By Steve Roberts. Recom Technical Support

DC/DC converters are not reverse polarity protected. Connecting them the wrong way round will destroy them almost instantly. So how can DC/DC converters be protected against reverse polarity connection?
There are three standard methods of reverse polarity protection: blocking diode, shorting diode and blocking FET.

	The blocking diode is the simplest protection circuit. If the supply is reversed, only -0.7V is seen by the converter and this will not cause any damage.
	The shorting diode must be used with an input fuse or a current-limited supply. If the supply is reversed the diode conducts, so short circuiting the input and blowing the fuse.
	The blocking FET is the most expensive solution, but offers a number of added advantages over the previous two solutions.

The advantage of the blocking diode method is that it is very simple, low cost and reliable solution. However, the forward voltage drop of the diode will alter the input voltage range of the converter.

A standard nominal 12V input converter will accept an input voltage from 9V to 18V (2:1 input voltage range). This range is designed to match the minimum and maximum terminal voltages of a 12V lead acid battery. With the blocking diode used as reverse polarity protection, the effective input range of the converter is shifted upwards to 9.7 to 18.7V. The 700mV shift may not seem very much, but the difference between 12V->9V and 12V->9.7V represents a 25% reduction in the amount of power that can be extracted from the battery.

On the other hand, this technique of adding diodes can be used to manipulate the input voltage range if so desired. For example, race cars often use a 16V battery, thus delivering a supply voltage range of 11V to 20V. Adding three diodes in series will bring this supply within the input range of a standard 12V converter.

It might be tempting to use second method, the shorting diode, as an alternative to the blocking diode solution. This method is also simple and low cost and has the advantage that the input fuse also protects the whole supply system during short circuit or fault conditions. Indeed, this solution is often recommended if the maximum possible energy needs to be extracted from a battery as it avoids the diode volt drop on the input. However, it is often overlooked that the fuse has an internal resistance that will also cause a significant volt drop. With medium-to-high power converters, this volt drop can even exceed that of a diode. Therefore, this method only offers real advantages for low power converters in the 1W to 5W range.

In addition, all fuses age with time and can also suffer mechanical failure due to vibration and mechanical shock. If the fuse blows, the circuit fails safe, but it is also inoperative until the fuse has been replaced. This may require a service visit from a technician before the application can be put back into operation.

Method 3: the blocking FET is the most expensive solution, but it is still inexpensive in comparison to the cost of the converter. The FET must be a P-channel MOSFET with an internal body diode otherwise this solution will not work. The maximum gate-source voltage VGSS should exceed the maximum supply voltage or reversed supply voltage. The FET should also have an extremely low RDS(ON) resistance - around 0.06 Ohms is an acceptable compromise between component cost and effectiveness. With the supply correctly connected, the FET is biased full on and even with an input current of over an amp it will exhibit a volt drop of only a few tens of millivolts.

With battery powered applications, the conversion efficiency of the power supply is an important consideration. The following table shows measured results using a Recom 12W RP12-1212SA converter which offers a stable 12V 1A isolated output with a standard input voltage range of 9V to 18V. The converter also features under-voltage lockout which shuts down the converter at an input voltage of 8.5V to protect the device from excessive input currents.

Looking at the measured results, several points are of note. Firstly, adding any form of reverse polarity protection will decrease the overall conversion efficiency. Although the 1N5400 series is rated at 3A, the forward voltage drop increases to over 1V when 50% loaded, thus the total voltage drop across the diode is a combination of the 0.7V forward voltage drop plus the ohmic volt drop. Finally, a 3A fuse drops 0.6V when passing 50% rated current. Replacing the 3A fuse with a 10A rated fuse made no difference to this volt drop.

In conclusion, the use of diodes for reverse polarity protection only makes sense if the converter is low power or if the conversion efficiency is not an important design factor. Otherwise, a P-channel MOSFET is always to be recommended.