = 1 MΩ) -> ~95 µA. At 58 V -> ~115 µA.
(Plus a few µA for EN pin leakage)
Questions
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Do I need to worry about leakage/noise at these resistor values (MΩ range) due to high impedance of SW1 line, and any suggestion to improve robustness?
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Is adding C54 at the EN node the right way to get ~500 ms startup delay when pressing SW1? If not, what RC placement/topology would you suggest to reach 500 ms without increasing standby draw?
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Should the circuit reference VIN_SAFE (post fuse/reverse polarity) or BAT_IN directly?
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Is a 63 VDC 3 A slow-blow fuse reasonable for a 15 Ah, 48 V (60 V max) battery pack, with board current ~1 A max? Is that rating and placement appropriate for protecting shorts on the 12 V/5 V rails (downstream of the buck), or should I consider a different rating/type or location?
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Are electrolytic bulk capacitors (e.g. 100 µF input, 10 µF output) recommended for inrush/spike protection on the LM5164 buck?
1 answer
Some general remarks:
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The reverse polarity protection won't work since D7 SMDJ58A is a unidirectional TVS. Upon reverse polarity, this TVS will start to conduct before the MOSFET switches off. You should use a bidirectional TVS for this to work as intended and prevent a current rush.
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Any noise not caught by D7 will indeed be present both to the regulator and through the 1M pull resistors, though I don't see any major problems there. I suppose you could add some additional protection diodes on SW1 for robustness - plain signal diodes, one from SW1 signal to its positive reference and another from ground to SW1.
Pull resistors in the megaohm range is questionable practice in general since they might lose to the energy in whatever EMI that is out there.
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A large input bulk cap somewhere to ensure supply stability is pretty much mandatory. It should sit before the regulator and you need to keep its inrush current in mind when picking the fuse. Somewhere around 470uF is a common value, depending on how much you trust the raw supply source. In this case it would depend on if there are other stuff supplied directly from the battery or not, like a DC motor which might cause surges when activated etc.
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A fuse is not suitable for actually protecting low voltage electronics - upon reaching 3A, the circuit is already fried since long. The fuse is just there to prevent catastrophic thermal runaway at that point.
Some manner of smarter overcurrent protection is what would actually save the parts, like a "smart" regulator or "smart" MOSFET with such features built-in.
Your current regulator is rated at 1A and got thermal shutdown - what that means in practice remains to be seen, maybe the regulator will survive until the fuse blows, maybe not. I suppose you can try this out with a lab supply, by adding a big load on the output to see which part that gives in first.
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