Electronic – Reverse polarity protection with MOSFET: low voltage drop, 1-50V supply

polaritypower supplyprotection

I have a circuit that tests DC PSUs. They are typically unregulated and unsmoothed, up to 50V. It performs tests to establish I/V curves, down to about 1V, and up to about 3A. I want a sub-circuit that protects the testing circuit from accidental reverse-polarity connection of the PSU under test, without dropping too much voltage. (i.e., a rectifier is definitely not ideal).

The testing circuit has +5V, -12V and +12V supplies which can be used for the protection sub-circuit.

What circuit would achieve this? Ideally I would use an N-channel MOSFET given its particularly low On-Rds. This question comes close to the answer, but it will not work for a PSU voltage of less than about 5-10V, or more than about 20V. (Although a potential divider is suggested for use for PSUs above 20V). I have tried scribbling some circuits but am not getting anywhere. Is what I'm asking even possible?

Best Answer

Consider using an LM311 comparator powered from the available +/- supplies and driving an SSR.

Divide the input voltage down by (say) 11:1 as shown. That gives a 45mV to 4.5V signal at the inverting input of U1 for a 0.5 to 50V input. The non-inverting terminal is held at about 23mV, so input voltages greater than +360mV (allowing for 10mV \$V_{OS (MAX)}\$) will turn the SSR on. Zero or negative input voltages leave the SSR off.

The circuit shown presents a small resistive (11K) load across the input. If that's objectionable, an additional high-impedance buffer could be added ahead of the comparator.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

A suitable SSR might be the 60V-rated CPC1907B, which will look like a ~0.06 ohm resistor when it is on and is okay for 3A (6A rating) (a bit more resistance worst-case as it warms up).

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The same principle could be used with the comparator driving a heftier SSR, or a mechanical relay, which would allow even lower series resistance. It's also permissible to parallel a few CPC1907Bs with the input LEDs either driven from individual 1K resistors or connected in series with single a lower value series resistor.

Related Solutions

Electronic – Reverse polarity protection

This is a very handy reverse polarity protection scheme.

A P channel MOSFET turns on when \$V_{gs}\$ is negative, probably starting around -3V to -5V. When power is switched on, \$V_{gs}=0V\$. This is where the parasitic diode drawn across the MOSFET in the datasheet comes into play (drawn for explanation purposes only, do not put a diode between the drain and source). It allows current to flow, drain to source, and drops about 1V.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Assuming a supply voltage of 10V:

$$V_{gs}=V_g-V_s=0V-9V=-9V$$

The MOSFET is driven into saturation. Since the MOSFET's on resistance, \$R_{ds(on)}\$, is around \$25m\Omega\$, the parasitc diode gets shorted.

If the polarity is reversed, the parasitic diode never conducts, thus never allowing the MOSFET to turn on.

You need to be mindful of the maximum \$V_{gs}\$ in the datasheet. The MOSFET may be able to switch 100V, but the maximum \$V_{gs}\$ may only be 20V. In that case, you must put some kind of voltage divider in place to protect the gate of the MOSFET:

schematic

^{simulate this circuit}

Electronic – Does an over voltage, over current and reverse polarity protection IC exist

Try this IC: http://www.linear.com/product/LT4356-1

LT4356-1 and -2 - Surge Stopper Features

Stops High Voltage Surges
Adjustable Output Clamp Voltage
Overcurrent Protection
Wide Operation Range: 4V to 80V
Reverse Input Protection to –60V
Low 7μA Shutdown Current, LT4356-1
Adjustable Fault Timer
Controls N-channel MOSFET
Shutdown Pin Withstands –60V to 100V
Fault Output Indication
Guaranteed Operation to 125°C
Auxiliary Amplifier for Level Detection Comparator or Linear Regulator Controller
Available in (4mm × 3mm) 12-Pin DFN, 10-Pin MSOP or 16-Pin SO Packages

You should note that the previously commended LT4361 IC is similar but does not support your input voltage range requirements; many ICs out there are intended to support protection of low voltage supply buses, less support the higher voltage ranges you seek. LTC4361-1/LTC4361-2 - Overvoltage/Overcurrent Protection Controller Features

2.5V to 5.5V Operation
Overvoltage Protection Up to 80V
No Input Capacitor or TVS Required for Most Applications
2% Accurate 5.8V Overvoltage Threshold
10% Accurate 50mV Overcurrent Circuit Breaker
<1μs Overvoltage Turn-Off, Gentle Shutdown
Controls N-Channel MOSFET
Adjustable Power-Up dV/dt Limits Inrush Current
Reverse Voltage Protection
Power Good Output
Low Current Shutdown
Latchoff (LTC4361-1) or Auto-Retry (LTC4361-2) After Overcurrent
Available in 8-Lead ThinSOT™ and 8-Lead (2mm × 2mm) DFN Packages

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Electronic – Reverse polarity protection

Electronic – Does an over voltage, over current and reverse polarity protection IC exist

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