What is Vampire Voltage? Better Reverse Voltage Protection

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Generally, “vampire power” conjures images of chargers left plugged into walls without anything to charge or appliances that merely idle instead of truly turning off. Both do waste energy in your home, but a much more subtle vampire lurks in the inner workings of these common devices and power supplies the world over. This vampire is ancient, yet continues to drain efficiency from modern power designs simply because it’s always been used, and often slips into a schematic as an afterthought.

Today, we conquer the ancient one.
Today, we reclaim our lost watts.
Today, we slay the 1N5404.

We all know this vampire. The bulky, thick leaded, tantalizingly cheap rectifying diode that we throw into designs as reverse power protection after the difficult work is done. We know it works, it clearly says it can take 3A and withstand upwards of 400V, and we want only the most trustworthy protection for our precious downstream power supply. Why not use what we’ve always used?

Because what we’ve always used is crippling our efficiency in a world where every milliwatt matters.

Most circuit protection works in a way that does not affect the circuit unless a stress condition is present. A zener or capacitor across power and ground does nothing to a DC voltage within the desired input range, but protects the circuit if a voltage transient or AC noise tries to enter the system. In normal operation, no power is lost. A rectifying diode is most commonly used to prevent power from flowing backwards out of the circuit into the power source, and is therefore inserted directly in the main current-carrying line. Any current that would try to pass backward into the supply is blocked, but even current flowing the correct direction (into the circuit) must pass through this diode. This is where the archaic vampire rears its ugly head by collecting a toll on every microamp that passes.

What is Forward Voltage?

The power lost to heat in a diode is equal to the current flowing through the diode times the voltage from the anode (positive end) to the cathode (negative end). This voltage is typically called the forward voltage, or Vf. Vf is usually given at a specific current, because even though diodes are nonlinear devices, they behave linearly up to a certain point and Vf will actually change slightly based on the amount of current flowing. The current requirements of your circuit are usually not arbitrary and cannot be adjusted, so we must minimize the forward voltage of the diode to minimize the power lost in protection circuitry.

Ground Zero

Let’s explore our options for slaying the efficiency vampire using a fictional circuit with nice round numbers. Say we wish to give our circuit 1A at 5VDC. The diode is the only protection we are using in the circuit, and the circuit itself has no relevant losses. If the diode were ideal, our load would only have to supply the 5W needed by the circuit, the diode would dissipate 0W, and efficiency would be 100%.

What are we really dealing with?  If we check the datasheet for Fairchild’s 1N540x family (from 1990) we see that at a current of 1A, the Vf is about 0.75V.

Power = Voltage * Current
Power = 0.75V * 1A
Power = 0.75W

Using this diode adds 750mW of heat to a 5W system, bringing the efficiency down to 87%. Are we really going to tolerate a 13% efficiency hit from a single component?

 

Using a Through-Hole Diode

Beyond being vicious efficiency vampires, the standard through-hole diodes now dwarf the majority of components on the typical PCB. Smaller, surface-mount diodes like the MBRD5H100T from ONSemi or the PDS5100 from Diodes INC. offer a performance improvement over the thru-hole versions and can be run through a reflow-solder oven rather than hand soldered. The MBRD5H100T has a lower Vf of 0.47V at 1A. It would still dissipate almost half a watt (470mW) but this improvement brings the system efficiency up to 91%. Typically, this is a wise switch based on board size and assembly savings alone - the extra bump in efficiency is just a bonus.

See related product

MBRD5H100T4G

onsemi Rectifiers View

 

See related product

PDS510013

Diodes Incorporated Rectifiers View

Reverse Protection Diode

Sometimes, it is okay to reinvent a classic. If you’re going to use a through-hole diode for reverse protection, do it right. The FSV2060L from Fairchild’s 2015 FSV family of ultra-low-Vf-rectifiers requires only 0.3V at 1A. This improved diode would dissipate 300mW in our fictional system and bring the efficiency up to 94%. Not bad!  However, this leaded part cannot be reflow soldered like a surface mount component, incurring the same additional placement and board costs as the original 1N5404.    

MOSFET Reverse Current 

94% is a perfectly respectable efficiency, and is about as good as we can get with a 1A current to be delivered through a diode. In many cases, this is as far as we will need to go.  However, there is another way – another way that blows 94% out of the water. Kids, don’t try this at home.

High current MOSFET technology has pushed the internal resistance of FETs down into the milliohms so they can act as nearly perfect switches. P-channel FETs have an internal blocking diode from drain to source to prevent current from flowing when the device is not biased. In order to bias a P channel device, the gate voltage must drop below the source voltage. We can exploit these last two facts by placing a P-channel FET in our circuit backwards with the gate tied to ground through a resistor.

1015 Slaying the Vampire IMAGE 1

If connected in the correct polarity, the gate of the FET will be tied to ground while the now-conducting body diode brings the source voltage up to that of the drain, biasing the FET.

1015 Slaying the Vampire IMAGE 2
If connected in the incorrect polarity, the source voltage will never go below the gate voltage, the device will never be biased, and the downstream circuitry is safe.

1015 Slaying the Vampire IMAGE 3

Once the device is biased, our 1A of current is inhibited only by the tiny internal resistance of the FET. In the case of International Rectifier’s IRLTS2242TRPBF, this is only 25mOhm. This method of protection only burns 25mW of power in our 5W system, giving us a system efficiency of over 99%.

Vampire: slain.


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