Download Reverse polarity and overvoltage protection

Reverse polarity and overvoltage protection
Reverse polarity
You can use a diode in series and zener diode (or better – a transient voltage suppressor) in parallel in
combination with a fuse. But you can do it better. First, the diode in series can be replaced by Pchannel MOSFET (Q1). Drain connected to input and source to output. Gate is connected to ground. The
transistor appears to be reversed and it is. But on purpose. Suppose the voltage is reversed. Then, gate
is on V+ so the transistor is closed. Its intrinsic diode is reverse biased so no current flows.
During normal operation, the current flows through its intrinsic diode but the gate, being grounded,
has also lower voltage than the source so the transistor conducts in active mode. It is fully open and its
resistance is the Rds(on) value. Which can be very low. In this way, the diode drop is eliminated and
energy loss on transistor is negligible.
If you need to protect against voltages that are larger than the gate-source breakdown voltage (Vgs),
then a voltage-limiting zener diode (D1) and a current-limiting resistor (R1) have to be added to the
gate.
Overvoltage
It is possible to automatically disconnect a circuit when the input voltage exceeds a predefined level.
This is different from parallel transient voltage suppressor and fuse. Blow fuses have the obvious
problem of being a one-time devices. PTC resettable fuses tend to be slow and they can conduct
significant amount of current even in tripped state (to stay tripped, the fuse needs to maintain itself
hot). You can use electronic fuses for overcurrent conditions though.
With just two P-MOSFETs and a few components, it is possible to automatically limit the input voltage.
The principle is this: A zener diode (D2) is chosen with value slightly lower than the maximum allowed
voltage on the output. Then, when the voltage is higher than the zener voltage, D2 starts conducting
and voltage drops across resistor R2. This lowers the voltage on the gate of Q2, which will open, raising
the voltage drop across R4. The gate of Q3 sees higher voltage and Q3 thus closes, disconnecting the
protected circuit from excess voltage. All without tripping any fuses or causing excessive current flow.
Neat! Because you can get P-MOSFET with very low Rds(on), the losses can be minimal.
Resistor R3 and two zeners, D3 and D4, are there for gate protection just like D1 and R1.